Among mammals, primates exhibit a trend toward increasing encephalisation. Attempts to explain the development of this trend focus on the energetic and metabolic trade-offs required to increase brain mass. The most widely discussed are variants of the Expensive Tissue Hypothesis (ETH) which proposes for any increase in brain mass other metabolically expensive tissues must decrease in size. The brain is metabolically costly with primates having larger brain sizes than other mammals and devoting up to 20% more basal metabolic rate to brain maintenance. Brain maintenance relies on aerobic cellular respiration processes, thus requiring oxygen to efficiently function. In a resting-state, up to 90% of brain maintenance is sourced from aerobic respiration. The brain does not source oxygen directly but relies on aerobic cellular respiration, converting glucose into adenosine triphosphate (ATP) to produce energy. In humans, the brain consumes, on average, around 30% of total glucose allocation. Skeletal muscle is another expensive tissue type. Muscle consumes up to 30% of resting energy expenditure with nearly 100-fold increase during high activity. Mammals have nearly 50% of their total body mass accounted for by muscle mass while primates have only 35% of total body mass accounted for by muscle mass. Of primates, humans possess 50% less muscle mass than expected for body size. Skeletal muscle comprises a mixture of fibers known as Type I (slow-twitch for prolonged activity) and Type II (fast-twitch for short, sudden activity). Both fiber types require constant oxygen supply and glucose to convert to ATP via mitochondria. Although Type II fibers consume a higher net-amount of glucose than Type I, this is done for short periods of time. Type I fibers used for prolonged activities possess greater capillary density and more mitochondria than Type II, potentially allowing significantly more efficient conversion of glucose to ATP. This could suggest muscle mass is in direct competition with the brain through glucose requirement and that any increase in brain size could require a corresponding decrease in muscle mass as evidenced in primates, especially humans.
Muchlinski et al. (2018) examined the potential trade-off between muscle mass and brain size in non-human primates. Several skeletal muscles were dissected from primate cadavers and immunohistochemistry used to isolate muscle fiber types. Body mass strongly influenced endocranial volume and muscle mass in the primate species so variables were size adjusted. Results indicated an increase in endocranial volume was associated with a decrease in muscle mass. Type I muscle fibers were negatively correlated with endocranial volume but a positive correlation between Type II and endocranial volume was not statistically significant. In general, the primates sampled possessed more Type II than Type I muscle fibers. These results are encouraging but potential bias could be introduced from the small sample size and muscle selection with larger postural and locomotor muscles, erector spinae and scalenes, not examined as the minimum sample content for immunohistochemistry could not be dissected in very small primate species. The use of published literature for endocranial volumes and body mass may introduce additional issues. Despite this, the assumption that muscle may be in direct competition with the brain appears metabolically and energetically viable and a potential avenue for proper consideration in evolutionary primatology.