The Evolution and Development of Molar Size Among Exudate-Feeding Lorises and Bushbabies

A challenge for linking evolution and development is to find proxies for developmental processes that can be studied from the types of material preserved in the fossil record (i.e., teeth). The inhibitory cascade, an activator-inhibitor developmental model said to explain variation in mammalian molar proportions, provides insight to such processes. However, past studies have generally failed to make a connection between the inhibitory cascade and diet, particularly among primates.

We examined this model and molar proportions in a sample of living (n = 52) and extinct (n = 6) lorisoid primates that vary in the proportions of exudates (e.g., gums, nectar) they consume. We use the reduced major axis regression to test for the ICM.

Taxa that consume a large proportion of exudates are characterized by a distinctive pattern of molar proportions (molars decreasing in size moving distally) from those that do not (molars increasing in size moving distally). Among fossil taxa, galagids are reconstructed as intensive or moderate exudativores; the fossil lorisid, Nycticeboides simpsoni, is inferred to have been non-exudativorous. The oldest toothcomb-bearing primate, Karanisia clarki, had molar proportions of a non-exudativore.

Our results demonstrate that we can explain patterns of phenotypic variation, as they relate to diet, using a developmental framework, thereby providing a link between genotype and phenotype. Furthermore, our results weaken the case for a connection between the evolution of the toothcomb and exudativory as it is unlikely that the earliest strepsirrhines (e.g., Karanisia) were exudativorous.