Lizards Re-Evolve To Lay Eggs, After Having Already Evolved To Give Birth To Live Young

This Liolaemus tenuis from Biobio, lays eggs, which is not unusual for a lizard. What is remarkable is its ancestors did not - it recovered a capacity lost when the species it evolved from lived at higher altitudes. Damien Esquerré

A family of lizards has achieved something very unexpected, evolving to give birth to live young, before going back to egg laying. Most remarkably, the zoologists who observed this think it is possible they rediscovered laying eggs multiple times.

“Dollo’s Law states that once you lose something it is really hard to regain it. For example, species that adapt to living in caves and lose their eyesight are extremely unlikely to get it back,” explained Damien Esquerre, a PhD student at the Australian National University, in a statement. As with most biological “laws”, this one isn't as rigorously enforced by nature as those of physics, but it is still a surprise to see it violated.

However, Esquerre reports in Evolution that this is what the Liolaemus lizards have done, possibly quite often.

Laying eggs is a common reptilian trait, but in colder climates, it's a bad evolutionary strategy. Internal gestation makes it easier to keep the next generation warm. As the Andes mountain range rose, an ancient lizard found this out, and the Liolaemidae are among their descendants.

Liolaemidae populations became separated on different mountain tops, or "sky islands", and evolved into many new species, some of which then made their way downhill and colonized warmer lowlands. Indeed, Esquerre writes, the Andes has become a “species pump”, with 300 known species of Liolaemidae living there and in surrounding regions today, and presumably many more now extinct.

High in the Andes Liolaemus valdesianus gives birth to live young to fend off the cold. Damien Esquerre

Of these species, around half live in warmer low altitude climates and lay eggs. Esquerre explained to IFLScience that, since Liolaemidae don't incubate their eggs like birds, egg laying for them requires far less investment than carrying around live young. This lets them produce far more offspring, allowing for much more rapid growth. Female lizards probably appreciate not having to spend so much time heavily pregnant as well.

Evidence has been produced previously that certain snakes may have also re-evolved egg laying having lost the capacity, but Esquerre told IFLScience this remains controversial.

The most intriguing aspect to the work is the possibility that egg-laying Liolaemidae don't all come from a single development, instead having 3-8 branches of the family develop it independently. However, Esquerre stressed to IFLScience this conclusion is very tentative, and needs far more research to confirm.

Equally, Esquerre said, while we know which came first, the lizard or the egg, it is unclear how egg-laying reappeared. After all, he noted, no mammal has ever re-evolved egg-laying (monotremes having always had it), despite the benefits, so it is clearly not an easy process. 

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