AbstractsBiology & Animal Science

Macropods exhibit the second largest sexual dimorphism in body size for any vertebrate, suggesting there is a major role of sexual selection, with the potential for different mating tactics amongst male kangaroos. It is suggested that dominant males invest heavily in large forelimb muscles, which aids in male-male competition, female coercion and increased attractiveness to females, gaining them the majority of matings. A subdominant male may not invest in muscle mass development, but rather rely on other reproductive tactics such as sneaky matings and investment in competitive spermatozoa characteristics; for example more spermatozoa (larger testes) and faster or longer spermatozoa. We examined whether there was a trade-off between investments in forelimb muscle development and sperm competition in the western grey kangaroo (Macropus fuliginosus). Sperm traits, body mass, and forelimb muscle masses were measured for 35 male M. fuliginosus and an index of ‘muscularity’ was calculated using the residuals of individual muscle masses against body mass. There were no significant relationships between forelimb muscle investment and sperm competition traits. Significant relationships were recorded between relative testes mass and relative epididymis mass (r2 = 0.26, p = 0.003), total motility and progressive motility (r2 = 0.31, p = 0.001) and spermatozoon velocity and spermatozoon head (r2 = 0.47, p < 0.001) and tail lengths (r2 = 0.19, p = 0.011) with the longer spermatozoa swimming slower. These results suggest that there is no evidence for a significant relationship between sperm competition and forelimb muscles. There was no evidence for a trade-off between these two measures of sexual selection, suggesting that these traits are independent of each other.