Dissection of 1,941 specimens provided data on reproductive cycles in six genera of skinks and three genera of agamids from the Alligator Rivers Region of Australia's Northern Territory. Comparative data on lizards from the temperate zone were gathered by dissecting specimens of three genera, and by reviewing published studies. The Alligator Rivers Region climate exhibits uniformly high temperatures but extremely seasonal rainfall. By analogy with studies on tropical herpetofaunas in other parts of the world, we hypothesised that most species would breed during the wet-season.
Instead, a great diversity in the seasonal timing of reproduction in tropical lizards was observed. For example, among the skinks, Cryptoblepharus breeds year-round, Carlia and Sphenomorphus breed in the wet-season, whereas Lerista, Morethia and most (but not all) Ctenotus breed during the dry-season. Among the agamids, Diporiphora and Gemmatophora breed in the wet-season, and Chelosania in the dry-season. Temperate-zone lizards in Australia show less interspecific variation: all species breed in late spring and summer.
Hypotheses concerning the evolutionary determinants of reproductive seasonality are reviewed in the light of these data. Thermal tolerances of developing embryos are unlikely to be important in determining breeding seasons of the Alligator Rivers Region herpetofauna, as there is little seasonal variation in temperature. Differences in reproductive timing between microsympatric species are inconsistent with hypotheses giving a major role to the seasonality of fire, flooding or intensity of predation. There is no clear association between food habits and reproductive timing. The best predictor of breeding seasonality seems to be the biogeographic history of the taxon. Alligator Rivers Region representatives of arid-zone taxa tend to breed in the dryseason, whereas representatives of mesic-adapted lineages tend to be wet-season breeders. A species from one cosmopolitan genus breeds year-round. We hypothesize that embryonic moisture tolerances may be an important determinant of breeding seasonality in this region, although some cases do not support this conclusion.