Little People: Will Climate Change Shrink the Species?

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Illustration by Danielle Byerley / University of Florida

In this illustration, "Sifrhippus sandrae," right, is shown next to a modern Morgan horse that stands about 5 ft. (152 cm) tall at the shoulder and weighs about 1,000 lb. (454 kg)

If you think there are no new reasons to get freaked out by climate change, try this: there’s at least a theoretical possibility that a warmer and warmer world could lead to tinier and tinier humans. That’s the inevitable conclusion of a just-published study of Sifrhippus sandrae, the littlest horse that ever lived.

The Sifrhippus sandrae first appeared about 56 million years ago, and it was never a terribly prepossessing creature, weighing no more than 12 lb. (5.5 kg). While it was not exactly built for steeplechase running, its skeletal architecture — not to mention its unmistakably equine face — mark it as a great-great-granddaddy of the modern horse.

But while the little critter had a grand evolutionary future in front of it, it came into the world at an unfortunate time — an era known as the Paleocene-Eocene Thermal Maximum (PETM), when a sharp spike in atmospheric CO2 led to a 10ºF (5.5ºC) rise in global temperature. In geological terms, the PETM didn’t last long — just 175,000 years — but biology moves faster than geology, and plenty of species felt the effects, with some existing ones dying out and other new ones emerging. The Sifrhippus sandrae, it turns out, did something else, shrinking by about 30% to 8.5 lb. (3.9 kg) — or about the size of a house cat — during the first 130,000 years of the PETM.

Those findings, reported by a team of vertebrate paleontologists in the current issue of Science, were based on studies of successive generations of Sifrhippus sandrae fossils, and at first, the investigators couldn’t believe what they were seeing. “I thought [we] had to be wrong,” says co-author Jonathan Bloch of the Florida Museum of Natural History. “But the pattern became more robust as we collected more fossils.”

Bloch and his collaborators could not be sure what was behind the biological downsizing, but they had a pretty good idea it was Bergmann’s Rule at work. First promulgated by the eponymous German biologist Christian Bergmann in 1847, the rule states that there ought to be a greater number of small species in warm climates, since a body with a high surface-area-to-volume ratio dissipates heat quickly; cold climates would encourage large species since their lower surface-area-to-volume ratio retains heat. This is, indeed, the broad pattern you see as you range north and south of the equator, which explains why you find moose in Alaska and tree shrews in the tropics and not vice versa.

To confirm that the hand of Bergmann was at work, Bloch and his collaborators conducted geochemical analyses of Sifrhippus sandrae teeth, looking for the presence of oxygen isotopes that, depending on their concentration, indicate a higher or lower global temperature. As they expected, the isotopes moved in lockstep with the animals’ size. “It was absolutely startling,” Bloch says. “We realized that it was exactly the same pattern we were seeing with the horse body.”

So if horses shrink when you put them in the dryer, could the same thing happen to humans? The short answer is yes — and the scary answer is, sooner than you think. Projected temperature increases of about 7ºF (4ºC) over the next 100 years would be more than enough to get the miniaturization going. Indeed, Ross Secord of the University of Nebraska at Lincoln, one of the paper’s other authors, says that ornithologists are already seeing evidence of dwindling body size among some species of birds.

Alas, the Bergmann rule does not discriminate, which means it would affect all of us equally — both greens who have long fought the environmental fight and climate-change deniers who have long pushed back. This likely comes as a disappointment to enviros, who were no doubt looking forward to wearing their very own James Inhofe lapel pin — featuring the actual James Inhofe. Still, it’s probably safe to assume that even as the world warms, the human species will maintain its current stature.

“We would only worry if we were still living in the wild and not wearing clothes,” says Secord. “Since we have ways of keeping the temperature around us at a comfortable level, we would not be directly affected.”

As for the horses? Well, if a 23rd century Kentucky Derby is held in a dog run, don’t say you weren’t warned.