CRANIODENTAL ALLOMETRY AND HETEROCHRONY IN 2 HOWLER MONKEYS - ALOUATTA-SENICULUS AND A-PALLIATA

Cranial dimensions were collected from growth series for two sexually dimorphic congeners: Alouatta seniculus, the red howler monkey, and Al ouatta palliata, the mantled howler monkey. In both A. seniculus and A . palliata, ontogenetic series for males and females were compared to evaluate if sexual dimorphism in skull form results from the different ial extension of common patterns of relative growth. Subsequently, gro wth series for both species were compared to investigate whether morph ological differences between species also result from the ontogenetic scaling of cranial proportions. Analyses indicate that cranial proport ions for both sexes of Alouatta palliata are ontogenetically scaled. I n mantled howlers, males apparently reach larger terminal size by grow ing for a longer duration and, to a lesser extent, at a faster rate th an females. Data for both sexes of Alouatta seniculus indicate that cr anial proportions are also ontogenetically scaled. In particular, male red howlers apparently reach larger adult size by growing at a faster rate and, perhaps, to an equivalent or longer duration than females. The Alouatta seniculus data underscore apparent differences in the rat e and timing components of sex dimorphism, possibly due to sexual diff erences among dental eruption patterns, cranial development, somatic g rowth, and socioecological factors. Results for both species indicate that intrasexual selection for size differentiation has a minimal effe ct on brain size and postcanine tooth size dimorphism. Lastly, compari sons of allometric trajectories for both species further demonstrate a strong pattern of ontogenetic scaling of cranial proportions. (C) 199 4 Wiley-Liss, Inc.