Global Warming Shrinks Mammals

Illustration of early horse ancestor. Photo via Heinrich Harder.

Climate change deniers still cling to the delusion that Earth is happily homeostatic, but the rest of us have to move on to the next dizzying set of questions raised by civilization's largesse. Among the most pressing is determining what kind of effect turning the thermostat up is going to have on extant species—especially on our evolutionary home team, team mammal.

As so often happens, prehistoric fluctuations provide a good roadmap for what to expect in the future. In this case, there is a strong link between higher global temperatures and smaller mammalian body size. These findings were presented last Friday in Los Angeles at the annual meeting of the Society of Vertebrate Paleontology.

A research team led by University of Michigan paleontologist Philip Gingerich looked mammalian fossils from the hyperthermal period that occurred about 53 million years ago. Also called as the Eocene Thermal Maximum 2 (ETM2), for about 80,000-100,000 years, Earth's temperature was hiked up about 5 degrees Fahrenheit.

Gingerich and his team found that, like wool in the dryer, the heat lead to species actually shrinking. Judging by jaw and teeth bones, the ugulate Diacodexis experienced a 20 percent decrease in size. The adapiform primate Cantius shrunk by 8 percent during the same period.

What's particularly exciting about these findings is that they are not the first of their kind. Paleontologists have long known that mammalian dwarfism occurred during the Paleocene-Eocene Thermal Maximum (PETM), which preceded the ETM2 by about two million years. Over those 160,000 years, global temperatures rose from nine to 14 degrees Fahrenheit, and the response among mammals was also a dramatic decrease in body size. With the new data collected from the ETM2 fossils, Gingerich's team has proven that the effect occurred multiple times in mammalian history.

“The fact that it happened twice significantly increases our confidence that we're 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 Gingerich in a press release about the findings.

Another telling correlation the team discovered was that the hotter, longer PETM's affected animal size more strongly than the ETM2. For example, the early horse ancestor Hyracotherium shrank by 30 percent during the PETM, whereas it only decreased by 19 percent during the ETM2. “Interestingly, the extent of mammalian dwarfism may be related to the magnitude of the hyperthermal event,” said Abigail D'Ambrosia of the University of New Hampshire in the team's press release. It's also telling that the Hyracotherium bounced back to its pre-sauna size after both PETM and ETM2.

One of the Hyracotherium specimens used in the study via Abigail D'Ambrosia, University of New Hampshire.

The environmental forces that gave rise to the PETM and ETM2 hyperthermals are a little trickier to figure out than our own period of pollution-based warming. Gingerich speculated that the prehistoric steamy cycles may have been brought on by the release of oceanic methane deposits called clathrates, similar in structure to ice.

Research into why mammals responded to these periods by getting smaller is also an ongoing topic of debate, but Gingerich has suggested that plants exposed to increased carbon dioxide levels may have become less nutritious for the mammals that relied on them. Smaller members of a species would have been selected for their ability to sustain themselves with less nutrients, in the same way modern three-toed sloths have compensated for the low energy yields of leaves by being endearingly lazy.

The study's findings are based on fossils of land mammals found primarily in Wyoming's Bighorn Basin, but the effects of global warming on other types of mammals – especially oceanic ones – can be observed in real time. Many of our seafaring cousins are threatened with habitat changes brought on by the greenhouse effect, and are unable to adapt fast enough to survive. As reported in Oryx, marine mammals that live either full-time or part-time in polar regions are particularly at risk of habitat loss, and ultimately extinction.

Perhaps the hyperthermal event we're currently entering will take humans down a notch figuratively, highlighting over-dependence on non-renewable energy. Failing that, it looks like it will at least bring us down a size literally.