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17 - Gas exchange across reptilian eggshells
- Edited by D. Charles Deeming, University of Manchester, Mark W. J. Ferguson, University of Manchester
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- Egg Incubation
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- 16 November 2009
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- 05 December 1991, pp 277-284
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Summary
Introduction
In contrast to the open nests of birds, nests of reptiles are generally located in the soil or in decaying vegetation where, effectively separated from the atmospheric gaseous environment, eggs may be exposed to relatively low partial pressures of oxygen and correspondingly high partial pressures of carbon dioxide (Ackerman, 1977; Seymour & Ackerman, 1980; Ferguson, 1985; Packard & Packard, 1988). The deviation from atmospheric oxygen and carbon dioxide tensions may be greater during the later stages of incubation due to a high rate of respiration of the embryos (Booth & Thompson, Chapter 20). In addition, the humidity of these nests is high. Consequently, problems of desiccation are reduced thereby allowing eggs to have high conductances to respiratory gases which result in smaller gradients of respiratory gases across the eggshell. These presumably help to maintain the internal gaseous environment within physiological limits even in nests with low oxygen and high carbon dioxide tensions (Booth & Thompson, Chapter 20). In this chapter, we discuss conductance of different types of reptilian eggshell to water and to respiratory gases, making comparisons, where possible, with avian eggs. As eggshell structure is highly variable (Packard & DeMarco, Chapter 5) conductance and allometric relationships are described separately for each major type of shell.
The eggshell as a mediating barrier
Water vapour
In avian eggs, water vapour, oxygen and carbon dioxide all diffuse across the eggshell through the same pathway – discrete pores in the calcareous eggshell; the relative conductance of the shell to these gases is related to their different rates of diffusion (Paganelli, Ackerman & Rahn, 1978).
19 - Reasons for the dichotomy in egg turning in birds and reptiles
- Edited by D. Charles Deeming, University of Manchester, Mark W. J. Ferguson, University of Manchester
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- Book:
- Egg Incubation
- Published online:
- 16 November 2009
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- 05 December 1991, pp 307-324
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Summary
Introduction
The need for egg turning during incubation is one of the most dramatic differences between the incubation requirements of birds and reptiles. Almost all avian eggs need to be turned throughout much of incubation (Poulsen, 1953; Drent, 1975) and although there are many studies of the egg turning behaviour in incubating birds (Drent, 1975), these are not discussed here. By stark contrast, reptilian embryos are usually killed by turning during incubation (Ferguson, 1985). Egg turning in birds is thought to prevent deleterious adhesions between the embryo and the shell membranes (New, 1957) but in reptiles such adhession is normal (Ewert, 1985; Ferguson, 1985). This simplistic view of the phenomenon of egg turning is widely accepted yet if the requirement and effects of turning are examined further the situation is not so clear cut. In this chapter, the effects of egg turning on avian and reptilian development are described and some suggestions are made for the physiological basis of turning in birds. The possible evolutionary relationships between tgg turning behaviour in birds and its absence in reptiles, are discussed.
The effects of egg turning on hatchability in birds and reptiles
Avian eggs
The majority of studies of egg turning on avian embryos are concerned with effects on hatchability in poultry (Landauer, 1967; Lundy, 1969). Eycleshymer (1907) first showed that turning rate affected hatchability, although by modern standards the results were poor; 58% of fertile eggs of the fowl (Gallus gallus) hatched after being turned five times a day compared with 45% of eggs turned twice a day and only 15% hatchability of unturned eggs.
10 - Physiological effects of incubation temperature on embryonic development in reptiles and birds
- Edited by D. Charles Deeming, University of Manchester, Mark W. J. Ferguson, University of Manchester
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- Book:
- Egg Incubation
- Published online:
- 16 November 2009
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- 05 December 1991, pp 147-172
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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.