Multivariate Craniodental Allometry of Tarsiers

Abstract

Evolutionary allometry describes size and shape differences across taxa matched for developmental stage (e.g., adulthood). Allometric studies can identify subtle differences among species, and therefore help researchers interested in small-bodied, cryptic species such as tarsiers. Recent taxonomic revision has emphasized size differences among three possible tarsier genera inhabiting different island regions: Sulawesi (genus: Tarsius), Borneo (genus: Cephalopachus), and the Philippines (genus: Carlito). We examined seven craniodental measures of 102 museum specimens of adult tarsiers representing these three regions. We found that the allometric patterns within groups do not predict the observable differences among groups. Crania of the largest-bodied genus, Cephalopachus, are characterized by relatively short skulls and small orbits, with wider palates and molars than predicted by allometric increase from the smaller-bodied Tarsius. Overall, we found tarsier skulls stay the same shape as they increase in size. This may reflect shared developmental and biomechanical adaptations across tarsier groups filling an extreme leaping, faunivorous niche with hypertrophied orbits and subtle dietary differences in prey selection. These shared adaptations of tarsiers may severely limit the range of body sizes in tarsiers and impose further constraints on cranial shape. Despite their deep divergence times in the Miocene, living tarsier groups are united by a common craniodental form across a limited size range. Adaptations to extreme niches might result in a hyperconservatism of the cranium. Future primate allometric studies should explore cranial variation in other taxa to determine how adaptations to specific niches affect the size and shape of the cranium.