| Description |
In many species, male reproductive fitness is dependent upon the ability to physically compete for access to mates. The direct link between performance in male contests and reproductive success has led to the evolution of male-biased sexual dimorphism in traits that improve fighting performance. However, species variation in social structure often leads to differences in the relative importance of intraspecific aggression and corresponding variation in the degree of sexual dimorphism. Traits such as body mass and canine size have received much attention because they have a clear impact on male fighting performance. However, additional musculoskeletal adaptations may also be under selection. Traits that improve strength, agility, and maneuverability (i.e., whole-organism performance capacities) may improve aggressive performance and this may lead to the evolution of sexual dimorphism throughout the musculoskeletal system. Because the postcranial anatomy also functions as the primary locomotor system, morphological specialization for aggression may directly conflict with locomotor performance, resulting in a functional trade-off. Given that locomotion represents a substantial proportion of total energetic expenditure in many species, compromises resulting from an aggression-locomotion trade-off may be vital to understanding the evolution of behavioral and phenotypic diversity. In this dissertation, I first investigated the prevalence of sexual dimorphism in skeletal morphology in three subspecies of gray wolf (Canis lupus). I then expanded this study to 26 species of carnivores and 11 species of primates. I found male-biased sexual dimorphism in skeletal traits that are predicted to improve aggressive performance, making males better equipped for intraspecific competition. Across species, the degree of dimorphism increased with the intensity of male competition. Consistent with sexual selection theory, the evolution of this dimorphism was best explained by mating system. To test for evidence of a functional trade-off between aggressive performance and locomotion, I measured male competitive ability through social competition trials in semi-natural enclosures and locomotor economy through running trials in an enclosed treadmill and open-flow respirometry. I found evidence for an aggression-locomotion functional trade-off. Together, this work improves our understanding of the role of aggression in the evolution of vertebrates and the impact that specialization for aggression may have on locomotion. |
