Evolutionary adaptations in the flexor digitorum profundus muscle in Tamandua mexicana (Xenarthra,Myrmecophagidae)

The northern tamandua (Tamandua mexicana) is a neotropical mammal of the order Pilosa, suborder Vermilingua, and family Myrmecophagidae. This species has anatomical and functional adaptations in its forelimb for semiarboreal quadrupedal locomotion. Several studies have reported that the medial head of the triceps brachii and flexor digitorum profundus muscles are fused in species belonging to the family Myrmecophagidae. However, there is no reference to the innervation in these. The triceps brachii muscle is commonly innervated by the radial nerve and the flexor digitorum profundus muscle by the ulnar and median nerves. This study aims to describe the gross anatomy of the flexor digitorum profundus muscle in Tamandua mexicana with respect to the shape, origin, insertion, innervation, and arterial supply. Both forelimbs of nine specimens were used, which were dissected from superficial to deep layers. The formalin-fixed caudomedial forearm muscles were weighed, and the weight percentages of individual forearm muscle specimens were calculated. The flexor digitorum profundus had the highest weight among the forearm muscles and consisted of five heads (three humerals, one radial, and one ulnar). These heads were innervated by median and ulnar nerves; therefore, based on the innervation pattern, we concluded that the medial head of the triceps brachii muscle is not fused with the flexor digitorum profundus. Therefore, the flexor digitorum profundus muscle is highly developed in Tamandua and occupies the caudal part of the arm and forearm, which is an evolutionary adaptation that could have occurred during evolution from the common ancestor of Tamandua and Myrmecophaga.