Tiny horses and other mammals adapt to warming by getting smaller

Miniature horses are always a favorite at petting zoos, and they may be an evolutionary favorite as well. New research shows that 56 million years ago, early horses shrank from about 12 pounds to 8.5 pounds during a period of intense global warming. This dwarfing trend supports other evidence that climate change can affect mammalian body size and may have implications for modern animals given current warming.
The study, led by scientists from the University of Florida and the University of Nebraska, looked at the tiny horse Sifrhippus using fossilized teeth collected in Wyoming's Bighorn Basin.
Sifrhippus specimens in North America date back to the start of an intense warming period called the Paleocene-Eocene Thermal Maximum (PETM). During this 175,000-year span, global temperatures increased from 5-10 degrees Celsius because of rising carbon dioxide levels in the oceans and atmospheres.
While organizing and evaluating their Sifrhippus fossils from the PETM, the scientists were surprised that the horse had originally been larger before becoming smaller. They also tested carbon isotopes in the fossils and plotted temperature data about the era. When they compared the warming data to the body size plot, they realized that it corresponded almost exactly.
"What immediately hit me was that the oxygen isotope data was a mirror image of the body size," Jonathan Bloch, associate curator of vertebrate paleontology at the Florida Museum of Natural History, said. "It's probably the only interval of time on the planet where mammals are alive and you get that rapid change in a warm interval as opposed to a cold interval."
This unique property of the PETM makes it a crucial analogue to today's climate change. The researchers concluded that Sifrhippus's size changes may be indicative of animal transformations to come. The study noted, though, that scale is a crucial difference between the PETM and modern warming. Though 175,000 years is very brief in evolutionary and geological terms, the PETM still had a longer duration than current global warming, in which temperatures are projected to rise significantly over hundreds, rather than thousands, of years.
This incredibly rapid increase may not give mammals time to respond at all in an evolutionary sense. But other human impacts on the environment may also reduce mammalian ability to adapt through migration. "Habitat destruction [could] be a big problem in terms of survivability then just purely a thermoregulatory response," Bloch said.
Though the study may be predictive, and the correlation between body size and temperature is very strong in the research data, it is hard to be assured of temperature readings from millions of years ago.
"It's a pretty good argument that the oxygen record is indicating temperature, but there's always the possibility that there's something else going on," Benjamin Passey, a Hopkins paleoecologist and paleoclimate scientist, said. "The challenge when you're working in a big area is knowing the relative age of each layer. They're to be commended for the hard work that went into collecting the samples to put the record together."
Regardless of how other mammals respond, humans are probably not at risk of developing a permanent Napolean complex. "I don't think that humans are going to be susceptible to the kinds of conditions we're seeing because humans regulate their temperature to some degree," Ross Secord, the lead author and a researcher at the University of Nebraska-Lincoln, said.
"There are some mammals that are more sensitive to temperature change than others. Some seem to cruise right on through, while some are more susceptible and get smaller."