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Could Rising Temperatures Mean Smaller Mammals?

Teeth of miniature dog-sized horse that lived in Wyoming 56 million years ago.
Teeth of miniature dog-sized horse that lived in Wyoming 56 million years ago.
Joe DeCapua

Scientists say in the ancient past, higher temperatures meant smaller mammals. They’re studying how a brief, but dramatic climate change event affected body size.

It’s called the Paleocene-Eocene Thermal Maximum or PETM for short. It took place 56 million years ago and lasted about 175,000 years. That’s a long time in human terms, but a blink of an eye in the geological record.

Jonathan Bloch said a lot happened back then.

“We had known it was a really unique event for a while in the sense that it was a very rapid, large scale global warming event. And it marks one of the most important moments in mammalian evolution in the sense that we see the first occurrence of several modern orders of mammals, including the primates that are clearly traceable as the direct ancestors of the group that we’re a part of, as well as the ancestors of horses, the ancestors of cows and hippos and camels,” he said.

Jonathan Bloch of the Florida Museum of Natural History
Jonathan Bloch of the Florida Museum of Natural History

Bloch is associate curator of vertebrate paleontology at the Florida Museum of Natural History at the University of Florida. He and colleagues from eight institutions were collecting fossils in the state of Wyoming’s Bighorn Basin.

Tiny horses

“For the past 9 years, we’ve been slowly, slowly collecting teeth, and sometimes more than teeth, fragmentary jaws, of the first horses to come in. And what we started to find was something pretty surprising to us. We had known that the horses that came in initially with that event 56 million years ago were very small, about the size of a small dog. But what we didn’t realize was that in fact when they came in they were a little bit larger than we had expected; and that through the climate event they became about 30 percent smaller and then became larger again,” he said.

Then, Bloch said, fellow researcher Ross Secord, now at the University of Nebraska, took a closer look at what are called oxygen isotopes. These were found in the teeth of the horses. The relationship between oxygen and carbon in these isotopes can provide much information.

“What he showed was that exactly coincident with this body size change that we had documented there were shifts in the oxygen isotope that showed it was getting warmer as the horses were getting smaller. And then as the horses became larger again it became cooler,” he said.

They concluded that temperature change resulted in smaller horses.

Climate itself is changing through this interval by as much as 10 degrees [Celsius] at high latitudes and perhaps as low as 5 degrees in lower latitudes. So that’s a large scale event and it starts to put us in the range of the kind of climate shift that is being predicted by climate models today say for the next 100 years.

Looking to the past, not future

But paleontologists, like Bloch, don’t try to predict future climate change. They look to the past to try to understand the present.

“Because the Earth went through substantial climate change in the past – some of it very rapid and large scale – there’s a record in the rocks for exactly how animals and plants responded. And so we can go back as paleontologists and just reap the benefits of those experiments. We document that by collecting fossils and studying them. And then we can report them to the world with regards to how we should think about the reaction of plants and animals to the potential future climate change. With regards to how much we know about future climate change, that’s really a round for climate scientists and climate modelers,” he said.

Now, although the focus was on tiny horses 56 million years ago, the question still arises as to whether rising temperatures will mean smaller people in the future? Bloch says that’s possible. But there are a lot of factors involved. Right now, humans are getting bigger and that’s generally due to better nutrition. Humans could also adapt to rising temperatures by spending more time in air conditioned spaces.

There’s evidence today that temperatures and mammal size are linked.

“What you’re referring to is an observation that’s been coined Bergmann’s rule. And essentially what this rule says is that mammals of smaller size live in warmer environments and mammals of larger size live in cooler environments. And this has been documented in many different species of mammals,” said Bloch.

So maybe the lesson for future humans is to eat well and stay cool.

In the meantime, Bloch and his colleagues will continue to collect fossils in the Bighorn Basin. He says their future discoveries may be of interest to climate scientists.

Their latest findings can be found in the February 24th issue of Science magazine.

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