The differential expression of dental sexual dimorphism in subspecies ofColobus guereza

Zygomatic arch root position in relation to dietary type in haplorhine primates

The zygomatic root, along with other key craniofacial features, is hypothesized to play a crucial role in strengthening the face in response to stresses and strains related to feeding. As such, it has been cited as indicative of dietary specialization among fossil taxa, although it remains unknown how variable zygomatic arch root position is among living primates, and whether its positioning predicts differences in diet. We test whether primates that consume more mechanically challenging foods possess more anteriorly positioned zygomatic roots compared to those consuming less challenging foods. Zygomatic root position, as defined by the zygomaxillare landmark, was identified and recorded from digital images and physical specimens of adult primate crania. Data were collected from 33 haplorhine species (n = 722). Published data were used to assign species to a dietary type based on patterns of overall consumption along with reliance on especially challenging foods. Pairwise comparisons between mechanically challenging (hard and/or tough) and less mechanically challenging (soft) consumers found significant differences (p < 0.05) in the position of the zygomatic root in 17 of 20 pairs, 11 of which supported the prediction that a more mechanically challenging diet is associated with a more anteriorly placed zygomatic root. PGLS analysis found no significant effect of phylogeny on root position. This suggests that a more anteriorly positioned zygomatic root is useful for identifying dietary specialization in some taxa but is not required for consuming a mechanically challenging diet given that other craniofacial and behavioral factors can facilitate the consumption of such foods.

Correlated response, competition, and female canine size in primates

Recently, comparative analyses of female canine tooth size in primates have yielded two hypotheses to explain interspecific variation in female relative canine size. Greenfield ([1992] Int. J. Primatol. 13:631-657; [1992] Yrbk. Phys. Anthropol. 35:153-184; [1996] J. Hum. Evol. 31:1-19) suggested that covariation in male and female canine size across species indicates that female canine size reflects correlated response (in which the expression of a trait in one sex causes the expression of the same trait in the other sex). Plavcan et al. ([1995] J. Hum. Evol. 28:245-276) noted that female canine size in primates is associated with variation in categorical estimates of the intensity of female-female agonistic competition, suggesting that selection favors large female canine size in many species. While it may seem that the two models are in conflict, they are not. To simultaneously evaluate these two models, this analysis examines the joint relations between male canine size, female canine size, and estimates of female-female competition in a sample of 108 primate species. Overall, female canine size is correlated with variation in male canine size. Controlling for variation in male canine size, female canine size is also correlated with estimates of the intensity of female-female agonistic competition. The relation between these variables differs strongly between anthropoid and strepsirhine primates. In anthropoids, the data suggest that selection for the development of large canines in females is not constrained by any affect of correlated response. In strepsirhines, the evidence suggests that sexual selection may affect male canine size but that correlated response affects female canine size, resulting in monomorphism for most species. These observations help reconcile the observations of Greenfield ([1992] Int. J. Primatol. 13:631-657; [1996] J. Hum. Evol. 31:1-19) and Plavcan et al. ([1995] J. Hum. Evol. 28:245-276) and provide a more precise model for understanding interspecific variation in female canine size and hence canine dimorphism.