Forearm muscles in the black lion tamarin (Leontopithecus chrysopygus)

<i>Leontopithecus chrysopygus</i> (black lion tamarin) is a platyrrhine primate found in the Brazilian Atlantic Forest. This species is threatened by human activities that encroach on its habitat. Studies on <i>Leontopithecus</i> myology provide insights into ecological variables and support primate medicine. This study aimed to analyze the anatomy of the forearm muscles of <i>L. chrysopygus</i>. For this purpose, six thoracic limbs from five adult male <i>L. chrysopygus</i> specimens were dissected. The muscles were described by their attachment points (origin and insertion), and their masses were measured using a precision scale. Functional muscle groups were established for comparison purposes. The positioning and skeletal attachments of the muscles resembled existing descriptions for primates. However, intraspecific variations were noted, such as a recess for the radial nerve between the origin of the tendon of the brachioradial muscle in one-third of the samples. The deep digital flexor muscle had the highest mean mass (2.34 ± 0.43 g), whereas the extensor muscle of the second digit had the lowest mean mass (0.03 ± 0.01 g). The carpal and digital flexor muscles showed a significantly higher mean percentage mass (<i>p</i> < 0.05) than the others (58.4 ± 2.7%), possibly due to the strength required for hand grip during locomotion on branches and climbing trees. Further studies on the muscles of other regions of the thoracic limb and other <i>Leontopithecus</i> species could enhance our understanding of how these muscles adapt to their lifestyles.