INFLUENCE OF THE EVOLUTION OF VIVIPARITY ON EGGSHELL MORPHOLOGY IN THE LIZARD, LERISTA-BOUGAINVILLII

In reptiles, the evolutionary transition from egg-laying to live-beari ng is thought to involve a gradual increase in the duration of egg ret ention, with progressively more development occurring prior to oviposi tion, and culminating in the birth of fully developed offspring. Howev er, prolonging the retention of fully-shelled eggs within the oviducts may pose serious gas-exchange problems for the embryos. Thus, evoluti onary increases in the period of intrauterine retention may require co rrelated decreases in the thickness of eggshells and/or their degree o f calcification to allow for adequate embryonic gas exchange. To test this evolutionary model, eggs of three distinct reproductive forms of the scincid lizard Lerista bougainvillii were examined to determine th e evolutionary relationships between the thickness of the shell membra ne, degree of eggshell calcification, and the duration of uterine egg retention. These comparisons revealed the predicted pattern of correla ted shifts in eggshell morphology and embryonic stage at oviposition. Evolutionary increases in the duration of egg retention were accompani ed by decreases in the thickness of the eggshell membrane and degree o f eggshell calcification. This evolutionary model suggests that there may be a tradeoff between the advantages of extended egg retention and the disadvantages of a thinner eggshell. On the basis of this tradeof f, I propose that oviparous taxa with relatively thin eggshells may be preadapted to evolve viviparity. Comparative examination of the limit ed data available on eggshell thickness in lizards supports this possi bility. (C) 1996 Wiley-Liss, Inc.