Comparative Myology of the Forelimb in Capuchin and Night Monkeys

Abstract

Background: The capuchin monkey (Sapajus apella) and the night monkey (Aotus azarae infulatus) are primates with broad distribution in South America. Sapajus apella is best-studied species in the genus Cebus, and is commonly found in captivity in parks, zoos, and sorting centers in Brazil. Species in the genus Aotus have a primitive appearance with large eye sockets, and their nocturnal habits are unique among the apes today. Because these two species belong to the same order, they have similar morphological characteristics. In this work, we conduct a comparative study of the forelimb musculature between these two species to produce data for use in veterinary procedures.

Materials, Methods & Results: Three capuchin monkey and three night monkey specimens were used, two females and one male for each species. Specimens were acquired from the National Center for Primates under approval from the Animal Research (CEPAN/IEC/SVS/MS) No. 008/2010. Specimens were frozen and donated after death by natural causes to the LaPMA (Animal Morphology Research Laboratory) at the Federal Rural University of Amazonia (UFRA). After being thawed in running water, the animals were fixed by intramuscular injection with 10% formaldehyde in aqueous solution and dissected to display thoracic limb muscle groups. Dissections were performed using basic instrumentation by creating a medial incision from the carpal region to the axillary region. After incision, the skin was folded and muscles were separated in order to identify muscle groups. The nomenclature adopted was based on the veterinary anatomy, and books about human anatomy for comparison of forearm muscles. The muscles that compound the shoulder was the supraspinal and infra-spinal. The arm was composed by the teres major and minor, the triceps brachii (long head, lateral head, and medial head), brachialis, biceps brachii, tensor muscle of the antebrachial fascia, coracobrachialis, and anconeous. The forearm included the extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorus, extensor digiti minimum, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis brevis, flexor carpi ulnaris, flexor digitorum profundus, flexor digitorum superficialis, flexor carpi radialis, flexor pollicis longus, extensor carpi radialis brevis, and brachioradialis muscles. In general, the two species have the same muscle groups, except for the absence of the teres minor and brachioradialis in Capuchin monkey, and the absence of the long head of the triceps brachii and extensor carpi radialis brevis at night monkey.

Discussion: The forelimb musculature in the capuchin and night monkey is similar that described in the literature for both domestic and wild species such as coati, paca, and bush dogs, with little variation among the studied species. Regarding the muscles of the forearm, the capuchin monkey and the night monkey have muscular anatomy similar to that of humans, with some variations. The anatomical variations in common traits do not impede function. Interspecific variation is a common occurrence in nature, and can be explained by species divergence which occurs over many years, for various reasons. Knowledge of the comparative miology of the capuchin monkey and the night monkey is still nascent, thus future research is needed to better understand the structure of these animals, which can then serve as a basis for studies of other ethological aspects of the species biology.