Size, scaling, and sexual size dimorphism in wild South African thick-tailed greater galagos (Otolemur crassicaudatus)

Abstract

The developmental bases of sexual size dimorphism vary across primates, with important implications for understanding the evolution of dimorphism. Here, we explore adult sexual size dimorphism and its developmental bases in Otolemur crassicaudatus. We aim to understand the anatomical pattern of adult sexual size dimorphisms and their developmental bases through allometric analyses of somatometrics. We caught and released wild subadult and adult animals annually at Lajuma Research Centre, South Africa from 2013 to 2023 (excepting 2020), and measured body mass and up to 23 body measurements. Among adults, males (mean body mass = 1242.89 g ± SD = 137.63 g, n = 91 observations of n = 52 individuals) are 1.21 times larger than females (mean body mass = 1027.55 g ± SD = 94.03 g, n = 85 observations of n = 44 individuals), possibly representing the highest body mass sexual dimorphism among extant strepsirrhines. The skeletal system shows limited sexual size dimorphism, suggesting decoupling of body mass size dimorphism and skeletal size dimorphism. Allometries lead to variation in adult sexual size dimorphism throughout the body, with high levels of dimorphism in circumferences, especially in the torso and proximal limb elements. Sexual selection, attributable to some level of intermale competition, probably accounts for sexual size dimorphism in this species. The conservatism of the skeletal system, combined with high body mass size dimorphism, may be related to generalized quadrupedalism and declining rates of leaping through ontogeny in the species. These findings complicate reconstructing and interpreting primate sexual size dimorphism in the fossil record.