Biol. Lett. (2008) 4, 486–489
Published online 15 July 2008
Apparent density patterns
in subchondral bone of the
sloth and anteater forelimb
Biren A. Patel1,*and Kristian J. Carlson2
1Department of Biomedical Sciences, Ohio University College of
Osteopathic Medicine, Athens, OH 45701, USA
2Department of Anatomy, New York College of Osteopathic Medicine,
Old Westbury, NY 11568, USA
*Author for correspondence (firstname.lastname@example.org).
Vertebrate morphologists often are interested
in inferring limb-loading patterns in animals
characterized by different locomotor reper-
toires. Because bone apparent density (i.e. mass
per unit volume of bone inclusive of porosities)
is a determinant of compressive strength, and
thus indicative of compressive loading, recent
comparative studies in primates have proposed
a structure–function relationship between appa-
rent density of subchondral bone and locomotor
behaviours that vary in compressive loading.
If such patterns are found in other mammals,
then these relationships would be strengthened
further. Here, we examine the distal radius
of suspensory sloths that generally load their
forelimbs (FLs) in tension and of quadrupedal
anteaters that generally load their FLs in
compression. Computed tomography osteoab-
sorptiometry was used to visualize the patterns
in subchondral apparent density. Suspensory
sloths exhibit relatively smaller areas of high
apparent density than quadrupedal anteaters.
This locomotor-based pattern is analogous to
the pattern observed in suspensory and quad-
rupedal primates. Similarity between xenar-
thran and primate trends suggests broad-scale
applicability for analysing subchondral bone
apparent density and supports the idea that
bone functionally alters its material properties
in response to locomotor behaviours.
Keywords: functional morphology; quadrupedal;
radius; suspensory; wrist
Apparent density of bone is a primary determinant
of its compressive strength both through the amount
of mineral (i.e. tissue density) and the degree of
porosity exhibited by the bone (Currey 1984; Keller
1994). Compressive strength of subchondral bone
is particularly revealing of habitual compressive load-
ing in a limb because it serves as the site of force
transmission across diarthrodial joints (Radin et al.
1970). When joint surfaces are relatively congruent
(i.e. predominantly experience compressive loading
rather than bending), apparent density can be visual-
ized to estimate habitual loading regimes (Mu ¨ller-
Gerbl et al. 1992; Eckstein et al. 1999). Although
extrapolating behavioural differences from compari-
sons of bone material properties is not without
its share of complexities, these properties still
provide valuable insights for biologists interested in
structure–function interactions (e.g. Demes et al.
2001; Lieberman et al. 2003; Pearson & Lieberman
2004; Ruff et al. 2006).
Compressive loading of a joint is predominantly
a function of (i) joint reaction forces arising from
muscle–tendon complexes crossing the joint and
(ii) gravitational forces operating on body mass that
is supported through the joint (i.e. body weight). A
recent ‘natural experiment’ comparing primates that
experience compressive forelimb (FL) loads falling
into both categories (i.e. quadrupeds) versus primates
restricted to the former category (i.e. suspensory
and bipedal) suggested that body weight may take
priority in establishing subchondral apparent density
(Carlson & Patel 2006). Radiodensity of subchondral
bone at the distal radial articular surface differed
between these two primate locomotor groups. Old
World monkeys (Cercopithecoids) and African apes
(Pan and Gorilla) had relatively broader areas of
high apparent density compared with suspensory
Asian apes (Hylobates and Pongo) and bipedal humans
(Carlson & Patel 2006). Asian apes habitually restrict
compressive forces generated through their wrist
to those arising from muscle contractions (i.e. joint
reaction forces) since they suspend their body mass
below superstrata (and increase tension through their
FL; Swartz et al. 1989). In addition, as Biewener
(1989) has shown for terrestrial mammals, suspen-
sory support may reduce muscle force requirements,
and thus compressive bone loading, owing to align-
ment of the FL and its joints with superstratum
reaction forces associated with suspensory support.
Humans do not support body weight through their
FLs during locomotion and thus they too will
habitually limit compressive forces across the wrist
joint to muscle contractile forces. Since monkeys and
African apes, unlike the other primate groups, typi-
cally are required to support body mass above
substrates during quadrupedalism, Carlson & Patel
(2006) suggested that their more expansive areas of
high apparent density corresponded to superimposed
compressive loads in the wrist joint due to muscle
contractile forces and body weight support. Others
have noted additional links between locomotor
behaviour and bone material properties in the primate
postcranial skeleton (e.g. Schaffler & Burr 1984;
If the relationship between subchondral apparent
density patterns and habitual locomotor modes can
be extended beyond primates, it may indicate the
presence of a broad-scale trend among vertebrates. It
also would bolster the argument that bone modulates
its properties according to functional input (e.g.
locomotor behaviour). Here, we investigate xenar-
thrans as an independent test of a previously ident-
ified primate trend. Others have demonstrated
functional and morphological convergences between
extant xenarthrans and some non-human primates
(Mendel 1979; Orr 2005). We also selected xenar-
thrans because they are phylogenetically distant from
primates (Springer et al. 2004) and exhibit beha-
vioural diversity (e.g. quadrupedal and suspensory
species; Nowak 1999). Specifically, we compare
suspensory sloths (Bradypus and Choloepus), which
Received 1 June 2008
Accepted 24 June 2008
This journal is q 2008 The Royal Society
probably habitually load FLs in tension, to quadrupe-
dal anteaters (Myrmecophaga and Tamandua), which
probably habitually load FLs in compression.
2. MATERIAL AND METHODS
Distal radii were examined in 33 sloths (Choloepus didactylus,
Choloepus hoffmanni, Bradypus tridactylus and Bradypus variegatus)
and anteaters (Myrmecophaga tridactyla, Tamandua mexicana,
Tamandua tetradactyla). We selected only wild-shot specimens
with epiphyseal plate fusion of all long bones (skeletally adult),
no/minimal signs of degenerative joint disease or traumatic injury
and no post-mortem abrasion to the distal radius (Carlson &
Computed tomography osteoabsorptiometry was used to visual-
ize apparent density (i.e. radiodensity) of subchondral bone at the
distal radius (Mu ¨ller-Gerbl et al. 1992; Carlson & Patel 2006).
Radii were serially CT scanned (GE lightspeed 16; slice thickness,
0.625 mm; tube voltage, 120 kV; tube current, 70 mA) in para-
sagittal planes. Elements were scanned individually to maximize
the accuracy of grey values for each specimen. Reconstruction of
raw CT data used a soft tissue algorithm. Reconstructed digital
images were saved as DICOM files and imported into IMAGEJ
software (http://rsb.info.nih.gov/ij) for subsequent evaluation. From
each image stack, two-dimensional maximum intensity projection
(MIP) maps were generated by visualizing brightness values of the
three-dimensional data volume (see Carlson & Patel 2006). Bright-
ness values of pixels in the MIP were partitioned into eight distinct
colour bins by creating a false colour map. The lowest bin (white)
included pixels with minimal radiodensities, while the highest bin
(black) contained pixels with maximal radiodensities (figure 1a).
Pixels in each bin were counted and calculated as a percentage
of the total pixels in the articular surface, although we emphasize
only the maximum bin (i.e. black pixels) during comparisons
within and between locomotor groups. By using percentages of
total articular surface area, we accounted for size effects (body
size). Group differences were evaluated using non-parametric
3. RESULTS AND DISCUSSION
The amount of maximal apparent density in the
articular surface of the distal radius is greater in
xenarthrans that habitually load their FLs in com-
pression (i.e. quadrupedalism) compared with those
that reduce compression in their FLs (i.e. increase
tension by adopting more suspensory postures). Quad-
rupedal anteaters (Myrmecophaga and Tamandua) have
expanded areas of high apparent density (i.e. greater
black pixels relative to total pixels in the articular
surface) compared with suspensory sloths (Choloepus
and Bradypus; figure 1 and table 1; UZ10; p!0.001).
Significant differences, however, do not extend to taxa
within locomotor groups (figure 1b). The quadrupedal
anteaters—the giant Myrmecophaga and the smaller
Tamandua—do not differ from one another in the
relative areas of high apparent density (UZ33;
pZ0.694), nor do the suspensory sloths—the two-
toed Choloepus and the three-toed Bradypus (UZ8;
pZ0.079). Considering that both groups probably
have strong digital flexor muscles for either digging
(anteaters) or hanging below superstrata (sloths), the
added weight support by quadrupedal anteaters could
be a probable explanation for their increased area of
maximal apparent density.
It is intriguing that xenarthrans in both locomotor
groups exhibit relatively more expansive high appa-
rent density areas than those reported in the respect-
ive suspensory and quadrupedal primates (i.e. 6.0%
and 16.2% of articular surface area in the suspensory
and quadrupedal primates, respectively; table 1;
Carlson & Patel 2006). Because compressive loading
should impact apparent density of subchondral bone,
potential differences in locomotor kinetics between
xenarthrans and primates may be relevant. Primates
generally experience lower peak vertical substrate
reaction forces (SRFs) in their FLs relative to their
hind limbs (HL) than most other mammals during
linear quadrupedal locomotion on runways and
simulated arboreal substrates (Demes et al. 1994).
The relatively unique primate interlimb SRF pattern
could be related to a more posterior location of the
centre of mass in primates, or to a dynamic caudal
shift of the body weight towards the HL during
primate locomotion (Reynolds 1985). While SRF
data from xenarthrans have not been published,
trends in interlimb SRFs among non-primate mam-
mals appear robust (Demes et al. 1994). If xenar-
thrans were found to support a greater proportion
of their body weight on their FLs, much like
non-primate mammals in general, FL/HL kinetic
distinctions from primates would be consistent with
non-primates exhibiting relatively greater amounts
of high apparent density in their wrist joint. There
are caveats, however, to such a scenario. Behavioural
variability of free-ranging animals is probably more
dynamic than often modelled in the laboratory, and
thus the overall variability in SRFs for any one
phylogenetic group (e.g. primates and xenarthrans)
percentage area of
highest apparent density
suspensory primatesquadrupedal primates
B. tridactylus M. tridactyla
T. tetradactyla C. hoffmanni
Figure 1. (a) Representative MIPs for each xenarthran
genus. Scale bar, 1 cm. (b) Percentage area of highest
apparent density for each xenarthran genus and each
locomotor group (suspensory and quadrupedal xenarthrans
and primates). Primate data adapted from Carlson & Patel
(2006). Boxes, means; whiskers, 1 s.d.
Forelimb loading in xenarthrans
B. A. Patel & K. J. Carlson487
Biol. Lett. (2008)
remains poorly understood. Additionally, differences
in high apparent density areas between xenarthrans
and primates could also be a phylogenetic effect
(i.e. genetic difference).
Despite the relatively greater areas of high apparent
density in xenarthrans relative to primates, it is
important to note the parallel between suspensory
versus quadrupedal xenarthrans and between suspen-
sory and quadrupedal primates (Carlson & Patel
2006). Similarities between the locomotor groups
that cross phylogenetic boundaries suggest a robust
structure–function signal. Functional morphologists
could use patterns in apparent density of subchondral
bone to evaluate habitual joint loading when experi-
mental data are untenable. A widely applicable signal
also supports the notion that bone modulates its
material properties, at least partially according to
behavioural activities (e.g. locomotor behaviour).
Therefore, studies of subchondral bone (Ahluwalia
2000; Carlson & Patel 2006; Patel & Carlson 2007;
Polk et al. 2008) augments the expanding breadth of
research into cortical and trabecular bone structure
by functional morphologists (e.g. Lieberman et al.
2003; Pearson & Lieberman 2004; Pontzer et al.
2006; Ruff et al. 2006).
No live animals were used in this study and all data were
obtained from museum specimens.
We thank the American Museum of Natural History (Eileen
Westwig), National Museum of Natural History (Linda
Gordon) and Museum of Comparative Zoology (Judy
Chupasko) for access to specimens. CT scanning was
facilitated by Stony Brook University Hospital with help
from Lou Caronia, Justin Georgia and Andrew Farke.
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% max % second% third % fourth% fifth
aPercentages calculated as the number of pixels in a colour bin divided by the total number of pixels in the articular surface. Colour scale in
figure 1a provides bin definitions.
bGenus means from Nowak (1999).
cData adapted from Carlson & Patel (2006).
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