Abstract:
Allen's rule (1877) predicts ecogeographical anatomical variation in appendage
proportions as a function of body temperature regulation. This phenomenon has
been tested in a variety of animal species. In macaques, relative tail length (RTL)
is one of the most frequently measured appendages to test Allen's rule. These
studies have relied on museum specimens or the invasive and time‐consuming
capturing of free‐ranging individuals. To augment sample size and lessen
these logistical limitations, we designed and validated a novel noninvasive
technique using digitalized photographs processed using LibreCAD, an
open‐source 2D‐computer‐aided design (CAD) application. This was used to
generate pixelated measurements to calculate an RTL equivalent, the Tail to
Trunk Index (TTI) = (tail [tail base to anterior tip] pixel count/trunk [neck to tail
base] pixel count). The TTI of 259 adult free‐ranging toque macaques (Macaca
sinica) from 36 locations between 7 and 2,087 m above sea level (m.a.s.l.) was
used in the analysis. Samples were collected from all three putative subspecies
(M. s. sinica, aurifrons, and opisthomelas), at locations representing all altitudinal
climatic zones where they are naturally distributed. These data were used to test
whether toque macaque tail length variation across elevation follows Allen's rule,
predicting that RTL decreases with increasing elevation and lower temperature.
Our results strongly supported this prediction. There was also a statistically
significant, negative correlation between elevation and annual average
temperature. The best predictor for the TTI index was elevation. Significant
subspecies differences in RTL are linked in part to their ecological and altitudinal
niche separation, but overall the variation is seen as the species' adaptation to
climate. The method developed for the quick morphometric assessment of
relative body proportions, applicable for use on unhabituated free‐ranging
animals, widens the range of materials available for research studying
morphological characteristics and their evolution in primates
