A Muscle‐fiber Comparison in New 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 with a specific focus on differential muscle fiber‐type distribution. Data from these studies can be used to address evolutionary questions in relation to the evolution of energetics and potential influences from body size and locomotory style. Primates have much larger brains than other non‐primate mammals, yet their overall metabolic requirements for body size are similar. When active, skeletal muscle is in direct competition with the brain for glucose and is an expensive tissue to maintain. Type I skeletal muscle fibers convert glucose to ATP using oxygen making them more energetically expensive. Previous studies demonstrated a negative correlation between brain size and type I fibers in lemuroidea (N=7) and hominoid primates (N=7), suggesting that as brain mass increases, type I fiber composition decreases. Based on these observations we hypothesized that platyrrhine primates would correspondingly have an increase in type II fibers in accordance to brain mass to account for more expensive tissue. Our goal was to measure type I and type II fiber distribution in New World primates using immunohistochemistry to develop a muscle fiber distribution profile. Two upper‐limb (deltoid and pectoralis major) and two hind‐limb muscles (gastrocnemius and soleus) were sampled from 5 New World primate taxa as well as the Philippine tarsier (N=7) that represented a range of body size and locomotory style. All samples were FFPE and sectioned (10um) for immunohistochemistry with a focus on type I and type II myosin heavy‐chain antibodies (abcam, 1:750). Stained sections were accessed for fiber‐type distribution using light microscopy and Image J, and multiple linear regression statistical analyses were performed. Overall larger‐brained primates displayed fewer type I fibers suggesting a negative correlation between brain mass and type I fibers. Deltoid fiber compositions quantified from T. syrichta (3.14cc), S. habiatus (cc), A. azazae (20.65cc) and S. apella (73.21cc), demonstrated a reduction in Type I fibers; 39.55%, 31.50%, 28.45% and 18.35% respectively, as endocranial volumes (ECV) increased. This observed trend was consistent across gastrocnemius and pectoralis major muscles. Conversely, the soleus of larger primates displayed a higher percentage of type I fibers (A. azazae; 71.61% and S. apella 81.76%) when compared with smaller primates (T. syrichta; 22.00%). Regression analysis of the platyrrhine primates produced a predictive model for ECV (R2 = 0.99 and RMSE = 2.3248) while the platyrrhine and hominoid data combined also demonstrated good prediction capabilities (R2 = 0.81 and RMSE = 83.074). The model found body mass (g), %fast‐twitch fibers and taxa to be the greatest explanatory variables in predicting brain size with body mass having the strongest effect. The present fiber distribution results are consistent with results from previous studies, and with the Expensive Tissue Hypothesis, however, the effects of body size and locomotory style cannot be discounted.Support or Funding InformationNational Science Foundation