The blood of ectothermic animals is far from cold, despite their common name. Fine-tuning their body temperature in shady nooks when overheated and basking in the sun when chilly, ectotherms successfully maintain their optimal body temperature range. But the future may not be so sunny for ectotherms as environmental temperatures continue rising. ‘Heat waves have become more common in recent decades and are predicted to increase in frequency in the future’, says Jonathan Webb, from the University of Technology, Sydney, Australia. While Webb suggests that adult reptiles will be able to weather whatever the climate can throw at them, he is more concerned for embryos and young hatchlings. ‘Developing lizard embryos cannot thermoregulate and may experience thermally stressful temperatures in natural nests during summer’, he suggests. Knowing that flies that develop in high temperatures seem to cope better in extreme conditions, Webb and his colleagues, Buddhi Dayananda and Brad Murray, incubated the eggs of tiny velvet geckos (Amalosia lesueurii) at high (14–37°C) and moderate (10–33°C) temperatures to find out how heat waves might affect the developing geckos.

Not surprisingly, the eggs that had been incubated at higher temperatures hatched first – 27 days earlier – but their head start didn't give them an advantage when the temperature fluctuated. The trio tested the youngsters’ ability to right themselves after being warmed or cooled and then gently turned on their backs, and realised that the geckos that had been incubated in hot nests were more impaired at extreme temperatures than the geckos that had been incubated at lower temperatures. The hot-incubated geckos became incapacitated at temperatures below 6.2°C and above 38.7°C, while the animals that had been incubated at lower temperatures were more robust, only failing to right themselves below 5.7°C and above 40.2°C. And when the team measured the temperatures under the rocks that the youngsters prefer to use for shelter, they could reach a sizzling, and potentially fatal, 50°C. However, crevices in which the hatchlings shelter only reached 33°C.

Webb points out that this will pose a problem for hatchlings that emerge early: ‘Shuttling between rocks and shady crevices may expose them to predators, compromising their survival’, he says. So gecko embryos that were incubated during a heat wave are less well prepared for scorching temperatures and will likely be more vulnerable to predation.

B. R.
J. K.
Hotter nests produce hatchling lizards with lower thermal tolerance
J. Exp. Biol.