Adapting to change Scientists find fossils of ancient horses that show they evolve during temperature changes
About 56 million years ago, horses were about the size of small dogs. But during a period when the global climate warmed substantially, they shrank to the size of average house cats.
According to recent studies, the dwarfing was in response to globally warming temperatures.
Research results appeared in the journal Science and the study was led by scientists from the University of Florida and the University of Nebraska-Lincoln as well as six other institutions.
The little horse was Sifrhippus (pronounced Sif-RIP-us), which appeared in North America at the beginning of the Eocene Epoch 56 million years ago.
In the early days of their evolution, their growth was challenged when global temperatures increased during a time known as the Paleocene-Eocene Thermal Maximum (PETM), a short (geologically speaking) climate event that lasted about 175,000 years.
Massive amounts of carbon dioxide and methane poured into the atmosphere causing average global temperatures to rise by 6 C. What triggered the PETM remains unclear.
“Part of what happened at that time was that there was a release of billions of tonnes of carbon into the atmosphere,” said Ross Secord, assistant professor with the Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln and lead author of the study.
An area known as Cabin Fork in Wyoming’s southern Bighorn Basin is rich in Sifrhippus fossils.
The oldest specimens had an average body size of 5.6 kilograms. They shrank about 30 percent in size over 130,000 years to about 3.9 kg, about the size of an average house cat. At this point, they became the smallest known horse.
Then, during the last 45,000 years of the PETM as the climate gradually cooled, the horse rebounded in size by 76 percent to an average size of seven kg.
The phenomenon in which mammals and birds are generally smaller at lower, warmer latitudes and bigger at higher, cooler latitudes is known as Bergmann’s rule and in a broad sweep, it defines animals living in widespread geoclimatic zones.
The theory is that mammals in warm climates can rid themselves of body heat more efficiently with a smaller body size while animals in cooler climates need that heat to keep warm.
It’s all about adapting to changing conditions, migrating to a new home range, or risk going extinct.
Apart from heat regulation, the reasons for a smaller body size in warmer climates are complex and are intertwined with the resources upon which animals depend.
“I did a calculation and showed that Bergmann’s rule … would not be enough to explain the change in size we see in dwarfing,” said Philip Gingerich , researcher with the University of Michigan and director of its Museum of Paleontology, who is credited with first showing a link between dwarfing and the PETM.
He has proposed that in a high carbon dioxide atmosphere, plants are less nutritious because they don’t require and don’t produce as much of the enzyme Rubisco that they normally use to break down carbon dioxide.
“I speculated that if mammals are on any kind of a calendar driven reproductive cycle, one way to cope is reproduce in a smaller size because of less nutrition in the plants. Interestingly, once the event was over, the mammals went back to their normal size previously.”
Other influences to favour dwarfism could be changes in prey-predator relationships, drought conditions resulting in fewer food resources, changes in home ranges, or natural selection.
But it is still a mystery why the similar correlations between body size and warming climate haven’t shown up in subsequent global warming periods.
Secord thinks it may be simply a matter of not enough available information.
“It may be we don’t have good enough records. Climate records are not strong enough or the fossil records are not dense enough,” he said.
However, he did refer to other studies, including one that calculated how wood rats got significantly smaller as the glaciers melted and the climate substantially warmed.
The research raises questions about how mammals might respond to future global warming events.
“The carbon isotope record was very important because it tied what we saw in continental Wyoming with horses and other mammals to what some colleagues in California had observed in deep sea cores in the south Atlantic,” said Gingerich.
“All of a sudden, it was clear whatever this was, it was short-lived and it was global. We have a very good record of the horse species both before and after the event and it is only in the warming event that they could see dwarfing species. It is still our best model for what we fear we are going into now.”
Secord wrote in the report that the PETM warming was similar in magnitude to that predicted by some global models over the next century. But it happened more slowly and it started from a warmer baseline.
He warned not to discount the possibility of shrinking body size of modern mammals and birds along with changes in ecology and physiology in the future.