||The articular facets of interosseous joints must transmit forces while maintaining
relatively low stresses. To prevent overloading, joints that transmit higher forces
should therefore have larger facet areas. The relative contributions of body mass
and muscle-induced forces to joint stress are unclear, but generate opposing hypotheses.
If mass-induced forces dominate, facet area should scale with positive allometry to
body mass. Alternatively, muscle-induced forces should cause facets to scale isometrically
with body mass. Within primates, both scaling patterns have been reported for articular
surfaces of the femoral and humeral heads, but more distal elements are less well
studied. Additionally, examination of complex articular surfaces has largely been
limited to linear measurements, so that 'true area' remains poorly assessed. To re-assess
these scaling relationships, we examine the relationship between body size and articular
surface areas of the talus. Area measurements were taken from microCT scan-generated
surfaces of all talar facets from a comprehensive sample of extant euarchontan taxa
(primates, treeshrews, and colugos). Log-transformed data were regressed on literature-derived
log-body mass using reduced major axis and phylogenetic least squares regressions.
We examine the scaling patterns of muscle mass and physiological cross-sectional area
(PCSA) to body mass, as these relationships may complicate each model. Finally, we
examine the scaling pattern of hindlimb muscle PCSA to talar articular surface area,
a direct test of the effect of mass-induced forces on joint surfaces. Among most groups,
there is an overall trend toward positive allometry for articular surfaces. The ectal
(= posterior calcaneal) facet scales with positive allometry among all groups except
'sundatherians', strepsirrhines, galagids, and lorisids. The medial tibial facet scales
isometrically among all groups except lemuroids. Scaling coefficients are not correlated
with sample size, clade inclusivity or behavioral diversity of the sample. Muscle
mass scales with slight positive allometry to body mass, and PCSA scales at isometry
to body mass. PCSA generally scales with negative allometry to articular surface area,
which indicates joint surfaces increase faster than muscles' ability to generate force.
We suggest a synthetic model to explain the complex patterns observed for talar articular
surface area scaling: whether 'muscles or mass' drive articular facet scaling is probably
dependent on the body size range of the sample and the biological role of the facet.
The relationship between 'muscle vs. mass' dominance is likely bone- and facet-specific,
meaning that some facets should respond primarily to stresses induced by larger body
mass, whereas others primarily reflect muscle forces.