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Comparative and functional morphology of hominoid fingers.

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Comparative and Functional Morphology of
Hominoid Fingers
RANDALL L. SUSMAN
Department of Anatomical Sciences, Health Sciences Center, State University of
New York at Stony Brook, Stony Brook, New York 1 1 794
KEY WORDS Ape hands . Hominoid morphology . Knucklewalking
. Ape locomotion . Morphometrics
ABSTRACT
Comparisons of hominoid metacarpals and phalanges reveal
differences, many of which are closely linked to locomotor hand postures. The
African apes display features of the metacarpals and phalanges which distinguish them from t h e other Hominoidea. These features are most evident in digits 111 and IV. The orangutan hand is demonstrably less well adapted to
knuckle-walking and is distinctive in its adaptation to power and hook grasping of vertical and horizontal supports, respectively. Orangutan fingers possess
a "double-locking'' mechanism (Napier, '601, and a slight ulnad shift in the axis
of the hand which results in lengthened phalanges of ray IV. Hylobatid apes
are more like orangutans in their finger morphology than any of the other
Hominoidea, but exhibit unique features of their own. These include elongate
phalanges of fingers 11-V. Human metacarpals 11-V form two sets composed of
11-111,and IV-V. The heads of both metacarpals I1 and 111are characterized by
axial torsion. This reflects the enhanced manipulatory role of the third finger
in humans. Human distal phalanges are unique in the development of pronounced apical tufts.
Multivariate analysis of metacarpal 111and proximal I11 yields variables that
array the extant apes along an arboreal-terrestrial axis, from hylobatid apes to
male gorillas. The positions of taxa on this discriminant concur with observations on the locomotion of free-ranging apes.
The comparative morphology of the hand
has been a central topic in discussions of
human phylogeny for over a century (Huxley,
1863; Gregory, '28; Osborn, '28; Wood-Jones,
'42; Straus, '40, '49; Marzke, '71). More recently, the discovery and accounts of hominoid hand fossils have spurred interest in the
functional and evolutionary morphology of
hominoid hands (Napier, '59; Napier and
Davis, '59; Zapfe, '60; Leakey, '60; Musgrave,
'71, '73; Day and Scheuer, '73; Lewis, '73; Johanson, '76).
A number of workers have studied the functional morphology of ape and human hands
(Napier, '60, '62; Tuttle, '67, '69; Lewis, '69,
'72a,b, '73, '771, Others, primarily interested
in the wrist, have also considered aspects of
the descriptive and evolutionary morphology
AM. J. PHYS. ANTHROP. (1979) 50: 215-236.
of hominoid hands (Schon and Ziemer, '73;
Corruccini et al., '75; Etter, '74; Jenkins and
Fleagle, '75; O'Connor, '76). Tuttle ('67) and
Lewis ('69) have identified features of wrist
morphology that distinguish the hominoids
and that separate hominoid from cercopithecoid primates, but functional interpretations
of wrist morphology are almost as numerous
as the studies themselves (e.g., Lewis, '72a,b;
Tuttle, '67, '74; Jenkins and Fleagle, '75;
Schon and Ziemer, '73). Correspondingly,
inferences on hand use in fossil primates have
varied. Lewis ('72a,b) identified certain suspensory features in the wrist of Dryopithecus
africanus while others have suggested cercopithecoid-like quadrupedalism (Preuschoft,
'73a; Morbeck, '75; Corruccini et al.; '75;
O'Connor, '76), cebid-like quadrupedalism
215
216
RANDALL L. SUSMAN
(Schon and Ziemer, ’731, and knuckle-walking
(Conroy and Fleagle, ’72). In spite of these differences of opinion, few workers have a t tempted t o extract quantitative, functional
information from the morphology of the manual rays.
The present study investigates the relationship of metacarpal and phalangeal morphology to locomotor hand use in the Hominoidea.
The data from the manual rays complements
earlier work on the primate wrist and provides
a firmer basis for determining hand use in
fossil Hominoidea.
hancing mechanical properties of long bones
(Currey, ’68). In some instances approximations to the ideal representation of these features had to be made. A series of measurements were made on each individual metacarpal and phalanx of rays 11-V.
Figure 1 illustrates the points from which
the variables are taken. Variables for metacarpals include length (c-h), radio-ulnar midshaft diameter (r-s), dorso-palmar midshaft
diameter (b-el, dorso-palmar head diameter
(d-f), radio-ulnar head diameter (n-o), biepicondylar diameter (p-q), dorso-palmar diameter of the medullary cavity, and set of the
metacarpal head. l Other variables taken from
these points were also tested. The phalanges
were similarly characterized by a set of linear
and computed variables. For the proximal
phalanges these include: length ( a d , radioulnar midshaft diameter (h-i), dorso-palmar
midshaft diameter (b-d),radio-ulnar diameter
of the base (f-g),radio-ulnar diameter of the
trochlea (j-k),“biepicondylar” diameter (1-m),
and flexor sheath ridge height (across points
h-i). Other variables including cortical thickness and longitudinal curvature were computed from these points. Middle and distal
phalanges were described by a set of measurements taken from points illustrated in figure
1. The variation in hominoid fingers is thus
described with multivariate procedures that
employ sets or subsets of the above variables.
Dissections were performed on hands of four
Pan troglodytes, two Gorilla, two Pongo, two
Hylobates lar, and two Homo sapiens. Other
dissection data from a n extensive series of ape
forelimbs was provided by Professor Russell
H. Tuttle.
MATERIALS AND METHODS
This work is based on observations and measurements of a sample of 208 adult hominoid
hand skeletons (table 1). In all but a few cases
specimens are wildshot. The human sample is
drawn from the Todd Collection housed a t the
Cleveland Museum of Natural History.
Articulated hands were used for determining identifiable patterns in individual ray segments and for determining ray segment formulae. Such skeletal material was then disarticulated and cleaned. After a method for
recognizing individual phalanges was determined, loose finger bones were entered in the
study. Measurements were taken on individual bones and from radiographs. The latter
were made with an Oralix 415 portable X-ray
unit. Bones were placed directly on the film
plane and exposed with the tube head a t a distance of 74 cm. Lateral and frontal exposures
were made on all specimens. Radiographs permit inspection of internal bony features and
provide an accurate permanent record of
specimens studied. Data attesting to the
accuracy of the radiograph technique are presented in table 2.
For statistical comparisons a set of variables were selected that represents morphological responses to patterns of stress in loading.
Features such as curvature, cortex thickness,
joint articular surface area, and linear dimensions have been discussed as means for en-
Descriptive morphology of metacarpals II- V
Gorilla
The metacarpal of gorillas are comparatively short and stout, with heavily developed secI
This was computed as the central angle of triangle abc using the
B‘ + C’ A‘
= COS t) where flis the central angle.
2BC
~
fUl-IIIUla:
TABLE 1
Specimens (adult, mostly wildshot)
Pan
troglodytes
Pan
paniscus
M
F
M
F
M
F
M
F
M
F
M
F
Total
19
19
5
6
20
17
13
24
20
20
21
24
208
H. concolor, H. lar, H.hoolok,
Gorilla
Homo
sapiens
Pungu
H.syndactylus, H. funerur, H.
alhrminus, If. klossi, Ii. s p p ?
Hylobates
SPP.
217
FUNCTIONAL MORPHOLOGY OF HOMINOID FINGERS
TABLL 2
Comparison o f measurements f r o m bones and radiographs
..
v, - J ,
Mean difference lmmr
so.
Length proximal phalanx IV
Diameter base proximal phalanx V
Length metacarpal V
Diameter, dorso-palmar metacarpal 111
Diameter, dorso-palmar distai phalanx I1
Ri-epicondyle diameter middle phalanx TI
Length middle phalanx V
Length distal phalanx I1
Diameter, radio-ulnar middle phalanx V
1
12
12
10
12
5
12
r y.yA'
0.999
0.998
0.998
0.996
0.959
0.995
0.998
0.994
0.998
0.358
0.125
0.700
-0.016
- 0.200
0.117
0.275
-0.740
0.067
12
5
~
12
r, correlation coefficient; y , . meamrements on art,ual hones; y l . mrasurements taken from radiographs
b
=&+9,
0.
a
C.
4
c
i
F
f.
Fig. 1 Measurement points for variables on metacarpals and phalanges. Linear and computed variables
are taken from these points and from radiographs.
ondary features in the adult. The predominant
metacarpal length formula is 11-111-IV-V
(82x1, but in 11 specimens metacarpals IV
and V are equal in length. This equality is in
marked contrast to the length pattern in the
other Hominoidea. The metacarpal shafts are
robust and essentially triangular in cross section. The metacarpal heads are large with well
defined secondary features such as epicondyles, articular surface relief and capsular
impressions.
The head of metacarpal I1 displays a vari-
218
RANDALL L. SUSMAN
A
C
F
Fig. 2 Distal end view of metacarpal heads 11-V.A. Siamang (AMNH 106584 Lt. d). B. Modern Human
(S.U.N.Y. 15 Lt. d). C. Bonobo (Tervuren 29042 Lt. 7). D. Chimpanzee (AMNH 89354 Lt. 7). E. Gorilla
F. Orangutan (AMNH 61584 3 ) .
(AMNH 1673384 6).
able dorsal articular ridge (fig. 2E) but the
radio-ulnar diameter of the articular surface
does not equal that of the third or fourth
fingers. The head is twisted axially so that the
dorsal aspect is oriented toward the third ray.
The epicondyles and excavations for the collateral ligaments are strongly represented.
The head of metacarpal I1 is distinct from that
of 111 and IV in having its maximum radioulnar diameters on the palmar surface in 27 of
30 cases,
Metacarpal I11 is particularly well developed. The shaft is heavily constructed with
pronounced impressions resulting from the
219
FUNCTIONAL MORPHOLOGY OF HOMINOID FINGERS
origins of the second and third dorsal in- ridge. Asymmetry of the head when viewed
terosseous muscles. A strong anterior crest dorsally is not marked in Pan troglodytes as in
often separates the anterior extent of these Gorilla, indicating reduced conjunct rotation
muscles and ends in a tubercle for the glenoid and radial deflection of the finger during hyligament of the metacarpnphalangeal joint. perextension of t h e metacarpophalangeal
The dorsal articular ridge is marked and spans joint.
heavily developed epicondyles. In 76%of speciThe fourth metacarpal in Pan troglodytes
mens (n = 37), the maximum radio-ulnar di- recalls the shape of metacarpal I11 in features
ameter of the head is on the dorsal surface.
of the head and shaft (fig. 2D). It is always
Metacarpal IV resembles 111in the morphol- shorter than I11 and less robust. The greatest
ogy of its shaft and head. The base may have breadth of the articular surface is dorsal, and
either one or two facets for contact with a well developed, continuous dorsal ridge is a
metacarpal 111. The hamate surface extends consistent feature.
anteriorly onto the surface for metacarpal V
Metacarpal V is small and does not apand may continue distally to a variable proach the length of metacarpal IV to the exdegree. The anterior interosseous crest and tent observed in gorillas. The head is broader
glenoid tubercle are conspicuous. The head is on the palmar than on the dorsal surface in
broad and resembles that of 111. In 77% of 89% of specimens (n=37). The head lacks a
cases (n=35), the dorsal articular surface is dorsal ridge and displays an axial twist which
broader than the palmar surface. In profile, mirrors that of metacarpal 11.
the heads of both metacarpals I11 and IV flatPan paniscus
ten prior to the dorsal ridge. In dorsal view the
The metacarpals of Pan paniscus differ
distal articular surfaces of these metacarpals
are extended on their ulnar aspect. This pro- from those of chimpanzees in a number of subvides for a slight axial rotation and radial tle ways. The present bonobo sample, although
deflection upon hyperextension of the meta- small, indicates that females and males have
carpophalangeal joints.
Metacarpal V is proportionately longer in
TABLE 3
gorillas than in other Hominoidea. In gorilla
males i t attains 97%of the length of metacarLength proportion of metacarpal V in the apes
and man (adults)
pal IV (table 3). The shaft is broad and flat
and often possesses a lateral curvature, conLength metacarpal
V/rnetacarpal IV
cave radially. The head is relatively narrow
with an axial twist t h a t mirrors the condition
Males
Female8
of metacarpal 11. In 29 of 30 specimens the
Gorilla gorilla
palmar surface of the head is broader than the
22
20
!!
dorsal surface.
X
Pan troglodytes
Chimpanzee metacarpals are long and thin
compared to those of gorillas, but they resemble the latter closely in features of the
metacarpal head. The head of metacarpal I1 is
asymmetrical, as in Gorilla, and lacks a dorsal
articular ridge in 42%of cases. The epicondyles are well developed as are the excavations for the metacarpophalangeal joint collateral ligaments.
Metacarpal I11 displays a marked dorsal
ridge on the distal articular surface and this
ridge spans well developed epicondyles (fig.
2D). The maximum radio-ulnar diameter of
the third metacarpal head is always located
dorsally (n=33). In profile the head recalls
the pattern observed in gorillas, with less pronounced dorsal flattening prior to the dorsal
S
97.33
2.97
95.18
2.34
17
91.18
2.63
19
91.21
2.20
5
88.95
3.85
89.19
0.67
13
92.31
2.10
24
91.92
2.30
12
88.08
1.83
15
88.20
1.74
20
93.00
2.29
20
92.35
1.93
Pan troglodytes
n
X
S
Panpaniscus
11
X
8
Pongo pygmaeus
n
-
X
S
Hylobates andsymphalangus
n
-
X
8
6
Homo sapiens sapiens
!!
x
S
220
RANDALL L. SUSMAN
similar mean metacarpal lengths (table 5 ) . If
this is indeed the case, then the pattern is
unique among the great apes. The metacarpal
heads are long and narrow compared to those
of the other apes (fig. 2C). The pattern of dorsal articular ridges on metacarpals 11-V differs in bonobos from the pattern observed in
chimpanzees. Slight dorsal ridges are found in
only 6 of 11 adult specimens. In two cases
slight ridges occur on all four metacarpals. In
two other instances dorsal ridges are found
only on metacarpal 111, and in two cases faint
ridges occur on 111and IV. All second metacarpals display a wider palmar articular surface.
but only 7 of 11 fifth metacarpals retain this
condition. Metacarpals 111 and IV display the
expanded dorsal surface observed in chim
panzees and gorillas. In 2 third and 2 fourth
metacarpals the palmar articular surface is
broader than the dorsal aspect.
Pongo
The metacarpals of orangutans differ from
those of the knuckle-walking apes in a number of features of the shaft and head (fig. 2F).
The morphology of the bases of metacarpals
11-IVis for the most part similar to that of the
African apes (Lewis, '77: p. 173). Orangutan
metacarpals are elongated with less pro
nounced muscle and ligament impressions on
the shaft and head. The shafts of metacarpals
11-IVare greater in antero-posterior diameter
than in radio-ulnar breadth. The sides of the
shafts diverge more abruptly than in the
African apes, and they terminate in relatively
reduced epicondyles. The cortices of the shafts
are thicker in orangutans than in the other
great apes or humans. The metacarpal heads
of all four metacarpals (11-V) are normally
broader on their palmar surface, and the most
dorso-proximal extent of the articular surface
of III and IV does not end in a dorsal ridge but
rather it ends in bipartite extensions of the a r
ticular surface.
The head of metacarpal I1 is invariably
wider on the palmar surface (n=31). The
ulnar moiety of the distal articular surface
commonly extends more dorsally than its radial counterpart, contributing to the axial torsion of the second metacarpal head. Metacarpal 111 is more symmetrical than 11, lacking torsion of the head. In 27 of 32 cases the
maximum breadth is palmad in contradistinction to the African apes. In four cases the dorsal surface is broader and in one specimen the
two surfaces are equal. Dorsal articular ridges
are absent, and in dorsal view the epicondyles
are not pronounced. Metcarpal IV is similar t o
I11 in morphology of the shaft and head. In 27
of 30 cases the palmar aspect of the articular
surface exceeds the dorsal aspect in breadth.
Metacarpal V mirrors I1 in asymmetry of the
head, while the shaft cross-section attains its
greatest diameter in the radio-ulnar plane,
similar to the other great apes and humans.
Metacarpal V in orangutans may possess a
marked lateral curvature, concave ulnad.
Hylobatid apes
The metacarpals of the hylobatid apes are
long and thin with nondistinctive heads. Muscle markings for the intrinsic hand muscles
are slight in gibbons, but may be moderately
developed in siamangs. The metacarpals are
lacking in curvature for the most part, although metacarpal V often displays a lateral
(radio-ulnar)curve, concave ulnad. The cortex
of the metacarpal shafts is relatively thin,
especially when compared with that of Pongo
and Pan (see below). In some individuals
metacarpal I1 has the most robust shaft, while
in others metacarpal 111 is more robust.' In
both gibbons and siamangs the dorsal extent
of the distal articular surface on metacarpals
11-V is more restricted than that of the great
apes (fig. 2A). The palmar portions of the
metacarpophalangeal joint capsules contain
sesamoid bones in contrast to the other apes
(Calhoun, '77).
Homo sapiens
The metacarpals of humans are characterized by well developed muscle markings,
globular heads and a short, stout appearance.
The third metacarpal in humans may exceed
the second metacarpal in length (27%of cases)
due to the development of a prominent basal
styloid process in the latter bone. Metacarpals
I1 and I11 resemble each other and differ from
those of IV and V in being long and robust. All
four metacarpals exhibit thin cortices.
Metacarpal I1 in humans is normally the
most robust in contradistinction to the great
apes. The base is well developed and variations
in carpometacarpal and inter-metacarpal articulations have been noted by Singh ('59).
The metacarpal shaft is marked by the
impressions for the first two dorsal and first
palmar interosseous muscles. The metacarpal
Shaft robusticit" is defined as: radioulnar
ameter a t m i d s h a f t k n g t h x 100.
X
antervpvsterivr di-
FUNCTIONAL MORPHOLOGY OF HOMINOID FINGERS
head is markedly twisted and bears no trace of
a dorsal articular ridge. The flexion set of the
head is slight compared to the great apes.
The third metacarpal in humans is distinctive in its asymmetric head (fig. 2B). It re.,qembles the second metacarpal in that the head i s
axially twisted. During flexion both the second and third fingers rotate and deviate
radially. Thus in humans, not only does the
index finger undergo rotation and radial
deviation during flexion (Landsmeer, '55;
Lewis, ' 7 7 ) , but the medius also endorotates
and radially deviates during metacarpophalangeal joint flexion. This is a clear accommodation to pollex-indicus-mediusopposition
during precision grip. The third metacarpal
head in humans, like that of metacarpal I l l in
Pongo and the hylobatid apes, maintains its
greatest radio-ulnar diameter on the palmar
surface.
The length and robusticity of metacarpal IV
is substantially less than that of I1 or 111. In
the human hand metacarpals IV and V form a
functional unit t h a t provides ancillary support in the precision grip and plays a more
prominent supportive role in the power grip.
This functional pairing is unlike the 11, 111-IV,
V grouping of the fingers in the great apes, or
the 11, 111, IV, V sequence in the hylobatid
apes. Metacarpal IV is more symmetrical in
humans with less torsion of the head than
either 11, 111, or V. The fifth metacarpal is diminutive. The base lacks t h e anterior lipping
of the hamate surface seen in the great apes.
In some specimens there is moderate torsion of
the metacarpal head while in others torsion is
only slight.
Summary of metacarpal differences
Differences in the metacarpals of rays 11-V
can be summarized as follows:
(1) The metacarpals of the African apes
possess more strongly developed heads than
other hominoids with a variable occurrence of
a dorsal articular ridge. Within African apes
the expression of the dorsal ridge ranges from
slight in the bonobo to marked in the gorillas;
it is more pronounced on metacarpals I11 and
IV than on I1 or V.
( 2 ) Widening of the dorsal aspect of the distal articular surface is common in metacarpals I11 and IV in the African apes, but this expansion is normally lacking in metacarpals I1
and V. Expansion of the dorsal articular surface is absent in other Hominoidea.
(3) The strongly developed secondary fea-
22 1
tures observed in the metacarpals of the
African apes are less well developed in Pongo
and lacking for the most part in humans, and
the hylobatid apes.
(4)The metacarpals of Pongo possess
thicker cortices than those of the other
Hominoidea while the shaft cross sectional
areas assume an ovoid shape with the long
axis in the dorso-palmar plane.
(5) Human metacarpals differ in two
unique and pronounced ways from the other
Hominoidea: (a) the head of metacarpal I11 is
twisted relative to the long axis of the shaft,
and (b) the pairs 11-111and IV-V form two distinct size groups which reflect their different
roles in precision and power grip.
Descriptive morphology ofphalanges 11-V
Gorilla
The proximal phalanges of gorillas are
stout, heavily constructed, and notable in
their lack of longitudinal curvature (fig. 3).
All articulated gorilla hands adhere to a
I I I > I V > I I > V length formula for the proximal phalanges. On the dorsal aspect of the
base of proximal phalanx I1 there is an extension of the palmar radial tubercle which receives the insertion of the pennate portion of
the first dorsal interosseous muscle. This
tubercle imparts a characteristic asymmetry
to the base (fig. 3). Basal tubercles for the collateral ligaments are also marked. In gorilla
males the flexor sheath ridges are extremely
well developed. The trochlea is flattened on its
distal end and raised well above the anterior
surface of the body.
Proximal phalanx I11 is notable in its symmetry in dorsal view. Proximal phalanx IV is
distinguished from I11 by its reduced length
and basal asymmetry induced by the insertion
of the fourth dorsal interosseous muscle. The
flexor sheath ridges are considerably more
pronounced on phalanges I11 and IV than on I1
or V. The anterior portion of the phalanx base
projects well above the surface of the body
increasing the moment of the long flexor tendons as they course over the metacarpophalangeal joint. The fifth proximal phalanx
is markedly asymmetrical and considerably
smaller than the others. The base protrudes on
its ulnar side where the abductor and flexor
digiti minimi muscles insert. The radial side
of the body bows out a t the flexor sheath ridge.
The middle phalanges of Gorilla are very
robust with marked antero-posterior flattening. The bases are extremely well developed
222
RANDALL Id. SUSMAN
1cm
m
Fig. 3
Proximal phalanges 11-Vat. to rt.) of gorilla (adult d!
TABLE 4
Antero-posterror diameter of distal phalanx baselradio-ulnar diameter of distal
phalanx base in the Hominoidea
Hominoid
No.
I
I1
111
IV
V
Chimpanzees
Gorilla
Humans
Orangutans
Hylobatids
6
6
6
67
58
58
80
71
76
69
79
70
60
60
60
6
68
73
6
57
74
80
71
72
70
80
69
62
73
71
and the bodies lack longitudinal curvature.
The rugose surfaces for the insertions of flexor
digitorum superficialis are marked, particularly in phalanges I11 and IV. Immediately laterally are well defined flexor sheath
ridges.
All but two specimens of gorilla display a
TI1 > IV > I1 >V length formula for the middle
phalanges. The trochleae of middle phalanges
11, IV, and V often incline toward the third
ray. The bases of IT-V are broad and deep with
well developed protuberances for the col-
lateral ligaments of the first interphalangeal
joint. The trochlear circumference, in lateral
view, is not as extensive as that of the proximal phalanges.
The distal phalanges do not conform to a
regular size sequence in Gorilla. However, the
base of distal phalanx I is distinctive in its
“flatness” and kidney-shaped appearance.
This obtains in all of the apes, and t o a lesser
extent, humans (table 4).As a result, the pollical distal phalanx may be distinguished from
that of the other rays.
FUNCTIONAL MORPHOLOGY OF HOMINOID FINGERS
223
1cm
Fig. 4
Proximal phalanges 11-V flt. to rt.1 of chimpanzee (adult d).
The apical tufts of distal phalanges 11-Vare
well developed, second only to humans. The
apical tuft of the pollical distal phalanx, however, is slight compared t o the other four.
Pan troglodytes
The proximal phalanges of chimpanzees
have a constant length formula of III>IV> I I > V . As with the proximal phalanges of
gorillas, subtle asymmetries of bones, centered around the axial third ray, permit identification of individual phalanges. The development of secondary features such as flexor
sheath ridges, basal protuberances and longitudinal curvature are variable in adult chimpanzees. Flexor sheath ridges range from
marked to very slight in adults of both sexes
(fig. 4).
Lateral radiographs reveal a proportionate-
ly thicker cortex in the proximal phalanges
than in the metacarpals of chimpanzees and
other great apes. The proximal phalanges develop the greatest amount of cortex in the distal portion of the body.
Asymmetry of the trochlear surface is not
consistent, but often the radial moiety of proximal phalanx I1 and the ulnar portion of V are
larger than their counterparts.
The length formula for chimpanzee middle
phalanges is like that for the proximal phalanges; however, the lack of consistent asymmetry in individual bones makes their identification difficult in the absence of a complete set.
The distal phalanges are similar t o those of
gorillas. The expression of the apical tuft and
overall robusticity are less than in gorillas,
but the distal phalanges are relatively longer
224
RANDALL L. SUSMAN
in chimpanzees. Distal phalanges I1 through
V adhere t o the III>IV>II>V pattern in all
cases (n = 17), and the pollical distal phalanx
can be distinguished on the basis of its basal
index (table 4).
Pan paniscus
The overall shape of the proximal phalanges
is similar in Pan paniscus and Pan troglodytes.
However, the proximal phalanges of bonobos
differ in a number of respects from those of
chimpanzees. The flexor sheath ridges are
faint and may be absent in adult bonobos.
Moreover, the palmar surface of the body, in
some cases, is raised above the margins where
the fibrous flexor sheaths insert. This condition recalls that often observed in orangutan
proximal phalanges. The trochlear sulcus is
deeper in bonobos than in chimpanzees.
Pongo
The phalanges of Pongo differ markedly
from those of the African apes both in the
comparison of individual bones and as a set.
This morphological divergence is observed in
proximal, middle, and distal phalanges.
The length formula of the proximal phalanges varies in Pnngo and reveals a pattern
of sexual dimorphism. In 58% (n=36) of
orangutans the length pattern of proximal
phalanges is III>IV>II>V, while 28% display a IV > I11> I1 >V formula. Fourteen percent possess a I11= IV > I1 >V pattern. Fifty
percent of articulated male orangutan hands
display the IV > I11 > I1 >V pattern while only
5%of females (1 specimen) show this pattern
of proximal phalangeal length. The proximal
phalanges in both sexes are long, thin, and ex-
Fig. 5 Proximal phalanges 11-V (It. to rt.) of orangutan (adult d).
FUNCTIONAL MORPHOLOGY OF HOMINOID FINGERS
tremely curved. The bones widen distally and
achieve their greatest radio-ulnar diameter a t
the flexor sheath ridges.
Curvature of the proximal phalanges contributes in large part to the exaggerated flexion set of the orangutan hand (fig. 5). The flexor sheath ridges are located opposite the point
of maximum arc, increasing the sheath's
ability to prevent bowstringing of the long
flexor tendons and perhaps also providing a bilateral buttress for the body of the bone. The
radio-ulnar concavity of the anterior surface
of the body is lacking in Pongo for the most
part. The anterior surface is, on the contrary,
often raised above the height of the flexor
sheath ridges. The trochlear sulcus is deep.
Asymmetry of the second and fifth proximal
phalanges of orangutans is similar to the
other Pongidae. The third and fourth phalanges are variable in their relative degree of
asymmetry. Often, when proximal phalanx IV
exceeds 111 in length, the former bone is also
more symmetrical. In this case the third phalanx takes on a pattern normally seen in I1
wherein the tubercle for the insertion of the
pennate portion of the second dorsal interosseous muscle is enlarged and the ipsilatera1 flexor sheath ridge may extend more distad. In these specimens, however, the overall
robusticity of proximal phalanx I11 is still
greater than that of IV. The third phalanx
maintains a greater proximal articular surface, greater radio-ulnar diameter of the body,
and a broader trochlea. Thus, the hypertrophy
of proximal phalanx IV (mainly in male
orangutans) is primarily a linear development.
The middle phalanges of Pongo reflect the
size relationships of the proximal phalanges.
The most notable feature of the middle phalanges is the relatively strong longitudinal
curvature. Only 32%of orangutans possess the
basic 111>IV > I1 > V length pattern in the
middle phalanges. In 40% phalanges I11 and
IV are equal in length, and in 28% the pattern
is IV > I11 > I1 > V. Thus the middle phalanges
deviate more frequently from the I11 > IV> I I > V sequence than the proximal phalanges. The middle phalanges constrict immediately beyond the base, but expand to their
maximum breadth a t the midpoint of the insertions of m. flexor digitorum superficialis.
These areas are well defined and bordered by
moderately developed flexor sheath ridges.
There is no striking asymmetry of the middle
225
phalanges and, unlike t h e proximal phalanges, the overall robusticity of IV may exceed that of 111. Often the distal articular surfaces of I1 and V incline toward the midline of
the hand.
The conical tip of orangutan fingers is reflected in the reduced apical tufts of the distal
phalanges. Often the pollical tuft is no wider
(radio-ulnad) than the body itself (29% of
cases). Malformations including lateral splaying and resorption of the apical tufts, or fusion
of the pollical distal phalanx are frequently
observed in orangutans. Fifty-four percent of
specimens deviate from the 111> IV> I1 > V
length pattern. Although distal phalanges are
highly variable the basal indices (table 4)
subscribe t o the pattern observed in other
apes.
Hylobatid apes
The individual proximal phalanges of the
lesser apes are similar in shape, and it is sometimes difficult to identify individual bones.
However, subtle variation in basal morphology normally permits identification of I1 and
TV. These two bones are the most similar in
size. The bodies of the phalanges are concave
medio-laterally on the anterior surface and
the flexor sheath ridges are well defined. Longitudinal curvature is pronounced and the
flexor sheath ridges lie opposite the point of
maximum arc as in Pongo (or slightly distal).
Lateral radiographs reveal a constriction of
the anterior posterior cortical moieties a t
about the point of maximum arc. Overall,
however, the cortex is relatively thin. The
sulcus which divides the trochlea is deep, particularly in siamangs. The distal articular surface is narrow (radio-ulnad) but it retains a
large circumference that extends well onto
the anterior aspect.
Asymmetry of the individual middle phalanges is not demonstrable, but the length formula of I11 > IV > I1 > V is constant among the
hylobatid species sampled (n =45).
The middle phalanges, together with the
proximal, are responsible for the curved character of hylobatid fingers. The intrinsic longitudinal curvature of the middle phalanges is
marked in both gibbons and siamangs. Well
developed insertions for the m. flexor digitorum superficialis are bounded by extensive
flexor sheath ridges. The bodies achieve their
' H klossr (4). H. eoncolor (41, H Zar (3). H hooiok (31, H ,spp.
(221, H. syndoctylus (91, H.spp. includes the ahove plus H. funerus,
H. albirnanus, but excludes H. klossi.
226
RANDALL L. SUSMAN
greatest breadth a t the flexor insertions. The
bases are characterized by well developed biconcavities for the trochlae of the proximal
phalanges and a deep antero-posterior diameter.
The distal phalanges possess very distinctive apical tufts compared to the other apes
especially on distal phalanx I.
Homo sapiens
langes. These segments are distinctive in the
shape of the base and in the development of
the apical tufts. Distal phalanges 11-V have
flat bases and bodies. with large, spade-like,
apical tufts. The pollical distal phalanx is recognized by its enlarged area of insertion for
the tendon of flexor pollicis longus.
Summary
of
phalangeal differences
The proximal phalanges of the African apes
The proximal phalanges of modern humans are shorter in relationship to the metacarpals
differ from the other hominoid primates in than those of orangutans or hylobatid apes.
lacking well defined secondary features such The proximal phalanges of gorillas are short
as curvatures, concavities, and muscle mark- and stout while those of orangutans and lesser
ings. The bones have no longitudinal cur- apes are long, thin, and markedly curved. The
vature and bear only faint flexor sheath proximal phalanges of chimpanzees, and esperidges. The trochlae are less circumferential cially bonobos, represent an intermediate
than in the apes. The short, straight appear- morphology between gorillas and orangutans
ance and reduced extension of the trochleae with respect to length, curvature, and robusaccount for the reduced flexion set of human ticity. In the African apes the I11 >IV > I1 > V
fingers.
length pattern is constant, while 50%of male
In 36 of 40 individuals the proximal pha- orangutans and 5%of female orangutans devilangeal length pattern is III>IV>II>V. In ate from this pattern.
The middle phalanges follow a similar trend
three cases phalanx I1 exceeds IV, while in one
individual phalanges I1 and IV are equal in to that of the proximal phalanges from
gorillas through the orangutan and hylobatid
length.
The second proximal phalanx is identified apes. The size sequence of middle phalanges in
by its relatively robust body and enlarged African apes is I11 > IV > I1 >V, while that of
radial tubercle, the dorsal portion of which Pongo deviates from this pattern in 68% of
accommodates the prominent first dorsal in- cases.
Human distal phalanges are distinguished
terosseous muscle. This feature characterizes
living (Landsmeer, ' 5 5 ) and fossil (Musgrave, from those of other Hominoidea by their well
'71) Hominidae. Although length alone may developed apical tufts and flat bases. Gorillas
not distinguish proximal phalanges I1 and IV, have the most well developed finger tips
the stouter base, asymmetry, and linear body among the apes, while orangutans often lack
identify the second phalanx. The third prox- apical tufts altogether.
imal phalanx is notable among the four for its
Multivariate analysis
combined length and its stout base and basal
tubercles. Proximal phalanx IV is long, thin,
Multiple discriminant analysis is one of the
and does not display the basal asymmetry of approaches employed to isolate and describe
11; however, the flexor sheath ridges are functionally related morphological differusually more distinct on IV than on 11. The ences in the fingers of the Hominoidea. This
fifth proximal phalanx is small and weakly de- statistical technique is well described in the
veloped. The insertions of the hypothenar morphometric literature (Howells, '73; Oxmuscle give the base a robust appearance rela- nard, '73; Creel and Preuschoft, '76). It distinguishes known groups from each other on the
tive to the body.
Human middle phalanges have broad bases basis of multivariate variables constituted
and well defined insertions for m. flexor dig- from weighted linear combinations of original
itorum superficialis. The bones lack the other variables. The weights are calculated so as to
secondary features seen in the middle pha- maximize the variance between groups relalanges of adult apes. The I11 > IV> I1 > V pat- tive to that within groups (for a detailed distern obtains in 36 of 40 cases. In four in- cussion see Tatsuoka, '71).
The hominoid groups in this study are morstances phalanges I11 and IV are equal in
phologically quite distinct. Discriminant analength.
The most striking feature of human fingers lysis can be expected to find combined variais the distinctive character of the distal pha- bles along which the positions of the groups
227
FUNCTIONAL MORPHOLOGY O F HOMINOID FINGERS
differ significantly. It is not so certain that
these combined variables will be readily interpretable with respect to their functional
significance. This is particularly true if the
original variables are not carefully chosen to
reflect functional differences. Even so, there
is no guarantee that some unknown, or
unrecognized, functional adaptation is not reflected in the weighting of variables. Seemingly important aspects of morphology may
represent the evolutionary history of particular taxa and this may or may not reflect functional affinities. Natural selection is not constrained t o effect the same solution to a particular problem each time it operates.
Presumably a discriminant reflecting morphological differences among taxa caused by
differing locomotor adaptations will array the
centroids for the taxa in a way similar to their
functional ranking. This ranking may reflect
a continuum (rather than discrete categories)
as in the case of locomotor differences in the
apes and in humans. Moreover, weightings
given to individual variables on the discriminant axis should lend themselves t o “meaningful (biomechanical) interpretation.
The analysis of metacarpal I11 and proximal
phalanx I11 was conducted on the linear and
calculated variables listed in tables 5 and 6.
These elements of ray I11 were chosen because
they represent the principal weight bearing
finger, and earlier studies and this work demonstrates the intimate relationship of locomotor hand posture and the metacarpophalangeal complex. Prior to the discriminant analysis the variables were adjusted for ‘‘size”
according to a procedure designed by N. Creel.
Metacarpal length and proximal phalanx
I.000
LOO0
2.000
LOO0
*Hylobates $
Hylobates
I
-4.000
**
I
-3.009
-2.000
1.on0
-1.000
Q+** R P
*
P.ta
3400
i
4.000
PP.?
*pmeo*
P.t-4
=+*
2.000
*pone0 d
-1.000
-2.000
-3.000
- -I 0 0 0
Fig. 6 Plot of hominoid group centroids on discriminant axes I (horizontal) and I1 from analysis of
metacarpal and proximal phalanx 111. Hs., modern humans; Gg., gorilla; P.P., bonobo; P.t., chimpanzee; Hylobates, Hylobates spp.; and Pongo. Scale, std. dev. units.
228
RANDALL L. SUSMAN
length for each animal were summed to form a
combined length variable. The combined variable and the other original variables were
then logarithmized. A pooled within groups
covariance matrix was calculated for the
transformed original variables and combined
length variable. From this a series of regression equations were calculated with each of
the original measurements being used in turn
as the dependent variable and the combined
length measurement as the independent variable. These equations were used to calculate
estimated values for each animal and each
variable from the length variables. The estimated values were then subtracted from the
measured values yielding residuals. The estimated value of each original variable group
mean on the length variable grand mean was
computed from the resultant regression equation. The residual of each individual in the
group was then added to the estimated value
for the group mean to yield a “size corrected”
variable in which all variance attributable to
variation in the combined length variable has
been removed. In other words, the combined
length variable is used as a covariate for each
of the measured variables.
A plot of the groups on the first discriminant reveals an axis from the hylobatid apes
to gorillas (fig. ti). This axis represents an
arboreal-terrestrial trend with orangutans
closest of the pongid apes to the lesser apes.
The position of man is, of course, difficult to
interpret in a locomotor context. Gorillas and,
to a lesser extent, humans and chimpanzees
are distinguished by the dorso-palmar diameter of the metacarpal shaft, dorso-palmar
diameter of the metacarpal head, a wide
trochlea on the proximal phalanx, and a stout
phalangeal base. Hylobatid apes are distinctive primarily in features of the proximal phalanx. These include flexor sheath ridge development on the proximal phalanges, the dorso-palmar diameter of the phalangeal body,
the “biepicondylar” diameter, and the enlarged medullary cavity. Axis two separates
the apes from humans. The basal development
of the proximal phalanx, wide medullary cavit y of the metacarpal, and dorso-palmar diameter metacarpal shaft are prominent variables
on axis two.
Features used in the discriminant analysis
can be viewed in the context provided by the
plot of group centroids (fig. 6) and the relevant behavioral data on horninoid locomotion
and posture. Sexes are combined and groups
are thus arranged in table 8. Row one expresses the proportion of the metacarpal and
proximal phalanx, and reveals the lengthened
proximal phalanges of the Asian apes. Humans have a high value for this index, but this
reflects the relatively shortened metacarpals
TABLE 5
Variable statistics-meta~arpalIII
Mean
Length
Radio-ulnar midshaft diameter
Dorso-palmar midshaft diameter
Dorso-palmar head diameter
Radio-ulnar breadth head
Biepicondylar diameter
Medullary cavity diameter
Dorsal ridge height
Set metacarpal head
(in radians)
Standard deviation
Length
Radio-ulnar midshaft diameter
Dorso-palmar midshaft diameter
Dorso-palmar head diameter
Radio-ulnar breadth head
Biepicondylar diameter
Medullary cavity diameter
Dorsal ridge height
Set metacarpal head
@
cf
@
Bf
Gm
c
f
03
of
18
16
5
6
I8
17
11
23
90.35 87.00
8.41
8.19
9.50
8.64
15.78 14.71
13.99 13.07
16.86 15.70
3.34
3.34
3.39
2.47
2.65
2.65
6.02
0.89
1.05
1.11
1.35
1.58
1.14
0.91
0.05
6.64
0.67
0.78
0.81
0.71
1.00
0.79
0.89
0.04
&
€
15
Hylf
H z
19
20
83.27 82.57 97.98 82.51 109.77 93.18 57.45 58.89
7.01
7.22 12.27
9.67
8.36 6.80
3.93
3.96
8.03
7.72
8.64 14.30
9.89 7.76
4.06
4.19
13.80 13.26 20.65 16.07 17.45 14.19
6.84
6.98
11.39 11.42 20.40 15.09 14.87 12.03
6.46
6.47
13.70 13.83 23.26 17.90 18.17 15.00
7.06
7.05
3.41
3.48
7.18
5.17
3.28 2.63
1.85
1.88
3.12
3.05
5.02
3.89
1.72 1.48
0.89
0.92
2.63
2.66
2.56
2.59
2.71 2.73
2.79
2.79
1.01
1.05
1.04
1.05
1.10
1.04
1.09
1.16
1.01
1.01
1.09
1.06
1.04
1.08
1.06
1.23
1.15
1.01
6.81
1.00
1.47
1.35
1.40
2.24
1.20
0.92
0.04
4.50
0.93
1.03
1.11
0.89
1.39
1.08
0.97
0.04
C, chimpanzee; B, honoho; G, gorilla; 0. orangutan; Hyl, hylnhatid apes, Hn,modern humans
8.01
0.92
1.47
2.01
1.50
1.99
1.39
1.04
0.04
5.55
0.71
0.83
0.99
0.84
1.18
0.76
0.87
0.05
6.49
0.61
0.63
0.86
0.96
0.83
0.37
0.34
0.03
4.42
0.47
0.44
0.82
0.71
0.68
0.32
0.31
0.02
HBf
20
69.82 65.71
8.81
7.87
10.08
8.85
14.28 13.38
13.54 12.39
14.74 13.48
4.87
4.37
0.58
0.56
2.59
2.64
5.33
0.69
0.81
0.90
0.91
1.33
0.95
0.38
0.03
3.49
0.40
0.68
1.00
0.83
0.89
0.74
0.35
0.04
229
FUNCTIONAL MORPHOLOGY OF HOMINOID FINGERS
of humans rather than the elongation of the
proximal phalanges. The cortex thickness reveals a linear trend in the Pongidae from
Gorilla to Pongo with Homo between Pan
troglodytes and Gorilla. The low value for
hylobatid apes violates this trend. This may
reflect a scaling problem related to decreased
body proportions. The flexion set of the
metacarpal head is pronounced in gorillas and
most reduced in the hylobatid apes. Lastly, the
proportionate depth of the metacarpal head
and the length of the proximal phalanx also
displays a trend from the knuckle-walkers,
through the orangutan and hylobatid apes.
This latter index is of critical importance
for maintaining joint stability in knucklewalkers (fig. 7).
Functional interpretations o f observed differences in the metacarpophalangeal joints
The observed differences in the size and
shape of hominoid ray segments are inter-
TABLE 6
Variable statistics-proximal phalanx 111
Mean
Length
Radio-ulnar midshaft diameter
Dorso-palmar midshaft diameter
Radio-ulnar diameter base
Radio-ulnar diameter trochlea
Biepicondylar diameter
Flexor ridge height
Longitudinal curvature
(in radians)
Standard deuiation
Length
Radio-uln a r midshaft diameter
Dorso-palmar midshaft diameter
Radio-ulnar diameter base
Radio-ulnar diameter trochlea
Biepicondylar diameter
Flexor ridge height
Longitudinal curvature
@
17
cf
B
m
Bf
Cfl
c
f
Om
of
Hylf
Hsm
Ef
16
5
6
1R
15
10
23
13
15
20
20
60.84
12.71
7.54
16.74
12.45
12.50
1.08
3.81
58.73
11.53
7.05
15.52
11.96
12.06
1.03
3.84
51.04
9.82
6.27
3.64
0.50
0.09
1.16
2.54
52.74
14.99
7.08
18.34
12.90
13.90
3.38
3.84
78.83
12.63
8.62
18.08
13.25
13.95
0.79
3.64
69.10
10.50
6.75
14.93
11.20
12.26
0.83
3.68
43.77
5.93
3.50
7.59
6.15
6.65
1.13
3.83
44.42
6.07
3.52
7.72
5.98
6.58
3.34
1.99
0.86
1.21
0.74
1.22
0.94
0.07
4.94
0.87
0.54
1.01
0.85
1.27
0.83
0.07
5.01
2.21
1.09
1.69
0.82
1.32
0.98
0.15
3.93
0.94
0.73
1.03
0.56
1.32
0.70
0.10
5.41
0.81
0.57
1.14
0.76
0.77
0.67
0.06
3.84
0.89
0.35
0.72
0.82
0.98
0.36
0.08
1.02
1.02
1.10
1.04
1.07
1.10
1.27
1.01
51.77 64.89
9.84 21.08
7.83
6.27
13.11 23.56
10.00 16.88
9.78 19.14
1.40 4.88
3.87
2.53
1.02
1.06
1.08
1.05
1.09
1.04
1.30
1.01
4.02
1.90
0.77
1.26
1.02
2.67
1.07
1.01
3.18
1.20
0.67
1.53
1.11
1.45
1.02
1.01
47.77
10.88
7.36
16.86
12.14
11.35
0.09
1.10
3.92
3.83
45.14
9.31
6.47
15.01
10.97
10.23
0.24
3.99
4.23
0.88
0.62
1.10
0.88
0.80
0.27
0.09
2.33
0.61
0.40
0.78
0.46
0.51
0.36
0.04
C. chimpanzee; B, bonabo, G , gorilla; 0, orangutan; Hyl, hylobatid apw: Hs, modem humans.
TABLE 7
Standardized dicriminant function coefficients from combined metacarpal-proximal phalanx III analysis
Metacarpal III
Metacarpal length
Midshaft diameter (r-u)
Midshaft diameter (d-p)
Head diameter (d-p)
Head diameter (r-u)
Biepicondylar diameter
Medullary cavity (d-p)
Dorsal ridge height
Set of metacarpal head
Proximalphalanx 111
Midshaft diameter (r-u)
Midshaft diameter (d-p)
Base diameter (r-u)
Trochlear diameter (r-uj
Biepicondylar diameter
Flexor sheath ridge height
Curvature
Relative percentage of trace
Function 1
Function 2
Function 3
-0.14568
- 0.17168
- 0.32612
- 0.24684
0.06767
-0,00998
0.10538
0.02479
- 0.00978
-0.11582
-0.03444
0.41738
- 0.47640
-0,02361
- 0.59205
0.30149
- 0.35861
0.03804
- 0.05832
- 0.33390
0.17629
0.04319
1.11748
- 0.09529
-0.27868
-0.03689
0.29549
- 0.45279
-0.11805
0.11449
- 0.19440
-0.20097
0.10438
0.11091
-0.08199
68.10
-
14.92
0.08678
0.12927
- 0.64757
0.52773
-0.33771
- 0.24783
0.30293
0.48359
0.92843
- 0.01545
- 0.02542
-0.24824
-0.31837
11.55
230
RANDALL L. SUSMAN
pretable in t h e divergent contexts of knucklewalking and suspensory hand postures. Figure
7 illustrates t h e third rays of Pun troglodytes
and Pongo in knuckle-walking and hypothetical knuckle-walking postures, respectively.
I t h a s been demonstrated above t h a t i n
the African apes features associated with
knuckle-walking are more pronounced in rays
111 and IV. Several additional features of
metacarpal and phalangeal morphology, some
of which are underscored by t h e discriminant
function coefficient values, are noteworthy:
(1) In t h e African apes the proximal phalanges are short relative to t h e length of t h e
metacarpal, and moreover, in relation to t h e
dorso-palmar diameter of t h e metacarpal head
(table 8, row 2 ) .
(2) In chimpanzees and gorillas there is a
PAN
FgC = FppA
f
well developed fibrocartilaginous p a l m a r
plate (=glenoid plate) and ligament on t h e
ventral surface of t h e joint capsule (fig. 8 ) .
This structure is absent for the most part in
Pongo.
(3) The epicondyles on t h e metacarpal
heads are pronounced in t h e African apes and
are most often connected by a dorsal articular
ridge on metacarpals 111 and IV. Adult Pun
paniscus, may altogether lack this feature on
metacarpals 11-V.
(4) Although variable, t h e widest radio-uln a r dimension on the heads of metacarpals 111
and IV is usually dorsal in the African apes
and palmar in Pongo and humans.
(5) The cortex in the metacarpal shafts is
thicker in the orangutan and bonobo than in
either chimpanzees or gorillas. Thus, this fea-
PONGQ
FmmB+ F j O + F c O
Fig. 7 Ray I11 of Pun and Pongo in knuckle-walking and hypothetical knuckle.walking postures, respectively. F,
and F,, counteract the torque imposed by the ground reaction force, F,. Moments of F,
and F,, are determined by
depth of the metacarpal head and the thickness of the palmar plate. Moment of Fg is determined by length of the proximal phalanx.
FUNCTIONAL MORPHOLOGY OF HOMINOID FINGERS
ture also displays a pattern of intergroup variation that follows a suspensory to knucklewalking continuum (table 8).
These morphological differences are elements of a system that in part functions to
reduce compressive stress in the metacarpophalangeal joints of the knuckle-walking
apes, and helps to balance the torque that
tends t o hyperextend the proximal phalanges
(fig. 7). The metacarpal heads are deeper and
more flexed in the African apes. This increases the moment of the long digital flexors
and the palmar plate (Fmm and Fr respectively) for resisting the torque of t e ground
reaction force (FgI. The hypertrophy of the
palmar plate in the knuckle-walking apes also
increases the moments (A and B) of the long
flexors. Concomitantly, the short proximal
phalanx (especially pronounced in gorillas),
reduces the moment of the ground reaction
force (FJ.
The result of these differences between
knuckle-walkers and non-knuckle-walkers is
231
that the potential influence of the long flexors
and the palmar plate to counter force F g , and
to provide an active (or passive) component of
resistance (Fmm and Fpp), is increased in the
African apes. The metacarpophalangeal joints
in Pongo, with long proximal phalanges,
shallow metacarpal heads and reduced palmar
plates, are less well suited to balance the
forces imposed in a knuckle-walking mode.
The “power armiload arm index” given in
table 8 further illustrates this point. It should
be noted, however, that this index is an underestimation of the difference between Pongo
and the African apes because i t does not include the thickness of palmar plate (fig. 8).
In the African apes the predominant occurrence of expanded dorsal articular surfaces on
metacarpals 111and IV reduces the joint compressive stress in normal loading. The dorsal
articular ridge is coincident with the epiphyseal line and does not reach its full expression until well into adulthood. The enlarged
dorsal ridge and expanded epicondyles serve to
Fig. 8 Metacarpophalangeal joint capsule of ray III in gorilla. Proximodistal incision of the capsule exposes the palmar surface of the metacarpal head. Note t h e extreme development of the fibrocartilagenous
palmar (=glenoid) plate.
232
RANDALL L. SUSMAN
TABLE 8
Proportions and indices of ray 111 in the Hominoidea
Gorilla
n=15
Relative length PPhIII
(pphllIimetaIlI X 100)
Power arm: load arm index
Metacarpophalangeal
joint I11 '
Cortex thickness index
Meta. I11
"Set" of the metacarpal
head (in")
Dorso-palmar midshafti
Metacarpal length
x 100
Biepicondylar diameter]
length X 100
Pan troglodytes
n-16
Pan panisrus
n=ll
Pongo
Iiylohates spp.
n=15
11-20
77
Humans
c-15
65
67
62
73
69
16.0
13.3
13.2
10.2
7.6
15.8
48.9
65.3
69.2
70.5
54.7
54.4
147.6
152.1
151.6
155.9
160.1
150.2
12.16
9.36
8.57
7.47
6.78
12.31
22.80
18.36
16.60
16.34
12.14
20.82
' This index is determined by one-half the dorso-palmar diameter of t h e metacarpal headflengh
Cortex
IS determined
from lateral radiogmpha. The index
18
of the proximal phalanx.
computed as cortexldorswpalmar diameter a t midshaft x 100
buttress the joint (Preuschoft, '73a) while the
flattened dorsal extent of the surface provides
a mechanism to tauten the collateral ligaments and inhibit axial rotation of the proximal phalanx while it is hyperextended. An
additional function of the protuberant epicondyles may be to help prevent bowstringing of
the interosseous and lumbrical tendons in the
hyperextended position of the joint.
Suspensory grasp and finger morphology
Attenuated ray segments and lack of strong
secondary features of the metacarpophalangea1 joints characterize the manual rays of the
Asian apes. Muscle force t o counter the external force on the fingers is increasingly necessary as the metacarpophalangeal and interphalangeal joints are fle ed (Preuschoft,
'73b). Lengthening of the ray segments increases the compass of the hand while markedly longitudinal curvatures are a remodelling response to strong bending moments
imposed by the lengthening of the fingers. In
orangutans and the hylobatid apes this results
in pronounced phalangeal curvatures. The
metacarpals are sometimes also subjected to
strong bending moments, especially during
combined flexion of the three finger joints
(Preuschoft, '73b). The metacarpals in Pongo
resist increased bending stresses by increased
longitudinal curvature and increased cortical
thickness (and perhaps increased density as
well). These features are individually variable, especially with regard to shaft curvature, but together they form a pattern distinct from chimpanzees and gorillas.
An additional feature of the orangutan
hand (especially in males) is the increased
length and concomitant high degree of variation in the fourth ray. The pattern of asymmetry suggests that the axis of the hand has
shifted ulnad. The increased emphasis on the
ulnar side of the hand seems to be related
more to grasping vertical supports (as in
climbing) rather than horizontal superstrata.
I t is interesting to consider this possibility,
especially in light of recent suggestions that
climbing per se may be the basic locomotor
adaptation in the hominoid radiation (e.g.,
Fleagle, '76; Stern, '76). In animals that climb
vertical supports it is the ulnar side of the
hand that provides friction against the downward force of the individual's weight. The
ulnar rays provide force against the branch or
vine, while the radial fingers, I1 and I11 (and
occasionally I) provide ancillary support
against the upward displacement of the fourth
and fifth fingers. The latter component would
not be available if the main muscular force
opposing the branch or trunk was exerted by
the radial side of the hand.
DISCUSSION
Fleagle ('76) and Tuttle ('77) have summarized the locomotor activities of the extant
apes. Gorillas a r e primarily terrestrial
knuckle-walkers, but occasionally climb trees.
Chimpanzees are also terrestrial knucklewalkers but frequently climb and knucklewalk in trees. Orangutans are versatile climbers when in the canopy and only adult males
are frequently found on the ground. When on
FUNCTIONAL MORPHOLOGY OF HOMINOID FINGERS
the ground orangutans do not knuckle-walk
but instead place the proximal phalanges or
palms on the ground. The lesser apes climb,
brachiate and, to a lesser extent, walk bipedally in the trees. They are rarely observed on the
ground.
The morphology of the manual rays in living
apes is intimately related to positions of the
hand in locomotor and postural modes. These
morphological differences, however, are subtle
and continuous, and grouping extant apes on
the basis of metacarpals and phalanges (11-V)
is best accomplished hy the use of multivariate variables. Morphological patterns of one
ray cannot be generalized to the others as both
the pattern of loading and the manipulatory
role of each ray differ within a single hand.
The human hand emphasizes the pollex
along with rays TI and IIT. The increased
robustness of metacarpal I1 and the torsion of
the third metacarpal head help identify the
human pattern. Torsion of the head of the
index metacarpal provides for pronation
(endorotation) of the proximal phalanx during
flexion (Lewis, ’77: p. 169); however, this
action also occurs a t the third metacarpophalangeal joint. Rays IV and V comprise a functional unit that contrasts with the index and
medius (Kaplan, ’ 6 5 : p. 6). The bones of rays
11-Vare notable in their lack of secondary features and the reduced proportions of the
metacarpus.
The hand of the African apes emphasizes
rays TI1 and IV. Features associated with
knuckle-walking hand postures are observed
in the metacarpals and proximal phalanges.
However, rays I1 and V commonly lack such
features as a bilaterally expanded dorsal articular surface of the metacarpal head, a
prominent dorsal articular ridge, and large
epicondyles. In fact, metacarpals of these rays
frequently resemble those of Pongo, and to a
lesser extent humans, in these isolated features. In Gorilla metacarpal V achieves a
length approaching t h a t of IV. In all three
African apes (gorilla, chimpanzee, and
bonobo), the proximal and middle phalanges
are individually recognizable on the basis of
relative length and pattern of asymmetry. Of
the distal phalanges only that of the pollex is
distinctive.
The fingers of Pongo are notable in their
length and curvature. The latter characteristic is primarily a result of proximal and middle phalangeal curvature. Suspensory features are especially marked in the proximal
233
phalanges. In Pongo the phalanges of ray IV
display increased length relative to the other
rays. The pattern of asymmetry also shifts
(especially in males) whereby proximal phalanx IV appears to be more symmetrical than
111. This increased emphasis on the fourth ray
and the occurrence of the “double locking”
mechanism in orangutan fingers (Napier, ’60)
reflects the increased importance of vertical
climbing in Pongo and an enhanced capacity
for grasping thin, vertical supports. Orangutan metacarpophalangeal joints are demonstrably less well adapted for knuckle-walking
hand postures owing to the reduced dorso-palmar dimension of the metacarpal head, absence of a developed palmar plate, and a proportionately long proximal phalanx.
Differences in finger morphology between
Pongo and the hylobatid apes reflect distinctive differences in locomotor patterns and effects of disparate body size. The characteristic
features of the pollex in the lesser apes are
most distinctive (Lorenz, ’71; Van Horn, ’72;
Tuttle, ’69, ’72). Because of the reduced body
size of the lesser apes, they do not fit into the
arboreal-terrestrial, or suspensory knucklewalking continuum, when the criterion is a
single character (e.g., thickness of the metacarpal shaft cortex). Notable in hylobatid apes
is the increased emphasis on the index finger.
The second metacarpal of hylobatid apes is the
most robust of the five. A metacarpal length
formula of IT > TI1 > TV> V is found in all gi bbons and siamangs sampled (n = 22, and n = 9,
respectively). It is noted above (table 8 ) that
gibbons have the greatest proximal phalanx IIIimetacarpal I11 index among the
Homoinoidea, and Midlo (’34) also noted that
the lesser apes possess the highest phalangealipalm length ratio (ray 111) among the
Hominoidea. The bones of rays 11-V are notable in their lack of secondary features (save in
some specimens of Symphalangusl. The flexion set of the fingers is due to strongly curved
proximal and middle phalanges.
Pan paniscus
It is interesting to note the position of Pan
paniscus among the other apes in the discriminant analysis of metacarpal I11 and proximal
phalanx 111. The relative position of bonobos
between chimpanzees and orangutans suggests a more arboreal finger morphology in
bonobos than in chimpanzees. Little has been
reported on the behavior of bonobos (Pan
paniscus) either in the wild or in captivity.
234
RANDALL L. SUSMAN
Fig. 9 Pan troglodytes versus (left) and Pan paniscus (right) in similar knuckle-walking postures. Note
the angle of the trunk to t h e horizontal (both animals are captive, young adult females).
Recent reports, however, of pilot studies and
work in progress indicate differences between
bonobos and chimpanzees in locomotion and
posture. Early reports (Schouteden, '31;
Yerkes and Learned, '25) and recent brief reports of free-ranging bonobos (MacKinnon,
'76; Badrian and Badrian, '77) suggest that
they are more arboreal than are chimpanzees.
My observations of captive animals a t the
Antwerp Zoo, San Diego Zoo, and Yerkes Regional Primate Research Center, support this
impression.
Captive bonobos adopt knuckle-walking
hand postures similar to chimpanzees and
gorillas. The employment of knuckle-walking
obtains, as in chimpanzees, both on the ground
and on firm arboreal supports. The hands
assume the same variety of angular placements and finger supports reported for other
African apes (Tuttle? '67, '69. '72, '74, '77).
Bonobos, however, appear to assume a more
pronograde attitude than either chimpanzees
or gorillas in normal quadrupedal stance and
gait (fig. 9). This difference may reflect the
lower intermembral index in bonobos than in
chimpanzees (101 ' and 107. respectively).
Badrian and Badrian ('77) report that in the
wild bonobos are more arboreal than chimpanzees. Small groups of one t o four animals
were observed t o nest high in the canopy (Badrian and Badrian, '77; MacKinnon, '76). Nests
are similar to those of Pan troglodytes but
bonobo nest heights exceed those of chimpanzees and even orangutans (MacKinnon,
'76). Mean nest heights of 82 and 78 feet were
reported by MacKinnon and the Badrians, respectively. Like chimpanzees, bonobos normally travel from one feeding source to
another along terrestrial routes, but bonobos
were observed to trek more than one kilometer
via arboreal routes (Badrian and Badrian, '77:
p. 466).
In a number of important features bonobos
fall between chimpanzees and orangutans
while retaining their overall relationship with
Pan troglodytes. One such feature is cortical
thickness index of the metacarpal shaft (table
8). As noted earlier, this is probably related to
large bending moments induced by strong
power grasp. Bonobos reveal a value for the
cortical thickness index very close to that of
Pongo (wildshot specimens) and distinct from
that of Pan troglodytes. Other features of the
metacarpal head are less pronounced in
bonobos than in chimpanzees and recall the
unspecialized condition seen in Pongo. These
include the lack of well defined dorsal articular ridges and absence of broad flat dorsal surfaces of the metacarpal heads. The phalanges
also appear to be somewhat more "suspensory
adapted." The curvature of the proximal phaThis difference is significant a t the 0.05 level (n= 13, d.f. = 11).
'This difference between honohos and chimpanzees IS significant
at 0.05 level tn=24, d.f.=22).
FUNCTIONAL MORPHOLOGY OF HOMINOID FINGERS
langes and the attenuation of the middle and
distal phalanges are suggestive of a more
suspensory hand. Future field studies are necessary to confirm the position of bonobos in
the hominoid locomotor spectrum.
ACKNOWLEDGMENTS
I thank R. H. Tuttle for his advice and help
throughout the course of this work. Benjamin
Beck provided numerous suggestions and behavioral insights during the early stages of
this project. J. T. Stern provided helpful criticisms of biomechanical points, and N. Creel
offered much statistical advice and many
hours of enlightening discussion. J. G. Fleagle
and W. L. Jungers also read and commented
on the manuscript. Funds for this study were
provided by the Henry Hinds Fund of the Committee on Evolutionary Biology, University of
Chicago. I am grateful to L. Throckmorton and
the Committee for their support throughout
my graduate studies. Ms. Lucille Betti prepared the figures and Ms. Eileen Petite typed
the manuscript.
Special thanks are due to R. Thorington
(National Museum of Natural History), S. Anderson (American Museum of Natural History), P. Helwig (Cleveland Museum of Natural
History), and T. Van Den Audenaerde (Musee
Royal de Lafrique Centrale, Tervuren) for
access to specimens in their care.
LITERATURE CITED
Badrian, A,, and N. Badrian 1977 Pygmy chimpanzees.
Oryx, 23: 463-468.
Calhoun, T. P. 1977 Sesamoid structures in primate
hands. Yearbook of Phys. Anthrop.. Vol. 20: 525-536.
Conroy, G. C., and J. G. Fleagle 1972 Locomotor behavior in
living and fossil pongids. Nature, 237: 103-104.
Corruccini. R. S., R. L. Ciochon and H. M. McHenry 1975
Osteometric shape relationships in t h e wrist joint of
some anthropoids. Folia Primat., 24: 250-274.
Creel, N., and H. Preuschoft 1976 Cranial morphology ot
the lesser apes. In: Gibbon and Siamang. D. M. Rumbaugh, ed. S . Karger, Basel, Vol. 4, pp. 219-283.
Currey, J. D. 1968 The adaptation of bones to stress. J.
Theoret. Biol., 20: 91-106.
Day, M. H., and J. L. Scheuer 1973 SKW 14147: A new
hominoid metacarpal from Swartkrans. J. Hum. Evol., 2:
429-438.
Etter, H. F. 1974 Morphulogisch und metrisch-vergleichende Untersuchung am Handskelet rezenter Primaten, 111. Morph. Jahrb., 120: 299-322.
Fleagle, J. G. 1976 Locomotion and posture of the
Malayan Siamang and implications for hominoid evolution. Folia Primat., 26: 245-269.
Gregory, W. K. 1928 Were the ancestors of man primi67: 129-150.
tive brachiators? Proc. Am. Phil. SOC.,
Howells, W. W. 1973 Cranial Variation in Man. Papers
Peabody Mus. Arch. and Ethol. No. 67. Harvard University Press.
235
Huxley, T. H. 1893 Man’s Place in Nature. D. Appleton
and Co., New York.
Jenkins, F A,, and J. G. Fleagle 1975 Knuckle walkingand
t h e functional anatomy of the wrist in living apes. In:
Primate Functional Morphology and Evolution. R. H.
Tuttle, ed. World Anthropology, Mouton Publishers, The
Hague, pp. 213-228.
Johanson, D. C. 1976 Ethiopia yields first “family” of
Early Man. National Geographic, 150: 791-811.
Kaplan, E. B. 1965 Functional and Surgical Anatomy of
t h e Hand. Second ed. Lippincott, Philadelphia.
Landsmeer, J. M. F. 1955 Anatomical and functional investigations of t h e articulation of the human fingers.
Acta Anatomica Supplementum 24-2,25: 1-69.
Leakey, L. S. B. 1960 Recent discoveries a t Olduvai
Gorge. Nature, 188: 1050-1052.
Lewis, 0. J. 1969 The Hominoid Wrist Joint. Am. J.
Phys. Anthrop., 30: 251-269.
1972a Osteological features characterizing the
wrists of monkeys and apes, with a reconsideration of this
region in Dryopithecus Proconsul) ufricanus. Am. J.
Phys. Anthrop., 36: 45-58.
1972b Evolution of the hominoid wrist. In: The
Functional and Evolutionary Biology of Primates. R. H.
Tuttle, ed. Aldine-Atherton, Chicago. pp, 207-222.
1973 The hominoid 0s capitatum, with special
reference to the fossil bones from Sterkfontein and Olduvai Gorge. J . Hum. Evol., 2: 1-11.
1977 Joint remodelling and the evolution of the
human hand. J. Anat., 123: 157-201.
Lorenz, R. 1971 The functional interpretation of t h e
thumb of t h e hylobatidae. In: Proceedings of the 34th
lntl. Cong. Primat. Biegert and Leutenegger, eds. S.
Karger Basel, pp. 130-136.
MacKinnon, J. 1976 Mountain gorillas and bonobos.
Oryx, 13: 372-382.
Marzke, M. W. 1971 Origin of the human hand. Am. J.
Phys. Anthrop., 34: 61-84.
Midlo, C. 1934 Form of t h e hand and foot in primates.
Am. J. Phys. Anthrop., 19: 337-389.
Morbeck, M. E. 1975 Dryopithecus africanus forelimb. J.
Human Evol., 4: 39-46.
Musgrave, J. H. 1971 How dextrous was Neanderthal
man? Nature, 233: 538-541.
1973 The phalanges of Neanderthal and Upper
Paleolithic hands. In: Human Evolution. Symp. Soc.
Study Hum. Biol. M. H. Day, ed. Taylor and Francis, Ltd.
London, Vol. XI, pp. 59-85.
Napier, J. R. 1959 Fossil metacarpals from Swartkrans.
Fossil Mamm. Africa, No. 17. Brit. Mus. Nat. Hist.
1960 Studies of the hands of living primates.
London, 134: 647-656.
Proc. Zool. SOC.
1962 Fossil hand bones from Olduvai Gorge.
Nature, 196: 409-411.
Napier, J. R., and P. R. Davis 1959 The forelimb skeleton
and associated remains of Proconsul africunus. Fossil
Mamm. Africa, No. 16. Birt. Mus. Nat. Hist.
OConnor, B. L. 1976 Dryopithecus (Proconsul/ ufricanus: Quadruped or non-quadruped? J. Hum. Evol., 5:
279-283.
Osborn, H. F. 1928 Present status of the problem of
human ancestry, Proc. Am. Phil. SOC.,67: 151-156.
Oxnard, C. E. 1973 Form and pattern in human evolution. University of Chicago Press, Chicago.
Preuschoft, H. 1973 Body posture and locomotion in
some East African Miocene Dryopithecinae. In: Human
Evolution. Symp. SOC. Study Hum. Biol., M. H. Day, ed.
Taylor and Francis, Ltd., London, Vol. XI, pp. 13-46.
236
RANDALL L. SUSMAN
1973h Functional anatomy of the upper extremity. In: The Chimpanzee. H. Bourne, ed. Karger,
Basel, Vol. 6, pp. 34-120.
Schon. M. A., and A. Ziemer 1973 Wrist mechanism and
locomotor behavior ofDryoithecus Proconsul) ufricanus.
Folia Primat., 20: 1-11,
Schouteden, H. 1931 Le chimpanze de la rive gauche du
Congo. Bull. du Cercle Zool. Congolais, 7: 114.119.
Singh, 1. 1959 Variations in the metacarpal bones. J.
Anat. London, 93: 262-267.
Stern, J. T. 1976 Before bipedality. Yearbook of Phys.
Anthrop., 19: 59-68.
Strauas, W. L. 1940 The posture of the great ape hand in
locomotion and its phylogenetic implications. Am. J.
Phys. Anthrop., 27: 199-207.
1949 The riddle of man’s ancestry. Qtr. Rev.
Biol., 24: 200-223.
Tatsuoka, M. M. 1971 Multivariate Analysis. Techniques for Educational and Psychological Research. John
Wiley & Sons, New York.
Tuttle, H. H. 1967 Knuckle-walking and the evolution of
hominoid hands. Am. J. Phys. Anthrop., 26: 171-206.
1969 Quantitative and functional studies on t h e
hands of the anthropoidea. J. Morph., 128: 309-364.
1972 The functional evolutionary biology of
hylohatid hands and feet. In: Gihhon and Siamang. D. M.
Rumhaugh, ed. S. Karger, Basel. Vol. 1, pp. 136-206.
1974 Darwin’s apes, dental apes, and the descent of man: normal science in anthropology. Curr. Anthrop., 15: 389-398.
1977 Naturalistic positional behavior of apes
and models of Hominoid Evolution, 1929-1976. In: Progress In Ape Research. G. H. Bourne, ed. Academic Press,
New York, pp. 277-296.
Van Horn, R. N. 1972 Structural adaptations to climbing in the gibbon hand. Am. Anthrop., 74: 326-334.
Wood-Jones, F. 1942 Principles of Anatomy as Seen in
t h e Hand. Second ed. Williams and Wilkins Co.,
Baltimore, Maryland.
Yerkes, R. M., and B. W. Learned 1925 Chimpanzee Intelligence and i t s Vocal Expression. Williams and
Wilkins, Co., Baltimore, Maryland.
Zapfe, H. 7960 Die Primatenfunde aus der miozanen
Spaltenfullung von Neudorf an der March (Devinska
Nova Ves), Tschechoslowakei Schwciz. Paleont. Abhand.,
78: 1-285.
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