Body mass in lowland gorillas: A quantitative analysis

Abstract

Body proportions and tissue composition (e.g., relative contributions of muscle, skin, bone, and adipose to total body mass) were determined through dissection of four adult captive lowland gorillas. The relative contribution of bone varies little among the four animals (10.2-13.4%) despite considerable range in body weights (99.5-211 kg). In tissue composition, three animals have on average 37.3% muscle relative to body mass. Maximum estimates of body fat range between 19.4-44%. Differences in age, sex, and life history events partially explain the observed variation in body proportions and tissue composition among the four animals. Although gorillas are considered extremely sexually dimorphic in body weight and canine size, differences in tissue are not as dramatic as body mass differences suggest. This study found sex differences mostly in the upper body; males have relatively heavier forelimbs, including heavier deltoid, trunk-binding, and deep back muscles compared to the younger female. The old, obese female had one half the muscle tissue of the other three animals (16% vs. 37.3%), and twice the body fat (44%); forelimbs and upper body musculature were relatively well-developed to compensate for the restricted hip-joint movement due to arthritis. Data on the variation in tissue composition and body proportions in gorillas provide a basis for comparison with other hominoids, including humans. For example, compared to highly dimorphic orangutans, gorillas have more muscle, less adipose tissue, lighter forelimbs and heavier hindlimbs. Such analyses complement studies of the skeleton and contribute to our understanding of human evolution and adaptation.

Body composition in Pan paniscus compared with Homo sapiens has implications for changes during human evolution

“…The negligible measurable fat in all seven P. paniscus males was unexpected, overriding captivity, age, and body mass. Among wild chimpanzees, there is little indication of an ability to mobilize fat stores during times of caloric restriction, a key adaptive feature found in orangutans and possibly to a lesser degree in gorillas (24,52,53). Without selection pressure for storage fat, and with over half of body mass in muscle, the male P. paniscus does not easily accumulate body fat, even under optimal circumstances of captivity.…”

“…Individual paniscus female 3 (PF3) had no offspring or pregnancies during her life, had remarkably low levels of dissectible fat, and stood out as having the highest percentage of muscle mass among the females (44.1%, Table 1). Body fat is also sexually dimorphic in some monkeys, as well as in gorillas and orangutans (23)(24)(25), and has a demonstrated role in reproduction (54)(55)(56). The same is true for H. sapiens (19,(43)(44)(45)(46)(47)(48).…”

“…Few such comparative data are available on apes (but cf. [22][23][24][25]. The rarity of apes in captivity, their long lives, and the logistics of obtaining relatively complete bodies for dissection pose challenges for accumulating a comparative sample.…”

Functional Anatomy and Adaptation of Male Gorillas (Gorilla gorilla gorilla) With Comparison to Male Orangutans (Pongo pygmaeus)

“…He is the youngest, leanest and most muscular of the sample and was in the prime of life. Some data from these individuals were published in Zihlman and McFarland (2000). Some data from this individual was published in Morbeck and Zihlman (1988).…”

“…We use standardized methods (Grand, 1977;Zihlman, 1984;Zihlman and McFarland, 2000). On one side of the body, referred to as the segment side, we separate the entire forelimb at the shoulder joint by cutting the skin around the joint, releasing trunk and rotator cuff muscles from the humerus, and the arm muscles from the trunk.…”