Adaptation of skeletal myofiber types to arboreality

Abstract

Skeletal muscles of primates are composed of fast-twitch/glycolytic (FG), fast-twitch/oxdative/glycolytic (FOG), and slow-twitch/oxidative (SO) myofibers, which are classified by differences in histochemical reactivity of reduced nicotinamide adenine dinucleotide dehydorgenase, mitochondrial glycerol-3-phosphate dehydrogenase, and myosin ATPase. The postural muscles have many SO myofibers. The locomotory muscles have many fast-twitch (FG and FOG) myofibers. The limb muscles of Japanese macaques, rhesus macaques, lesser bushbabies, and gray lesser mouse lemurs contain FG, FOG, and SO myofibers, whereas the muscles of slow lorises have FOG and SO myofibers and no FG myofibers. SO myofibers are involved in postural maintenances and function in preventation of fall from trees. FOG myofibers are used for continuous movements and locomotary and FG myofibers for short bursts of high activity. The SO and FOG myofibers are necessary for climbing a tree and living on a tree, adaptation to aloreality. Although the FG myofibers are dispensable in arboreal monkeys, they are developed greatly in primates' muscles that generate a large propulsive force for running, climbing, jumping, and leaping. The composition of myofiber types differs in analogous muscles. The differences are correlated with locomotory patterns of primates.