The relationship of food size can be seen throughout time, in many groups of vertebrates

The U-shaped relationship between diet and body size in modern terrestrial mammals may also mean ‘universal’, according to a new study, which found that the relationship lasts at least 66 million years and a diverse group of vertebrate animals.

It has been decades since ecologists realized that planning the size of the food relationship in terrestrial mammals would provide a U-shaped curve when these mammals were aligned on a plant-protein gradient. As illustrated by this curve, the herbivorous herbivores on the far left and the carnivorous carnivores on the far right are likely to grow to a larger size than all the omnivores and invertebrate-eating invertivores in the middle.

Currently, however, there is hardly any research looking for the pattern beyond mammals or modern times. In a new study, researchers from the University of Nebraska-Lincoln and institutions on four continents concluded that the pattern actually dates back to ancient times and applies to land birds, reptiles and even fish from salt water.

But the study also suggests that human-related extinction of larger herbivores and carnivores has disrupted what appears to be a fundamental part of past and present ecosystems, with potentially unpredictable consequences.

“We don’t know what’s going to happen because it hasn’t happened before,” said Will Gearty, a Nebraska postdoctoral researcher and co-author of the study, published April 21 in the journal Ecology of nature and evolution. “But since the systems have been in such a stable state for a long time, it’s about what could happen if they leave that state.»

Size up, size down

The evolutionary and ecological stories of animal species can be told in part through the interconnected influences of diet and size, Gearty said. The diet of a species will determine its energy consumption, which stimulates growth and ultimately helps dictate its size. But this size can also limit the quality and quantity of food available to a species, even if it sets the standards for the quality and quantity needed to survive.

“You can be as big as your diet allows,” Gearty said. “At the same time, you’re always as big as you need to get and process your food. So there’s an evolutionary interaction there.»

Because the diet of plant -based herbivores is relatively poor in nutrients, they can often be large enough to cover a lot of soil to search for more food – and to accommodate the long, complex digestive tracts that take up most nutrients from them. Carnivores, on the other hand, need to grow large enough to track and eradicate these herbivores. Although omnivores ’buffet-style menus often fill their stomachs, their high energy needs often focus on nuts, insects, and other small, hearty foods. And while invertivores primarily enjoy protein-rich prey, the small nature of this prey, combined with fierce competition from many other invertivores, relegates them to the smallest size of all. .

The end result: a U-shaped distribution of mean and maximum body size in mammals. To analyze the generalizability of this model in modern times, the team compiled body size data for several living species: 5,033 mammals, 8,991 birds, 7,356 reptiles and 2,795 fish. .

Even in the absence of the pattern of marine mammals and sea birds, likely due to the unique needs of aquatic life, they can be found in other vertebrate groups – reptiles, marine fish, and land birds. – reviewed by the team. The pattern is set even in different biomes – forests versus grasslands versus deserts, for example, or the tropical Atlantic Ocean versus the moderate North Pacific – when analyzing terrestrial mammals, land birds, and fish in the water.

“Showing that it is present in all of these different groups suggests that it is something fundamental about how vertebrates acquire energy, how they interact with each other, and how that way they coexist, ”said co-author Kate Lyons, assistant professor of biological sciences in Nebraska. . “We don’t know if it’s necessary – there may be other ways to organize vertebrate communities in relation to body size and diet – but it’s enough.”

But the researchers also wanted to know how long the U curve was. So they examined the fossil record of 5,427 mammal species, some of which date back to the first Cretaceous 145 to 100 million years ago. Lyons and his colleagues originally collected fossil data as part of a 2018 study on the extinction of large mammals at the hands of humans and their new ancestors.

“To my knowledge, this is the most comprehensive study of the evolution of body size and especially the diet of mammals over time,” Gearty said.

He found that the U-curve dates back at least 66 million years, when non-avian dinosaurs were recently extinct, but mammals had not yet branched out into the dominant animal class. that they are now.

“It’s very interesting and very shocking,” Gearty said, “to see this relationship continue even when there are other dominant animals around.

“We suspect that it has actually existed since the creation of mammals as a group.»

The shape of things to come

After cataloging the present and past of the U-curve, Gearty, Lyons and their colleagues focus on their visions of the future, or potential lack thereof. The median length of herbivores and omnivores has decreased about 100 times since the emergence of Neanderthals and Homo sapiens over the past hundreds of thousands of years, the team reported, with the size of carnivores falling about 10 times over the same period. As a result, the U-curve that has been going on for a long time is starting to flatten, according to Gearty.

In this vein, the team predicts more than a 50% chance that many large and medium -sized mammals – including the tiger and the Javan rhino, which both count humans as their predators – will disappear within the next 200. years. These expected extinctions can only exacerbate the destruction of the U-curve, the researchers say, especially since the loss of large herbivores could cause or accelerate the disappearance of large carnivores that eat it.

“It is quite possible that by removing some of these animals from the top (of the U-curve) and reducing some of these orders in body size, we have changed the way we distribute the energy, ”Gearty said. mentioned. “It could have fundamental effects on the environment and the ecosystem as a whole.”

It is also possible, the researchers concluded, that the future reduction in mammal body size will exceed the unprecedented reduction observed over the past hundreds of thousands of years.

“You keep seeing, in the ecological literature, people speculating on how ecosystems are less resilient today, less resilient, and more vulnerable to collapse,” Lyons said. “I think that’s just a source of evidence to suggest that it could really happen in the future.»

Gearty and Lyons co -authored the study with Robert Cooke, UK Center for Ecology & Hydrology; Amanda Bates, from the University of Victoria (Canada); Abbie Chapman, of University College London; Jillian Dunic, Simon Fraser University (Canada); Graham Edgar and Rick Stuart-Smith, from the University of Tasmania (Australia); Jonathan Lefcheck, Smithsonian Environmental Research Center; Craig McClain, Louisiana Universities Marine Consortium; and Gil Rilov of Israel Limnological and Oceanographic Research.

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