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Elemental and isotopic analyses of mammalian fauna from Southern Africa and their implications for paleodietary research.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 76:49-60 (1988)
Elemental and Isotopic Analyses of Mammalian Fauna From
Southern Africa and Their Implications for Paleodietary Research
ANDREW SILLEN
Department of Archaeobgy, University of Cape Town,Rondebosch, 7700
South Africa
KEY WORDS
Strontium, Carbon isotopes, Bone, Diet, Mammals
ABSTRACT
Elemental analyses of mammalian bone (e.g., strontium-calcium ratios, or Sr/Ca) distinguish between herbivores and carnivores; however,
the relationships among herbivores are unclear. To study this question, a
modern faunal sample from the Nagupande Tsetse Control Area (Zambezi
drainage, Northwestern Zimbabwe) was used. This collection has the advantage of well-established geographical controls in addition to a varied fauna,
which includes both bovids and suids.
The grazinghrowsing dietary status of each species was ascertained by
means of isotopic analysis of carbon. Clear differences were seen in the 613C of
grazing and browsing animals; a specialized grazer was found to have significantly lower Sr/Ca than less specialized grazers and browsers.
In this study it was also possible to examine differences in Sr/Ca by sex;
female warthogs were found to have significantly lower Sr/Ca than males. The
variation in certain animal groups was found to be abnormal.
Implications for reconstruction of prehistoric human diets using trace-element techniques are discussed.
The application of trace-element relationships in food chains, such as strontium-calcium ratios (Sr/Ca)and multielement studies,
to dietary reconstruction has been hampered
by the lack of background information on
modern animals from natural settings. The
physiological rationale for using Sr/Ca as a
paleodietary technique is well-established
and documented in a large biochemical literature (for reviews, see Wasserman and
Comar, 1961; Wasserman, 1977; Sillen and
Kavanagh, 1982);yet these data are no substitute for measurements of wild animals in
the environment. For example, recent studies of North American mammals have demonstrated that the SdCa of wild specimens of
white-tailed deer are more variable than that
of laboratory animals (Price et al., 1985).
Moreover sex differences among some wild
animals do not appear to conform to expectations based on laboratory models.
Although some data exist on the Sr/Ca of
wild animals, they were generally not collected with the aim of developing background information for paleodietary research. A prominent example is the study of
0 1988 ALAN R. LISS, INC.
Elias et al. (19821, who examined the Pb/Ca,
Sr/Ca, and BdCa of herbivores and carnivores from Thompson Canyon, a subalpine
ecosystem in the High Sierras. The principal
aim of the study was to establish the input of
industrial lead into this remote ecosystem.
As part of the research strategy, the biopurification of calcium in this foodchain was established not only for lead, but also lead in
comparison to strontium and barium. For example, there was a fourfold reduction in Sr/
Ca from sedge leaves to voles; pine martens,
feeding on voles, had a further sixfold reduction in Sr/Ca. The reduction in BdCa in the
food chain was even greater.
Paleodietary reconstruction depends upon
the development of specialized data, which
such studies do not provide. In Sr/Ca research, fauna is used in two ways: the demonstration of ideal “herbivore” and “carnivore” diets, against which humans may
be compared, and in the assessment of diagenetic change after interment. The rationale
for the first category is the development of
Received May 13, 1987; accepted August 13, 1987.
50
A. SILLEN
TABLE 1. SrlCa in selected masses and legumes’
Grasses
Sr (fieq/gm)
Ca ( w k m )
Sr/Ca
1.49
1.31
1.19
1.19
1.07
1.01
.95
.95
348
366
390
352
306
235
352
235
4.28
3.58
3.05
3.38
3.50
4.30
3.70
3.96
4.70
3.81
3.57
3.51
3.27
3.20
3.09
2.53
1500
995
1105
1020
995
940
976
718
3.13
3.83
3.23
3.44
3.29
3.40
3.17
3.52
Bromus inermis, Manchar
Lolium perenne, Oregon Commercial
Lolium perenne, A.lO1
Festuca elatior, Late Otofte
Festuca arundinacea, Aka
Phleum pratense, Amer. Commercial
Phalaris Tuberosa, Harding Grass
Dactylis glomerata, Potomac
Legumes
Trifolium subterraneum, Tallarook
Medicago sativa, Caliverde
Trifolium fragericum, Salinas
Trifolium repens, SlOO
Trifolium repens, Dutch White
Trifolium pratense, Kenland
Trifolium repens, Ladino
Lotus corniculatus. Los banos
‘Source:Vose and Koontz, 1959
standards based on species whose diets are
well understood against which to compare
humans or other species whose diets are unknown. Since background levels of Sr/Ca differ from one terrestrial environment to the
next, it is necessary to provide these data
wherever the Sr/Ca technique is applied. The
second category stems from the observation
that bone Sr/Ca may change after interment
(Sillen, 1981a,b; Klepinger et al., 1986; Nelson et al., 1986; Pate and Brown, 1986; Price
et al., 1986). One way to monitor and correct
for this phenomenon is to plot changes in the
Sr/Ca through time of species whose diets are
understood to be invariate.
The documentation of faunal relationships
other than herbivore to carnivore comparisons has been largely overlooked. In particular, there is uncertainty regarding the
relative Sr/Ca of grazing and browsing mammals, and of omniverous species like certain
suids.
In the first publication to suggest that Sr/
Ca would be of value in the reconstruction of
fossil food chains, Toots and Voorhies (1965)
suggested that grazers would have lower Sr/
Ca than browsers for the simple reason that
grasses contain less Sr than browse. These
authors did not attempt to demonstrate the
point with modern animals, yet the notion
that dietary strontium alone determine the
Sr concentration in skeletons has persisted.
However, ample evidence exists that grass
also contains less Ca than browse, with the
result that the Sr/Ca of the two categories
may not differ in any given soil. With few
exceptions, the SdCa of pIants are identical
to that available to the plants in the form of
ground water and soil nutrients. The best
data for dicotyledonous plants is on legumes
(Menzel and Heald, 1959; Vose and Koontz,
1959), as summarized in Table 1. A review of
the biochemical and physiological literature
on Sr suggests that dietary SdCa, rather
than Sr alone, determines skeletal levels of
Sr (Sillen and Kavanagh, 1982). Following
this logic, no difference is expected between
the Sr/Ca of grazers and browsers from any
given environment.
To date no study has set out to establish
the relative Sr content of grazing and browsing mammals. This research has become feasible particularly for tropical African
ecosystems since it is now possible to discriminate very clearly between grazers and
browsers using measurements of carbon isotopes (Ambrose and DeNiro, 1986; Vogel,
1978). These measurements provide a n additional quantitative control for browsing and
grazing behavior beyond the conventional
assessments based on dental morphology,
field observation, and stomach content analysis.
In order to provide this information, it is
necessary to work with a varied faunal collection derived from a well-documented geographic locality. The Nagapande’ faunal
collection housed at the Zimbabwe Natural
History Museum in Bulawayo meets this
specification, and was chosen for this study.
’Current maps adhering to the Names (Alteration)Act of 1983
(Zimbabwe) use the spelling “Nagapande”; thls spelling is used
throughout this article except where referring specifically to the
historical “Nagupande Tsetse Control Area.”
51
PALEODIETARY RESEARCH AND MAMMALIAN FAUNA
/
Fig. 1. Map of Zimbabwe showing the Nagupande Tsetse Control Area.
Historically, the Nagapande fauna dates
from the early 1960s and was collected incidentally to a tsetse-fly control program. The
program established a number of hunting
stations (of which the Nagupande station was
only one), which were intended to control or
eliminate game vectors of the tsetse fly (Child
and Wilson, 1964). The Nagupande Tsetse
Control Area covered an area of 209 square
miles at 3,000' in the Zambezi drainage of
North Western Zimbabwe (approximately
18"15'S, 27'40'E) (Smithers, personal communication; Fig. 1).Child and Wilson (1964)
describe the area:
. . undulating country on the headwaters of the Nagupande (sic)
~
river. . .(with) . . . an uneven overlay of
windblown Kalahari sand obscuring
areas of Karoo soils. The annual rainfall was about 25 to 30 inches. An open
mixed species savannah woodland with
sparse perennial grasses occurred on
the Kalahari sand ridges. Elsewhere
the dominant vegetation was mopane
woodland in which grass growth was
poor and consisted mainly of annual
species. Interspersed through the mopane there were areas of scrub thicket
dominated by Combretum elaeagnoides
while along water courses Acacia spp.
were well represented. The eastern end
of the area took in part of the open
perennial grassland of the Matabolo
52
A. SILLEN
TABLE 2. Specimens from Nagapande used in this study
Species
English name
Redunca arundinum (Boddaert, 1785)
Trageluphus strepsiceros (Pallas, 1766)
Equus burchelli (Gray, 1824)
Potumochoerus porcus (Linnaeus, 1758)
Phucochoerus aethiopicus (Pallas, 1766)
Reedbuck
Kudu
Zebra
Bushpig
Warthog
flats where trees such as mopane or
Acacia spp. were restricted to water
courses. There were also a few dambos
on the contacts between Kalahari sand
woodland and mopane veld where perennial grasses were well represented.
Whereas the tsetse control operations in
the former state of Rhodesia were ecologically questionable, the mammalian fauna assembled provides various research opportunities. The varied fauna from this one drainage includes reedbuck (Redunca arundinum,
Boddaert, 1785), Burchell’s zebra (Equus
burchelli, Gray, 1824), kudu (Tragelaphus
strepsiceros, Pallas, 1766), bushpig (Potamochoerusporcus, Linnaeus, 17581,and warthog
(Phacochoerus aethiopicus, Pallas, 17661,
most of which are of known sex. Approximately 90 specimens were examined in this
study; a summary is provided in Table 2.
In order to evaluate the dependence of each
species on grazing or browsing, stable carbon
isotopic analyses were performed. The rationale for using carbon isotopes to distinguish
between grazing and browsing animals in
tropical environments is by now well established (Vogel, 1978; van der Merwe, 1982;
Ambrose and DeNiro, 1986). Briefly, plants
that fix atmospheric carbon dioxide during
photosynthesis initially into a three-carbon
molecule (the Calvin, or C3 photosynthetic
pathway) fractionate carbon isotopes more
than those using a four-carbon molecule (the
Hatch-Slack, or C4 pathway). As a result, the
two groups have different 13C/12C,or 613C,
and the ranges do not overlap. Tropical and
savannah grasses follow the C4 pathway,
whereas trees, most shrubs, and temperate
grasses follow the C3 pathway. Because less
significant fractionation occurs in mammals,
these differences are perpetuated in mammalian food webs: the practical application is
that the carbon of browsing and grazing animals in tropical ecosystems contain distinctive isotopic signatures.
In examining the Nagapande fauna, the
following questions were addressed: 1)Is it
Dossible to detect a difference in the Sr/Ca of
Number
10
28
6
13
31
grazing and browsing bovids, and how do any
differencesrelate to their grazing and browsing status as indicated by the carbon isotopic
signature? 2) Is a similar pattern seen in the
Sr/Ca of grazing and browsing suids? 3) Does
the Sr/Ca of free-ranging animals, e.g.,
equids, differ from those of territorial ones?
4) Are there differences in Sr/Ca that are due
to sex, and can these differences be related to
previously reported field and laboratory observations? 5) How does the pattern of variation within populations compare to those
previously reported? Answers to such questions have direct bearing on the research design of paleodietary studies using trace
elements. Following a presentation of the
methods and results, these implications are
discussed in the final section.
METHODS
For the carbon isotope analyses, diaphyseal
cortical bone specimens were demineralized
in dilute HC1; the total remaining organic
component was recovered, defatted, and converted to pure C02 by combustion and cryogenic separation according to methods
described elsewhere (Sealy, 1986).Mass spectrometry was performed using a VG Micromass 602E light-isotope mass spectrometer.
Replicate analyses of the same samples give
6I3C values within 0.2%0.
Specimens were prepared for elemental
analysis of strontium and calcium according
to methods described previously (Sillen,
1981a; Sillen and Smith, 1984). Briefly, 4-5
mg diaphyseal cortical bone fragments were
weighed with a Perkin-Elmer A D 4 autobalance and digested with 100 pl concentrated
nitric acid (Aristar) in 12 x 75 mm borosilicate glass culture tubes at 150°C. The samples were brought to dryness and redissolved
in 1 ml 0.2 N HNO3. These solutions provided stocks from which aliquots could be
removed for analysis. The digestion procedure was performed in duplicate for each
specimen. In 6% of the specimens, the difference in Sr/Ca between duplicates was greater
than 10%; in these cases repeat duplicate
digestions were analyzed.
53
PALEODIETARY RESEARCH AND MAMMALIAN FAUNA
TABLE 3. 613C in Nagapande faunal sample
Animal
Grazers
reedbuck
zebra
warthog
Browsers
kudu
bushpig
N
613~0/00
S.D.
3
3
6
- 8.4
0.3
3
8
-21.1
- 19.6
0.6
0.9
Concentrations of strontium were determined with a Perkin-Elmer model 5000
atomic absorption spectrophotometer (AAS)
fitted with an HGA 500 graphite furnace;
concentrations of calcium were determined
with a Varian AAS using an airlacetylene
flame and a matrix consisting of 0.2 N HN03
and 0.1%lanthanum (as LaC13).
RESULTS AND DISCUSSION
Herbivore 613C
Carbon isotopic values for the herbivores
are presented in Table 3. A clear difference
is seen between the 613C of the grazers, zebra, and reedbuck, and that of the browsing
kudu. The mean value of -8.5960 for zebra
closely agrees with that reported previously
for this species in East Africa (-8.3960,Ambrose and DeNiro, 19861, and is slightly more
positive than those reported from southern
Africa previously ( x = -9.3960, Vogel, 1978).
Reedbuck values have not been reported and
it is of interest that the mean carbon isotopic
value for this specialized grazer, -6.4%0, is
more positive than that of zebra. Vogel(1978)
reports a value of -6.0960 for related species,
Redunca fulvorufula, the mountain reedbuck. Since C4 grasses average - 12.5960,and
the positive shift in 613C from food to consumer collagen is approximately +5.3%0for
ungulates2 (Vogel, 19781, the value of -6.4960
is at or near the theoretical maximum for a
total grazer.
These isotopic data confirm behavioral and
other field observations that zebra are somewhat less than specialized grazers (Ansell,
1960; Darling, 1960; Smithers, 1983). For example, dicotyledons have been identified in
zebra faeces (Stewart and Stewart, 1970); detailed observations of feeding zebras have
also documented the inclusion of herbiage
*Although the exact enrichment value for collagen is the subject of continuing discussion (see Bumsted, 19841, I have used
the value previously reported specifically for large African mammals. Minor differences in this value do not affect the interpretation of the results.
-
6.4
8.5
0.8
0.2
and the pods of at least nine species of shrubs
or trees (Brynard and Pienaar, 1960; Smuts,
1972).With minor exceptions, reedbuck have
by contrast been observed to be virtually exclusive grazers (Brynard and Pienaar, 1960;
Jungius, 1970,1971).
Kudu are understood to be somewhat generalized browsers, eating leaves, shoots, and
the reproductive parts of woody plants. Only
small quantities of grass are accepted (Brynard and Pienaar, 1960; Wilson, 1970; Novellie, 1983).
Suid 613C
Carbon isotopic values for the suids are
also presented in Table 2. The mean values
of -19.6960 for the bushpig and -8.9960 for
the warthog also agree with recently published values for East Africa (Ambrose and
DeNiro, 1986). Bushpigs are omnivorous,
generalized feeders. Underground rooting is
directly largely at bulbs and tubers (Smithers, 1983).The diet includes ferns, other monocotyledons, and dicotyledons(Phillips, 1926),
with extensive utilization of fruit in the summer (Maberly, 1950; Brynard and Pienaar,
1960; Breytenbach and Skinner, 1981).
Bushpigs also eat earthworms and the pupae of defoliating insects, carrion, reptile
eggs, and young birds (Thomas and Kolbe,
1942; Williemse, 1962; Skinner et al, 1976).
One study identified reedbuck hoof and hair
in dung (Breytenbach and Skinner, 1981).
The omnivorous aspect of the diet may be
significant in contributing to the relatively
low Sr/Ca (discussed below) for this species.
In contrast, warthogs are more strictly vegetarian, living on annual and perennial
grasses (Brynard and Pienaar, 1960; Field,
1970; Cumming, 1975). Like zebra, they are
partial to freshly sprouting grasses after a
burn (Smithers, 1983). Inspection of stomach
contents of 32 animals collected in October
1962 a t the Nagupande station revealed
grass roots (Loudetia superba) to the almost
complete exclusion of other matter (Child,
A. SILLEN
Fig. 2. Frequency distribution of Sr/Ca in grazing and
browsing bovids. The specialized grazer (reedbuck) have
slightly lower Sr/Ca than the generalized browser (kudu).
The ranges overlap considerably.
Fig. 3. Frequency distribution of Sr/Ca in grazing and
browsing suids. Browsing, omnivorous bushpigs have
lower Sr/Ca than grazing warthogs. Neither distribution
is normal (see text for discussion).
TABLE 4. S d C a in Nagapande faunal sample
cv
Animal
N
Sr/Ca
(%)
Reedbuck'
Kudu (all)'
male'
female'
Zebra
Bushpigs3
Warthoes (all)3
male4female4
10
28
7
20
6
13
31
18
11
1.15
1.74
1.78
1.75
3.27
1.86
2.26
2.53
1.79
37
34
22
38
71
27
34
32
26
'Reedbuck and kudu (Student's t test) p = ,0089.
'Male and female kudu (Mann-Whitney U testl p = ,7028 (no significant diff.).
"Bushpigs and warthogs (Mann-Whitney U test) p = .0640.
4Male and female warthogs (Mann-Whitney U testl p = .0022.
1965; see also Cumming, 1975). Dicotyledonous plants have been rarely observed; these
and perhaps some C3 grasses account for the
6I3C values seen in this study, which are
slightly more negative than that of the pure
C4 grazing reedbuck they are, in fact, comparable to that of zebra.
SrlCa
Grazing us. browsing bovids: A significant
difference was observed between the Sr/Ca of
the specialized grazer, reedbuck, and the
generalized browser, kudu (Table 4); however, considerable overlap exists in these distributions (Fig. 2). Clearly Sr/Ca is not as
sensitive to grazingbrowsing dietary status
as carbon isotopes, where no overlap exists.
However, these data support the view that in
wild animals total dietary Sr (as opposed to
Sr/Ca) is at least a partial determinant of
bone mineral Sr/Ca. This suggestion could be
confirmed in the future by analysis of the Sr
and Ca content of specific plant species and
plant parts utilized by these animals.
Grazing us. omnivorous suids: In this case
the grazing warthog was found to have significantly higher Sr/Ca than browsing, omnivorous bushpigs. Neither distribution was
normal (Fig. 3); the demonstration of significance is therefore based on the Mann-Whitney U test rather than the t distribution
(Table 4). The relative Sr/Ca of these animals
are reversed when compared t o grazing and
browsing bovids. Given the discussion of
bushpig diets in section 2, the reason is likely
to be that bushpigs are also omnivorous: this
would have the result of reducing dietary Srl
Ca.
55
PALEODIETARY RESEARCH AND MAMMALIAN FAUNA
Sr/Ca i n nonruminants: The Sr/Ca of zebra,
the only nonruminants in this study, are
broadly comparable to those of the bovids
(Table 4); however, the mean values are significantly higher than that of the specialized
reedbuck. The likely interpretation is that
the slight browse component of the zebra diet,
indicated by 613C values 2 per mil more negative than reedbuck, provides enough additional dietary Sr to appear in the skeleton.
In other words, the difference seen in SrICa
between grazers and browsers may occur only
when dealing with extremely specialized animals. This hypothesis could be further tested
by sampling other sensitive grazers, e.g.,
roan and sable.
Although the sample size is relatively
small, the zebra SdCa is characterized by a
very wide range (Sr/Ca = 0.97-7.12) when
compared to other species reported in this
study. There are at least two possibilities to
account for this variation, which may also
contribute to the high mean SrICa. First, the
relatively fast transit time of food in the gut
of nonruminants such as a zebra may result
in nonrepresentative absorption of dietary
Sr/Ca. That is, only the most available alkaline earths may be absorbed, and these may
not reflect the total Sr/Ca of the plant material. This explanation seems unlikely, however, when the relative transit time of foods
among ruminants themselves is considered.
As specialized grazers, reedbuck have a relatively large rumen with very slow transit
time. In contrast, kudu have a relatively
small rumen, which is adapted to a rapid
throughput of food (Hofmann and Stewart,
1972; Giesecke and Van Glyswyk, 1975).The
CV in Sr/Ca of these two species is nevertheless similar.
A more likely explanation concerns the
ranging behavior of the species under consideration. Reedbuck are territorial animals
that restrict themselves to well-defined
ranges (Burt, 1943; Hediger, 1949; Jungius,
1971). The same may be said for kudu, although there are some significant variations
on this theme, which are discussed below
(Simpson, 1972). In contrast, zebra are not
territorial and range widely over large areas
(Smuts, 1972; Smithers, 1983). For the present it seems that the ranging behavior of
zebra, coupled with likely differences in Sr/
Ca entering various potential habitats is contributing to the high CV of this species.
Differences i n SrlCa by sex: Potential differences in SrICa between sexes of the same
species were examined in both kudu and
warthogs. From Figure 4 it may be seen that,
whereas no significant difference exists in
the mean SrICa, female kudus are more variable than males. The CV for males in 22%;
that for females is 38%. Among warthogs,
males have an average higher SrICa than
females, although there is considerable overlap and neither distribution is normal (Fig.
5). In contrast to kudus, males are slightly
more variable than females (male CV = 32%
female CV = 26%). Since these observations
have not been reported before, some discussion of the question of sex differences is provided here.
Laboratory studies under controlled conditions have repeatedly demonstrated that
0.10
om
110
1.m
zoo
zw
ldo
530
am
sr/m I imo
Fig. 4. Frequency distribution of Sr/Ca in male and
female kudu. Females are more variable than males.
1.m
1.m
zm
240
P/ca
ldo
am
am
Am
64
x lmo
Fig. 5. Frequency distribution of Sr/Ca in male and
female warthogs. Female Sr/Ca is significantly lower
than male Sr/Ca.
56
A. SILLEN
pregnant or lactating animals have higher
SrICa than nonpregnant or lactating controls
(Kostial et al., 1969; Blanusa et al., 1970;
Price et al., 1986). During pregnancy there is
an increase in the absorption of all alkaline
earths, and the maintenance of discrimination against strontium in the transport of
ions across the placenta (Hartsook and
Hershberger, 1973;Jacobson et al., 1978).The
result is a significant net increase in the Sr/
Ca ratio of the maternal plasma and, in turn,
maternal skeletons. A similar phenomenon
occurs during lactation inasmuch as the
mammary glands also act as a relative barrier to Sr (Lough et al., 1963; Twardock,
1963).
Similar sex differences do not appear to
occur in natural human and other animal
populations. No difference in SrICa by sex
has been seen in studies of modern Western
human populations (Turekian and Kulp,
1956). A study of white-tailed deer from midwestern North America has documented decreased Sr/Ca among females (Price et al.,
1985).
In certain instances it was possible in the
present study to measure the SrICa of individual animals observed to be pregnant or
lactating at the hunting station itself. A lactating reedbuck was found to have the lowest
SrICa among specimens of that species (Sr/
Ca = 0.72). A pregnant kudu was similarly
found to be at the lower margin of the respective range (Sr/Ca = 1.39).
What explanations may account for the apparent disparity betewen laboratory animals
and natural populations? First, all laboratory studies on pregnancy and maternal Srl
Ca have been based on a rat model. Rats
have significantly faster bone mineral turnover than larger mammals (Bronner and Lemaire, 1969),with the result that any potential effect of pregnancy on the skeleton may
be more pronounced in this species. Second,
laboratory studies generally include controls
for age, diet, and drinking water; these factors may indeed vary for wild populations.
For example, male and female kudu have
been shown to differ in important details of
range exploitation (Simpson and Cowie,
1967). Specifically, males may range less
widely on a seasonal basis than females, and
this observation has been related to: 1)the
greater water requirements of females during the winter dry season, and 2) the need to
shelter calves particularly during the same
period. Females may thus be more pressured
to follow water and cover with the change of
seasons, whereas males are not so dected
by these constraint^.^ The greater dispersion
of females in a mosaic of habitats is thus one
hypothesis to explain the greater variability
in Sr/Ca seen in the Nagapande specimens.
Moreover, it would also explain the similarity in CV between male and female warthog
Sr/Ca; in contrast to kudu, there is little difference in the range size of male and female
warthogs (Cumming, 1978).
As mentioned above, the elevated Sr/Ca
seen in pregnant and lactating rats is likely
to be related to the high rate of turnover
typical of rat skeletons. Rat skeletons may
more sensitively reflect elevated serum Sr/
Ca during these physiological conditions. At
least one study using rabbits has shown that
the turnover of Sr is greater than for Ca
(Kshirsagar, 1966);in larger animals Sr may
thus be preferentially mobilized during periods of skeletal demineralization as in lactation. This coupled with greater renal
clearance of Sr (Spencer et al., 1960; Walser
and Robinson, 1963) would result in lower
skeletal Sr/Ca in lactating animals.
Variability in Sr/Ca: With the exception of
zebra, the CVs seen in this study range between 30-35%. These values are broadly
comparable to the scant data available elsewhere. It should be noted that the variability
seen in these populations is somewhat
greater than that seen in laboratory animals
and other well-controlled studies (Table 5).
This issue has recently received attention
elsewhere (Price, 1987).
What remains unexplained is the peculiar
skewed distribution seen for suids in this
study. A search of the biochemical literature
on
indicates that such distributions are
in fact the norm for this element in bone
(Eckelmann et al., 1958; Kulp et al., 1959;
Kulp, 1960,1961).Similar patterns have been
reported for common Sr in the skeletons of
Australian sudden-death victims (O’Connor
et al., 1980). The skewing of Sr distribution
curves to the right received some comment
in the epidemiological studies of the 1950s;
the phenomenon was ascribed to the different rates at which individuals may equilibrate their bones with diet (Eckelmann et
3This is one possible interpretation, but it should be kept in
mind that ranging behavior may be quite variable from one
environment to the next. Female kudu live and breed in the
Kalahari with no water for months at a time (Child, personal
communication).
57
PALEODIETARYRESEARCH AND MAMMALIAN FAUNA
TABLE 5. Coefficients of variation (CV in natural, modern human, controlled, and
archaeological populations reported previously
Natural populations
cv (%)
Species
Price et al., 1985
Sillen, unpublished data
wild deer
wild gazelle
Modern human studies
Population
U. K. Med. Res. Council, 1959-1970
Thurber et al., 1958
Turekian and Kulp, 1956
Great Britain
New York City
worldwide
Controlled diet studies
Schoeninger, 1988
Kyle, 1986
Price, 1985
Sillen, 1981b
Sillen, 1981b
32.8
32.0
40.0
Animal model
Schoeninger, 1979
Price et al., 1986
Archaeological studies
35.0
45.0
farm mink
lab. rats
Sample
Dutch whalers
New Guinea
Late Archaic
Natufian
Aurignacian
al., 1958). Moreover, it was noted that it is
possible to increase the amount of "Sr per
gram of calcium by factors of 100 to 1,000 in
soils, but to decrease it by factors of only 2 to
10 (before reaching the effective detection
limit). Whereas natural strontium certainly
varies far less in a given environment than
did "Sr in 1955, the same principle may
apply. That is, the potential for increased Srl
Ca may be greater than for decreased Sr/Ca
in a given environment.
A related possibility is that, barring conditions of physiological demineralization as in
lactation, Sr accumulates in skeletons with
age. The elevated SrICa seen in some individuals may therefore be related to the age curve
of the population. (It may be possible to test
this idea further since the criteria exist for
aging warthog up to 48 months if time of
death is known [Child et al., 19651. Examination of the early biochemical literature on
common Sr, as well as related data on lead
(Pb), provides some evidence for this suggestion. One study of over 400 human samples
from England and Wales noted an increase
in Sr concentration with age (Bryant et al.,
1958; Bryant and Loutit, 1961; see also Thurber et al., 1958). Similar data exist for mule
deer from Colorado (Farris et al., 1967). Related data exist for the heavy metal Pb; this
element behaves vis a vis calcium in much
the same way as Sr (Elias et al., 1982). When
controls for pathological demineralization as
19.3
22.0
Date
400 BP
300-1000 BP
3000-700 BC
c.10,OOO BC
c.17-20,000 BC
25.0-30.0
18.0
11.0-21.1
13.0
9.6
in osteoporosis were incorporated, Pb concentrations in skeletons from ambient exposure
have indeed been shown to increase with age
(Barry and Mossman, 1970; Gross et al., 1975;
O'Connor et al., 1980).
These considerations provide a framework
for understanding the skewed distributions
seen in the suid data. Suids have been shown
to have unusual population structure because they are multiparous with a potentially very high reproduction rate; in this
respect they differ from other large ungulates (Cumming, 1975). Indeed, one study
from western Rwanda has reported warthog
longevity to be 17 years, but with a mean
length of life of only 2.62 years and a mean
life expectancy of 3.03 years (Spinage, 1972).
Whereas hunters tend to select large targets,
and hence adult animals (Child, personal
communication), the sample of warthogs may
contain relatively more young adults than
the sample of kudu; coupled with the observation that skeletal Sr may increase with
age, this would explain the skewed distributions seen in skeletal Sr/Ca for warthogs but
not for kudu.
IMPLICATIONS FOR PALEODIETARY STUDIES
Sr/Ca as atrophic indicator is of interest to
prehistorians because it can potentially address the central issue of the origins of human omnivory. The application of SrICa and
other trace-elemental relationships to pa-
58
A. SILLEN
leoanthropological issues has been hindered,
however, by the recognition that the biological signals may become obscured or obliterated after interment. This phenomenon is
called diagenesis; efforts to understand it
have to some extent dominated the Sr literature (Sillen, 1981a,b, 1986, 1988; Lambert et
al., 1982, 1985; Klepinger et al., 1986; Pate
and Brown, 1986; Price, 1988)at the inadvertent expense of sufficient attention to the
biological assumptions on which the technique is based.
The results of the present study do not show
that Sr is insensitive to diet and other biological phenomena, but rather that it may be
more sensitive than previously thought. This
sensitivity implies that it may be necessary
to devote more attention to the details of
wildlife ecology than has been the case until
now.
In the past comparisons of human to faunal
skeletal material have not considered the
natural distributions of Sr in animal species;
a consideration of such distributions would
be useful firstly in clarifying the theoretical
basis for biopurification in food-webs.For example, some confusion has existed over
whether the biopurification of Sr is due to
the reduced Sr/Ca in meats (from intestinal
discrimination by herbivores) or due to the
absolute low levels of Sr in certain foods,
notably meats and grasses. The answer is
probably both the reduced Sr seen in specialized grazers in this study provides evidence
that total dietary Sr may well be a factor,
but this conclusion needs to be strengthened
by analysis of wild plant species from the
region?
With regard to the research design of paleodietary studies, the Nagapande data bears
directly on the selection of appropriate species against which to compare humans, on
the detection of diagenesis, especially where
carnivores are unavailable, and on the necessary sample size.
It is clear from the Nagapande data that
zebra are troublesome animals from the point
of view of Sr because of their high variability, and this may be related to their ranging
behavior. Reedbuck are virtually exclusive
grazers with low Sr values, which are not
representative of other less specialized bo4This issue should not be confused with the purely methodological one of whether to report bone assays in terms of Sr or SrICa.
Since bone ash has a constant Ca value of approximately 30%
(Price, 19871,it is necessary to report SrICa values only when
whole bone (rather than bone ash) is analyzed.
vids. On the other hand, kudu vary similarly
to humans and have Sr values more representative of a generalized herbivore. Kudu
are therefore an attractive candidate against
which to compare human bones at African
archaeological and palaeontological sites.
It has been previously asserted that measurement of both carnivore and herbivore
bones provides the best control for diagenesis
(Sillen, 1981a; Sillen and Kavanagh, 1982).
One problem with this approach is that carnivores are usually quite scarce at archaeological sites (Price et al., 1985). For critical
applications, there is no real substitute for
the measurement of carnivores, since this is
the only way to ascertain the spacing between known herbivore and carnivore diets;
this spacing has been shown to change with
diagenesis (Sillen, 1981a,b).Nevertheless, the
examination of herbivore bones alone can
provide an indication of the presence of
diagenesis, if not its extent. That is, if the
extent and shape of the biological variability
for given taxa in a given environment are
understood, they may be compared to fossil
taxa. Any deviation from the variability seen
in analogous modern taxa would be a clear
danger signal. In general, older material is
found to have lower variability (Table 5),and
this is presumably a diagenetic phenomenon.
Finally, the variability seen in the modern
fauna in this study makes it possible to estimate the kind of sample numbers that would
be required to make confident statements
about the diets of prehistoric humans. An
approximation for sample number is based
on the equation:
where: 1 - /3 = the specified power (in this
case .go), cx = the level of significance (in this
case .05), p1 and ~2 are hypothetical population means, and i? = the estimate of variance
(taken from the kudu sample in this study).
Thus
n =
2('6)2
(1.8 - 1.0)
n = 12
(1.96
+ 1.29)2
A sample number of at least 12 is therefore
appropriate.
In summary, the Sr/Ca of modern faunal
animals from southern Africa have been
shown to vary in relation to diet and sex.
PALEODIETARY RESEARCH AND MAMMALIAN FAUNA
Ranging behavior may also contribute to
variability i n Sr/Ca. Further attention t o the
details of wildlife ecology will enhance the
application of trace-element techniques to the
study of prehistoric human diets.
ACKNOWLEDGMENTS
I wish to thank Dr. H.D. Jackson, director
of the National Museum of Zimbabwe, and
Mr. A. Kumirai, the cadet curator of mammalogy, for making specimens available for
this study; my thanks also to Ms. J. Sealy,
Professor N.J. van der Menve, and Dr. Graham Child for useful suggestions and comments. Mr. J. Lanham, Ms. M. de Rocha
Mille, Cedric Poggenpoel, and Dr. R.S. Sparks
(Department of Mathematical Statistics,
UCT) provided valuable technical support.
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