Hallucal grasping in Nycticebus coucang: further implications for the functional significance of a large peroneal...

Euprimate grasping feet are characterized by a suite of morphological traits, including an enlarged peroneal process on the base of the first metatarsal, which serves as the insertion site of the peroneus longus muscle. In prosimians, a large process has typically been associated with a powerful hallucal grasp via the contraction of the peroneus longus to adduct the hallux. Recent electromyography (EMG) studies have documented that peroneus longus does not contribute substantially to hallucal grasping in lemurids (Boyer et al., 2007). However, non-lemurid prosimians have a I-V opposable grasp complex that is morphologically different and phylogenetically more primitive than the I-II adductor grasp complex of the lemurids previously studied. Therefore, it is possible that peroneus longus did function during grasping in early euprimates, but lost this function in large-bodied lemurids. The present study tests the hypothesis that a large peroneal process is related to powerful grasping ability in primates displaying the more primitive I-V grasp complex. We use EMG to evaluate the recruitment of peroneus longus, other crural muscles, and adductor hallucis in static and locomotor grasping activities of the slow loris (Nycticebus coucang). Results show that peroneus longus is active during grasping behaviors that require the subject to actively resist inversion of the foot, and likely contributes to a hallucal grasp in these activities. Peroneus longus activity level does not differ between grasping and power grasping activities, nor does it differ between grasping and non-grasping locomotor modes. Conversely, the digital flexors and hallucal adductor are recruited at higher levels during power grasping and grasping locomotor modes. Consequently, we reject the hypothesis that an enlarged peroneal process represents an adaptation specifically to enhance the power of the I-V grasp, but accept that the muscle likely plays a role in adducting the hallux during grasping behaviors that require stabilization of the ankle, and suggest that further work is necessary to determine if this role is sufficient to drive selection for a large peroneal process.