Although rare, there are three identified (so far) amphibian species where females can engage in either egg-laying (oviparity) or give birth to live young (viviparity).
Zootoca Vivipara – The Viviparous Lizard
The lizard Zootoca vivipara is one species that demonstrates this capability. Viviparous and oviparous populations are almost always geographically separated.
Viviparous females commonly have a larger body size. This makes sense considering that they will carry eggs with an increasing volume for a longer period of time than their oviparous counterparts.
Oviparous females generally experience higher energy expenditure during vitellogenesis (yolk deposition) and the first phase of development prior to egg-laying. The shells of oviparous females are eight times thicker than those of viviparous females. This makes sense based on the fact that the latter part of development will happen within the egg outside of the female’s body.
The energy saved by viviparous females is spent during the later phases of embryonic development, where offspring require a greater amount of maternal investment.
Why Do Some Lay Eggs While Others Give Birth To Live Young?
The fact that two modes of reproductive development occur in these species is perplexing.
It could represent a late-stage step in a speciation event (i.e., they are well on their way to becoming a separate species), as the transition from oviparity to viviparity has occurred multiple times in diverse vertebrates.
Or perhaps we can look at this as an extreme case of parental plasticity. It would be interesting to investigate whether oviparous females are also capable of viviparous development. Perhaps some biologists somewhere are currently trying to make that happen.
This post is an adapted excerpt from my book “Wild Sex: The Science Behind Mating In The Animal Kingdom“