Side Effects: Sifrhippus, the First Horse, Got Even Tinier as the Planet Heated Up

Scientists have known that many mammals appear to have shrunk during the warming period, and the phenomenon fits well with what is known as Bergmann’s rule, which says, roughly, that mammals of a given genus or species are smaller in hotter climates.

Although the rule refers to differences in location, it seemed also to apply to changes over time. But fine enough detail was lacking until now.

In Science, Ross Secord, of the University of Nebraska-Lincoln; Jonathan Bloch, of the Florida Museum of Natural History at the University of Florida in Gainesville; and a team of other researchers report on the collection and analysis of Sifrhippus fossils from the Bighorn Basin.

They report that the little horse got 30 percent smaller over the first 130,000 years, and then — as always seems to happen with weight loss — shot back up and got 75 percent bigger over the next 45,000 years.

The fossils indicate that at its smallest Sifrhippus weighed about eight and a half pounds, and at its largest about 15 pounds.

Using fine-grained detail on both climate and body size, the researchers concluded that the change in size was, as suspected, driven primarily by the warming trend.

“It seems to be natural selection,” said Dr. Secord. He said animals evolved to be smaller during warming because smaller animals did better in that environment, perhaps because the smaller an animal is, the easier it is to shed excess heat.

Paul L. Koch, head of the department of earth and planetary sciences at the University of California, Santa Cruz, and a specialist in reconstructing ecosystems and climates from many millions of years ago, said, “The paper lets us see the effect of warming on mammals where the climate change is really large.”

Dr. Koch, who was not involved in the study, said he thought that the question of whether natural selection was the cause of the changes was still open, and that the disruption of ecosystems during the warming period might have led smaller animals to migrate to new locations.

The current warming period is occurring on a scale of hundreds of years, not thousands, and scientists can only speculate on whether modern mammals will shrink.

“It’s difficult to say that mammals are going to respond in the same way now,“ Dr. Secord said. “If I had to guess,” he said, he thinks some will get smaller. And, he said, some studies have shown some birds to be getting smaller in response to warming.

If warming continues at the highest rate projected, he said, there’s another question: “Can mammals keep up?” 

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