Scientists find a reason rabbits aren't as big as a horse
If you've ever wondered why the rabbits in your garden aren't as big as horses, wonder no more. According to a team of scientists led by Kyoto University's Primate Research Institute, competition with larger, hoofed herbivores puts a cap on bunny size.
In 1972, MGM released the low-budget horror feature called Night of the Lepus that asked the question, are giant rabbits scary? The answer is, no, not at all. Even if magnified to the size of a Buick, they still come off as cute and cuddly.
Fast forward to 2021 and the Kyoto University team asked another question, why aren't rabbits gigantic? At first that may seem an odd thing to ask, but there are very serious reasons for doing so.
If you look at many animal groups, you find a wide range of sizes. Domestic dogs, for example, can range from teacup poodles to Saint Bernards, and there are many other groups that routinely include dwarfs and giants. Even sloths can grow to gigantic size.
However, the order Lagomorpha, which includes rabbits, doesn't get very big at all. This is especially perplexing because rabbits are relatives of rodents, which include 4-g (0.14 oz) pygmy mice and 50-kg (110 lb) capybaras. Why do rabbits remain so small? Why aren't they big enough to saddle?
"The largest living wild lagomorphs weigh only about 5 kg (11 lb) on average, a tenth of the largest living rodent, the capybara," says team leader Susumu Tomiya. "But some breeds of domestic rabbits and other extinct species can weigh up to 8 kg (18 lb). We were surprised by this, and so began to investigate what sort of external forces keep wild lagomorphs across the world from evolving larger body sizes."
To find out, the Kyoto team looked at the evolution of lagomorphs in the fossil record in North America, which they combined with an analysis of ecological factors. What they found was a link between size and competition with ungulates, or hoofed herbivores. When lagomorphs become larger than 6 kg (13 lb), they become less energy efficient than the ungulates, giving the latter a competitive advantage.
As a result, the smallest ungulates in an area control how big the largest lagomorphs can grow, which has consequences outside of big bunnies.
"An ongoing debate in evolutionary biology concerns whether biological or environmental processes are more important in shaping biological diversity, as characterized by the 'red queen' and 'court jester' hypotheses," says Tomiya. "For some time, the court jester model – ascribing diversity to abiotic forces such as the climate – has been dominant, due to the difficulty of studying biological interactions in the fossil record."
The team says their work shows that, within the larger context of physical influences, interactions between living organisms play a vital role in guiding the evolutionary paths of some species.
The research was published in Evolution.