Friday, 27 February 2015

Introduced Wall Lizard Colonies in England: How Do They Survive?

Over the past hundred years or so, Wall Lizards (Podarcis muralis, formerly Lacerta muralis) that are native to many parts of continental Europe have been released deliberately or accidentally in southern Britain. Some have survived as colonies and some colonies have thrived, especially in the extreme south.

The usual explanation for why non-native oviparous reptiles do not survive in colder climes is that ground temperatures are insufficient for the embryos to develop and hatch either at all or before winter. Clearly, there are parts of England where Wall Lizards can breed and such lizards, the results of a ‘natural’ experiment, have been the subject of a very recent study published in Proceedings of the Royal Society.

The results are clear and show how Wall Lizards in England have adapted to colder conditions. The duration of incubation is reduced and the embryos are allowed to develop longer in the mother before oviposition (i.e. the mother gives the young a head start before subjecting them to a colder life in the egg). The mother can thermoregulate behaviourally even on relatively cold sunny days and bring the embryos to basking temperature. Hatching early, by an estimated two weeks, would allow the young to feed before hibernation and maintain a higher body temperature by behavioural thermoregulation.

That much seems clear. However, I am by no means convinced by the authors’ suggestion and thrust of their argument that the adaptive phenotypic shift they have recorded is the result of rapid selection (some of the colonies have only been in existence for a few decades). I am not convinced at this stage that their studies cannot be explained by physiological plasticity; in other words, that Wall Lizards can adapt to a wide range of external thermal conditions as a normal physiological response. The trigger to retain embryos for longer could be subtle, night-time body temperature for example, while the embryos/eggs could be programmed for early hatching by incorporating a chemical signal at the time of oviposition.

I would suggest that two key experiments need to be done. The first is that continental Wall Lizards be subjected to the conditions that obtain in southern England. The question then is: are the responses the same in the first year as in those lizards from introduced colonies? The second is the reverse: do introduced lizards kept in continental European conditions maintain the decreased incubation duration in the first year or not? If there has been a genetic adaptive shift, the effect should be maintained. Those and further longitudinal studies should enable the results of selection to be distinguished from the results of phenotypic plasticity, and by swapping eggs, to distinguish possible genetic from epigenetic effects.

It’s another case of more work needs to be done.