A Muscle‐fiber Comparison in New World and Old World Monkeys Based on Brain Size, Body Mass and Locomotor Style

Abstract

This study is part of a larger one (Muchlinski et al., 2018) focusing on the relationship between muscle mass and brain mass in primates. Previous studies have demonstrated a negative correlation between brain size and type I fibers in lemuroideea(N=7) and hominoid primates (N=7), suggesting that as brain mass increases type I muscle-fiber composition decreases. Based on these observations we hypothesized that platyrrhine and cercopithecidae primates would correspondingly have an increase in type II fibers in accordance to their brain mass to account for other more expensive tissue. Our goal was to measure type I and type II fiber distribution in New and Old World primates using immunohistochemistry to develop a muscle fiber distribution profiles and analyze the effect of fiber distribution on endocranial volume (ECV). Two upper-limb (deltoid and pectoralis major) and two hind-limb muscles (gastrocnemius and soleus) were sampled from 7 platyrrhine, 4 cercopithecidae as well as the Philippine tarsier (N=12), representing a range of body size (240-7765g) and locomotory styles. FFPE samples were H&E stained and immunostained using type I and type II myosin heavy-chain antibodies (abcam, 1:750). Pectoralis Major fiber compositions quantified from C. syrichta (3.36cc), A. azazae (20.67cc), C. hamlyni (62.5cc) and S. apella (65.72cc) demonstrate a reduction in Type I fibers; 57.27%, 33.85%, 20.59 and 18.17% respectively as ECV increases, consistent across upper and lower-limb muscles and supporting a negative correlation between brain mass and type I fibers. Predictive modeling using multiple linear regression found muscle mass (g), locomotory style, body mass (g) and % muscle fiber composition to be the greatest explanatory variables in predicting brain size (R²= 0.99 and RMSE = 4.0882). Data from these studies can be used to address evolutionary questions in relation to the evolution of energetics and potential influences from muscle mass, body size and locomotory style.