What are the advantages and disadvantages of egg laying versus live birth?

Oviparity and viviparity involve some trade-offs of parental resources. In egg-laying species, all the nutrition for the developing embryo is supplied in the yolk. This means there is no continuing drain of parental resources as the embryo develops. One advantage of oviparity is that a female can often produce more offspring (since she devotes fewer resources to each one). Clutch size is often larger in oviparous species of lizards and snakes than in closely related viviparous species, although there are exceptions. Because each offspring in oviparous species receives a smaller parental investment compared with offspring of viviparous species, hatchlings tend to be smaller than the offspring of viviparous species.

Why is it sometimes advantageous for a species to commit more resources to fewer offspring? Developing embryos are retained in females in viviparous species, so females can control the temperature of development by behavioral thermoregulation. In high elevation and other cool environments, for example, females can bask in the sun during the day, and then retreat to cover at night. In addition, sometimes survival of offspring may depend on larger size and more parental investment. For example, in areas with short growing seasons and relatively cold temperatures, producing a few large offspring may yield significantly more descendants than producing a large number of small offspring that are less likely to survive. In contrast, in a favorable environment with abundant resources, producing many smaller offspring is likely to produce the most descendants. Indeed, there is a correlation between reproductive mode and elevational range in many groups of squamates, with oviparous species occurring at lower elevations and viviparous species at higher elevations.

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Future directions

It is comparatively easy, from evolutionary and physiological points of view, for viviparity to evolve from oviparity in squamates. The membranes of the amniote egg can be modified to provide maternal nutrition, as well as gas and waste exchange (see Figure 32.19). Evolutionary reversal—from viviparity to oviparity—is more difficult, since complex modifications for shell deposition, independent growth, and hatching from the shell need to re-evolve. Nonetheless, a few candidate squamate lineages (including the huge venomous bushmasters of Central and South America) have been identified in which such reversals appear to have occurred. Biologists are focusing on these exceptional reversals to learn how lineages may redevelop structures and physiological processes that were lost in ancestors that lived millions of years earlier.