Craniodental allometry and heterochrony in two howler monkeys: Alouatta seniculus and A. palliata

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Abstract
Cranial dimensions were collected from growth series for two sexually dimorphic congeners: Alouatta seniculus, the red howler monkey, and Alouatta 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 differential extension of common patterns of relative growth. Subsequently, growth series for both species were compared to investigate whether morphological differences between species also result from the ontogenetic scaling of cranial proportions.Analyses indicate that cranial proportions for both sexes of Alouatta palliata are ontogenetically scaled. In mantled howlers, males apparently reach larger terminal size by growing for a longer duration and, to a lesser extent, at a faster rate than females. Data for both sexes of Alouatta seniculus indicate that cranial 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 rate and timing components of sex dimorphism, possibly due to sexual differences among dental eruption patterns, cranial development, somatic growth, and socioecological factors. Results for both species indicate that intrasexual selection for size differentiation has a minimal effect on brain size and postcanine tooth size dimorphism. Lastly, comparisons of allometric trajectories for both species further demonstrate a strong pattern of ontogenetic scaling of cranial proportions.