Environmental physiology of reptilian breeding cycles: Role of temperature

Abstract

Phenological data indicate that most reptiles, even in tropical regions, exhibit some seasonality in reproductive activity; in most temperate species, a single, relatively brief breeding period alternates with a period of sexual quiescence. In lizards, the most studied of the Reptilia, it appears that exogenous, climatic factors act to synchronize these cycles, although some species may also exhibit endogenous ( circannual) rhythmicity. Rainfall may influence egg laying in some tropical lizards. Photoperiodism probably also affects certain aspects of the gonadal cycle in a few species, but many reports of photoperiodism have been questioned on the basis of inadequate experimental design. Of all the environmental factors, temperature appears to be the single most important and widespread of the timing cues for saurian reproduction. Available data on the basic reproductive endocrinology in reptiles are reviewed and it is suggested that reptiles may possess only a single (FSH-like) gonadotropin (GTH). Studies on the mechanisms by which changes in temperature generate seasonal reproductive cycles indicate that several loci may be involved. They may act at the level of the central nervous system in relation to GTH-releasing factors and, likewise, may modify the response of the CNS to photoperiodic changes. The physiological responsiveness of the pituitary gonadotropes per se may be altered by temperature but this hypothesis is untested. The possibility that the peripheral metabolism (e.g., half-life) of GTH may also vary with temperature has also not been tested. In both sexes, the peripheral target tissues (gonads) are highly temperature sensitive. More importantly, gametogenesis and steroidogenesis may be differentially affected by changes in temperature; their relative temperature sensitivities vary among species. These two processes also show different sensitivities to the level of circulating GTH.