Beyond Body Mass, Beyond Adulthood: The Ontogeny of Sexual Size Monomorphism

Contest competition for mates and female reproductive energetics influence body size and sexual dimorphism across many primates; nevertheless, some monomorphic species defy these patterns. These deviations may reflect biological anthropology's focus on body mass and adult size as defining features of sexual dimorphism. Using ontogenetic data, we test whether sexual monomorphism in adult body mass necessitates monomorphism across all traits and evaluate whether mate competition and female energetics explain sex-specific developmental patterns and adult morphology in a monomorphic primate.

We built sex-specific growth curves using generalized additive mixed models to examine levels of sexual dimorphism in 14 morphological measurements across development in Verreaux's sifaka (Propithecus verreauxi) at Ankoatsifaka Research Station in western Madagascar.

Sifaka exhibited male-biased dimorphism in upper arm and thigh circumference at adulthood and across growth, but not in other contest-related traits. Adult females had significantly longer hindlimbs and thighs and exhibited bimaturism and increased growth rate in these and several other traits.

We found limited support for our hypotheses that female reproductive energetics or male-male contest competition drive adult size, sex differences, and growth trajectories in Verreaux's sifaka. However, male-male contest competition likely drives male-biased dimorphism in muscle mass, and longer female hindlimbs may represent an adaptation to infant carrying, reflecting a species-specific suite of dimorphic traits. This study demonstrates that sexual dimorphism exists at finer scales even in monomorphic species, and that adult size and sex differences are the result of a mosaic of selective pressures acting on individual traits.