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American Zoologist 1989 29(3):953-971; doi:10.1093/icb/29.3.953
© 1989 by The Society for Integrative and Comparative Biology
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Effects of Incubation Temperature on Crocodiles and the Evolution of Reptilian Oviparity1

GRAHAME J. W. WEBB and HARVEY COOPER-PRESTON
G. Webb Pty. Limited P.O. Box 38151, Winnellie, N.T., 0821 Australia and Conservation Commission of the Northern Territory P.O. Box 496, Palmerston, N.T. 0831, Australia

Crocodylus porosus is a mound-nesting crocodilian in which incubation temperature influences the rate of embryonic development, the probability that embryos will survive to hatching, post-hatching growth rates and the probability of hatchlings surviving to 2 yr of age. Similar responses have been described in Alligator mississippiensis (Joanen et al., 1987) and C. niloticus (Hutton, 1987), and they reflect a suite of "non-sexual" effects of incubation temperature. Temperature-dependent sex determination allocates sex on the basis of these "non-sexual" effects. In C. porosus, it results in maleness being assigned to embryos with high probabilities of surviving and good potential for post-hatching growth. Within the limits of survival, effects of the moisture environment on embryological development rate and hatchling fitness seem minor relative to those of the temperature environment.

Reptilian orders have either obligate oviparity (chelonians, crocodilians and rhynchocephalians) or facultative oviparity (squamates), depending on the extent of embryonic development within the oviducts. The distinction is equally one between embryos which are buffered from thermal effects within a female's body (facultative oviparity) and those that are not (obligate oviparity). Facultative oviparity and internal thermal buffering may be the primitive condition within the Class Reptilia, and the "shell-less" eggs of extant squamates may reflect the original amniote egg. Obligate oviparity, which also exists in birds, appears to have been a specialized development, and is a blind end in the evolution of viviparity among vertebrates. The significance of thermal buffering being lost in obligate oviparous reptiles remains unclear.


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