Patterns of breeding in two species of agamid lizards in the arid subtropical Pilbara region of Western Australia.

The reproductive cycles of two annual species of desert agamids have been studied over a 7-year period in the arid Pilbara region of Western Australia in an effort to identify proximate factors influencing their disparate modes of reproduction. Amphibolurus (= Ctenophorus) nuchalis breeds regularly in spring (September-October in the southern hemisphere) in the southern parts of its range, following reliable winter rainfall which stimulates insect abundance sufficient to sustain their reproductive effort. In the Pilbara region, however, winter rainfall is small and erratic and total rainfall is influenced primarily by summer cyclones. A. nuchalis breeds in spring in the Pilbara in those years when winter rain is substantial, but defers breeding until after the cyclonic rains in late summer in very dry years. The physiological condition of A. nuchalis in spring in such years is depressed, suggesting that resources sufficient to sustain the reproductive effort are lacking, but they show no obvious signs of stress. In years where adequate winter rainfall has been followed by early cyclonic rains, the A. nuchalis population breeds continuously for a 6-month period between October and March of the following year. Although circulating levels of androgens rise significantly in spring in the second species, A. caudicinctus, the gonads remain regressed and this species displays an inflexible pattern of reproduction, breeding only following cyclonic rain in March-April each year, no matter what rains fall in the preceding winter. A. nuchalis thus appears to be an opportunistic vernal breeder, limited only by the availability of resources, whereas A. caudicinctus, which apparently displays the greater reproductive effort of the two species, has a typical aestival pattern of breeding which is uniquely attuned to the enormous burst of productivity occurring in this arid region following the substantial and predictable summer cyclonic rains. Although environmental temperature has often been seen as the primary factor determining reproductive cycles in reptiles, this study suggests that temperature is a necessary but not a sufficient condition for successful reproduction and that the availability of adequate resources may assume an overriding importance, especially in arid habitats where annual rainfall may be highly unpredictable.