Exudativory in the Asian loris, Nycticebus: Evolutionary divergence in the toothcomb and M3

Slow lorises (Nycticebus spp.) are obligate exudativores that gouge tree bark. Dental adaptations for gouging within marmosets, the only other known primate obligate exudativore, are well-known but dental adaptations in Nycticebus are largely unidentified. In an effort to more completely understand potential dental adaptions within Nycticebus and the evolution of this dietary niche within Primates as an order, the present study examined dental morphometrics in the Asian lorises (Nycticebus and Loris).

We compared dental morphometrics between Nycticebus and the insectivorous slender lorises (Loris). Measurements from the toothcomb and select other teeth were taken from 92 specimens. Each variable was scaled by the geometric mean and resulting mean ratios were statistically compared between groups. A biomechanical shape variable was also calculated to estimate the ability of the toothcomb to resist bending that may be experienced during gouging.

Toothcombs in Nycticebus were significantly (P < 0.05) more narrow, shorter, and thicker than those in Loris and had a higher calculated ability to withstand bending forces. Nycticebus also had reduced size in the last lower molar relative to Loris.

The more robust, "squared off" toothcomb in Nycticebus matches behavioral observations that these primates gouge to access exudates. Results of the present study indicate that the toothcomb is the likely candidate for the dental tool used in gouging. The size reduction of the lower last molar in Nycticebus, a trait also found in a previous study in exudativorous galagos, may indicate that there is reduced selective pressure in a diet where little mastication would be needed to mechanically process exudates. These results may indicate that reduction in molar size could be a potential dental signature for exudativory, but further studies on a wider phylogenetic range of exudativorous primates would be necessary.