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Femoral mechanics in the lesser bushbaby Galago senegalensisStructural adaptations to leaping in primates.

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T H E ANATOMICAL RECORD 202:419-429 (1982)
Femoral Mechanics in the Lesser Bushbaby (Galago
senegalensis): Structural Adaptations to Leaping in Primates
DAVID B. BURR, GEORGE PIOTROWSKI, R. BRUCE MARTIN, A N I )
P. NONG COOK
Department of Anatomy and Orthopedic Research Laboratory, West Virginia
University Medical Center, Morgantown, West Virginia 26506 (DB.B.,
R B.M.1; Department of Mechanical Engineering, University of Florida.
Gainesuille, Florida 526'11 (G.P.);and Department of Diagnostic Radiology,
University of Kansas Medical Center. Kansas City. Kansas 66103
ABSTRACT
One method used to examine the relationship between behavioral
strategies and anatomical adaptation is to study the results of mechanical stress
associated with a given behavior and compare this with skeletal adaptations to
other behaviors. This comparative approach is appropriate for highlighting combinations of features that are specializations to specific types of behavior. The purpose of this paper is to compare femoral mechanics in Galago senegalensis with
previously collected data for macaques and humans as a basis for discussing structural adaptations in the primate hindlimb to leaping. The stiffness and load carrying capabilities of the femoral diaphyses of 27 G. senegalensis were analyzed using
the SCADS computer program. The data suggest that the galago femur is well
adapted to sustain large sagittal plane compressive loads rather than large bending loads. The straightness of the femoral shaft and large midshaft area moments
of inertia prevent buckling from these large compressive loads. Calculations indicate that the ratio of critical buckling load to body weight in galago is 31 times
that in macaques and 55 times that in humans. The femur of this saltatory primate
is morphologically adapted to resist buckling when subjected to large compressive loads, while those of macaques and humans are better adapted to resist bending moments caused by ground reaction forces acting on the extended limb. The
differences between galago on the one hand and macaques and humans on the
other suggest that relatively smaller moments about the hip and relatively larger
moments about the knee accompany more quadrupedal and bipedal walking, while
habitual leaping is associated with relatively larger moments about the hip. These
data reinforce the apparent similarity of the mechanical effects of quadrupedal
and biuedal locomotion on the femur and dissimilarity with femoral mechanics in
habitually saltatory primates.
The focus of recent investigations of primate
skeletal morphology has been the characterization of a relationship between behavioral strategies and anatomical adaptation (Fleagle,
1977a,b, 1978; Fleagle and Mittermeier, 1980;
Rodman, 1979; Morbeck et al., 1979). One
method used to examine this interaction is to
study the results of mechanical stress associated with a given behavior and compare these
with skeletal adaptations to other behaviors.
This comparative approach may be the most
appropriate for highlighting combinations of
features that are mechanical specializations to
specific types of behavior. The functional association between locomotor behavior and
0003-276X/82/2023-0419$03.500 1982 Alan R. Liss, Inc
anatomy can be shown best by investigating
living species in which both behavior and anatomy can be established.
The purpose of this paper is to further clarify
functional patterns of mechanical adaptation
in skeletal morphology to patterns of locomotion. This study concentrates on the adaptive
correlates to leaping in the lesser bushbaby
(Galago senegalensis), a primate well adapted
to saltation. Movement patterns and musculoskeletal anatomy of G. senegalensis have been
reasonably well established (Nayak, 1933; Hill,
1953; Hall-Craggs, 1964, 1965a.b; Stevens et
Received February 17. 19x1: accepted November 2. 1981
420
D.B. BURR, G. PIOTROWSKI. R.B. MARTIN, AND P.N. COOK
al., 1971, 1972; Jouffroy e t al., 1974; Jouffroy
and Gasc, 1974; Petronis, 1978), but the mechanical, as opposed to morphological, adaptation of the skeletal system to these movement
patterns has not been examined.
Hindlimb dominance has been carried to an
extreme in the lesser bushbaby (54%-60%
forelimb lengthihindlimb length ratio) (Jouffroy and Lessertisseur, 1979). This represents
an entirely different adaptive strategy than
that studied previously in macaques, which are
pronograde primates that exhibit little difference in limb proportion. Although both animals are capable of leaping, saltation in
galagos is more specialized, subjects the skeleton to different temporal and force thresholds,
and consequently must exert qualitatively and
quantitatively different mechanical demands
on the skeleton than leaping or quadrupedalism in macaques. Unfortunately, there is no
clear idea of what these demands are because
the bushbaby skeleton has not been examined
in terms of its load-carrying capability.
This study concentrates on the stiffness and
load-carrying capability of the femur for two
reasons. First, i t satisfies the constraints of
the comparative method. Previous work done
on the macaque femur (Burr e t al., 1981) provides some comparative background. Second,
if it is true that skeletal adaptation is sensitive
to temporal and stress-related thresholds
(Preuschoft, 1979; Oxnard, 1979a).then combinations of features that are mechanical specializations to leaping should be most evident
in the femur. This is the primary load-carrying
bone in the hindlimb and probably the one subjected to the largest joint-reaction forces. One
need only observe the massive musculature
surrounding the femur in galagos to realize
that the thigh is the prime motivator in leaping
and that large loads will therefore be sustained
by the femur.
Leaping in Galago:
Behavioral observations
Galagos are among the most saltatory of primates (Oxnard e t al., 1981a,b).They have been
known to jump fourteen times their body
length both vertically and horizontally (Jouffroy e t al., 1974) a t take-off velocities more
than twice those found in other leaping animals (e.g., 380 cmisecond compared with 180
cmkecond in a frog) (Calow and Alexander,
1973).
Hall-Craggs (1965a) has provided the most
detailed observations of the mechanics of
leaping in G. senegalensis. He states (pp. 2223):
The stance prior to jumping can be described as a
Although the neck is extended the
back is flexed and the longitudinal axis of the trunk
forms an angle of approximately 30" to horizontal.
The hind limbs are acutely flexed. the thigh lies
alongside the trunk and the thigh, leg and foot are
apparently tightly compressed against each other.
low crouch.
Movement begins as a progressive extension of
the back and an extension of the trunk on the thigh,
the thigh and the rest of the lower limb remaining
virtually stationary during the first 0.03 second or
so of the jump.
The first apparent movement of the hind limb is a
rotation forwards of the flexed thigh, leg and tarsus
about the distal tarsus as dorsiflexion at the tarsometatarsal joint occurs.
extension of the hind limb does not begin until
0.05 second after the trunk begins to rise. Extension
of the trunk on the thigh continues and is joined at
first by extension of the leg on the thigh. Extension at hip, knee and ankle joints lead to an alignment of trunk and lower limbs in the direction of the
trajectory and the animal leaves the ground with
trunk and hind limb lying almost along a straight
line.
Jouffroy e t al. (1974:823) provide additional
observations on hindlimb mechanics during
the symmetrical leap:
The whole movement lasts 0.35 sec. It can be
divided into a first, or preparatory phase of relatively long duration (0.26 sec.), and a very quick
springing phase that lasts 0.09 sec.
During the preparatory phase, the animal
crouches with the feet in full plantigrade contact
with the ground. The main movements occur at the
femur and tibia levels. The femur undergoes a rotatory movement that diminishes the femoro-tibia1
angle in such a way that the pelvicifemoralarticulations are lower than the knees.
before the end of
this phase, the tibia moves towards the ground.
The outcome of this movement is that it narrows
even more t h e femoro-tibia1 angle.
The springing phase straightens, in perfect alignment, all the hindlimb bone segments. 0.09 sec. before the launching, the heads of both femurs begin
the ascent; at 0.05 sec.
the tibiae follow the same
ascending movement.
Gross morphology of the femur
The head of the femur in galago differs from
that in nonsaltatory primates in being a cylindrically enlarged extension of the femoral neck
rather than a well-defined spherical caput (Fig.
la). The most spherical portion of the head projects anteriorly from the neck, while the posterior articular surface blends imperceptibly
with the femoral neck. The femoral neck is
rather short and indistinguishable from the
head posteriorly and forms nearly a right angle
FEMORAL MECHANICS IN T H E LESSER BUSHBABY
with the axis of the femoral shaft. The head
and lesser trochanter project about the same
distance medially, the lesser trochanter fading
gradually and disappearing about a quarter of
the way down the shaft. There is no evidence of
a third trochanter, such as is found in some lemurs and tree shrews, although Hill (1953) reports this as a distinguishing feature of the
Galagidae. However, the distal portion of the
greater trochanter projects laterally a good
distance from the femoral shaft, and this may
indicate a blending of the third trochanter with
the greater trochanter. The diaphysis of the
femur is very smooth, with no development of
a linea aspera. The diaphysis thickens noticeably toward midshaft and narrows slightly distal to this (Fig. lb). The posterior aspect of the
femur is quite flat, as are the medial and lateral
aspects proximally, such that the cross section
of the proximal femur appears to be nearly rectangular, rather than cylindrical. The distal
portion of the galago femur appears nearly
cylindrical, in contrast to the anteroposterior
thickness and mediolateral narrowness reported by Stevens et al. (1971). The distal joint
surfaces are elongated anteroposteriorly more
than that found in nonsaltatory primates and
tend to project sharply anterior to the shaft
(Fig. lc).
MATERIALS AND METHODS
The femora of 27 Lesser bushbabies (Galago
senegalensis) were mechanically cleaned and
air dried. The femora were each embedded in
plaster of Paris in neutral rotation such that
the major anatomical axes were parallel to the
edges of the block mold. Blocks were cut out of
the diaphysis at 26, 38, 50, 62, and 74% of
femoral length using a Buehler Isomet lowspeed saw. The specimens were bathed in mineral spirits during these procedures.
Each block was magnified 1 2 X and photographed under a 30 X 30 grid calibrated in 0.25
mm squares. The grid served as a scale with
which the digitizer was calibrated and also permitted orientation of the photograph on the
digitizing table. The x and y coordinates of
each bone section were digitized using a Graf
Pen digitizer interfaced to a PDP 11/04 minicomputer. Data were recorded on disc and
later transferred to cards for input into the
stress analysis program.
The SCADS computer program permits the
analysis of the mechanical properties of long
bones based on cross sectional geometry of
serial sections. The analyses performed, algorithms used and output generated have been
421
extensively described by Piotrowski and Wilcox (1971) and Piotrowski and Kellman (1978).
The program calculates the following variables:
Cortical area (A) is one indication of the
bone’s resistance to axial loads and can be used
to measure the increase or decrease of bone tissue throughout the diaphysis.
The area moments of inertia (Ixx, Iyy) are directly related to the stiffness of the bone in
bending. The SCADS program defines Ixx as
stiffness in the mediolateral plane and Iyy as
stiffness in the anteroposterior plane. These
definitions follow Crandall et al. (1972), although other investigators may reverse the
meaning of these terms.
The principal moments of inertia indicate the
stiffest (Imax) and most flexible (Imin) axes
through the cross section.
The effective polar moment of inertia (Jeff)is
related to the stiffness of noncircular cross sections of bone subjected to twisting moments
and is computed from torsional rigidity calculations using the membrane analogy described
by den Hartog (1952).
The application of torque to a bone creates
shear stress within the bone with the largest
stresses along the periosteal border. Likewise,
the application of a bending load to a bone
creates compressive and tensile stress within
the bone with the largest stresses found furthest from the axis about which bending occurs.
Torsional shear stresses under a constant
torque of 100 kgf.cm and bending stresses
under a constant moment of 100 kgf.cm were
computed for each cross section. Bending moments were applied in both anatomical planes
and maximum bending stresses in each quadrant were calculated. Bending stresses calculated along the anterior and posterior aspects of
the femur were based on bending about a mediolateral neutral axis through the cross section; bending stresses calculated along the
medial and lateral aspects were based on bending about an anteroposterior neutral axis
through the cross section.
Outer and inner diameters in the anteroposterior and mediolateral planes were measured
from data provided by the SCADS program.
This permitted an assessment of the effects of
the morphological variability in cortex thickness on the mechanical specialization of the femur in G. senegalensis.
RESULTS
The cortical area available to support compressive axial loads in the galago femur declines sharply in the distal two thirds, such
422
D.B. BURR, G . PIOTROWSKI, R.B. MARTIN, AND P.N. COOK
a
b
C
Fig. 1. The femur of G. senegalensis. (a)Anterior view of the proximal femur; (b)anterolateral view of the whole femur; (c)
lateral view of the distal femur.
423
FEMORAL MECHANICS I N T H E LESSER BUSHBABY
that there is 20% less cortical bone at 74% of
femoral length than at 26% of length (Fig. 2).
This suggests an enhanced ability to sustain
axial loads proximally and decreased ability to
withstand such loads distally. This variation
in cross sectional area has no direct effect on
the structural strength of the femur in bending
or torsion, as shown below.
The distal decline in cortical area can be attributed primarily to relative medullary expansion in the sagittal plane (Fig. 3). Widening of
the marrow space is accompanied by cortical
thinning in the sagittal plane. Cortical thickness in the coronal plane, however, does not
change markedly throughout the femur since
increased inner width is paralleled by increased
2:
21
-
n
-E 2(
E
<
W
2 19
10
17
Fig. 2. Variation in cross sectional area throughout the femoral diaphysis of G. senegalensis
8.0
Outer
7.0
.14
-
.15
-
.13
.11
AP
.14
-E 6 . 0
Le
.13
ML
.11
w
c
w
5.0
1
n
4.0
.12
Inner
3.0
i
26
38
50
LEVEL
62
74
( X )
Fig. 3. Variation in anteroposterior and mediolateral outer and inner diameters proximal to distal in the femoral diaphysis. Numbers above and below symbols refer t o standard errors.
424
D.B. BURR, G. PIOTROWSKI, R.B. MARTIN, AND P.N. COOK
outer diameter at most levels. A slight decrease in both outer diameters occurs between
62% and 74% of femoral length and further
contributes to the decline in cross-sectional
area. At the most distal level, coronal and
sagittal plane outer widths are the same and
inner diameters differ by less than 10%. The
anteroposterior thickness and mediolateral
narrowness of the distal femur of Galago reported by Stevens et al. (1971) was not observed.
Examination of Figure 4 indicates that the
femur of G. senegalensis is much stiffer in
bending in the anteroposterior plane (Iyy)than
the mediolateral plane (Ixx) in the proximal
diaphysis, and that this difference in stiffness
gradually declines until stiffness in the two
planes is nearly the same in the distal diaphysis. Stiffness in the anteroposterior plane is
highest proximally, declines slightly a t midshaft, and declines considerably distally. In
contrast, stiffness in the mediolateral plane is
highest at midshaft and lowest proximally,
along with a small decline in mediolateral rigidity below midshaft. Large torsional stiffness
(Jeff) is also found at femoral midshaft, with
low stiffnesses both proximal and distal to
this. Maximum bending rigidity (Imax)within
each section parallels patterns found in the an-
teroposterior plane. Largest values are found
in the proximal 50% of the diaphysis, but decline somewhat between 62% and 74% of
length. Lowest values of minimum stiffness
(Imin) are found at both proximal and distal
ends of the shaft, with the highest values of
Imin found at midshaft. Large values of Imax,
Imin, and Jeff at midshaft indicate that the
stiffness of the bone as a whole in all planes is
largest at midshaft.
The cross sections of the galago femur tend
to be elongated, with widely discrepant bending stiffnesses in each plane. Imax is more
than 40% greater than Imin in the proximal
quarter of the diaphysis and is 24% greater in
the distal femur. The much larger bending
ridigity in the anteroposterior than the mediolateral plane proximally is illustrated by the
fact that Iyy is about 35% greater than Ixx in
the most proximal section and 15% larger at
midshaft.
Although the cortical area available to support loads in the galago femur declines sharply
from 38% down the shaft distally, this variation in area is not closely correlated to structural properties of the femoral shaft in bending
or torsion (c.f. Fig. 4). Stiffness in the anteroposterior plane shows very little decline from
proximal to distal and mediolateral stiffness
17
IS
12
t
E
E
10
7
5'
26
38
50
62
74
LEVEL (XI
Fig. 4. Variation in cross sectional moment of inertia in the mediolateral (Ixx)and anteroposterior (Iyy)planes, principal
moments of inertia (Imax, Imin). and the effective polar moment of inertia (Jeff)in the femoral diaphysis of G. senegalensis.
Numbers above and below symbols refer to standard errors.
425
FEMORAL MECHANICS IN THE LESSER BUSHBABY
actually increases in this range. The slight decline in outer diameter and thinning of the cortex in the sagittal plane between 62% and 74%
of femoral length does contribute to the decline
in anteroposterior rigidity in the distal femur.
The relationship between femoral rigidity
and cortical width and thickness is well
demonstrated by these data. A 17% increase in
anteroposterior medullary cavity diameter
coupled with no change in outer width accounts for a 7% decline in Iyy between 2690
and 6290 of femoral length. However, only a
4.8370decrease in outer diameter coupled with
but a slight increase in inner diameter accounted for greater than 20% decline in Iyy between
62% and 74% of femoral length. Changes in
outer diameter have larger effects on the bending stiffness, as characterized by the area moments of inertia (I),than equally large changes
in inner diameter. What appears on first inspection to be a minor change in geometry actually has a significant mechanical effect on
femoral function and underscores the fallacy
behind using cortical width alone as an indicator of the mechanical adaptation of bone. The
increasingly tubular nature of the galago
femur, which results in similar stiffnesses in
each plane at the most distal level, is illustrated in Figure 3 as well.
The maximum bending stresses within each
quadrant a t each of the five sectional levels
under constant moment of 100 kgf.cm are
shown in Figure 5. These data indicate that the
femur is configured to reduce bending stresses
in all planes at femoral midshaft. Higher
stresses are found both proximally and distally. Highest stresses in the mediolateral plane
are found proximally, and although stresses
distally are higher than at midshaft, they don’t
approach the magnitude of those found proximally. The opposite pattern is found anteroposteriorly, i.e., highest stresses are found distally, with somewhat lower stresses proximally. This again indicates the adaptation of the
femur to support bending loads in the anteroposterior plane proximally and the adaptation
to support higher mediolateral bending loads
distally than proximally. Because Ixx is smaller than Iyy, higher stresses are found mediolaterally than anteroposteriorly a t all levels,
900
399
F
-
goo
I*
E
Q
=a
u)
u)
W
a
I-
n
700
302
600
26
38
50
62
74
LEVEL
Fig. 5. Maximum bending stresses developed proximal to distal within each quadrant (m.1.a.p)of the femoral diaphysis
under a constant moment of 100 kgfxm. Numbers above and below symbols refer to standard errors.
426
D.B. BURR, G . PIOTROWSKI, R.B. MARTIN, AND P.N. COOK
with one minor deviation from this posteriorly
at the most distal section. The adaptation of
the femur in G. senegalensis is toward a reduction of anteroposterior bending stresses.
A similar pattern of torsimal stress is found
in the femoral diaphysis of G. senegalensis
(Fig. 6 ) .Lowest stresses are found at midshaft
with somewhat higher stresses both proximally and distally. The trend is not so pronounced mediolaterally, however. Torsional
stresses along the medial aspect remain at low
levels distally, while stresses along the lateral
aspect compensate by marked increase just
distal to midshaft. Maximum torsional stresses are found along the posterolateral aspect of
the femur at all cross sectional levels.
DISCUSSION
There are two fundamental differences in the
mechanics of leaping and walking locomotion.
First, leaping is a much more dynamic activity
requiring much larger accelerations of the
body center of gravity. This implies that peak
forces acting on and within the body will be far
greater than those that would be required to
walk at a nearly constant velocity. Second, the
trunk is flexed on the thigh during leaping so
that the body's center of gravity is placed forward of the ground support point. This increases the horizontal component of the
ground reaction force, which must act through
the body's center of gravity. This causes the
ground reaction force to have a longer moment
arm about the hip in leaping than it does in bipedal or quadrupedal walking. Thus, mechanical principles imply that leaping produces
larger muscle and ground reaction forces, and
larger moments about the hip joint, than does
walking.
These differences in the mechanics of leaping
and walking are supported by the structural
adaptations found in the femora of leapers
and walkers. The cross sectional moments of
inertia of the galago femoral diaphysis are
large both proximally and in the sagittal plane,
indicating that sagittal plane bending moments are relatively greater near the hip than
near the knee. This also corresponds to behavi-
306
65Ol
- 5501
(*
E
-s
x
01
01
u1
c
I01
4501
3501
26
38
50
62
74
LEVEL ( X )
Fig. 6. Torsionasl shear stresses developed proximal to distal within each quadrant Im.1.a.p) of the femoral diaphysis and
maximum stresses (M) under a constant torque of 100 kgf.cm. Numbers above and below symbols refer to standard errors.
FEMORAL MECHANICS IN T H E LESSER BUSHBABY
oral observations that leaping in galagos is
achieved with a two-legged symmetrical
stance in which sagittal plane bending
moments in the femur should be created by hip
and knee extension (Hall-Craggs, 1965a; Jouffroy et al., 1974). In contrast, cross sectional
moments of inertia in macaque and human femurs are largest in the frontal plane and increase distally (Miller and Piotrowski, 1977;
Burr et al., 1981), suggesting relatively larger
moments near the knee. The principal bending
loads in human and macaque femurs are in the
frontal plane and, in humans, result from abductor forces necessary for balancing on one
leg during the swing phase of locomotion (McLeish and Charnley, 1970).
When the distal joint reaction force acts at
an angle to the femur it applies a bending moment to the bone causing it to deflect. Although it is clear that large anteroposterior
bending moments are developed proximally in
the galago femur during leaping the diaphysis
of G . senegalensis is not nearly as curved anteroposteriorly as that of humans and other
nonsaltatory primates. Frost (1964) has hypothesized that if a long bone bears significant
radial loads (i.e., loads perpendicular to the
longitudinal axis of the shaft)that create bending moments in the shaft, the shaft will
remodel into a curved shape. The curvature is
such that compressive loads are eccentric to
the midshaft and create bending moments opposing those of the radial loads. The degree of
curvature is inversely proportional to the
amount of compressive stress. This would explain the anteroposterior curvature in the human femur since, during several portions of the
gait cycle, the ground reaction force acts
through or anterior to the knee (Elftman, 1951)
bending the femur in the anteroposterior
plane. This bending by perpendicular loads at
the knee is opposed by compressive muscle
loads acting in conjunction with the anteroposterior curvature. Hall-Craggs (1965a) showed
that in galago, however, ground reaction forces
pass behind the knee and across the femoral
shaft during the leap. Consequently, bending
loads perpendicular to the shaft should be
relatively small. Also the leaping galago generates large forces in its hamstring and quadriceps muscle groups as it extends the hip and
knee simultaneously from the initial flexed
position. This must produce compressive
forces in the femur that are relatively much
greater than those in nonsaltatory primates.
Because of the large compressive loads less
curvature of the femoral shaft is necessary to
compensate bending loads.
427
High compressive loads in the galago femur
would be more likely to cause buckling if the
femoral diaphysis were curved. In general,
bones of smaller animals are more slender than
those of larger animals because of the allometric relationship of body weight and cross sectional moment of inertia. The shaft of the
galago femur is, however, proportionately
more robust than human and macaque femora.
The ratio of periosteal diameter to length is
0.10 in galago and 0.065 in both macaques and
humans. The critical buckling load (Pc)may be
defined as
where E is the modulus of elasticity, Imin is
the mean value at femoral midshaft, and L is
femoral length. The ratio of critical buckling
load to body weight in Galago is 31 times that
in macaques and 55 times that in humans.
(These calculations assume the elastic moduli
of femoral bone to be equal in the three species
and use mean data for males and females.)
Thus, the femur of this saltatory primate is
morphologically adapted to resist buckling
when subjected to large compressive loads,
while those of macaques and humans need not
be as well adapted to compressive loads since
compressive forces required for walking are
much less. The cortical walls of the femur in
Galago are thin in an absolute sense but are
thick relative to body weight and femoral
length. The absolutely thin cortical walls and
small body weight allow speed of movement,
while the allometric relationships of cross sectional geometry, femoral length, and body
weight prevent buckling when the femur is
subjected to large loads. These data reinforce
the apparent similarity of the mechanical effects of quadrupedal and bipedal locomotion
on the femur and dissimilarity with femoral
mechanics in habitually saltatory primates.
The mechanics of leaping in arboreal and terrestrial species differ (Oxnard, 1979b),and it is
therefore not surprising that femoral mechanics in G. senegalensis are markedly different
than femoral mechanics in macaques (Burr et
al., 1981). Although some of these differences
could be taxonomic, the very specialized mechanical adaptation of the bushbaby femur to
its particular type of locomotion suggests that
these differences are basic to its saltatory
mode of progression. These data satisfy Oxnard’s (1979a) third method of separating
spurious morphology-behavior associations
from true functional complexes -the nature of
428
D.B. BURR, G . PIOTROWSKI, R.B. MARTIN, AND P.N. COOK
the morphological differences is biomechanically related to the function in question, i.e.,
leaping. Distribution of cortical area in both
macaques and bushbabies are superficially
similar in that area is high proximally and declines distally. However, the percentage
decline is more than twice as large in Galago as
in Macaca. While this distal decline in cortical
area was offset in Macaca by increases in bending stiffness distally, this is clearly not the
case in Galago, a t least in the anteroposterior
plane. While stiffness in the macaque femur
was greatest distally in most planes and was
generally lower at midshaft, the stiffness of
the galago femur was generally high a t midshaft, and decreased toward the distal end.
Moreover, stiffness is high in galagos in the
sagittal plane while stiffnesses were higher
mediolaterally in macaques. These patterns of
rigidity are reflected in stress patterns in each
group: stresses in the anterior and posterior
quadrants were highest in Macaca and lowest
in Galago. Also the midshaft decline in stress
found in Galago is not found in the femur of
Macaca.
This analysis indicates that there is a very
specific functional complex of traits that define the femoral mechanics of leaping primates
and that this complex of traits differs from
functional complexes found in animals less
specialized for leaping. As Fleagle (1979)
points out, paleoanthropological and paleoprimatological reconstruction is limited by our
ability to predict behavior from anatomy. This
analysis demonstrates that the biomechanical
techniques used here can be useful for discriminating anatomical specializations to different
behaviors based on a single aspect of morphology. These techniques can be expected to be
more sensitive to subtle behavioral differences
when applied to more than one morphological
structure and when used in conjunction with
other biomechanical techniques. Fleagle (1979)
suggested that the articular surfaces of the
long bones provide more information about the
mechanics of movement of an animal than do
other parts of the bone. While this may be true,
it is also true that other parts of the skeletal
anatomy may hold information which can only
now be extracted using techniques which have
not been applied to such analyses in the past.
This analysis of femoral mechanics in a leaping primate indicates that a great deal of information about the locomotion of a given animal
can be extracted from the femoral diaphysis
alone. I t further extends the analyses carried
out by Hall-Craggs (1965a.b)in demonstrating
the adaptation of one aspect of the skeletal system of G. senegalensis to a very specialized
form of behavior. Further comparative data
using these techniques on animals less dissimilar than Macaca and Galago are needed to demonstrate the sensitivity of the techniques in
functional anatomical investigations, and to
further improve the difficult task of interpreting the complexities of morphologyibehavior
interactions.
ACKNOWLEDGMENTS
This research was supported by a University
of Kansas Medical Center Committee on Research grant and by the Department of Anatomy. I am greatly indebted to Dr. Duane
Haines, Department of Anatomy, West Virginia University, for providing the galagos
used in this study.
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