Mammals Shrank As the World Warmed

By Tim Radford, Climate News NetworkThis piece first appeared at Climate News Network.

LONDON — A warmer world could be a smaller world if the testimony of the past is any guide. Palaeontologists have unearthed fresh evidence that species tend to shrink during periods of conspicuous warming.

Philip Gingerich of the University of Michigan in the US told the Society of Vertebrate Palaeontology at its annual meeting in Los Angeles that decreased body size seems to be a common mammalian evolutionary response to extreme global warming events.

Researchers have repeatedly observed that ancestral primates, horses and deer dwindled during a hot spell called the Paleocene-Eocene Thermal Maximum, 55 million years ago. But Gingerich and colleagues reported that they had found a second instance of mammalian dwarfing during yet another, but smaller, global warming event two million years later, around 53 million years ago.

During the first and more dramatic warming, temperatures rose 6°C or more during 160,000 years of climate change. The second event, called the Eocene Thermal Maximum 2, lasted between 80,000 to 100,000 years, with a much smaller average temperature rise.

And teeth and other fossil evidence from this period, collected in Wyoming’s Bighorn Basin, show that, once again, body size decreased during this second, milder event – but did not decrease as much. A horse ancestor, known as Hyracotherium, shrank by about 19% during the second event, but showed a 30% decrease in body size during the first, much hotter event.

Mounting evidence

The ancient warming events could possibly have been caused by the natural release of vast quantities of trapped methane from the seabed during these periods. There are fears that the same thing could happen again, as climate changes in response to the buildup of carbon dioxide and other greenhouse gases from factory exhausts and power station chimneys.

And three times this year, researchers have drawn the same conclusion. In January European scientists engaged in the Bighorn Basin project made a similar prediction, and proposed that even humans might dwindle to hobbit proportions as a successful evolutionary strategy. A second set of researchers identified a stunted generation in Brazil’s north-east that had been cruelly deprived of food during a climate-related famine.

And in the US, a Kansas biologist analysed the weight and size of 22 herds of wild bison in the US and found the same story: those that grazed the warmest grasslands were consistently smaller than animals from the same species further north.

The discovery of a second set of changes 53 million years ago confirms the pattern. “The fact that it happened twice significantly increases our confidence that we are seeing cause and effect, that one interesting response to global warming in the past was a substantial decrease in body size in mammalian species,” said Prof Gingerich.