Strong arm tactics: sexual dimorphism in macropodid limb proportions

Abstract

AbstractMacropodid (kangaroo and wallaby) males use their forelimbs as weaponry in fights for dominance and access to females. The choreography and intensity of these fights varies with species body size. It has been established that the largest species show stronger dimorphism in upper limb muscularity as a product of sexual selection via increased intermale competition. However, beyond muscularity, and across the other macropodid species, limb dimorphism is poorly understood. Here, we examine dimorphism in the relative proportions of the bones within the forelimb and between the fore‐ and hindlimbs. We used a large sample of bones from 15 promiscuous macropodid species to confirm that absolute dimorphism in the forelimb becomes greater with body size (indexed by cranial length), and to further explore whether dimorphism in relative forelimb proportions increases also. As macropodids are obligate bipedal hoppers, no dimorphism was expected in the lower limb. Phylogenetic generalized least‐square regression was used to resolve phylogenetic non‐independence of the data. Greater upper limb dimorphism was significantly correlated with longer crania, confirming established patterns, but we found it was the relative length of the male humerus that became increasingly exaggerated with species' body size, not the radius as previously suggested. Enlarged proximal limb segments are indicative of adaptations for powerful movements. This aligns with higher intensity and ritualization of male agonistic behaviour, and reflects an increased selection for strong male forelimbs in the larger promiscuous species. Contrary to our hypothesis, relative male pes length decreased with body size, possibly indicating selection for a stronger and sturdier pes required for powerful kicks. This study clarifies the patterns of sexual dimorphism in Macropodidae limbs and illustrates the power of sexual selection to influence anatomy that is fundamental to locomotion in both sexes.