Variation in mating systems and sexual size dimorphism between populations of the Australian python <Emphasis Type="Italic">Morelia spilota</Emphasis> (Serpentes: Pythonidae)

Abstract

Although adaptationist hypotheses predict a functional relationship between mating systems and sex- ual size dimorphism, such predictions are difficult to test because of the high degree of phylogenetic conservatism in both of these traits. Taxa that show intraspecific varia- tion in mating systems hence offer valuable opportunities for more direct tests of evolutionary-ecological hypothe- ses. Based on a collation of published and unpublished records, we document intraspecific geographic variation in mating systems (presence versus absence of male- male combat) within the widely-distributed Australian python Morelia spilota. Radiotelemetric monitoring of 19 free-ranging pythons in a population in north-eastern New South Wales showed that these animals display a mating system of female defence polygyny. Previous studies on a southern population of the same species found that males engaged in long mate-searching move- ments, showed no overt agonistic behavior, and formed long-term (>2 months) aggregations around reproductive females. In strong contrast, our adult male carpet py- thons (i) moved about relatively little (mean displace- ment <11 m/day) during the mating season, (ii) remained with females only briefly (<5 days), and (iii) engaged in male-male combat in the vicinity of females. This male- male combat included vigorous biting as well as rituali- sed "wrestling" matches, resulting in a high incidence of bite scars in adult males. In keeping with predictions from sexual selection theory, males attain larger body sizes than females in this population, whereas females grow larger than males in the previously-studied south- ern population where males do not engage in physical combat for mating opportunities. Key words Mating system - Sexual dimorphism 9 Geographic variation 9Reptiles 9Pythonidae R. Shine9M. Fitzgerald([5~) Schoolof BiologicalSciencesA08, The Universityof Sydney,NSW 2006Australia

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