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.