From elephants to tiny shrews, throughout evolution, plant-eating mammals have twice the belly of meat eating predators.
Four-legged vertebrates, otherwise known as tetrapods, have come in all shape and sizes as they adapt to their surroundings and food sources. They also ranged from pure herbivore to fierce carnivores with their bodies reflecting their dietary choice.
As plants are usually more difficult to digest than meat, herbivores are thought to need larger guts and more voluminous bellies. As they grind and masticate their food, they need longer guts to help digest it while meat eaters found having trimmer tums gave them an evolutionary advantage.
Yet this hypothesis had never been tested scientifically, and a new study surprisingly showed that this only applied to mammals, not dinosaurs.
Professor Dr Marcus Clauss of the University of Zurich said: "In broad terms, the diets of herbivorous animals are less easily digested than those of carnivores, and require both the presence of a large number of symbiotic gut microbes and time for these microbes to perform their digestive function.
"Therefore, in order to accommodate this large microbiome, and to delay digesta passage, the gastrointestinal tract of herbivores are typically considered to be particularly long and or voluminous.
"Differences in the length of the intestinal tract according to diet have been repeatedly shown for fish, lizards, and in other animal lineages such as invertebrates, but not convincingly in birds. In mammals, similar evidence is questionable and mostly limited to small body sizes."
European researchers headed by the University of Zurich and the Technical University Berlin studied the shape of the rib cage in more than 120 tetrapods - from prehistoric times up to the present day. With the aid of photogrammetry and computer imaging techniques, the scientists produced a 3D database for skeletons of dinosaurs, reptiles, birds, mammals and fossil synapsids (mammal-like reptiles).
Armed with this data they reconstructed the volume of the body cavity, which is delineated by the spinal column, the rib cage and the pelvis. The study found on average, herbivorous mammals have a body cavity that is twice as big as carnivores of a similar body size.
Prof Clauss said: "This is clear evidence that plant-eating mammals actually have larger guts.We were amazed that there wasn't even the slightest indication of a difference between herbivores and carnivores in dinosaurs."
The study was published in the Journal of Anatomy