Animal diet and body size have a U-shaped relationship

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It’s been several decades since scientists noticed the U-shaped relationship between diet and body size seen in modern land mammals: if you align mammals on a plant-protein gradient, you’ll find that herbivores and carnivores (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 studyrather, it is nearly universal among 24,000 species of vertebrates, including mammals, birds, reptiles, and saltwater fish.

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

But the researchers suggest that human-related extinctions of larger herbivores and carnivores are disrupting this U-curve – which 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?

This is due to the intertwined influences of diet and body size.

A species’ diet determines the amount of energy it consumes, which in turn helps stimulate growth and dictates its size. But this size can also limit the quality and quantity of food available to the species, although it simultaneously sets thresholds for what is needed to survive.

“You can be as tall as your food will let you,” says Will Gearty, postdoctoral researcher at the University of Nebraska-Lincoln (UNL) in the United States and co-author of the study published in Nature ecology and evolution. “At the same time, you’re often as big as you need to catch and process your food.

“So there’s an evolutionary interaction there.”

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

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

A figure illustrating the U-shaped relationship between diet and body size (or mass, in kilograms) in land mammals. The gray parts of the bars represent species currently threatened with extinction, the white parts representing species that have already disappeared. Credit: Nature Ecology and Evolution / Springer Nature

“Showing that it exists in all these different groups suggests that it’s something fundamental about how vertebrates acquire energy, how they interact with each other, and how they coexist,” says the 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 lasts, so they analyzed the fossil record of 5,427 species of mammals – some of which date back to the early Cretaceous ago. 145 to 100 million years old – and found that the pattern dates back at least 66 million years.

But the U-curve has begun to flatten noticeably, as the average size of herbivorous mammals has shrunk about 100 times, and carnivores 10 times, since the emergence of Neanderthals and Homo sapiens over the past hundreds of thousands of years.

This is due to the extinction of several species, including the disappearance of mammoths, ground sloths, short-faced bears and saber-toothed cats.

Looking to the future, the research team predicts there is a greater than 50% chance that several large and medium-sized mammals – including the tiger and the Javan rhino, both of which include humans as their only predators – disappear over the next 200 years.

This does not bode well, as these predicted extinctions would only exacerbate the disruption of the U-Curve and have unpredictable consequences for wildlife and humans.

“It’s certainly possible that by removing some of these animals from the top (of the U-curve) and reducing some of these ranges of body sizes, we’re changing the way the energy is distributed,” Gearty said. said. “It could possibly have fundamental implications for the environment and the ecosystem as a whole.”



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