Sperm competition in the Macropodoidea: a review of evidence

This chapter reviews the evidence for sperm competition in the marsupial superfamily Macropodoidea. The opportunity for sperm competition arises when a female is mated by more than one male during a single oestrous period. Previous studies suggest that the male that inseminates the female with the greatest number of sperm, or the highest-quality sperm, will have the highest probability of successfully fertilising the female's egg(s). If sperm competition is operating, selection should favour males with larger testes that produce more sperm, or males capable of producing higher-quality sperm. In a 'raffle' system, in which sperm numbers are crucial to success, males of species with polyandrous mating systems are expected to have larger testes relative to their body weight than males of species with monandrous mating systems. This relationship holds true for many taxa including marsupials. Hence, testes: body weight ratios have been used in conjunction with other morphological and behavioural data to predict the likelihood of sperm competition and the possible mating system. We have assembled testis data from numerous studies, both published and unpublished, resulting in testis weights for 17 species in eight genera, and data on scrotum width for 17 species in eight genera. The literature has been examined for information about the mating system and other correlates of sperm competition to determine if the expected relationships between testis weight, body weight and mating system hold true within the Macropodoidea. The paucity of data about mating systems, especially with respect to the degree of polyandry in females, prevents us from clearly stating that there is a positive relationship between the degree of polyandry and testis weight although none of our data contradict this relationship. There is some indication that seasonal breeding may increase the probability of sperm competition.