Attention lizards of the future: lay your eggs in shade

What’s the difference between a lizard and a lizard egg? This isn’t the start to a joke, and the answer could have dire implications for the future of lizards on our planet. The main difference between the two—the difference that will become increasingly important in a warmer (or colder) world—is that lizards can move while their eggs are stuck right where they are until the little lizard babies hatch.

Lizards, like other “cold-blooded” ectotherms, do not regulate their own body temperature. They rely instead on their environment to either warm them up or cool them down. Too hot? Find some shade, or nestle down under a few centimeters of dirt. Too cold? Find a nice sunny spot on a warm log or rock and let the sun do all the work for you. Seems simple enough, but lizard embryos can’t do that. Instead, they have to be able to withstand whatever changes in temperature come their way if they’re to survive long enough to become a baby lizard.

Most studies that have attempted to predict lizards’ ability to withstand climate change have looked at adult lizards. Ofir Levy, a postdoctoral fellow at Arizona State University, thinks that’s a problem. Adult lizards might be able to survive extremely hot or cold spells, but the eggs are another story. By only focusing on animals at a particular developmental stage, he argues, researchers are missing out on important information. If extreme variations in temperature kill most of the embryos, it really doesn’t matter if the adults can withstand them.

Levy and his colleagues used a combination of computer modeling to predict future temperature variations and experiments to see how lizard eggs would respond to those variations. The team focused on some of the most common lizards in the United States: spiny lizards, those of the genus Sceloporus. There are some 90 species, though they focused on two: the eastern fence lizard S. undulates and the plateau fence lizard S. tristichus. They rounded up gravid females from four locations at different latitudes in New Jersey, South Carolina, Colorado, and Arizona. Then they waited for them to lay their eggs, which were moved to artificial nests designed to mimic natural ones (lizards lay their eggs between 6 and 12 centimeters below the soil’s surface). Once settled into their artificial nests, the eggs and embryos within them were subjected to various temperatures.

In natural nests, embryos heat up gradually throughout the day as the air and soil become warmer. In the afternoon they reach their peak temperature and then begin to cool again as day turns into night. In the lab they were exposed to similar patterns, but with varying peak temperatures. All embryos died even after a brief exposure to a peak of 107.6oF (42oC), though most of them survived at the still-sizzling temperatures below that threshold. Embryos were most likely to survive if they were warmed to 100.4oF (38oC), and had lower but still reasonable survival at peaks of 95oF (35oC) or 104oF (40oC). There were no differences between species or among collection sites.

The trends were different for cooling, however. While the embryos had a threshold for survival in extreme heat, their response to extreme cool was much more graded. Some managed to survive even if cooled to a frigid 17.6oF (-8oC).

When those survival situations were compared to projections for future climate regimes, prospects seem pretty grim for American lizards. “Assuming that females lay eggs at a depth of 6 cm and no shade, lethal events spread from 77% to 96% of the USA between 2000 and 2100 and increased in frequency from 24 to 50 days per year,” write the researchers. Lizards lay a lot of eggs expecting that most of them won’t survive, but even so, this is still a dire prediction.

But if mother lizards are clever enough to create their nests in 100% shade, then their offspring have much better odds. In those circumstances, lethal events would increase from just 3% to 48% of the US and would increase in frequency from less than 1 to 4 days every year. Shady nests are perhaps not ideal, but they might be thought of as the better of two bad options.

Earlier estimates of future lizard populations based solely on experiments with adults actually predicted that lizards in the US would thrive under warmer conditions, because the extra warmth allows them more time to be active. But by taking the embryonic stage into account, Levy and his colleagues wound up with a very different conclusion, all because embryos can’t walk around. Despite performance increases in adults, decreases in survival at earlier stages can still lead to extinction or severe population declines.

The problem is, that’s not how lizards evolved. Up until now, even those in the warmest parts of the country could nest in sunny spots, because embryos there were more likely to survive, hatch earlier, and have more time to grow before the winter cold set in. In the future, lizard mothers will have to adopt a different strategy.

Levy’s prescription for maximum lizard survival is nesting in shadier locations, which would compensate for the effects of warming in 95% of the country, and in deeper soil, which would compensate for warming in some 99.9% of the US. Together, those two strategies should help provide a buffer against the most extreme heat waves and cold snaps. Now somebody just has to tell the lizards. – Jason G. Goldman | 04 September 2015

Levy O, Buckley LB, Keitt TH, Smith CD, Boateng KO, Kumar DS, Angilletta Jr MJ. 2015 Resolving the life cycle alters expected impacts of climate change. Proc. R. Soc. B 282: 20150837. DOI: 10.1098/rspb.2015.0837.

Header image: Eastern fence lizard via Flickr/Sarah Viernum.