U-shaped relationship between diet and body size is nearly universal among animals

It’s been several decades since scientists noticed the U-shaped relationship between diet and body size observed in modern land mammals: if you align mammals on a plant-to-protein gradient you’ll find that herbivores and carnivores (on the far left and right, respectively) tend to be much larger than omnivores and insectivores.

Now, we know that this relationship is more widespread than previously thought. According to a new study, it’s instead nearly universal across 24,00 vertebrate species – including mammals, birds, reptiles, and saltwater fish.

The pattern also holds consistently across global ecosystems, from rainforests to deserts, and even dates back to at least 66 million years ago.

But the researchers suggests that that human-related extinctions of the largest herbivores and carnivores are disrupting this U-curve – what appears to be a fundamental feature of past and present ecosystems – with potentially unpredictable consequences for the future.

What causes this relationship between diet and body size?

It’s due to the intertwined influences of diet and body size.

A species’ diet determines how much energy it consumes, which in turn helps drive growth and dictates its size. But that size can also limit the quality and quantity of food available to the species, even as it simultaneously sets thresholds for what’s needed to survive.

“You can be as big as your food will allow you to be,” says Will Gearty, a postdoctoral researcher at the University of Nebraska-Lincoln (UNL) in the US, and co-author of the study published in Nature Ecology & Evolution. “At the same time, you’re often as big as you need to be to catch and process your food.

“So there’s an evolutionary interplay there.”

The international and interdisciplinary team of scientists compiled diet and body size data for an impressive number of modern surviving species: 5,033 mammals, 8,991 birds, 7,356 reptiles and 2,795 fishes.

For the first time, they found that the U-shape pattern between diet and body size held for almost all species, but was absent in marine mammals and seabirds, probably due to the unique demands of living in water.

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A figure illustrating the U-shaped relationship between diet and body size (or mass, in kilograms) among land-based mammals. The gray portions of the bars represent species currently under the threat of extinction, with the white portions accounting for species that have already gone extinct. Credit: Nature Ecology and Evolution / Springer Nature

“Showing that this exists across all these different groups does suggest that it is something fundamental about how vertebrates acquire energy, how they interact with one another, and how they coexist,” says co-author Kate Lyons, assistant professor of biological sciences at UNL.

The shape of things to come

The researchers were also interested in how long this U-shaped relationship between diet and body size has persisted, so they analysed fossil records from 5,427 mammal species – some of which date as far back as the Early Cretaceous Period 145–100 million years ago – and found that the pattern stretches back at least 66 million years.

But the U-curve has begun to noticeably flatten, as the average size of mammalian herbivores has decreased by roughly 100 times, and carnivores by 10 times, since the emergence of Neanderthals and Homo sapiens over the past few hundred thousand years.

This is due to multiple species extinctions, including the disappearance of mammoths, ground sloths, short-faced bears, and sabre-toothed cats.

Looking to the future, the research team projects that there’s a greater than 50% chance that multiple large- and medium-sized mammals – including the tiger and Javan rhinoceros, both of which count humans as their only predators – will go extinct within the next 200 years.

This doesn’t bode well, as those predicted extinctions would only exacerbate the disruption of the U-curve and have unpredictable consequences for wildlife and humans.

“It’s certainly possible that as we take some of these animals off the top (of the U-curve), and as we collapse some of these ranges of body sizes, that we’re altering the way the energy is divvied up,” Gearty says. “That could perhaps have fundamental repercussions for the environment and ecosystem as a whole.”

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