10 - Physiological effects of incubation temperature on embryonic development in reptiles and birds

Summary

Introduction

Generally, the incubation temperature of bird eggs is conservative: within a species there is little variation in incubation temperature at which normal development can proceed. By contrast, the incubation temperature of oviparous reptiles is relatively labile; normal patterns of development in individual embryos can ensue at a wide range of temperatures. In addition, the incubation temperature of avian eggs is usually higher than for reptile eggs which, as a group, have a much wider range of viable incubation temperatures. Average incubation temperatures for birds are tabulated by Rahn (Chapter 21) and comprehensive reviews of the thermal tolerances of avian embryos have been prepared by Drent (1975) and Webb (1987). Comparable data for reptiles are available for turtles (Ewert, 1979, 1985; Miller, 1985a), crocodilians (Ferguson, 1985) and squamates (Hubert, 1985).

Incubation temperature is very important in determining rates of embryonic growth and development and to a large extent the length of the incubation period. It also has other effects, as yet predominantly observed in reptiles. Incubation temperature determines sex in many species of reptile and also affects the pigmentation pattern of hatchlings, post-hatching growth rates and moulting cycles as well as thermoregulatory and sexual behaviour patterns. These topics are reviewed for reptiles, using primarily the American alligator (Alligator mississippiensis) as the example, but the possible effects of temperature on avian development are also examined.