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Use of tusks in feeding by dugongid sireniansObservations and tests of hypotheses.

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THE ANATOMICAL RECORD 290:523–538 (2007)
Use of Tusks in Feeding by Dugongid
Sirenians: Observations and Tests
of Hypotheses
Laboratory of Evolutionary Biology, Department of Anatomy, Howard University,
Washington, DC
Anatomy, New York College of Osteopathic Medicine, Old Westbury, New York
Most living and fossil sea cows of the subfamily Dugonginae (Dugongidae, Sirenia, Mammalia) are characterized by large upper incisor tusks,
which are thought to play an important role (at least primitively) in feeding on seagrass rhizomes. Testing this hypothesis is difficult, because the
only extant tusked sirenian (Dugong dugon) is morphologically and perhaps behaviorally aberrant. The tests attempted here involve examination of stomach contents of wild Recent dugongs, experiments using plastic replicas of diverse tusks to harvest seagrasses, gross anatomical observations on tusks and skulls, measurements of tusk tip geometry, and
observations of microwear on tusks. We conclude that (a) male D. dugon
(with erupted tusks) do not consume more rhizomes than females
(without erupted tusks); (b) the tusks do not play a significant role in
feeding in the modern dugong; (c) larger, more bladelike tusks are more
effective at harvesting rhizomes, but the effect of shape was not experimentally separated from the effect of exposed tusk length; (d) some fossil
dugongines show apparent cranial adaptations for downward and backward cutting motions of their large, bladelike tusks; (e) geometry of wear
surfaces is consistent with use of at least the more bladelike tusks as cutting instruments; (f) preliminary observations of microwear in D. dugon
do not indicate more than occasional use of the tusks in purposeful harvesting of rhizomes, and then only opportunistically by large adult males.
The hypothesis of such tusk use by extinct dugongines (in contrast to the
living species) is so far corroborated, but available data and tests do
not suffice to establish this conclusively. Anat Rec, 290:523–538, 2007.
Ó 2007 Wiley-Liss, Inc.
Key words: Sirenia, Dugongidae; Dugong dugon; tusk; feeding;
seagrass rhizomes; stomach contents; dental
A quantitatively significant but poorly understood aspect of mammalian history is the long relationship
between herbivorous marine mammals and the plants
they have eaten. This history extends over the 50 million years of the fossil record of these mammals (mainly
those in the Order Sirenia: dugongs and manatees), and
chiefly involves the tropical-to-temperate, cosmopolitan
marine angiosperms or seagrasses (Hydrocharitaceae
and Potamogetonaceae), which today constitute one of
the Earth’s most productive ecosystems, as has presumably been true throughout the Cenozoic (Ivany et al.,
1990). The greatly diminished diversity of modern sirenians, however, is a major obstacle to exploring how
feeding adaptations, niche partitioning, and other
*Correspondence to: Daryl P. Domning, Department of Anatomy, Howard University, Washington, DC 20059.
Received 2 March 2007; Accepted 6 March 2007
DOI 10.1002/ar.20540
Published online in Wiley InterScience (www.interscience.wiley.
aspects of the order’s ecology evolved (Domning, 2001;
see also review in Uhen, 2007, this issue).
The typical seagrass is a submerged marine plant that
comprises parts both above ground (shoots and leaves)
and below ground (roots and rhizomes). In most seagrasses, the rhizomes or underground stems compose at
least 50% of the plant’s biomass and are a rich source of
carbohydrates in the form of sugars and starch,
although the exact distributions of biomass and
nutrients within the plant seem to vary seasonally
(Lanyon et al., 1989). Therefore, they are a potentially
significant resource for marine herbivores able to extract
them. Several seagrass herbivores (dugongs, sea turtles,
fish, etc.) often occur sympatrically and have to compete
for plant resources in a community typically comprising
fewer than a dozen angiosperm species. Therefore, a species’ ability to extract seagrass rhizomes may be critical
to its long-term survival in the system and to the overall
pattern of niche partitioning among these competitors.
Here, we examine this ability in the extant dugong
(Dugong dugon) and its extinct relatives.
Although green turtles (Chelonia mydas) are major
seagrass consumers, they do not disturb the rhizomes, and
they also eat large amounts of algae, in contrast to manatees (Trichechus spp.) and dugongs (Lanyon et al.,
1989; André et al., 2005; cf. simulation study by Aragones and Marsh, 2000); so it is doubtful whether they
compete significantly with dugongs or manatees at the
present time. With the possible exception of some members of the extinct mammalian Order Desmostylia, which
were restricted to the North Pacific Ocean (Domning,
1978), no other marine animals are known to extract
significant amounts of seagrass rhizomes (McRoy and
Helfferich, 1980; Klumpp et al., 1989).
Sirenians, then, are the only known herbivores that
have had the capacity to exploit rhizome resources in
the tropical seagrass beds of most parts of the world at
any time during the Cenozoic Era. Moreover, sirenians
are almost exclusively consumers of angiosperms and
eat algae in significant amounts only when angiosperms
are scarce (e.g., after storms, Spain and Heinsohn, 1973;
or in the range of the extinct Steller’s sea cow [Hydrodamalis], Domning, 1978); so far as is known, this has
always been true. Sirenians also eat animals on occasion
(e.g., Powell, 1978; Preen, 1995), but never, apparently,
as a major part of the diet. We, therefore, seem justified
in considering tropical marine sirenians as obligate seagrass consumers for all practical purposes, and as the
only potential consumers of seagrass rhizomes.
These conclusions apply in particular to the Dugongidae, which during the Holocene, and apparently throughout their history, have been exclusively marine and (except for hydrodamalines) tropical. Increasing evidence is
emerging for the sympatric occurrence of three or more
dugongid species at various times and places in the geological record (Domning, 1989a, 2001; Toledo and Domning, 1991; Aranda-Manteca et al., 1994). The presence
of more than one sirenian species in a tropical seagrass
community will likely prove to have been the rule
and not the exception throughout the Tertiary history of
This finding becomes problematical when viewed in
light of the low taxonomic diversity of present-day seagrasses (only 12 genera and 50 or so species worldwide;
Phillips and Meñez, 1988). Moreover, the fossil record of
seagrasses, scant though it is, gives no hint that this diversity was ever significantly greater: all known Tertiary
seagrasses, even as far back as the Eocene, are referred
to living genera, and many to living species (Larkum
and den Hartog, 1989; Ivany et al., 1990). There is evidence (Lumbert et al., 1984; Ivany et al., 1990) that the
Caribbean seagrass flora was more diverse in the Eocene
than it is today, but not more diverse than the Indopacific seagrass flora of today.
Thus, we are faced with the general problem of
explaining how it was possible for several sirenian species to coexist on such a narrow resource base. Several
potential kinds of answers present themselves: (a) seagrass communities were more diverse in the past than
their fossil record now reveals; (b) marine sirenians had
more varied diets (including more algae, freshwater
plants, and/or animals); or (c) it is in fact possible for
several sirenian species to stably partition a seagrass
assemblage of the level of diversity observed today.
The main methodological problem with any such venture into paleoecology is that of maintaining some link
with the realm of testable hypotheses. The more factors
allowed to vary from the present in a reconstruction of
the past, the fewer and more tenuous such links become.
It has, indeed, been suggested (Sickenberg, 1934) that
the extinct sirenian Miosiren fed on molluscs, but this
possible exception by itself does not threaten to change
the overall picture outlined above. This study will,
therefore, proceed on firmer ground by first exploring
(and, if need be, eliminating) the alternative that postulates the least divergence from modern conditions. An
initial working assumption is that tropical seagrass diversity (at least at the generic level) has not fluctuated
dramatically during the Cenozoic and that all the tropical dugongids of the past were like the modern Dugong
in their reliance on a seagrass diet.
Recent studies on the digestive system of the living
D. dugon, together with much else that we know about
their anatomy and physiology, have shown that modern
Sirenia are extremely efficient in minimizing energy use
(Aketa and Kawamura, 2001; Aketa et al., 2001, 2003).
Despite what is known about their internal digestive efficiency, however, little is known about their means and
efficiency of food acquisition.
Large tusks formed by the first upper incisor teeth are
a prominent feature of D. dugon and many of its extinct
relatives in the Family Dugongidae. These tusks vary
considerably within and among taxa in size, shape, and
degree of eruption and wear. In cases where several genera of dugongids occur sympatrically in the fossil record,
differences in tusk morphology are among their most
obvious points of contrast in presumably adaptive features and may offer some of the best clues to strategies
of niche partitioning among these animals (Domning,
1989a, 2001). However, little direct evidence of the
actual function of sirenian tusks has been reported. Display as a major function seems inconsistent with the
fact that most of the tusk is always embedded in (and
supported by) the premaxilla, with only a small portion
exposed, and even that portion mostly concealed by the
upper lip—a design better adapted for forceful use than
for show. The tusks’ past and/or present use in feeding,
specifically for the extraction of seagrass rhizomes, has
been conjectured (Domning, 1989a–c, 2001), but
attempts to directly observe tusk use for feeding by wild
dugongs (e.g., Vosseler, 1924–25) have not succeeded,
and this conjecture is still in need of empirical support.
This study seeks to provide this support by bringing a
diversity of indirect techniques to bear on the question.
Only a single species of tusked sirenian (Dugong
dugon) survives today to give us possible insight into
this problem. Unfortunately, the modern dugong is atypical of its family in that its tusks are sexually dimorphic:
although large in both sexes, they erupt and become
worn only in adult males and in a few old, possibly postreproductive females (Marsh, 1980; Domning, 1995).
This has led some writers (e.g., Anderson, 2002) to conclude that they serve only a social function. They are
used by the males when attempting to mate, as weapons
against rival males and possibly as instruments to roll
females into position for mating; the pairs of parallel
scars that are abundant on most dugongs are attributed
to the tusks of aggressive males (Anderson and Birtles,
1978; Anderson, 1979; Marsh et al., 1984; Preen, 1989).
This, however, leaves two questions unanswered: do
male dugongs also use their tusks in feeding, and did
the tusks of their extinct, nondimorphic relatives function in feeding or only in social interactions? The following investigations were performed to answer these questions: (1) examination of dugong stomach contents to see
whether adult males consume more or larger seagrass
rhizomes than adult females; (2) experiments to determine the relative efficiency of different shapes and sizes
of dugongid tusks as tools for rhizome extraction; (3)
observations of the gross morphology of the tusks, rostral architecture, and other oral structures of Dugong
and related fossil forms; (4) measures of tip geometry of
tusks to determine their relative utility as tools; and (5)
observations of microwear on Recent and fossil dugongid
tusks. Tasks 1–3 were carried out by Domning, and
tasks 4 and 5 by Beatty.
The Recent dugongs studied were from northeastern
Australia and Papua New Guinea, where there exists a
diverse flora of seagrasses with rhizomes of several different sizes. Dugongs in this region reportedly feed
largely on species of Halophila and Halodule (Johnstone
and Hudson, 1981; Marsh et al., 1982), whose rhizomes
are <2 mm in diameter. Less frequent in the dugong
diet are species of Cymodocea, Zostera, and Syringodium
(rhizomes 1–3 mm); Thalassia hemprichii (2–5 mm); and
Enhalus acoroides (10–15 mm; den Hartog, 1970; Meñez
et al., 1983; Phillips and Meñez, 1988).
Dugongs use at least two different modes of feeding:
cropping of seagrass leaves (e.g., when feeding on
Amphibolis; Anderson, 1986), and rooting up the whole
plant, during which their characteristic feeding trails
are produced (e.g., Anderson and Birtles, 1978; De Iongh
et al., 1995). These trails are typically approximately
10–25 cm wide (roughly the width of an adult dugong’s
facial disk) and several meters long; 1–3 trails are produced on a single dive (Heinsohn et al., 1977; Anderson
and Birtles, 1978; De Iongh et al., 1995, 1997; Anderson,
1998). Such trails appear to be made most often by
dugongs eating the smaller, more delicate seagrasses
(Halophila, Halodule) growing on soft substrates, and
less frequently on larger seagrasses (Zostera, Syringodium). Dugongs feeding in this manner typically stir up
conspicuous clouds of silt (Anderson and Birtles, 1978)
and are probably less vigilant for predators; they harvest rhizomes in this manner when predation risks from
sharks are absent, and for short periods (11% of foraging effort) if risk of predation by sharks is minimal but
present (Wirsing, 2005). It is thought that a protruding,
soft-tissue knob on the upper jaw is used to plow
through the upper 4–6 cm of sediment, turning up the
rhizomes together with the shoots and leaves, which are
then tucked into the mouth by the upper lip and its enlarged prehensile bristles (Gudernatsch, 1908; Marshall
et al., 2003; H. Marsh and A. Preen, personal communication). This plowing through sediment can presumably
cause incidental wear on the tusks, whether these are
intentionally used for digging or not.
Arguably, a third mode of feeding occurs as a limitcase of trail-making. While foraging on leaves and rhizomes of small seagrasses like Halophila and Halodule,
which offer little resistance to the animal’s forward
movement and require no tusk use for their excavation,
dugongs are able to maintain headway, making feeding trails on the bottom. As the seagrass bed becomes
denser, however, the feeding trails (each made on a single breath-hold dive) become shorter (Anderson and Birtles, 1978); and when dugongs feed on plants or invertebrates that are very resistant to excavation, they must
remain in one spot during each dive and, hence, make a
pit rather than a linear trail (Anderson, 1998; Domning,
As the rhizomes of their food plants vary toward the
larger and more fibrous (as well as more deeply growing), dugongs apparently tend to feed more exclusively
on the leaves. Harder substrates, and greater density of
rhizome mats, would also discourage extraction of rhizomes. Dugongs extracted 75% of rhizome-root biomass
from a Halodule-dominated meadow in one study (De
Iongh et al., 1995), and are probably capable of extracting even more, whereas Florida manatees (Trichechus
manatus latirostris) feeding on Syringodium beds can
remove up to 96% of the total biomass despite their complete lack of tusks (Packard, 1984). Therefore, it seemed
necessary, in examining the stomach contents (the first
study reported here), to concentrate on dugongs eating
still larger seagrasses (Thalassia) to determine the limits of Dugong’s ability to excavate rhizomes.
Materials and Methods
The hypothesis that male dugongs use their tusks in
extracting rhizomes from the bottom was tested by examining mouth or stomach samples from dugongs that
had been feeding on Thalassia. Johnstone and Hudson
(1981) reported analyses of such mouth samples from
Daru, southern Papua New Guinea. These samples are
now stored at James Cook University of North Queensland, Townsville, Australia (JCU). Thalassia was the
most abundant seagrass in the collection area and was
also the most abundant species in 52 of the 102 mouth
samples studied. Unfortunately, records no longer exist
to show which 52 animals these were. Neither were the
great majority of the mouth samples themselves preserved, nor were the skulls of most of the dugongs collected. However, stomach contents (preserved in 10%
formalin–seawater) were kept from most of these specimens. Therefore, all the adult (hence, presumably
tusked) males (n ¼ 35) from the list of 102 specimens
studied by Johnstone and Hudson (1981) were selected
for study. Of these, 27 are represented by stomach contents. Adults (of either sex) were defined as animals 240
cm or more in body length, because sexual maturity
appears to be reached by both sexes at variable body
lengths between 220 and 250 cm (Marsh et al., 1984). To
increase sample size, stomach samples were also examined from an additional 11 adult males in the same collection that had not been represented by mouth samples
in Johnstone and Hudson’s study (namely, those that
had been collected in 1978 and the first half of 1979).
This period was selected because none of the eight samples examined that had been collected later was found to
contain substantial amounts of Thalassia as defined
below. Therefore, the total sample of 38 was selected to
maximize the likelihood that the stomach samples examined would contain substantial Thalassia (Table 1).
Five smears (slides) of each sample were made and
scanned with a dissecting microscope at 310. Each
stomach sample was assigned to one of four categories
according to the approximate proportion of Thalassia
leaf fragments present: none, small amounts (<5%),
moderate amounts (5–10%), or substantial amounts
(>10%). These visual estimates were quantified and calibrated by measurements on 19 of the samples, using a
Weibel graticule and counting the leaf material seen at
50 points on each of the 5 slides. Thalassia rhizomes
were searched for on the slides and in the remainders of
the samples. Because this analysis produced little or no
evidence of feeding on Thalassia rhizomes by males (see
below), examination of samples from females was
deemed superfluous for testing the hypothesis.
To detect possible sexual differences in consumption of
the next smaller category of rhizomes, stomach samples
from Queensland, Australia, were examined. These were
collected from animals either drowned in shark nets or
killed by native hunters between 1968 and 1978, and
are also stored at JCU. From 95 of these samples that
were analyzed in detail by Marsh et al. (1982), the samples of adult males and females whose stomachs contained 10% or more of Cymodocea, Zostera, or Thalassia
were selected. Unfortunately, these amounted to only 8
individuals: 5 males (at least 4 and perhaps all with
erupted tusks) and 3 females (all with unerupted tusks).
Five smears from each were examined at 310; using the
Weibel graticule, plant material at 50 points per slide
was classified as either rhizome of <2 mm diameter, 2–3
mm diameter, >3 mm diameter, or nonrhizome.
The results of the analysis of stomach samples from
Daru, southern Papua New Guinea, are shown in Table
1. No dugongs showed evidence of having consumed significant amounts of Thalassia rhizomes, not even those
that had been eating substantial quantities of Thalassia
leaves. Of the 38 stomachs, 8 contained rhizome fragments that appeared to represent Thalassia, and such
fragments occurred most often in the stomachs containing the most Thalassia leaves, as would be expected if
the rhizome material was correctly identified. However,
these fragments were few in number, small in diameter
for Thalassia (3 mm or even less), and constituted a negligible fraction of the stomach contents.
The eight stomach samples from the Queensland,
Australia, collection contained various amounts and mix-
TABLE 1. Analysis of Thalassia content of stomach
samples of adult male Dugong dugon from Daru,
Papua New Guinea. Animals whose tusks are known
to have been erupted are indicated by T; tusks
are assumed to have been erupted in all cases.
Proportion of Thalassia leaves as a percentage of
plant material in the stomach is indicated where
this was measured; in other cases assignment
among the four general categories was by
visual estimate. Stomachs in which one or
more fragments of Thalassia rhizome were
identified are indicated by R
Specimen No.
Length (m)
% Leaves
tures of Cymodocea, Zostera, and/or Thalassia in addition
to smaller amounts of other seagrasses and algae. The
proportion of plant material in these samples previously
identified by the JCU workers as leaves of one or more of
these three relatively large seagrasses averaged 45.8% 6
30.88 (SD) in the five male samples and 44.3% 6 1.53 in
the three females. They also classified as rhizomes (of all
species) 40.2% 6 26.30 of the male stomach contents and
49.0% 6 3.61 of the female stomach contents. Domning’s
own classification of the rhizomes in these samples into
three categories gave the following results: rhizomes <2
mm in diameter: males, 15.7% 6 8.70, females, 19.7% 6
7.43; rhizomes 2–3 mm in diameter: males, 3.3% 6 3.69,
females, 5.6% 6 7.99; rhizomes >3 mm in diameter:
males, 0.3% 6 0.72, females, 0.5% 6 0.92.
Although Thalassia has more distinctive rhizomes
than most seagrasses, identification of chewed-up rhizomes in general is problematical, and it is conceivable
that unrecognizably macerated Thalassia material was
overlooked in the Daru, southern Papua New Guinea,
samples; however, if this was present to any important
extent, then recognizable fragments should have been
more in evidence. (Cymodocea rhizomes in the stomach
samples were readily recognized.) The fragments present
were consistent with what might have been ingested
incidentally to feeding on Thalassia leaves plus leaves
and rhizomes of other species in a mixed-species bed,
especially if current scour had exposed occasional
patches of Thalassia rhizomes on the surface. It was
concluded that the male dugongs collected at Daru were
not consuming significant amounts of Thalassia rhizomes, and, therefore, were not using their tusks for
this purpose.
Next considered was the possibility that Thalassia rhizomes might be beyond the ability of Dugong to extract,
but that feeding on seagrasses with somewhat smaller
rhizomes might reveal a difference between the sexes.
The collection of Queensland, Australia, dugongs provided a set of stomach samples whose species content
was approximately known. This allowed selection of
those with the desired species content (Cymodocea, Zostera, and/or Thalassia), but this yielded only a very small
sample of individuals (n ¼ 8), as the Queensland dugongs
had been feeding mostly on Halophila and Halodule.
The total proportion of rhizomes in the Queensland
stomach samples was consistently underestimated, in
comparison with the estimates of earlier workers, probably because of some difference in our working definitions of rhizome fragments. However, it is clear that the
males in no way surpassed the females in proportion or
size of rhizomes consumed; rather the contrary.
Therefore, neither the Australian nor the Papua New
Guinean samples of dugongs showed evidence of greater
consumption of large rhizomes by the males. Although
these samples are not as large or as well suited to
answering the question as could be desired, in the absence of other evidence, it appears that the tusks of the
males do not play a significant role in feeding. This, of
course, was to be expected from the fact that the females
lack erupted tusks (unless diet is sexually bimodal as
well). However, the failure of present-day dugongs to
use their tusks for feeding does not prove that they or
their relatives did not do so in former times. Hence, the
seagrass-harvesting experiments were designed to answer the question: If the tusks were to be used for rhizome removal, how well would they work?
Materials and Methods
To evaluate the relative effectiveness of different types
of dugongid tusks as tools for excavating seagrass rhi-
zomes, plastic casts of fossil tusks were used by hand as
digging tools. Casts (Fig. 1) were made of the following
specimens: one halitheriine, Metaxytherium floridanum
(USNM 356686; middle Miocene, Florida from U.S.
National Museum of Natural History [USNM], Washington, DC), and three dugongines, Crenatosiren olseni (holotype, UF/FGS V6094; latest Oligocene, Florida from former Florida Geological Survey [FGS] collection, now
housed in the Florida Museum of Natural History, University of Florida [UF], Gainesville), Dioplotherium
manigaulti (ChM PV2633; late Oligocene or early Miocene, South Carolina, from Charleston Museum, South
Carolina), and Corystosiren varguezi (USNM 425695;
early Pliocene?, Florida; broken tip restored to match
other specimens referred to Corystosiren and Rytiodus).
These taxa were selected to span the spectrum of size
and shape variation of dugongid tusks (cf. Domning,
2001). The original specimens are described in detail by
Domning (1988, 1997, 1989b, 1990a), respectively. On
the models, no attempt was made to reproduce the difference in hardness between enamel and dentine; only
the effect on efficiency of overall size and shape was
tested. An actual tusk of an Australian Dugong dugon
was also used for comparison in each experiment.
Although only one example of each of the fossil tusks
was available, resulting in pseudoreplication at the level
of sirenian species, this is unlikely to alter the results as
the interspecific variation across the range of tusk morphologies used was vastly greater than observed intraspecific variation.
The trials were conducted at two sites in northern
Queensland, Australia: one stand of a seagrass with relatively thick rhizomes (Thalassia hemprichii), and one
of intermediate thickness (Cymodocea serrulata). (As
indicated above, and as confirmed by a pilot study in
Florida [Domning, 1990b], all sizes and shapes of
dugongid tusks are equally effective at extracting rhizomes of Halodule size or smaller.) The Thalassia bed
was located on silty coral sand (7.4% silt, 89.2% sand,
3.5% larger material) on the reef platform at Clack
Island, northern Great Barrier Reef (148 040 S, 1448 150
E); the seagrass cover at this site was visually estimated
at 60% and the leaf height was approximately 7 cm. The
Cymodocea bed was located on coarser coral sand (0.0%
silt, 93.1% sand, 6.8% larger material) in Cockle Bay,
Magnetic Island, Townsville (198 100 S, 1468 500 E).
(Comparable beds of these species or genera growing on
the same substrate could not be located, although this
would have improved the design of the experiments.)
The Cymodocea beds sampled unavoidably contained a
mixture of C. serrulata and Halodule uninervis (wideleaved form).
In each trial, a tusk or tusk model was used to excavate all seagrass material from a 25 x 25 cm area. The
tusk was held in one hand, directed anteroventrad, with
approximately the same length of its tip exposed as
would have been outside the gum in the living animal
(here termed ‘‘effective tusk length’’: Metaxytherium, 1.5
cm; Crenatosiren, 2–3 cm; Dioplotherium, 3–5 cm; Corystosiren, 8–9 cm; Dugong, 3–4 cm). Backward and downward (i.e., posteroventrad with respect to the tusk’s morphology) strokes roughly 10 cm long or less were used to
penetrate the sediment and sever any rhizomes contacted. (The choice of backward rather than forward
movement is justified below under Cranial Architecture.)
Fig. 1. Plastic replicas of fossil sirenian tusks used in seagrassharvesting experiments: a: Metaxytherium floridanum (USNM 356686;
left tusk in premaxilla, medial view). b: Crenatosiren olseni (UF/FGS
V6094, right tusk, medial view). c: Dioplotherium manigaulti (ChM
PV2633, left tusk, medial view). d: Corystosiren varguezi (USNM
425695, right tusk, lateral view showing broad wear surface, partly
restored). Scale bar ¼ 15 cm.
The force applied was kept as constant as possible,
whether this sufficed to sever a rhizome with one stroke
or not. Shoots and rhizome fragments loosened by this
process were removed by hand and collected. Rhizomes
severed at one end that snapped off or pulled out of the
sediment easily when encountered by hand were also
removed; a rhizome felt to be still attached at both ends
was severed with the tusk before removal by hand. This
process was continued until all plant material in the
25 3 25 cm test quadrat had been removed to a depth of
8–10 cm; the number of tusk strokes needed to do this
was recorded. The seagrass material collected was
washed and sorted into above-ground (shoots þ leaves)
and below-ground (rhizomes þ roots) biomass; the number of shoots was counted; and the rhizomes þ roots
were blotted dry and weighed. Five such trials with
each tusk design were made.
At Clack Island, all of the test quadrats were located
within a 6-m radius of each other. A 10-cm diameter
core sample was also taken in this area, which revealed
Thalassia rhizomes at depths of 6–12 cm. Therefore, it
is likely that some of the deepest rhizomes in the other
sampling areas were missed. However, careful exploration by hand in the bottom of each quadrat before termi-
nation of sampling, and continuance of excavation until
no more rhizomes were felt, ensured that all those with
parts lying within approximately 10 cm of the sediment
surface were collected.
The aim of this study was simply to produce a rank
order of rhizome-extraction efficiency for the tusk
designs used, without any absolute values being
assigned to these efficiencies. To avoid misunderstanding, it is essential to emphasize that this was not an
attempt to model animal behavior, but rather to quantify
a physical property of certain physical objects. Apart
from the length of each tusk exposed, the position in
which it was held, and the direction in which it was
moved, there is no reason to think that the manner of
tusk use described above bears any relation whatsoever
to how any sirenian would have used these objects in
extracting seagrasses. It is better to think of the tusks
in these experiments not as parts of living organisms,
but simply as tools manufactured for the purpose of cutting or digging, and whose relative efficiency is being
tested under standardized, albeit artificial, conditions.
The results of these experiments were analyzed using
a two-way analysis of variance with sirenian species and
seagrass species as fixed factors. Input data were num-
Fig. 2. The mean wet weights of seagrass rhizomes removed per
stroke by each tusk demonstrate the differences among the tusks in
their ability to excavate Thalassia, but not Cymodocea þ Halodule.
The means for Thalassia have been adjusted to compensate for the
significant differences in the wet weight of rhizomes available to the
various tusks. The least significant difference between the means is
shown for each species of seagrass. MF, Metaxytherium floridanum;
CO, Crenatosiren olseni; DD, Dugong dugon; DM, Dioplotherium manigaulti; CV, Corystosiren varguezi.
ber of tusk strokes per quadrat and wet weight of rhizomes removed per stroke. The number of strokes was
log-transformed to equalize the error variance. Unfortunately, although the quadrats were selected to be as uniform with respect to seagrass shoot density as possible,
there was a systematic difference in the wet weight of
rhizomes in the Thalassia quadrats selected for excavation with the different tusks. However, there was no
such problem with the Cymodocea/Halodule quadrats.
Accordingly, the analysis of variance was reanalyzed to
test for the differences among tusks in their ability to
excavate Thalassia with wet weight of rhizomes as a
stroke. This was obviously the result of the much
greater depth of burial as well as the greater size and
strength of the Thalassia rhizomes. It was not attributable to sediment type, which was slightly finer and siltier in the Thalassia bed.
When the results from the trials on both the seagrass
types were considered together, there was a significant
difference among the sirenian species in mean number
of strokes per quadrat, but not in the weight of rhizomes
removed per stroke. However, as clearly shown in Figure
2, when the different seagrasses were considered separately, there was no significant difference among the
sirenian species in ability to remove Cymodocea þ Halodule. There was, in contrast, a marked difference in
the case of Thalassia (adjusted for the differences in the
weight of rhizomes available), with the larger and more
bladelike tusks being the most effective.
Results are summarized in Figure 2. The seagrass
beds harvested using the various tusks were uniform with respect to shoot density at both Clack Island
(Thalassia: F ¼ 0.76; df ¼ 4, 20; P ¼ 0.56) and Magnetic
Island (Cymodocea: F ¼ 0.70; df ¼ 4, 20; P ¼ 0.60;
Halodule: F ¼ 0.09; df ¼ 4, 20; P ¼ 0.98; Cymodocea þ
Halodule: F ¼ 0.20; df ¼ 4, 20; P ¼ 0.94). Cymodocea þ
Halodule was easier to excavate than the Thalassia, as
evidenced by the number of strokes needed per quadrat
(F ¼ 1074.26; df ¼ 1, 40; P < 0.001) and the wet weight
of rhizomes removed per stroke (F ¼ 24.70; df ¼ 1, 40; P
< 0.001). Combined results from both excavation sites
show there was a significant difference among the sirenian species in mean number of strokes per quadrat (F
¼ 12.25; df ¼ 4, 40; P < 0.001), but not in the weight of
rhizomes removed per stroke (F ¼ 1.68; df ¼ 4, 40; P ¼
0.1737). A difference was also observed in the wet
weight of rhizomes in the Thalassia quadrats selected
for excavation with the different tusks (F ¼ 5.39; df ¼ 4,
20; P ¼ 0.004).
Overall, the Cymodocea þ Halodule was easier to
excavate than the Thalassia (Fig. 2). This finding was
demonstrated in both the number of strokes needed per
quadrat and the wet weight of rhizomes removed per
Gross observations on tusks and skulls of Dugong
dugon were based on specimens collected by the dugong
research project at JCU and deposited in the Museum of
Tropical North Queensland, Townsville.
Cranial Architecture
The architecture of the skull provides further clues to
the possible mode of use of the tusks, at least among the
more derived dugongines having large, bladelike tusks.
Some of the distinctive characteristics and evolutionary
trends of the latter are: shortening of the nasal process
of the premaxilla, broadening and thickening of the posterior end of this process, and development of a relatively flat and vertical transverse joint surface between
this process and the frontal (Domning, 1989b,c). This
contrasts markedly with the primitive condition found
in most sirenians (including Dugong and Metaxytherium; see Domning, 1994 for phylogenetic analysis), in
which the premaxillary–frontal joint surface slopes
gently anteroventrad and the nasal process is relatively
long and thin and overlaps the dorsal surface of the
Fig. 3. Lateral view of skull, mandible, and masseter muscle of a
derived dugongine, illustrating the hypothesized manner of tusk use in
harvesting seagrass rhizomes. With the front of the lower jaw braced
against the substrate (a), contraction of the masseter to close the jaw
(b) drags the bladelike tusk backward (c), severing rhizomes with
the posterior edge of the tusk. The resistance of the substrate results
in a compressive force field along the dorsum of the skull (d), which
is countered in part by a butt joint between the premaxilla (PM) and
frontal (F).
The development of a premaxillary–frontal butt joint
reaches an extreme in Xenosiren, where it appears to
reflect a compressive stress field along the dorsal side of
the skull (Domning, 1989c) (Fig. 3). If it is true that
such a joint surface is better adapted to resist anteroposteriorly applied compressive stress than is a gently sloping oblique surface, then the progressive evolution of the
butt joint in some dugongines may well reflect a large
and unprecedented increase in such stresses in this
group of dugongids.
Posterad- or posteroventrad-directed stresses at the
premaxillary–frontal joint would most naturally result
from anterad- or anterodorsad-directed forces applied to
the tip of the sharply deflected rostrum. This is because
the premaxilla tends to pivot about its attachments to
the maxilla, as can easily be seen by manipulating a dry
dugong skull with loose sutures. Forward and upward
stresses on the rostrum would, in turn, result from
attempts to force it backward and downward against resistance, as would occur in closing the jaws on material
between the symphysial pads (Fig. 3), or in depressing
the head against resistance at the tip of the rostrum.
Because the progressive evolution of the premaxillary–
frontal butt joint in dugongines occurs together with
increase in size and in bladelike form of the tusks and
not with any other dramatic osteological change in the
oral region, it is logical to conclude that this stress
resulted from use of the tusks, which consequently were
being applied most forcefully in a downward and backward direction and with their posterior edges as the
leading edges. Such use of the jaw-closing muscles to cut
with the tusks would be analogous to the canine shearbite postulated for the sabertooth cat Smilodon by
Akersten (1985). This behavior can be envisioned as an
elaboration or special case of the third (pit-making)
mode of feeding described above for the modern dugong,
i.e., it would more or less require the animal to work in
one place during a dive. If efficient enough, however, it
might enable the animal to maintain some headway,
making at least a short feeding trail while extracting
larger rhizomes than Dugong dugon.
This conclusion in regard to the most derived dugongines, however, may not apply to other dugongids that
also have enlarged tusks but did not develop a premaxillary–frontal butt joint (e.g., Dugong, Metaxytherium subapenninum ¼ M. forestii ¼ M. gastaldi; for synonymy
see Pilleri, 1988). These (or in the case of Dugong, its
ancestors) conceivably used their tusks in a different
manner, i.e., in a forward and upward direction. More
likely, perhaps, the frequency of large rhizomes in their
diets may have been lower, although their large tusks
gave them some capacity to extract such rhizomes when
needed. In any case, this line of reasoning argues for a
frequently downward-and-backward direction of tusk
movement in at least one group, the advanced, bladetusked Dugonginae. For these reasons, this direction of
movement was chosen for use in the seagrass-harvesting
experiments described above.
Anatomy and Use of the Dugong
Feeding Apparatus
The intact oral region of Dugong dugon (Fig. 4) has
been most carefully described by Gudernatsch (1908),
Gohar (1957), and Marshall et al. (2003). In brief, a
broad, disk-shaped surface of the upper lip, covered with
tactile bristles, lies anterior to the mouth opening. Specially enlarged bristles at the posterolateral corners of
this oral disk have a prehensile function, much the
same as in manatees (Marshall et al., 1998). Against the
posterior margin of the oral disk lies a median knob-like
projection (Fig. 4, hp), which is clearly visible in the
intact animal when the mouth is closed, and which (as
noted above) is thought to be used in rooting for rhizomes. This knob is made of tough connective tissue and
is an anterior extension of the flat, trapezoidal pad that
covers the palatal surface of the premaxillae. It is, therefore, an integral part of the upper jaw, but is not directly
supported by bone. The tusks are longitudinally curved
so that they diverge distally, and are slightly flattened
in planes that diverge posteriorly, so that the posterior
edges of the worn surfaces are farther apart than the
anterior edges (Fig. 4). The wear surfaces, however, face
more laterad than anterad. Both anterior and posterior
edges of the tusk tips are usually sharp (see edge sharpness data given below). In animals with erupted tusks,
the knob lies posteroventral to the tusks and extends a
couple of centimeters beyond their tips. The posteromedial sides of the tusks lie against the knob, but are not
enclosed within it, as shown by the fact that the dark
staining that covers nonocclusal surfaces of the cheek
teeth outside the gums also covers all sides of the tusks
except the worn surface (Fig. 4). The limits of this staining accurately indicate the gumline, which lies less than
1 cm outside the margin of the tusk’s bony alveolus (see
than with the anteromedial side. Hence, a potentially
effective cutting edge is formed along the posterior edge
of the tusk much sooner than on the anterior edge. The
orientation of the cross-section in tusks of this type is
well suited to rapid development and maintenance of a
cutting edge useful in posterad tusk movement, but
much less suited to anterad cutting.
Materials and Methods
Fig. 4. Oral region of adult male Dugong dugon, ventral view,
showing part of lower jaw (lj), mouth opening (m), and knob-like horny
pad on upper jaw (hp), with oral disk (od) pulled forward to expose
tusks (t). Note prominent clusters of prehensile vibrissae (U2 bristle
fields of Marshall et al., 2003) on corners of oral disk just lateral to
tusks. Photo courtesy of Paul K. Anderson.
also Fernand, 1953). The anterior and posterior edges
and lateral side of the tusk’s tip are exposed to wear.
The cross-sectional shape of the unworn tusk is, in
some cases, suggestive of the direction of movement to
which the tusk was adapted. In Crenatosiren, Dugong,
Dioplotherium, and Corystosiren, the tusk is roughly
oval or polyhedral in cross-section, with two fairly distinct flattened surfaces on the medial side, respectively
facing anteromediad and posteromediad (cf. Domning,
1978, 1989b, 2001). These are the surfaces covered by
enamel if any is present, and they form approximately
equal angles with the median plane. The paper-thin
enamel, confined to the medial side, provides for a selfsharpening edge analogous to the chisel point of a
rodent incisor. However, because the posteromedial side
of the cross-section is (except in Crenatosiren) longer
than the anteromedial, the long axis of the cross-section
lies at an acute angle to the median plane. As a result,
as the lateral side begins to wear, the worn surface
forms a more acute angle with the posteromedial side
The study of tip geometry of tusks was based on tusks
of Dugong dugon (isolated and in skulls) in the University of Kansas (KU) Natural History Museum Mammals
Collection, Lawrence, Kansas (three isolated tusks cataloged as a lot, KU 130105) and the Mammals Collection
of the Department of Zoology, Field Museum of Natural
History (FMNH), Chicago, Illinois. A tusk referred to cf.
Corystosiren by Domning (1990a) from the Dept. of Vertebrate Paleontology, Florida Museum of Natural History, University of Florida (UF), Gainesville (UF 18826,
the only one with a complete tip) was also studied in
this way. Casts were made of opaque urethane resin
from microwear molds (described below) and sectioned
for the study of geometric profile.
Evans and Sanson (2003) have shown that dentitions
can be described in terms of geometric details that indicate their utility as tools in food comminution. Although
dugongid tusks are not brought into occlusion with
opposing teeth, they can be considered in an analogous
way with respect to a transverse plane parallel to the
long axis of the tusk, here termed the transverse axial
plane (this plane being analogous to the occlusal plane
of the carnassial teeth of a carnivoran). This is because
whether they are dragged through the substrate passively in anterad or anterodorsad motion during grazing,
or during forceful posterad or posteroventrad motions in
digging, their contact with other objects will be, on average, in a direction perpendicular to the long axis of the
tusk and, therefore, perpendicular to that transverse
axial plane. In the case of a tusk digging rhizomes, the
resistance is from root attachment in the substrate and
the mechanical properties of the rhizome itself, not from
the cusp of an occluding tooth passing by it. Tusks used
in either way function essentially like knives, so the
meaning behind tusk tip geometry is slightly different in
comparison with how occluding cheek teeth break food
(which is more like a mortar and pestle or pair of scissors). Therefore, tusk profiles were measured for variables important to blades (sensu Evans and Sanson,
2003) with respect to the transverse axial plane. Certain
details, such as rake and relief angles, are important in
slightly different ways (below) from the occluding cheek
tooth models of Evans and Sanson (2003), simply due to
the fact that clearance and orientation with respect to
an opposing tooth are not issues for tusks.
The variables measured included edge sharpness (measured in radians), rake and relief angles, and approach
angle (measured in degrees; Fig. 5). Edge sharpness, as
well as rake, relief and approach angles, were measured
for both anterior and posterior tusk ‘‘blade’’ edges in
Dugong to better assess effectiveness of tusks as cutting
tools in either anterad or posterad movements. Some
of these measures were not possible (or necessary) for
Fig. 5. A: Schematic of a right tusk of Dugong in lateral view, with
longitudinal axis of tusk (equivalent to transverse axial plane seen
edge-on, represented by the parallel vertical lines) oriented vertically,
depicting anterior and posterior approach angles between longitudinal
axis of tusk and long axes of blade edges. B: Schematic of right tusk of
Dugong in apical view, illustrating posterior relief (<08) and rake angles
(>908), as well as anterior rake (08 < rake angle < 908) and relief angles
(08< relief angle < 908). The vertical lines represent parasagittal planes;
the horizontal lines represent the transverse axial plane.
cf. Corystosiren, because only a single blade edge (the
posterior blade edge) actually exists.
The edge sharpness of a blade is the radius of curvature of the blade edge. A blade with a smaller radius of
curvature will have higher edge sharpness. Because the
tusks of Dugong tend to wear at an oblique angle, but
are pointed more or less ventrally when feeding, the
edge sharpness of the anterior, posterior, and apical
edges was measured for comparison of edge utility in
cutting. Higher edge sharpness will decrease the area of
contact and so increase stress in the food, and is indicative of a more effective cutting tool.
The rake surface is that which follows the blade edge
and is worn by occlusion or abrasion from food. Thus,
the rake surface is first identified as the worn surface of
the tusk. The rake angle is the angle at which this surface lies with respect to the transverse axial plane (Fig.
5B). The magnitude of a rake angle is indicative of its
effectiveness. If a rake angle is larger, food will have
greater clearance to pass by the tusk after being cut,
and edge sharpness will be more easily maintained. Just
as with a knife edge, the closer a rake angle is to 908,
the more effective it will be as a cutting tool (the difference between cutting with a knife made with thick
metal and one made of thin metal). If a rake angle is
larger than 908, the rake surface is not facing in the
direction of motion. In summary, a rake angle (RA)
greater than 08 and less than or equal to 908 characterizes a blade edge (with the most effective blade closest
to 908). A blade edge is not used at a rake angle greater
than 908; hence, the rake angle of an edge can indicate
whether the edge is used in a particular direction of
motion (Fig. 5B).
The relief angle is the angle between the surface of
the tusk following the blade edge and a parasagittal
plane abutting the blade edge; it is a measure of clearance posterior to the blade edge. A relief angle is
described as negative if the portion of the tusk following
the blade edge projects into the path of food. That is, if
the relief angle is negative (like the posterior relief angle
in Fig. 5B), after being cut a seagrass rhizome will still
need to pass by more dental materials, increasing friction. A positive relief angle is the state in which the
medial side of the tusk is deflected away from the path
of food, permitting food clearance once it is cut, without
wear on the medial side (like the anterior relief angle in
Fig. 5B). Thus, the relief angle can theoretically be used
to estimate whether tusks were used in an anterad or
posterad manner (cf. Fig. 3): optimally, the more effective cutting direction would be the one making use of
the tusk edge with the more positive relief angle. We
can compare these angles for the alternate feeding
modes and, thereby, estimate which mode is/was more
likely used in modern and fossil taxa.
The approach angle of a blade is the angle between
the long axis of the blade (the tusk’s leading edge) and
the transverse axial plane (Fig. 5A). The mechanical
advantage (MA) of a blade will depend on the approach
angle (a) of the blade, where MA ¼ 1/cos(a), so that a
larger angle will have a greater mechanical advantage.
This keeps a food item forced along the blade edge so
that it experiences point contact with the entire length
of the blade until it is cut completely or slips off the end
of the blade edge.
Additionally, the length of the blade edge (cutting
edge) was measured. A longer blade edge maximizes the
possible duration of a cutting event for every sequence
of ingestive movements, so a longer blade edge would
also enhance the effectiveness of a tusk as a cutting tool.
Likewise, a longer blade edge permits further efficiency
in cutting by permitting the blade edge to move along
the rhizome for a distance (as opposed to a short or
pointed blade that would only puncture or briefly jab at
a rhizome). This may seem obvious, but studies of the
mechanics of ‘‘pressing and slicing,’’ such as the motion
of cutting cheese with a knife while using a slicing
movement as opposed to simply pushing the knife in
like a wedge, have proven that this movement is
mechanically advantageous (Atkins et al., 2004).
Similarly, the rake surface area was measured (essentially the same as the area of the worn facet). Just as
with the rake angle and its role in minimizing friction
with the rake surface, minimization of the rake surface
area itself would also minimize friction. A more effective
blade should have a smaller rake surface area.
Tusk tip geometry comparisons show that the edge
sharpness varies greatly along the perimeter of worn
tusk tips of Dugong and cf. Corystosiren. The radius of
curvature for the anterior edge of the tusk tip is greater
for cf. Corystosiren (1.39 radians) than Dugong (0.524
radians), while the radius of curvature of the posterior
edge of the tusk tip in cf. Corystosiren (0.349 radians) is
much less than that of Dugong (0.576 radians). The edge
sharpness of the very extreme tip (apical end) is greater
in Dugong (radius of curvature ¼ 0.401 radians) than in
cf. Corystosiren (0.873 radians).
The anterior rake angles were recorded as 268 for
Dugong and 68 for cf. Corystosiren. The posterior blade
rake angle was 1448 for Dugong and 758 for cf. Corystosiren. The rake surface of the posterior blade of Dugong
is not facing in the direction of posterad motion (RA >
908). The anterior relief angles were 348 for Dugong
and approximately 658 for cf. Corystosiren. The posterior relief angles were 258 for Dugong and approximately 108 for cf. Corystosiren. Anterior approach
angles were 228 for Dugong and approximately 338 for
cf. Corystosiren. Posterior approach angles were 408 for
Dugong and approximately 558 for cf. Corystosiren.
Blade edge lengths for Dugong were 33 mm (posterior),
35 mm (anterior), and approximately 3 mm (apical). The
posterior blade edge of cf. Corystosiren was 71 mm. The
apical and anterior edges of the tusk of cf. Corystosiren
are not worn into blades and are, thus, difficult to differentiate and measure in this respect. The rake surface is
742 mm2 for Dugong and 870 mm2 for cf. Corystosiren.
Tusk tip geometry comparisons show that the worn
tusk tips of Dugong and cf. Corystosiren are geometrically very different. Edge sharpness varies greatly along
the perimeter of a tusk tip in both taxa. The radius of
curvature for the anterior edge of the tusk tip, presumably the edge that would encounter wear during passive
substrate encounters during grazing, is much greater for
cf. Corystosiren than for Dugong, meaning that the anterior edge of the cf. Corystosiren tusk is less sharp than
that of Dugong. In contrast, the radius of curvature of
the posterior edge of the tusk tip in cf. Corystosiren is
much less than that of Dugong. The edge sharpness of
the very extreme tip (apical end) is greater in Dugong
than in cf. Corystosiren. This finding may be in part
because only one edge (the posterior edge) of the tip in
cf. Corystosiren is used for cutting and consequently gets
sharpened. The apical edge of Dugong is significantly
sharper due to wear on its anterior edge, and shorter
because the overall wear produces a roughly elliptical
rake surface, of which the apex is one of the narrow,
more strongly curved ends. The straighter posterior
blade edge of cf. Corystosiren permits the self-sharpening effect to maintain a long blade edge.
The anterior rake angles of Dugong and cf. Corystosiren are drastically different, which reflects the betterdeveloped anterior-facing blade edge of the tusk of
Dugong as compared with lack of an anterior blade edge
in cf. Corystosiren. The posterior blade rake angle of
Dugong is large because the rake surface does not face
the direction of motion. This strongly indicates that the
posterior blade edge of Dugong is unlikely to have been
used in cutting. The rake angle of the posterior blade
edge of cf. Corystosiren is very positive, and clearly indicative of being a more effective tool when used in a posterad motion. This would provide not only slightly greater
fragment clearance for cf. Corystosiren, but also help in
maintaining the edge sharpness throughout its wear life.
The anterior relief angles appear to be different
between Dugong and cf. Corystosiren. The horny pad of
Dugong blocks the potential passage for foods in an anteroposterior direction, and, therefore, negates the importance of this angle to some degree. For cf. Corystosiren,
this angle indicates lesser use of the anterior edge for
cutting. Although it is uncertain how much of a horny
pad surrounded the tusk of cf. Corystosiren, it seems
clear that the tusk protruded proportionately farther
from the alveolus (and hence, presumably, from the gingiva) than in Dugong. The posterior relief angles between Dugong and cf. Corystosiren also appear to be different in magnitude, but are even more fundamentally
different because the rake surface of the posterior blade
of Dugong is not facing in the direction of posterad
motion (RA > 908), effectively increasing the relief angle
by pushing the entire posterior edge away from the sagittal plane. Ultimately, rake and relief angles of the posterior blade edge of Dugong are only important in illustrating their lack of use in cutting. The relief angles of the
posterior blade edge of cf. Corystosiren are very small
and would easily permit clearance of food past the tusk.
Anterior approach angles for Dugong are comparable
to that of cf. Corystosiren. Posterior approach angles for
Dugong are smaller than that of cf. Corystosiren,
although this could be easily modified by a small amount
by adjustments to head orientation. The larger posterior
approach angle would have given cf. Corystosiren a significantly enhanced mechanical advantage in cutting
seagrass rhizomes during digging.
Blade edge length is the other, more obvious, feature
that distinguishes the mechanical utility of these tusks.
Dugong has relatively short apical, posterior, and anterior blade edges. In contrast, the posterior blade edge of
cf. Corystosiren is much longer, reflecting not only the
depth to which it could dig during feeding, but also its
shape when considered along with its rake surface. The
apical and anterior edges of the tusk of cf. Corystosiren
are not worn into blades and were thus difficult to differentiate and measure in this respect.
The rake surface of Dugong is large in comparison
with its blade edge length when compared with that of
cf. Corystosiren. This finding reflects the narrow blade of
the latter, optimal for maximizing cutting and minimizing friction with food after cutting.
Materials and Methods
The same tusks (Dugong dugon, KU 130105, and cf.
Corystosiren, UF 18826) used in the study of tip geometry (above) were analyzed for microwear data. In addition, wear patterns were analyzed on a dugong skull
from Arabia (JCU uncataloged) and two specimens from
Fig. 6. Lateral view of Dugong dugon skull and tusk, with microwear features of different edges illustrated by micrographs at right (oriented with the skull, and all at 703, original magnification). Data are
reported as average numbers of pits (p), fine scratches (fs), and
coarse scratches (cs). Arrows indicate average orientations of fine
scratches (narrow arrows) and coarse scratches (thicker arrows) for
each edge location, presumably caused by grit particles and/or seagrass rhizomes crossing the tusk during feeding.
Queensland (MM 083 and MM116, from the James Cook
University marine mammal collection [MM], now housed
in the Museum of Tropical North Queensland, Townsville, Australia). Specimens were cleaned with acetone
and cotton swabs. Once dry, each tooth was molded twice
using a high-precision, polyvinylsiloxane dental impression material (President Jet Microsystem1; Coltene/
Whaledent). The first mold was discarded as a final
cleaning step, and in the case of the fossil, this included
remnant matrix left over from preparation.
Casts were made using clear urethane resin: CC200
Crystal/Water-Clear1 ultraviolet-stable urethane resin
and hardener from Eager Plastics, Inc., Chicago, IL.
Once the urethane was mixed and poured, the molds
were placed in a pressure pot to remove air bubbles and
were then left to cure for 2 days.
Casts were examined under a stereo light microscope
at 335 and 370 magnification using the Solounias and
Semprebon (2002) technique. A 0.3 mm 3 0.3 mm area
was examined in several locations along the perimeter of
the tusk tip (Fig. 6). On each tusk, four continuous
microwear variables were documented: the number of
pits, the number of light and coarse scratches, and the
orientation of each scratch with respect to the tangent of
the tusk edge. Pits were defined as those features that
are generally circular. Scratches were features with
greater lengths than widths and with parallel sides.
Scratches were categorized as fine (narrow and shallow)
or coarse (wider and deeper). Scratch orientations were
then adjusted to relate to the anteroposterior axis of the
whole tusk.
onto the posterior edge of the worn surface, whereas the
anterior edge is polished clean. On the other hand, MM
083 has apparent wear on the posterior edge of the
tusk’s medial surface. Likewise, MM 116 has the stain
removed from the posteromedial and not the anteromedial surfaces of both tusks. The majority of specimens
examined, however, show no such indications of preferential movement in either direction.
Dental microwear was not adequately preserved for cf.
Corystosiren (UF18826) due to its extremely thin enamel
and the effects of postdepositional processes on its dentine. Microwear for modern Dugong dugon (KU 130105)
was well preserved and supported some of our expectations (Fig. 6), although a larger sample size will be
needed to corroborate these conclusions. The number of
pits and coarse scratches was small in comparison with
the number of fine scratches on average for all three
locations on the tusk edge, although notable variations
exist in the proportions and orientations of those
scratches, the number of light and coarse scratches, and
the orientation of each scratch with respect to the tangent of the tusk edge. The average number of pits did
not significantly increase along the blade, although
scratches were on average different in number and orientation. Scratch orientation is reported here with
respect to the coronal plane, so that comparisons
between edges facing in different directions can be made
with minimal assumptions about feeding posture (one
can just reorient and add 908 for horizontal plane
Fine scratches showed an average bimodal distribution in orientation on the anterior and apical edges, and
a mostly unimodal distribution on the posterior edge. On
both anterior and posterior edges, the bimodal distributions of fine scratches appear to be biased toward those
from anteroposterior movements rather than those from
The dugong skull from Arabia (JCU uncataloged)
shows staining on the sides of the tusk that extends
dorsoventral movements. On the posterior edge the fine
scratches are oriented predominantly anteroposteriorly,
as would be expected if the fine scratches were being
created as the tusk coursed through the substrate during anteroposterior head movements during feeding.
Coarse scratches were much less numerous, but their
average orientations may be more informative. The few
coarse scratches found on the anterior and apical edges
were inclined almost dorsoventrally with a slight inclination dorsoposteriorly. In contrast, coarse scratches on
the posterior edge were, on average, oriented almost dorsoventrally but with a slight inclination inferoposteriorly. If indeed these coarse scratches result from forceful digging for rhizomes in the substrate, then their orientation appears to support the notion that this digging
motion could include anterior and posterior cutting
actions. If digging for rhizomes were a regular behavior,
presumably a much larger number of deep scratches
would be expected.
It is most unlikely that the macroscopic wear on tusks
of Dugong is produced merely by contact with the upper
lip, as suggested by Fernand (1953). Neither can the
(presumably gender-specific) social uses of the tusks
account for the wear, which also occurs on erupted tusks
of old females, as pointed out by Pocock (1940). It is,
however, plausible that the forward movement of the
snout through the substrate while making feeding trails
would account for the wear, without any purposeful use
of the tusks. This would suggest that the anterior edge
of the tusk tip is the leading edge during wear. This
appears to be supported by staining on the worn posterior edge and a cleanly polished anterior edge in a
dugong skull from Arabia (JCU uncataloged). In contrast, the posterior edge of the tusk’s medial surface has
apparent wear on a specimen from Queensland (MM
083), suggesting that the posterior edge was at least
sometimes used as the leading edge. Similarly, the stain
is removed from the posteromedial and not the anteromedial surfaces of both tusks in MM116. The majority of
specimens examined, however, show no such indications
of preferential movement in either direction. Given possibly idiosyncratic individual behavior, together with the
lack of evidence that tusk wear in Dugong is anything
but incidental to feeding, no firm conclusions could be
drawn from macroscopic tusk wear about the manner of
use of the snout in feeding by Dugong, let alone by
extinct dugongids.
Microwear was studied to see if directions of scratches
on the tusk surfaces could resolve this question. Major
scratches caused by sand grains were expected to be
roughly perpendicular to the tusk’s long axis. Incidental
wear was assumed to result mainly from forward movement of the head and tusk. If the tusks were deliberately used for rooting by forward movements of the
snout, or if tusk wear were simply incidental to such
movements, then the scratches should be in the same
general direction as those incidentally created while
making feeding trails. Rooting might also involve backward movements of the rostrum, leaving some incidental
scratches in the opposite direction. If, however, the
sharp posterior edges of the tusks were persistently and
forcefully used as cutting instruments, the tusk would
have to be moved backward, making scratches indicating
backward movement that might be deeper and more
numerous than any of the incidental scratches.
The data on dental microwear from Dugong dugon
seem to support the expectation of wear if tusks are
worn from passing through the substrate during normal
feeding bouts, with occasional events of vigorous cutting
of rhizomes by anterior and posterior movements of the
tusks in the substrate. The deep gouges on both the ventral and dorsal edges of the Dugong tusk suggest that
cutting may have been done by both motions, maybe
using neck or jaw-closing movements. The gouges on the
dorsal edge seem too large to be incidental to passing
through the substrate during normal grazing, like the
scratches on the tusk tip.
Tropical dugongids of the past likely resembled today’s
dugong in eating seagrass. How, on the other hand, did
these extinct dugongids differ from the modern one?
One category of differences with adaptive implications is
that of rostral deflection, which seems to reflect the average position of food items in the water column
(Domning, 1977, 1980, 1982, 2001). This is not very
helpful, however, in distinguishing among extinct tropical dugongids, all of which (like Dugong) seem to have
been bottom-feeders with more or less strongly downturned snouts (an observation that supports the assumption of seagrass diets).
Another category of conspicuous differences is tusks.
The most impressive development of tusks in the Sirenia
is found in the dugongid subfamily Dugonginae. Highly
derived, extinct dugongine genera, such as Rytiodus,
Corystosiren, and probably Xenosiren, have broad, mediolaterally compressed, bladelike tusks whose roots
extend the full length of the premaxillary symphysis
and whose medial surfaces are covered by thin enamel
which provides a self-sharpening edge. It seems impossible to explain the morphology of such tusks, or their evolution from more primitive, subconical incisors, apart
from the supposition that they were used to cut something. The narrower and less flattened but likewise selfsharpening tusks of Dugong are used as weapons
against other animals of the same species (or even,
reportedly, against sharks; Promus, 1937). However, it is
not clear why this use would require any more bladelike
tusks than Dugong now displays; cutting food items
seems a likelier use for large blades.
In opposition to this view, Anderson (2002), analyzing
the evolution of sirenian mating systems, argued that
‘‘[t]usks may have had a social function throughout
dugongid evolution and their social function(s) may have
been primary. . . .Evolution and retention of tusks exclusively, or even primarily, as foraging structures would be
unique among mammalian herbivores.’’ This hypothesis
would, however, lead us to expect sexual dimorphism of
tusks to have been prevalent among dugongids—which
the fossil record does not support. With specific reference
to D. dugon, he argues that ‘‘[i]f tusks were primarily
foraging structures, the sex with the greatest nutritional
requirements and the most to gain from a wide range of
foraging options (females) should have retained them’’
(Anderson, 2002:77). This would be true if those females
were capable of efficiently chewing and digesting fibrous
rhizomes harvested by the tusks; but (for reasons that
admittedly remain mysterious; Domning, 1995) modern
dugongs of both sexes lack the multicusped, enameled
cheek teeth that this would require (Lanyon and Sanson, 2006). So while it seems clear that neither sex has
a motive, at present, to use the tusks in any but social
roles, we consider this an exceptional and derived situation in Dugong dugon that tells us little about primitive
conditions among dugongids in general.
In our view, therefore, the social functions of dugongid
tusks are historically secondary to the more universal
and primitive use of incisor teeth in gathering food. And
if we conceive of dugongids in general as seagrass eaters, then the obvious and only food items needing to be
cut by large tusks are the more or less tough and deeply
buried rhizomes of the larger seagrasses.
How can this hypothesis be tested? First, by seeing
whether Dugong tusks today have alimentary in addition to social functions. The stomach-content data presented here suggest that males do not eat more rhizomes than females. This, of course, was expected simply from the fact that females lack erupted tusks; sexual
divergence of diet in large mammals would be highly unusual. However, the fact already noted that almost no
fossil dugongid is known to exhibit tusk dimorphism
shows that the sexual dimorphism (and, hence, possibly
the social role of tusks) seen in D. dugon is not representative of the family as a whole. While the stomach-content data do not rule out the possibility that D. dugon or
its relatives used their tusks for feeding in the past,
they do deprive us of the one uniformitarian key to the
past that a living species might provide, and reduce us
to evaluating the tusks on their own evidence: How, and
how well, might such instruments work as cutting and/
or digging tools if they were in fact so used?
As noted above, when dugongs feed on Halophila and
Halodule, they typically uproot and consume the entire
plant and make a linear trail. Because a trail is made in
the few minutes between successive breaths, it is necessarily made by an animal maintaining more or less
steady forward motion. Hence, such trails are usually
not made on seagrass beds that offer significant resistance to this sort of plowing or rooting, due to toughness
and/or deep burial of the rhizomes and/or hardness of
the sediment. If a dugong were to feed on rhizomes
under the latter conditions, it would have to spend its
available time on any one dive working on a comparatively small area of bottom, and would presumably clear
only a small patch or make a circular pit rather than a
linear trail (as is sometimes observed; Anderson, 1998).
It is not obvious a priori how a dugong might use
tusks in feeding on seagrasses with larger, stronger rhizomes such as Thalassia—whether cutting them with a
forward and upward stroke, using a toss of the snout, or
with a downward and backward movement, perhaps
using the jaw muscles, as suggested for Xenosiren by
Domning (1989c) (Fig. 3). Once again, Dugong dugon
fails us as a guide, partly because its mode of feeding is
poorly understood and partly because specimens can be
found to seemingly support either model. The evidence
from cranial architecture, tusk geometry, and tusk wear
adduced above, however, tends to support the downward-and-backward alternative.
The relatively crude seagrass-harvesting experiments
reported here did not separate the effects of tusk length
from those of tusk shape. In fact, it seems likely that
the advantage of the Corystosiren tusk over the smaller
tusks at excavating Thalassia rhizomes from depths of 6
cm or more was largely due to its effective length of 8–9
cm. When effective tusk length is less than the minimum depth of rhizome burial, several strokes are
required merely to penetrate and remove the sediment
before the rhizomes begin to be disturbed. As for the
smaller tusks that were within a centimeter or two of
each other in effective length and which differed mainly
in shape, the experimental technique was apparently
not sensitive enough to reveal significant differences in
digging/cutting efficiency if such exist.
Janet Lanyon (personal communication) has, in fact,
stressed sediment disturbance as a potential alternative
function of tusks, separate from cutting of rhizomes. She
visualizes the tusks being used to loosen and stir up the
sediment surrounding the rhizomes, shaking them free
and making them accessible to breakage by other
mouthparts. This could logically involve a forward and
upward toss of the snout, which would also serve to pull
intact rhizomes up toward the sediment surface. Small,
conical tusks with no cutting edges, such as those of
Metaxytherium floridanum (Fig. 1a), could well have
served mainly for sediment disturbance long before the
evolution of larger ones better suited for cutting. As for
D. dugon, sediment disturbance may now be the principal way in which tusks remain useful to the males for
feeding, if indeed they are still helpful in feeding at all.
Regarding the larger tusks, several possibilities
remain: their more bladelike shape may in fact make a
direct contribution to digging and/or cutting efficiency
distinct from the contribution of tusk length (by increasing cutting edge and minimizing drag-inducing lateral
bulk); or the mediolateral flattening of the tusk may
serve to stiffen it in the plane of its action (much as a
deep mandibular body prevents dorsoventral bending) or
anchor it more strongly in the alveolus (by increasing
surface area for periodontium); or the flattening may be
in some way a developmental or structural byproduct of
enlargement of the tusk. These alternatives are not
mutually exclusive. The last, however, can probably be
eliminated, because in at least one dugongid (Metaxytherium subapenninum), and in the miosirenine trichechid
Miosiren kocki, the tusk became lengthened to the full
length of the premaxillary symphysis without losing its
subconical shape, suggesting that tusk thickness is not
limited by lateral dimensions of the premaxilla or other
While our evidence generally supports the hypothesis
that the tusks of most large-tusked dugongids served in
feeding (most probably to excavate and sever large seagrass rhizomes, by means of a backward, jaw-closing
movement), there are intriguing exceptions: two dugongids, Dugong dugon and Metaxytherium subapenninum,
(a) independently evolved large tusks (extending the
length of the premaxillary symphysis) that are (b) subconical rather than conspicuously flattened and bladelike, (c) lack a premaxillary–frontal butt joint, and (d)
show (or possibly show) sexual dimorphism. This combination of character states is unique to these two forms.
Did they alone use the tusks primarily for social purposes? Other dugongids likely used their tusks socially
as well as for feeding, as Anderson (2002) emphasizes;
many mammals in which both sexes use tusks for feed-
ing, such as elephants, use them also for social purposes
(Spinage, 1994). Sexual dimorphism in elephant tusks is
mostly restricted to their age of eruption, which does little to alter their utility in feeding.
The nearest known relative, and possible ancestor, of
D. dugon is represented by an undescribed adult skull
from the late Pliocene of Florida (cast, USNM), which
has large, subconical, enameled and erupted, somewhat
worn tusks, together with enameled molars, comparable
to the conditions seen in M. subapenninum. If this specimen represents the ancestry from which D. dugon was
derived, then the latter’s somewhat more flattened, selfsharpening tusk evolved subsequent to the Pliocene, as
did its loss of functional enamel crowns on its molars (cf.
Lanyon and Sanson, 2006). Domning (1995) speculatively attributed the latter development to a shift away
from large rhizomes in the diet—rather than toward
such a diet, as the tusk modifications would suggest.
The relative timing and ecological context of these two
changes, seemingly adaptations in different directions,
remain to be clarified.
In contrast, tusk enlargement in Mediterranean Metaxytherium took place from the late Miocene to the middle Pliocene, was not associated with molar degeneration, and seems connected with the late Miocene Mediterranean salinity crises (Bianucci et al., 2004). Only in
the later (Pliocene) stages of this process did sexual
dimorphism possibly develop; in the single putatively
male specimen, the tusk is somewhat flattened and possibly self-sharpening, although it is unclear if enamel is
Do Dugong lack a premaxillary–frontal butt joint as a
result of evolutionary reversal? If so, this could have
come about through neoteny, which seems reflected in
other aspects of the feeding apparatus (Domning, 1995).
This hypothesis would presuppose that a butt joint was
absent in juveniles of taxa whose adults possessed it;
but juveniles of the latter fossil species have not yet
been found. In any case, such a joint does not seem to
have existed in any of the direct ancestors of Dugong (or
Metaxytherium; Domning, 1994).
In conclusion, available evidence seems to support
(albeit less than conclusively) our hypothesis that
extinct dugongines with large, bladelike tusks used
them to feed on robust seagrass rhizomes. The modern
dugong, however, is atypical of its subfamily in having
evidently evolved away from this sort of specialization;
it now appears to use its tusks only in historically secondary social roles. It is, thus, of only limited help in
resolving the complexities of dugongine feeding ecology.
Katherine Rafferty fabricated the model tusks. Warren
Lee Long and the Northern Fisheries Research Centre,
Queensland Department of Primary Industries, Cairns,
loaned field equipment and analyzed sediment samples.
Jeff Miller of the Department of Primary Industries
allowed Domning to join his field team and made possible his work at Clack Island. Access to collections under
their care was kindly provided by T. Holmes (University
of Kansas Natural History Museum Division of Mammals), W. Stanley (Field Museum of Natural History
Mammals Collection), and R. Hulbert (Florida Museum
of Natural History Dept. of Vertebrate Paleontology).
Helene Marsh very kindly did the statistical analysis of
the seagrass-harvesting experiments and gave other assistance. William Akersten, George Heinsohn, Janet
Lanyon, Helene Marsh, and Tony Preen provided valuable discussions and/or comments on earlier drafts of the
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