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<p>To evaluate alternative hypotheses for the role of mass and muscle-induced forces
in joint construction, the allometric scaling relationships of the articular facets
of the talus were estimated with phylogenetic regressions. Many articular surfaces
scale with significant positive allometry, suggesting that mass-induced forces are
an important influence for the bony architecture of synovial joints.</p><p>Using a
large sample of primates and their nearest living relatives, body mass prediction
equations were generated from the articular facet areas of the talus and calcaneus.
Those facets that scaled with positive allometry were both accurate and precise. Compared
to previously published prediction equations, the novel equations developed for this
study were substantially more reliable.</p><p>Several methodological debates for body
mass prediction were also evaluated. Prediction equations had their highest correlations
when species with greater than a 20% difference between sexes are represented by both
males and females. Using dental measurements from cercopithecoids housed at the National
Museum of Natural History, predictive accuracy was maximized when body mass was predicted
using a mean value estimated from a robust sample. Even when only a single individual
was represented, tests of predictive accuracy using primates with associated body
masses from several localities (Hacienda La Pacifica, Costa Rica; Gombe Stream National
Park, Tanzania; Amboseli Reserve, Kenya; and the Duke Lemur Center) demonstrated that
prediction equations provide more accurate predictions of species mean values than
individual-specific values.</p><p> The importance of longitudinal change in body mass
was evaluated by comparing coefficients of variation for individual and mean body
mass of the populations at La Pacifica, Gombe, and the Duke Lemur Center. Individual
coefficients of variation were significantly greater than the population coefficients
of variation, which suggests that mean body masses are more stable “targets” of prediction.</p><p>Finally,
the novel prediction equations were applied to a sample of sympatric primates with
associated dental and postcranial elements from the middle Eocene of Wyoming, including
Notharctus tenebrosus, Smilodectes gracilis, Omomys carteri, and Hemiacodon gracilis.
New body mass predictions suggest two pairs of similarly sized primates: N. tenebrosus
and S. gracilis (~2500g), and O. carteri and H. gracilis (~400g). Thus, niche partitioning
between closely related taxa was probably achieved through differences in diets, rather
than differences in body mass.</p>
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