11 - Cold torpor, diapause, delayed hatching and aestivation in reptiles and birds

Summary

Introduction

This chapter considers temporary suspension of active development within the oviposited eggs of reptiles and birds. These various processes appear to be ‘natural’ and adaptive; however, much of the study about them has occurred in the laboratory.

As presently known, all oviposited reptilian and avian embryos have advanced in development to blastulae or beyond (Bellairs, Chapter 23). In many, perhaps all, turtles embryonic development becomes arrested as a late gastrula within the oviducts and resumes normally only after oviposition (Ewert, 1985). In lizards, developmental arrest may occur in the oviducts in a few cases, e.g. Sceloporus jarrovi, which is ovoviviparous (Goldberg, 1971), but these must be very rare (Shine, 1983, 1985). The egg of the Tuatara (Sphenodon punctatus) is laid at an early embryonic stage, as in turtles, but whether this stage is also associated with pre-ovipositional arrest is unknown (Moffat, 1985). In snakes, crocodilians and birds, pre-ovipositional arrest is unknown and unlikely (Ferguson, 1985; Blackburn & Evans, 1986; Lance, 1987).

After oviposition, development within reptilian or avian eggs of many species can be divided into periods of active embryonic differentiation and growth and periods of relative inactivity, when as time passes there is little change. Such prolongations can be grouped as 1) cold torpor, 2) diapause, and 3) delayed hatching, which grades into aestivation in some reptiles. A previous survey of these topics (Ewert, 1985) focuses on turtles, but gives some reference to lizards and other vertebrates.