Spiny lava lizards spend less time wooing mates and choose their partners more hastily when exposed to warmer-than-usual temperatures.
Many animals shift their behaviour when it is hot out: some stay cool by moving less, sweating or going for a swim. Nicola Rossi at the National University of Córdoba in Argentina and his colleagues were curious how spiny lava lizards (Tropidurus spinulosus), a species native to South America, might change their courting behaviours to cope with the heat. “They do push-ups around the rocks they guard to signal their ability,” says Rossi. “It requires a lot of energy to do.” The lizards also bob their heads to attract mates.
The researchers began by testing two groups of spiny lava lizards living in nearby areas of a nature preserve in Argentina. One group lives on a rocky outcropping that receives lots of sunlight, while the others are in a zone with lots of shade trees. The cooler area was around 26.5°C (80°F) and the warmer area was 30°C (86°F).
They recorded the reptiles’ movement for 20 days and found those in the warmer area were less active during the hottest midday hours. Lizards in the warmer condition tended to move around most during mornings and late afternoon when temperatures were coolest, possibly missing out on mating opportunities with others.
Next, the researchers brought 42 male and 35 female lizards into the lab and gave them their choice of two mates behind a glass barrier. Some lizards were in their normal climate, while others were in environments 2 or 4°C warmer than usual, reflecting different end-of-the-century warming predictions by the Intergovernmental Panel on Climate Change (IPCC).
Male lizards did just as many push-ups regardless of temperature, but their head bobs were sporadic at hotter temperatures — while some kept bobbing, others gave up. Males also increased their use of shady refuges in the warmest condition by up to four times, reducing the time they spent interacting with females.
Females of this species typically opt for males with the most colourful flanks – a sign of sexual fitness – but at the highest temperature, their preference vanished. “If they are choosing their mates randomly, they’re not choosing the best males,” says Rossi. Females also did fewer head bobs when it was hotter.
All of this suggests the species will “probably not adapt fast enough to cope with the rate of temperature increase that is expected in the future”, says Daniel Ariano-Sánchez at the University of the Valley of Guatemala, who was not involved in the work. Because lizards are ectothermic, or “cold-blooded”, they are especially vulnerable to dramatic temperature swings that can push them beyond their physical limits.