[Animals] are shrinking. Blame climate change.

[Love Pulse]

 

This story is part of Down to Earth, a Vox reporting initiative on the science, politics, and economics of the biodiversity crisis.

One fall morning in 1978, David Willard, an ornithologist at the Field Museum in Chicago, walked to the nearby McCormick Place convention center — a hulking structure along Lake Michigan — to look for dead birds. He'd received a tip that birds were crashing into the building's many windows on their journey south.

He found a few birds lifeless on the concrete that morning. And as any good scientist might do, he brought them back to the museum to measure them and store the winged creatures in the museum's collection. His curiosity piqued, he returned to McCormick Place the next morning. He still found more birds and brought them, too, back to the museum.

Four decades later, Willard has helped collect more than 100,000 birds from window collisions in Chicago, with help from other scientists and volunteers. They now make up a stunning 20 percent of the museum’s ornithology collection.

Sign up for The Weeds newsletter
Vox's German Lopez is here to guide you through the Biden administration's burst of policymaking. Sign up to receive our newsletter each Friday.

While these birds represent a tragic loss of life, they've also helped reveal fascinating insights into how wildlife is changing. One especially striking finding from the collection is that these birds have been shrinking — and rising global temperatures are likely to blame, according to a 2019 analysis of Willard's measurements.

It's not just birds. A growing body of research suggests that global warming is messing with the body sizes of all kinds of creatures, from cold-blooded frogs to warm-bodied mammals, and often making smaller animals.

Wild animals are already facing a wide range of threats. If they shrink — and especially if they shrink at different rates, as researchers predict — that could push some species even closer to extinction. And it could throw a wrench into ecosystems that humans rely on.

The idea that warming is linked to smaller body sizes is also borne out by fossil evidence. During the largest warming event in the early Eocene, about 56 million years ago - when temperatures rose between 5 to 8 °C within 10,000 years - many animals became smaller, including mammals (which scientists learned by measuring fossilized teeth) . Another previous warming occurred called Eocene Thermal Maximum 2, which saw temperatures rise by 3°C, and was also linked to the animals shrinking.

Scientists are looking to these warming periods to understand what the future might look like. If current warming continues, we can expect the planet to warm by 1.5°C by 2040, compared to pre-industrial levels. And he will go up from there. "Reductions in body size in fossils are 'particularly informative of what we can expect in the next century,'" two ecologists wrote in a 2011 perspective on natural climate change.

Then again, nature is complex and tends to surprise even the smartest of minds.

Many animals are getting smaller, but it's not a global trend
In 2019, when scientists examined more than 70,000 bird specimens in the Field Museum's collection, they found that individuals from 52 bird species shrank an average of 2.6 percent between 1978 and 2016. It also became smaller on average.

said Brian Weeks, lead author of the study and assistant professor of ecology and evolution at the University of Michigan. "It has big implications for what happens in the future."

Other studies of birds, deer, rodents, insects, and fish show similar patterns. Research in 2017, for example, found that the body size of a small silverfish called menhaden, which is widely used for animal feed and bait, has shrunk on average by 15 percent over the past 65 years — likely due to a high temperature. Eugene Turner, study author and professor at Louisiana State University: "As the Earth's atmosphere and oceans continue to warm, it appears that the future of menhaden will be smaller."

What's interesting is that fish and other so-called ectotherms don't generate their own heat, so having a smaller body doesn't help them stay cool. Instead, it may be shrinking in response to rising temperatures for other reasons, said Jennifer Sheridan, assistant curator of amphibians and reptiles at the Carnegie Museum of Natural History and lead author of 2011 Perspective. Warm temperatures, for example, speed up the development stages of frogs, from eggs to tadpoles and so on, she said, but their growth rate does not keep up. As a result, they are smaller when they reach adulthood.

Dead beetles are stacked in rows in boxes in the drawer of the collection cabinet.
A 2018 study used these and other specimens from the University of British Columbia's Petty Biodiversity Museum to determine that beetle sizes are decreasing in response to climate change. Courtesy of University of British Columbia
But while there are plenty of examples that fit this trend, there are also many exceptions — many that we still don't know.

If Bergmann's rule were truly global, po[CENSORED]tions of species would be expected to be smaller in the warmer parts of its range — smaller polar bears in the south, for example. But a 2017 analysis of more than 950 species of birds and mammals found that "most species have similar sizes regardless of their environment temperature."

There is evidence that some animals get older, Sheridan said. It's not entirely clear how that happens, but one explanation is that warming hampers winter and lengthens the growing season, allowing plant-eating animals to reproduce. (Sheridan also said that museums with specimen collections are more commonly found in temperate and richer regions, which can lead to data gaps.)

 

The point is that natural systems are really complicated. Even if theory and lab research suggests that animals consistently shrink under warming, the exact result of climate change is messier in reality, she said. “It's almost always the case that some are getting smaller and some are not,” Sheridan said. “With climate change, there are so many other factors changing at the same time.”

There are also unanswered questions about how, exactly, animals are shrinking. The big one is whether body-size changes result from natural selection — meaning, they're passed down from one generation to the next — or occur within the lifetime of a single animal, which researchers call “plastic” changes.

What happens when animals miniaturize
These details aside, researchers are confident that global warming will mess with the size of animals and make many of them smaller. But is that a problem?

You could argue that Earth has been here before. It's gone through major periods of warming, and many animals were able to adapt to drastic changes — birds, after all, evolved from dinosaurs millions of years ago, and there are some 10,000 to 18,000 species of them today. But then again, today is nothing like the Eocene. We're warming the planet at an unprecedented rate — about 10 times faster than the average warming following ice ages — which means most animals have little time to adapt. “The idea that they're going to happily evolve is an oversimplification,” Weeks said.

And to be clear: Shrinking comes at a cost. For many species, a smaller size translates to fewer babies, said Sheridan. “The fact that they’re smaller has implications for their future reproduction, which, in turn, has implications for po[CENSORED]tion size,” she said. “This is one of the reasons why people care so much about body size.”

For some ectotherms, including amphibians, being small also makes you more likely to dry out during a drought. Body size has implications for species that have evolved specialized body types for long migrations. (Interestingly, the 2019 study on migratory birds found that their wings are actually getting longer, likely to compensate for their smaller body size, Weeks said.)

But perhaps more concerning is that warming will change body sizes in different ways for different species, said Sheridan, and that can screw up the relationships between animals. For example, if a predator shrinks more slowly than its prey, it might need more prey to fill its stomach in a warming world.

“If everything was getting smaller at the same rate, I don't think it would be that big of a deal,” Sheridan said. There would still be consequences, she said, such as higher extinction risks for some species, but you would likely just have “a miniaturized ecosystem that is still functioning because all of the elements are still in proportion to one another.” It's the mismatch, she said, that's “extra worrisome.”

Debates about the role of warming in shrinking animals are ongoing. Sheridan, for one, is working on an update of her 2011 article that will include more data, and it includes more exceptions to the rule. And Weeks is working to understand, among other things, whether the changes in body size he observed in birds were produced through evolution or during their lives: “If you warm up birds while they’re developing, do they actually get smaller?”

Meanwhile, David Willard, now collects manager emeritus at the Field Museum, is still spending some of his time looking for birds. Lately, there haven't been many to collect, he said, because the bright lights at the convention center that can attract and disorient birds have been off for much of the pandemic. That's one bit of good news that has come out of his research: It's possible to save the lives of birds just by turning off the lights.