The fire salamander Salamandra salamandra is an organism with a perplexing level of parental plasticity.
Females are generally oviviparous (not to be confused with oviparous), which means that somewhere between thirty and sixty larvae are produced and birthed at a developmental stage prior to metaphophosis (the transition from aquatic, free-swimming larvae to terrestrial adults). The only food source for these offspring is their yolk sac.
However, females are also capable of an alternative developmental mode: viviparity. Viviparous females produce between one and fifteen fully metamorphosed juveniles, who obtain nutrition from their yolks and from direct maternally derived sources.
Fire salamander mothers who are oviviparous versus viviparous are not geographically separated like the Zootoca vivipara. It’s not clear whether females are capable of individually switching their developmental mode.
It’s thought that evolutionary changes to the timing of development are called heterochronic shifts, and they are responsible for the shift to viviparity in this species. They are responsible for a wide variety of speciation events.
It’s important to remember that the process of evolution need not act exclusively on the adult for of any animal (or even at the level of the entire individual). Developmental phases are as likely to experience selection pressures that result in overall changes to adult morphologies.
In the case of the fire salamander, it’s thought that development of the cephalic and pharyngeal structures (the head parts that are involved in feeding) happens at an earlier time in viviparous individuals.
The fact that some embryos are capable of feeding at an earlier stage than others allows for them to cannibalize many of their siblings in utero, which contributes to their continued development to metamorphosis whilst still in their mother’s body.
It’s a wacky notion to wrap your mind around, but this kind of change in developmental timing could have easily facilitated the evolution of another reproductive mode (i.e. viviparity) in the fire salamander.
This post is an adapted excerpt from my book: “Wild Sex: The Science Behind Mating in the Animal Kingdom“