How bamboo has shaped the anatomy and physiology of Hapalemur

Purpose Hapalemur spps. and Prolemur simus (bamboo lemurs) stand out among the relatively homogeneous lemurids because they are bamboo feeders and vertical clingers and leapers. This unique diet presents equally unique challenges, like its verticality, toughness, and toxicity. The bamboo lemurs share the generalized anatomy of the other lemurids, but also display some well-documented adaptations in their limb proportions to overcome the problems presented by bamboo. Soft tissue adaptations, however, remain largely unexplored. Here we begin to explore possible soft tissue adaptations in Hapalemur griseus. Based on the available anatomical and physiological data obtained from other Hapalemur and Prolemur species, we predict that H. griseus will have differences in hindlimb morphology when compared to other lemurids. We further predict that H. griseus will have more hindlimb muscle mass and will amplify muscle mass differences with increased type II muscle fibers. Methods We compared regional muscle mass, relative to total muscle mass, in mainland African, Malagasy, and Asian prosimians and a tree shrew (n = 11). Raw regional muscle mass values (e.g., hindlimb and sural) were divided by total muscle mass for a particular species to compare relative muscle mass. We then used immunohistochemistry to evaluate the fiber profile (the relative amount of type I/type II fibers) of muscles of significance, based on the results of the comparison of muscle masses. Results Relative hindlimb muscle mass in H. griseus is no different than other lemurids (p = 0.26). However, relative sural muscle mass is significantly heavier (p H. griseus than other lemurids. When the fiber profiles of primary foot plantar flexors were evaluated, the soleus muscle of H. griseus displayed a higher amount of type II (fast) fibers than any other species. Conclusions These findings indicate that although H. griseus shares some generalized lemurid morphology, its diet of bamboo has pushed this generalized lemurid to an anatomical extreme. We suspect that based on the diameter of the bamboo stalk and the bamboo lemurs body size, bamboo lemurs may be leaping in a unique fashion that does not easily fit with small-bodied “foot-powered” leapers or large-bodied “hip-powered” leapers. Although the results are preliminary, we suspect additional bamboo-specific adaptations in their anatomy and physiology will be uncovered with further examination into the anatomy of the bamboo lemurs.