Primitive Horses Shrank Because of Global Warming

Modern and Prehistoric Horses
Illustration of a modern 500 kilogram horse face to face with a Sifrhippus sandrae, which lived 55 million years ago and was the size of a small cat, by Danielle Byerley, Florida Museum of Natural History.

Primitive Horses Shrank Because of Global Warming
55 million years ago, the ancestors of horses shrunk to the size of modern house cats
They grew in side when planetary temperature cooled

El País: Los caballos primitivos encogieron por el calor
Alicia Rivera reporting from Madrid February 23, 2012

More heat means smaller size, and lower temperatures mean more bulk. Some 55 million years ago, primitive horses followed this rule of thumb to evolve and adapt to the conditions of their environment, to the point that the Sifrhippus, the oldest ancestor of the horse known to the fossil registry, shrank to the size of a house cat (and less than 4 kg of weight) during a pronounced planetary warming phase. Then, when the temperature dropped again, it got bigger. The interesting thing about the investigation into this subject by American scientists is not only the variation in size of those remote horses but also its clear association with warming (up to five degrees centigrade) and cooling. Ross Record and his colleagues have explained this in the journal Science.

The Sifrhippus emerged in North American forests and weighed something less than six kilos. But in the climatic period called Paleocene-Eocene Thermal Maximum, during which the earth’s temperature rose up to five degrees Celsius (nine degrees Fahrenheit), this animal shrank in size by 30%. This hot interval was 55 million years ago and 17,500 years long. When the climate cooled, the primitive horse grew in size and gained weight by up to 75%, reaching seven kilos.

This correlation between size and temperature, called Bergmann’s Law, has been observed in many mammals, but it wasn’t clear if animals’ tendency to be more corpulent in cold climates and smaller in hot ones was linked to the temperature (large animals conserve body heat better than small ones) or other environmental factors like the availability of food.

Secord (University of Nebraska) and his colleagues have found an answer in the fossils, specifically horse teeth found in the state of Wyoming, whose size and chemical compositions (like oxygen and hydrogen isotope counts) give clues about the conditions of the animals’ environment.

Jonathan Bloch (Florida Museum of Natural History), coauthor of the research, said that another researcher, Stephen Chester, then a pre-doctoral student, entrusted himself with the task of measuring these horse teeth, and the data he presented was surprising: “It signaled that the first horses in the series were much larger than the later ones. We thought something was wrong, but no, that was correct, and the pattern was even clearer when we accumulated more fossils.” The surprise was even greater, Bloch continued, when Secord ran geochemical analyses of the oxygen isotopes in the teeth, and the data curve showed “exactly the same pattern the teeth’s size did.” Altogether, they measured the teeth of 44 horses and chemically analyzed fossils from 150 mammals.

For the first time, we were able to go back tens of millions of years and demonstrate that temperature was the essential cause of the change in body size of this species of horse,” concluded Bloch in a statement from the University of Nebraska. The investigation began seven years ago. They have ruled out the idea that the fundamental cause was quantity of food, since during that warm fluctuation, the climate seemed to be more humid and thus more productive, and yet the horses still shrunk.

Felisa A. Smith, a specialist at the University of New Mexico, wrote in Science that the tendency toward dwarfism has been verified for many animals which lived during that heating period 55 million years ago, but there wasn’t a quantitative evaluation of how size was influenced throughout the evolution of these animals’ bodies.”

Though the temporal divide is more than 50 million years wide, one can’t avoid looking for parallels between current climate change and the effect of the Paleocene-Eocene Thermal Maximum. Secord and his colleagues didn’t evade the subject, either: “These discoveries could be important to understanding mammals’ evolutionary response to future global warming,” he wrote in the scientific journal. They recalled that climatologists’ projections indicated that the average temperature of the earth could rise four degrees (Celsius) in this century, not far from the five degrees of that heating phase.

But there is a big difference, they say: the velocity of the change. During that thermal maximum, the 5-degree heating took between 10,000 and 20,000 years, while now we are speaking of a century or two. “So there’s a big difference in scale, and the question is if we would see the same kind of response, if the animals would be capable of readjusting their physical size in two centuries,” Secord commented. It’s not clear how animals would quickly react to warming, Smith points out, going on to say that we must be cautious extrapolating what happened in the past to what is happening today, since the average temperature then was much higher than it is now.

Superior Mandibles of Sifrhippus and Modern Horse
Scientist Jonathan Block shows the upper jaw of a Sifrhippus from 55 million years ago. Photo courtesy of Kristen Grace and the Florida Museum of Natural History.

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