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Hopping Down the Bunny’s Evolutionary Trail

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If Easter was celebrated 3 million years ago, the chocolate bunny in your basket would likely be six times larger, with stubby rat-like ears. Such was the body plan of Nuralagus rex, an ancient rabbit that roamed what is now the Spanish island of Minorca.

N. rex wouldn’t seem very rabbit-like at first blush—for one thing, it had a short, stiff vertebral column that means it wouldn’t have hopped along like its modern relatives. In fact, “N. rex would be a rather clumsy rabbit walking,” Josep Quintana, the paleontologist who discovered the giant extinct bunny, told National Geographic. “Imagine a beaver out of water.”

Other ancient rabbits would be equally strange to our eyes. The bones of a 53-million-year old rabbit ancestor found in India reveal a creature that was probably an accomplished hopper, but in this case, the creature would be not much bigger than a mouse.

A Bunny Tale: The Hoppers and the Hoofers

If the rabbit and hare lineage is so ancient, and once included very big and very small creatures, then why are rabbits today virtually all the same size? After all, the rodent group—which is closely related to the order that includes rabbits, has been evolving for just as long, and is just as widespread geographically—is incredibly biodiverse. It encompasses thousands of species from the tiny chipmunk to beavers and capybaras. In a 2014 paper presented at the annual meeting of the Society of Vertebrate Paleontology, one team of scientists fingered a potential perpetrator for the destructive influence on rabbit diversity: Ungulates, or hoofed mammals.

In their work, researchers Lauren Miller and Susumu Tomiya compared the fossil records of lagomorphs (rabbits, hares, and pikas) and ungulates in North America over 30 million years. At the beginning of that time period, big rabbits lived alongside big and small-hoofed mammals, but after a major climate shift 23 million years ago, lagomorphs started to get smaller, on average, and ungulates started to get bigger. As the hoppers and the hoofers were brought into closer competition, Miller and Tomiya theorize, lagomorphs couldn’t pay the metabolic price to maintain gigantic bodies.

“Much of the discussion about the current biodiversity crisis has revolved around saving species from extinctions,” Tomiya said in a statement. “From a paleontological perspective, we believe that it is also important to think about the future of what will turn out to be surviving lineages.”

Hooves, Humans, and Future Rabbits

The evolutionary future of rabbits will likely be as heavily influenced by humans as by ungulates. Rabbits came a bit later to the domestication game than cattle, sheep, and dogs—mostly likely beginning around the seventh century AD in French monasteries. Because so little time has elapsed since domestication, the rabbit is an ideal test subject to look at the changes wrought by domestication. In 2014, researchers compared genes from six kinds of domesticated rabbit to a wild rabbit species. They found that domestication couldn’t be pinned down to any one particular gene, but they did see that one of the greatest area of differences between the wild and the domesticated rabbit was in the brain.

“Our finding that many genes affecting brain and neuronal development have been targeted during rabbit domestication is fully consistent with the view that the most critical phenotypic changes during the initial steps of animal domestication probably involved behavioral traits that allowed animals to tolerate humans and the environment humans offered,” Carneiro and colleagues wrote in the journal Science.

So enjoy your little chocolate bunny this Easter. And if you’re disappointed that sweet treat isn’t as big as Nuralagus rex, maybe you can blame deer and horses for the difference.

Image: iStock.com/DamianKuzdak

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