Vivipary means “live birth,” which, as you probably know, humans ascribe to. There are alternatives, of course, like ovipary, which means “egg birth.” So, knowing that there are these two basic options, evolutionary biologists want to know which one is better?, and by that they mean “more derived,” which would suggest that it is selected for over time.
Of course, we, as eutheran mammals (placental), are tempted to suggest that vivipary is the more derived trait, and therefore advantageous in the long run. But birds, which are also considered to be pretty derived, are oviparous. And then there are oviparous mammals, the monotremes (like platypus!).
The interesting twist is that there are some groups of animals, like the reptiles, which exhibit both types of birth. Some species of reptiles (up to one fifth of all squamates in fact) have vivipary, and some have ovipary. And then there are live-bearing sharks! And live-bearing plants (particularly coastal plants, I wonder why…)!!! Now I’m really confused…
At a first glance, one would think that there must be some energetic cost differential between vivipary and ovipary. But it is difficult to compare them directly. Oviparous organisms invest a lot in egg production (shells etc.) and it comes at a huge energetic cost for them. In contrast, viviparous organisms spend relatively little on eggs, but must carry their young around for a longer period of time (gestation). This in itself is a slower, but more long term energy cost and may put the mother at more risk (i.e. due to encumbrance from carrying developing young).
Reptiles are a tempting study system, given that they span the full continuum from fully oviparous to fully viviparous. Given that some of the viviparous species are derived from oviparous ancestors, it seems that there must be some advantage.
Here are some potential ideas, which seem to suggest that vivpary might be advantageous in highly heterogeneous (variable) or unpredictable environments.
1. You can ensure the right environment for the optimal development of your offspring. (temperature, humidity, etc.) The environment outside of the uterus is a highly unpredictable and dangerous place!
2. Protection from predators. The adult female is much more capable of defending herself than an egg. The newly born offspring, however, require some care. Parental care seems to be almost obligatory in viviparous organisms, but it is hard to compare directly because some oviparous organisms also demonstrate parental care.
3. Protection from pathogens. Similarly, the adult female has a well developed immune system to combat fungal, bacterial, and viral pathogens.
Any other points I am missing? (I’d love it if there were a paper comparing the phylogenetics of these two options from an evolutionary standpoint. If you have one, send it my way!)