TESTES WEIGHT, BODY-WEIGHT AND MATING SYSTEMS IN MARSUPIALS AND MONOTREMES

Relationships between testes weight, body weight and mating systems we re examined in 40 marsupial species and in the extant monotremes. Rela tionships between relative testes weight and mating systems in marsupi als resemble those previously described for primates. Thus relative te stes weights are greatest in those marsupials where females mate with multiple males during the fertile period, i.e. polyandrous species (e. g. Antechinus flavipes, Isoodon obesulus, Perameles nasuta, Potorous t ridactylus, Macropus eugenii and M. agilis) and smallest in monandrous forms (e.g. Petauroides volans and Petaurus breviceps) where females usually mate with a single male. These findings are consistent with ef fects of sperm competition upon the evolution of relative testes sizes in marsupials. Where field studies on marsupial mating systems are la cking, we make predictions based upon examination of their relative te stes weights. Tarsipes rostratus, Acrobates pygmaeus, Macropus rufogri seus and Sarcophilus harrisii are predicted to engage in multiple mati ngs and sperm competition. Conversely, Lasiorhinus latifrons, Cercatet us concinnus and Pseudoantechinus macdonnellensis are predicted to be monandrous in their mating behaviour. The monotremes (Ornithorhynchus anatinus, Tachyglossus aculeatus and Zaglossus bruijnii) are character ized by possession of very large testes; monotremes are shown to have significantly greater relative testes weights than marsupials, primate s or avian species. This taxonomic difference is unlikely to be relate d to the occurrence of oviparity or to the abdominal position of the t estes in the Monotremata. Their mating systems are not known in detail , but some evidence for multiple matings (and hence for sperm competit ion) exists for Tachyglossus aculeatus so that their large testes may be adaptive in this context.