Muscles of facial expression in Otolemur, with a comparison to lemuroidea

Gross and histologic aspects of facial expression muscles are presented here for Otolemur spp. (suborder Prosimii, family Lorisidae) and are compared with those of lemuroids. Muscles of facial expression are involved in social signaling among primates, and are a primary means by which close-proximity nonverbal communication is achieved. These muscles have been well described in catarrhines and many of the lemuroids; however, their arrangement is not well known in the lorisids. In the present study we examined muscles of facial expression in Otolemur by dissecting preserved faces. The arrangement and appearance of the muscles were noted, and samples were gathered from each muscle for histologic processing. The results showed 17 muscles of facial expression in Otolemur, as compared to seven reported in previous studies. Histologically, muscles of the ear region were arranged in tight, dense fascicles, while muscles of the orbital region were arranged more loosely. Grossly, the facial expression muscles in Otolemur were very similar in morphology and attachments to those in the lemuroids, with some differences in the ear region. Otolemur garnettii had several muscles that appeared to be more robust than in the larger O. crassicaudatus. This may be due to dietary and/or social differences between the species. In previous studies it was concluded that, relative to lemuroids, Otolemur has a primitive arrangement of facial expression muscles. The current results do not support that conclusion, and in fact support a far greater similarity between Otolemur and lemuroids in general. These results underscore the need for a reexamination of facial musculature in prosimians in general, and may have taxonomic value as regards the position of Otolemur with lemuroids and other galagos.