Scientists warn that larger mammals are more vulnerable to extinction with rapid global warning than smaller ones.
Jessica Theodor, a mammalian paleontologist in the department of biological sciences at the University of Calgary, looks at ways in which animals of the past evolved and adapted through past climate changes.
She said, in general, the body sizes of land mammals go down when the planet is warm, and noted North America’s present day ecosystems don’t have near the diversity of animal species they should when looking at a 65 million year fossil record.
“By looking at the ways in which mammals have changed size in the past, we can understand which species are most vulnerable to extinction in the future,” she said at a recent Bow Valley Naturalists’ meeting in Banff.
“If we look at the last 65 million years as the record, we are still in recovery from a mass extinction, and we’ve started a new one – and that’s a really dangerous position to be in.”
Scientists can tell a lot about land mammals and their body sizes in the fossil record, often identifying species and their sizes with just one or two teeth, which correlate with body size.
The fossil records indicate mammals generally seemed to get smaller during warmer periods and bigger during cooler times. Mammals lived when dinosaurs roamed the earth, but never got much bigger than a small dog.
The dinosaur die-off 65 millions years ago, however, paved the way for the evolution of massive mammals which, according to the fossil record, grew to up to 18 tonnes at the top limit.
However, Theodor said a reduction in mammal sizes occurred almost 56 million years ago when the planet warmed at a rapid rate. It’s called the Paleocene-Eocene Thermal Maximum (PETM) and the most recent event scientists compare today’s global warming to.
PETM warming lasted 200,000 years and saw temperatures rise a further 5-8 C, and is thought to have been caused by a massive carbon release into the atmosphere over 20,000 years.
Theodor said scientists don’t know for sure what happened, but one theory is that melting of methane hydrates in the ocean seabed released, triggering atmospheric global warming.
“They were gaining two billion metric tons of carbon per year, and it took about 20,000 years,” she said, noting methane is a potent greenhouse gas.
“Our current rate of increase of C02 in the atmosphere is closer to 30 billion metric tons of carbon per year, and the timeframe we’re talking about is 500 years, or less.”
While the fossil record shows some mammals did get bigger during the PETM, Theodor said most mammals got a lot smaller.
“Rapid global warming caused dwarfing of a lot of mammals, particularly herbivores and, particularly, big herbivores,” she said.
One theory about the reduction of mammal body size at the PETM is that lineages got smaller through natural selection because being smaller was an advantage in a warmer climate.
Theodor said the second idea is that smaller species from elsewhere were able to move into new areas because their range changed following the warming, whereas larger mammals “don’t have that luxury.”
“Because of the granularity of the environment, a mouse, for example, can move 20 yards to a different microclimate and be OK, but the home range for an elephant is significantly larger and it can’t migrate away,” she said.
That said, there are small mammals vulnerable to rapid warming as well.
“One that is getting trapped right now, as we speak, are pikas,” said Theodor. “Because they are elevation-restricted and temperature restricted, as warming happens they go further up the mountain and they get isolated, and they will likely go extinct.”
Compared to reptiles or smaller mammals that can produce a lot of offspring per season, Theodor said larger-sized mammals reproduce very slowly, noting today’s African elephant has a two-year gestation.
“We can’t reproduce quickly if we’re in danger, and what that means is large mammals are at a very high extinction risk,” she said.
When scientists tested a series of variables thought to be important to mammal body size evolution, like atmospheric oxygen, the size of continents, and temperature, Theodor said the only one that was statistically significant was temperature.
“Mammal body size is inversely related to temperature. As temperature goes up, the maximum size goes down,” she said.
“Mammals can’t tolerate a lot of heat stress, as we’re very warm to begin with, and as animals get larger, their volume increases much faster than their surface area,” she added.
“This means that large animals have much more volume to generate heat than they have surface area over which to radiate excess heat.”
The largest mammals that ever lived include a hornless, rhino-like creature called Indricotherium and an elephant-looking mammal named Deinotherium, weighing about as much as three or four adult modern-day African elephants.
Throughout the last 65 million years, Theodor said land mammals have never gotten bigger than 18 tonnes, noting that limitation seems to be a consequence of metabolism and/or reproductive systems.
“It’s a pretty consistent pattern across multiple continents, which means that it’s probably biologically real that there is a limit on how big mammals can be,” she said.
Meanwhile, Theodor said North America was once home to an incredible diversity of mammals over different times, including primates, camels, and a much wider array of ungulates than exists today.
“There were more hoofed leaf-eating mammals around 17 million years ago in North America at single fossil sites than there are in any African ecosystem today,” she said.
Many of the large mammals from the last Ice Age also died out, such as mammoth, mastodon, and saber-tooth cats.
“They’re all extinct now, but they used to be normal components of the North American ecosystem,” she said.
“Diversity was good until right to the end of the Ice Age and then extinctions went way up and outpaced origination of new species.”
Theodore said as earth is in another period of rapid global warming, North America is still recovering from a “mass extinction.
“Even our baseline is fundamentally altered from what North America was like the very last 65 million years,” she said. “The more we break up habitat, in combination with the amount of CO2 we’re producing, the worse this is going to get.”
