Give Felisa Smith a time machine and she’d know exactly which era to visit, no questions. She’d set the dial back 15,000 years or so and head straight to modern-day New Mexico.
The scene in front of her would be unrecognisable to most of us today. On the plains around her, huge ground sloths would graze on fruits and plants, while armadillos the size of VW Beetles would stalk around on their squat limbs, avoiding the huge bison, camels, and multiple horse and pronghorn species.
And that’s just the herbivores. Late Pleistocene-era New Mexico was also home to a fearsome array of predators, including short-faced bears that could look a man in the eye while standing on all fours, then outrun a horse over short distances. Keeping a close eye on the browsing herbivores, a smattering of big cats – sabre-toothed smilodons and homotheriums, cave lions and cheetahs – would prowl the grasslands.
“North America would have supported a mammal fauna arguably more diverse than Africa today – it was a vastly different place,” say Smith, a paleoecologist at the University of New Mexico. To Smith, who spends her time trying to uncover the mystery surrounding the rise and fall of these terrestrial giants, the scene would be akin to seeing her career spring to life in front of her.
But in just a few thousand years – a blink of an eye, geologically speaking – all of them would be gone. Seventy-five giant species simply ceased to exist, causing the mean average mammal size to plummet from 98 kilograms to just 7.6 kg. In effect, the average American mammal was shrinking, and humans were to blame. And if we don't take action to stop the few giant animals we have left from going extinct, in 200 years the largest animal on Earth will be the humble domestic cow.
“When humans got here the megafauna just didn’t know what hit them – they had never seen anything like that,” says Smith. The American extinction is just one part of a much wider trend Smith has identified that took place across the globe from around 125,000 years ago. Wherever humans or their ancestors went, the average animal size plummeted.
Smith’s study, published in the journal Science, firmly points the finger of blame at hominins – the group that includes modern humans and their ancestors. In the 100,000 years after hominins migrated across modern-day Europe and Asia, the mean mass of land mammals dropped by half. When they migrated to Australia, around 50,000 years ago, the average mammal size there dropped by a factor of ten. “As we move out of Africa, we’re increasingly becoming more sophisticated and the extinction was taking a much shorter amount of time,” Smith says.
By the time homo sapiens made it to the Americas they were equipped with long-range throwing tools. The animals didn’t stand a chance. In the following few thousand years, North America lost 11.5 per cent of its non-flying terrestrial species while 9.7 per cent of South American species died out. The last time the world had seen such disruption of animal body sizes was 66 million years ago, when the dinosaurs were wiped out.
It’s not that hominins were only killing large animals, but they did have a certain predilection for hunting large prey. The average animal that went extinct in the Americas during this period weighed one tonne, about the same as a giraffe.
For hominins, large animals would have presented an almost irresistible target, says Todd Surovell, an anthropologist at the University of Wyoming. “Big animals are a lot of food – they provide you with loads of meat. Simply put, they provide the most bang for your buck,” he says. Although there’s a high degree of risk involved with hunting very large prey, armed only with throwing spears, the payoff seems to have usually been worth it. Surovell estimates that around 70 to 80 per cent of the meat eaten by early humans in the Americas came from megafauna.
Hunters bringing back a wooly mammoth carcass would also find themselves in possession of a useful bargaining tool. “If you kill a mammoth there’s far more meat than you can personally exploit, even with your family, so by sharing the results of your hunt you can gain favour with your kin and your non-kin.”
Although hunting was probably the main cause of megafauna extinction over this time period, it’s likely that hominins also killed some large animals by destroying their habitats. In Australia, hominins often burned back vegetation in order to improve foraging efforts, and this may have had the side-effect of reducing the amount of food available to animals, leading to slow declines in their numbers.
Smith’s study is mostly concerned with long-dead mammals, but biologists are also debating whether other animals today are still at risk of shrinking. One line of argument suggests that as global temperatures increase, the body size of mammals will decrease.
This all comes down to something called Bergmann’s rule, an ecological theory first described by the German biologist Carl Bergmann in 1847. The general idea is that, for animals within a particular genus or species, the further north they are found, the larger that animal tends to be.
“The idea is that this all has to do with temperature,” says Abigail D'Ambrosia Carroll at the University of New Hampshire in the US. Large animals tend to be good at conserving heat because they have a relatively small surface area in comparison to their size, which makes them well-suited to colder climates where heat conservation is essential. Small animals have the opposite problem. They need to get rid of heat, and their high surface area to volume ratio provides plenty of skin surface through which they can give off all that excess heat, making them suited to warmer environments.
The red fox, found all the way from the Arctic Circle to the southern tip of the Arabian Peninsula is the living embodiment of Bergmann’s rule. In the Arctic, it is plump and about the size of a dog while in hotter climates it’s an incredibly skinny almost feline creature with disproportionately huge ears. Simply put: bigger foxes live in colder climates.
Now biologists are trying to figure out if Bergmann’s rule applies to the fourth dimension: time. As an area heats up, do the animals that live there slowly get smaller too? The evidence for this is mixed, says Daniel Naya, an ecologist at the Universidad de la República in Uruguay. His analysis of 17 rodent species and a further 17 mammal species didn't find a strong link between temperature and the size of animals. There are some indications that animals get smaller – in the rodent study about half of the species got smaller while the other half didn’t change – but it’s not at all clear whether that change is down to temperature alone, or some other factor, such as food availability.
But the fossil records of our earliest horse ancestors tell another story. D’Ambrosia Carroll has studied the impact that two global heating events had on the size ancient horse species. In the first, around 56 million years ago, global temperatures increased by between five and eight degrees celsius in the space of between ten and 100,000 years. This is the perfect test case for Bergmann’s rule in the fourth dimension.
The horse ancestors D’Ambrosia Carroll studies are nothing like our modern day versions. Stumpy, with toes instead of hooves, Sifrhippus was around the size of a smallish dog. After tens of thousands of years of cooling, it became much smaller until the average animal was the size of a cat. “Horses over that time dwarfed by 30 per cent,” says D’Ambrosia Carroll, but as soon as temperatures started dropping again, their size went back up.
The same thing seemed to happen in a slightly smaller heating event where temperatures rose by between two to three degrees. In that case, the horses dwarfed again but only by 14 per cent. “This suggests that there’s some relationship between the warming event and the extent of the dwarfing,” D’Ambrosia Carroll says.
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The reason behind this dwarfing isn’t exactly clear. It could be a case of Bergmann’s rule in the fourth dimension, as horses with larger body sizes found themselves less likely to produce offspring in the newer, hotter climate, so over time populations became smaller. It’s also possible that this was a bog-standard case of Bergmann’s rule, and the size change was caused by larger horses moving a bit further north while smaller horses filled in the gaps they left behind.
The problem is that there’s no way for us to know that this change in body size was because of the change in temperature, and didn’t just happen at the same time as the temperature increase. Even for cold-blooded species – animals where their metabolism is directly linked to the environmental temperature because they can’t control it themselves – there isn’t a clear link between size and temperature change.
But that doesn’t mean that we shouldn’t stop searching for the link between animal size and temperature, says Jennifer Sheridan at Yale-NUS College in Singapore. If animals get smaller at different rates, then this could send ecosystems out of kilter as temperatures increase. “We don’t really have a good sense of what percentage of organisms are going to get smaller,” she says. “There are so many unknowns, but it is important to consider because it can impact ecosystems balances.”
Whether it’s hunting or climate change, it seems that humans – and our ancestors – have being playing god over animal size for most of our existence. “Once we became hunters we started to have very deleterious effects on these animal populations,” says Surovell. For Smith, and her study of some the strangest and most awe-inspiring creatures nature has ever created, climate change is just the last chapter in a long history of hominin-induced chaos. “The thought that largest thing in 200 years would be a domestic cow is really kind of frightening.”
This article was originally published by WIRED UK
