Craniometric variation in a population of mantled howler monkeys (Alouattapalliata): Evidence of size selection in females and growth in dentally mature males

A large body of work on monkey cranial metrics (involving conclusions about interspecific variation, sexual dimorphism, and ontogeny) depends on the a ssumptions that growth effectively ceases with dental maturity and that int raspecific variation is negligible. We test these assumptions by examining variation in 39 measurements of 166 dentally mature Alouatta palliata skull s from animals found dead on Barro Colorado Island (BCI), Panama. We also i nvestigate whether this population is under size-based selection, since our found-dead sample reflects the natural mortality in this population. The s ample was divided into three age stages by occlusal wear (A-C, least to mos t wear). Female stage A means are significantly smaller than female stage B means for three cranial measures. Female stage B means are significantly s maller than female stage C means for five cranial measures. Male stage A me ans are significantly smaller than male stage B means for 21 cranial measur es. Multivariate analyses confirm this trend of expansion between adult age stages. The dental metric and suture closure data suggest that the cranial expansion in females is due to size-based selection, while the cranial exp ansion in males is due to significant growth after dental maturity. Sexual dimorphism ratios are highly variable across different samples of A. pallia ta, indicating that dimorphism varies between populations of this species. These results provide insight into the selective forces operating on the BC I howlers and challenge the validity of the many studies which pool subspec ies and assume growth ceases with maturity. (C) 2000 Wiley-Liss, Inc.