Smaller Butterflies, Bigger Birds—a Warming World Is Changing Animals’ Size
There’s plenty of evidence that species are relocating in response to climate change. Tarpon now show up in summer as far north as Maryland. Humboldt squid have recently moved from South and Central America into California. But is climate change also affecting the size and shape of animals’ bodies or the way they function?
I first started thinking about the idea when I ran across a 2013 study of ocean acidification. It’s a subject I had scrupulously avoided until then because the words “ocean acidification” are, let’s face it, sleep-inducing. But stay with me a moment: The oceans have soaked up about a third of all the carbon dioxide put into the atmosphere by human activity over the past three centuries, with the result that marine creatures now live in water that is 30 percent more acidic than in preindustrial times.
Think of it this way: If you jump into a swimming pool where the pH has crept a little above the accepted range (say, from 7.6 to 7.7), you may notice that your eyes sting and your skin starts to itch. You can just jump out again and run to the shower, of course. But sea creatures can’t.
The study, by Australian and European researchers, looked at how increasing acidity might affect an Indo-Pacific conch snail living on coral reefs. (It’s got a pretty white shell with some brown stripes, but it goes by the unlovely name Gibberulus gibbosus.) The good news: Spending a week in an aquarium tank with the acidity tweaked to near future levels had no effect on the conch’s ability to right itself after being turned upside down. The bad news: It rapidly lost its ability to jump.
Right. I know. You never imagined that a snail could jump in the first place. But these conchs jump out of the way—by their own body length—to escape predators. (You can see a slow-motion video here.) They may leap five times in a row, and with good reason. The predator is a cone snail, which uses a lance to inject a deadly toxin. Not jumping would be a fatal mistake. But the research team, led by Sue-Ann Watson of James Cook University, found that increasing acidity altered the function of a key neurotransmitter, slowing down or stopping the escape response in many individuals. Species can adapt, so this change might not lead over time to extinction. In certain scallops, for instance, escape responses are heritable, meaning they are subject to natural selection. But many mollusks are key predators, shaping food chains, so changes to their behavior could also change or even undo entire ecosystems.
A more recent study looked at 11 bird species in Germany from 1889 to 2010 to see if the steadily warming climate had caused them to shrink a little. (Bergmann’s rule says bigger species are more common in the north, smaller ones in warmer regions.) The German researchers found no change, or at least no change consistent with climate. But another study, published this week in the journal Biology Letters, looked at two butterfly species collected at the Zackenberg Research Station in northwestern Greenland between 1996 and 2013 and found that their wings had become significantly shorter, meaning they have less ability to disperse by flying long distances.
Another study early this year looked at four parrot species in Australia from 1871 to 2008. Allen’s rule—and yes, there do seem to be a lot of rules for such an unruly topic as wildlife—argues that animals tend to have bigger limbs in warmer areas, as a means of shedding body heat. The Australian researchers found that the surface area of the parrots’ bills had increased by as much as 10 percent, apparently in response to the warming climate.
The most startling change, though, has to do with a new study of tongue size. I’m not sure there are any rules in play here, but tongue size makes a huge difference both for pollinators and the plants they pollinate, with tongue length and flower size often coevolving together. In one famous story, Charles Darwin received samples of an astonishing orchid from Madagascar in which the nectar was at the bottom of a 14-inch-long tube called the nectary. “Good heavens, what insect can suck it,” he wondered to a friend. He conjectured that there must be a moth with a proboscis that long to reach the nectar. When just such a moth turned up 41 years later, biologists named it Xanthopan morganii praedicta (“predicted”) in Darwin’s honor.
For the new study, researchers looked at plants and bumblebees that have coevolved at high elevation in the Rocky Mountains west of Denver. Comparing current specimens with others in museums, they found that the tongues in some bumblebee species had lost a quarter of their length over the past 40 to 50 years, a rate of half a percent per year. Nicole Miller-Struttmann of SUNY College at Old Westbury, New York, and her coauthors tracked the cause not to decreasing body size, competition from invasive species, or other factors. Instead, warmer summers had reduced the abundance of the bees’ coevolutionary partners: flowers with deep nectary tubes. The bumblebees are becoming generalists, visiting almost any flowers still available. That’s good news, in a way. “Evolution is helping wild bees keep pace with climate change,” the authors concluded.
But a large part of the wonder and variety of the natural world has to do with its quirky specialists, its praedicta moths. Those specialists must now often adapt, becoming generalists, or die. If so, wildlife in one place will increasingly become like wildlife in another, all around the world, the way a McDonald’s is much the same in Madrid and Tokyo. It promises to be a far more boring world, if it were not also so damn scary.