Tropical lizards might be able to adapt to climate change
An experiment in the Bahamas offers a slim ray of hope that some species could survive higher temperatures. James Mitchell Crow reports.
We’re in the midst of a sixth extinction. Species are succumbing to the combined effects of habit destruction, dislocation and climate change with extinction rates a 1000 times higher than before humans existed.
But a report published in Proceedings of the National Academy of Sciences this week suggests there may be hope for one highly susceptible group – tropical lizards. Michael Logan and colleagues from Dartmouth College, New Hampshire, found that some brown anole lizards living in the Bahamas were able to adapt when relocated outside their comfort zone, at least for one season.
The brown anole, Anolis sagrei, is spoiled by year-round balmy temperatures in its home in the leafy interior of the islands of the Bahamas. But it is susceptible to heat stress. All a cold-blooded creature can do when it starts to overheat is seek out a cool tree hollow or similar spot and wait for its body temperature to drop. Some scientists predicted this would lead to mass extinction as the temperatures rose and shady hollows heated up.
But Logan put his bets on the power of evolution. Like all species, some lizards would be fitter than others and might be selected to survive. “In the laboratory we know organisms can adapt quite quickly, so why are we assuming evolutionary change isn’t a significant factor in wild populations?” he asked.
'We saw really strong natural selection favouring those individuals
that could deal with that new environment.'
He and his colleagues set out to test that assumption. A key to lizard fitness is their sprint speed that allows them to hunt down insects or escape from predators. Would different individuals perform differently under heat stress, or would they all uniformly expire?
They tested a field of sprinters at temperatures ranging from 15 to 42 degrees. Each lizard was warmed up in a chamber to the required temperature, and then released on to a track, where its speed was clocked.
“I would release each lizard on the track, and then follow behind it with my hand, to mimic a predator trying to catch it,” Logan explains. “At most temperatures the individual would just run like a bat out of hell right after I let it go, but at the sub-optimal temperatures – really cold or really hot – they would usually wait until my hand almost ‘ate’ them before they tried to run,” he says. The team found that some individuals performed better than others in searing temperatures.
Then like a lizard version of the reality TV show Survivor, they were released on to the coast where temperatures were some 2.7 degrees warmer than their native habitat in the interior.
At the end of the season, the researchers collected the survivors. Sure enough, it was the high temperature sprint champions who made it. “We saw really strong natural selection favouring those individuals that could deal with that new environment,” says Logan. “If you assume the traits are heritable, it suggests these guys could respond quite rapidly, in an evolutionary sense, to global warming.”
By contrast, when the researchers looked at a team of lizards who’d been left in their native habitat, they found no link between the heat-tolerant sprinters and the odds of surviving the season. In other words, there had been no selection for heat tolerance.
“I think the fact this work was done on a wild population was one of the nice things about the study,” says Ary Hoffmann, who studies the adaptability of animals to climate change at the University of Melbourne. “It shows that selection can occur for thermal traits.” What it doesn’t show, he says, is the extent to which the successful lizards can pass on these useful traits to their offspring.
Establishing how well these traits are passed on to the next generation is what Logan is working on now.
And if it does turn out lizards can handle the heat, many other potential traps lie in wait for these species in a warmer world.
Plenty of other heat-related factors could influence survival other than their ability to run fast. New parasites, for instance, might gain the upper edge in the heat and wipe out the lizards.
“I don’t want to imply that what our work is saying is that all organisms in the tropics are going to evolve perfectly in response to temperature,” Logan says. “It is very likely that many species will experience big problems as the world warms. What our work is showing is we need to give species a bit more credit – there are many modes of response they can use to changes in the environment.”