Swedish sand lizards lay their eggs earlier during warmer years, and these offspring may be surviving better as a result, according to findings published in BMC Evolutionary Biology this week. 

While some wildlife species have been able to adjust to changes in climate by adapting or dispersing, others have suffered decreases in fitness and losses in numbers. One of the most widely observed responses to climate warming has been changes in the timing of lifecycle events like migration, breeding, and emerging from hibernation. “The crucial question is how this affects an animal or plant's fitness,” University of Gothenburg’s Gabriella Ljungström said in a statement. “Are these shifts adaptive and will thus help the population to persist under climate change, or not?”

Ectothermic animals like lizards are especially influenced by local temperature changes since they regulate their body temperatures using external sources of heat, such as a rock that’s been warming in the Sun. Tropical lizards live in climates that are close to their optimal body temperatures, so increasing temperatures might decrease their fitness. The opposite might be true for high-latitude lizards. 

To investigate, Ljungström’s team analyzed 15 years’ worth of data on 354 female Swedish sand lizards (Lacerta agilis) from the west coast of Sweden. The geographic range of this species stretches as far south as France and extends farther north than any other egg-laying lizard in Europe. The population studied here comes from the northernmost border where females lay just one clutch a year. 

These sand lizards appear to be responding adaptively to rapid annual changes. The females laid eggs earlier during years that were relatively warmer. An earlier egg-laying date has previously been linked to higher reproductive success for this lizard species: Hatching earlier improves the fitness and survival of the lizard babies.

While these findings do seem to suggest a positive effect of global warming (at least in the short term), the researchers caution that a whole suite of traits are likely affected – and their combined effects are unknown.