The peculiar carpal tubercles of male Marmosops parvidens and Marmosa robinsoni (Didelphidae : Didelphinae)

Adult males of several genera of Neotropical marsupials display sexually dimorphic carpal tubercles of two distinct kinds. In certain species of Marmosops and Gracilinanus, a prominent external carpal tubercle is located on the ulnar side of the palmar surface of the wrist and is supported internally by an enlarged and spoon-shaped pisiform. In certain species of Marmosa and Micoureus, a prominent external carpal tubercle is located on the radial side of the palmar surface of the wrist and is supported internally by an enlarged and modified prepollex. In female conspecifics, there are no obvious carpal tubercles nor is there enlargement or modification of the associated bones. We propose a copulatory function for these structures, which may be the first examples of sexually dimorphic bony limb characters thus far identified in mammals.