Metabolic Cost of Development in One of the World's Smallest Lizard Eggs: Implications for Physiological Advantages of the Amniote Egg

Abstract

tor and are distinguished from anamniotes by the form of their eggs (Carroll, 1982; Packard and Seymour, 1997). Instead of a naked egg, which must be incubated in an essentially aquatic environment, amniotes have an eggshell that provides some protection from desiccation (Little, 1983), and, instead of gas exchange by diffusion from the environment to the embryo, gas diffuses through the eggshell between the environment and extra-embryonic membranes where it is transported in the blood to the embryo (Packard and Seymour, 1997). Thus, the diffusion distance is essentially that of the thickness of the eggshell in eggs of amniotes. It is often postulated that the limitation on the size of the anamniote eggs of amphibians is the slow rate of diffusion of 02 from the outside of the egg to the embryo in large eggs (Seymour and Bradford, 1995). The maximum rate of 02 consumption in large amphibian eggs seems to be compromised by limitations of rates of diffusion of 02 through the gelatinous capsule of amphibian eggs (Packard and Seymour, 1997). Rates of 02 consumption and total energetic cost of development have been measured in a number of amphibian and reptilian eggs (Vleck and Hoyt, 1991; Seymour and Bradford, 1995; Packard and Seymour, 1997). However, all the reptilian eggs measured to date are larger than those of the amphibians so far measured. To make comparisons of the rates of 02 consumption and energetics of reptilian and amphibian eggs by using regressions beyond the size range for each taxon is not statistically valid. The largest of amphibian eggs has had their rates of 02 consumption and/or energetics measured (Packard and Seymour, 1997); larger eggs are not available to extend regressions of these parameters against egg mass. Thus, we chose to measure these parameters in the smallest of available reptilian (amniote) eggs, those of the Australian skink Menetia greyii, to test the null hypothesis that the maximum rate of 02 consumption during embryonic development is not greater in reptiles than in amphibians of equivalent size. Eggs of M. greyii, one of the smallest lizards in Australia (Greer, 1989), are smaller than the largest of amphibian eggs. We quantified the rates of 02 consumption and energetic cost of development to enhance understanding of the evolution of amniote eggs from anamniancestors. At the same time, we provide det iled information on the energy contents of eggs and neonates and quantify how much energy is consumed during embryonic development for comparison with other species of reptiles.