Supersized superbunny

It may not be Harvey (it’s visible), but paleontologists have found fossils of a giant rabbit — the largest ever described — on the Mediterranean island of Minorca.

Three to 5 million years ago, a rabbit species there grew about half a meter high with an estimated weight of 12 kilograms (about 26 pounds), researchers report in the March Journal of Vertebrate Paleontology. Six times the size of today’s wild European rabbit, the hefty extinct species outweighed not only all known rabbits, but all species in the broader group of rabbits, hares and pikas, say paleontologist Josep Quintana Cardona of Minorca and his colleagues.

The study of island fauna is interesting from a scientific point of view, Quintana Cardona says, because one can see in an especially clear way on islands how organisms adapt to the environment they live in, no matter how difficult it may be.

The new hunk of a rabbit, now named Nuralagus rex, shows the kind of unusual turn that evolution can take on islands. “Gigantism happens,” explains Brian Kraatz of Western University of Health Sciences in Pomona, Calif. When pioneer animals start colonizing an island, rates of evolution typically speed up at first, he explains. Small creatures can supersize, and big ones can shrink.

It’s particularly exciting to see a huge rabbit, since rabbits and their kin are fairly similar to one another compared with the diversity of body forms in closely related groups like rodents, comments paleontologist Lucja Fostowicz-Frelik of the American Museum of Natural History in New York City. Not a coauthor of the new paper, she studies the evolutionary history of rabbits and their relatives. “It works,” she says. “Despite the fact that they stick to one pattern, for over 40 million years they were quite abundant and widespread animals.”

So far no plausible rabbit-eaters have turned up among fossils from the same stretch of time on Minorca, so the big bunnies could have evolved larger and larger body size without pressure to maintain speed and agility to escape predators. The relatively short, stiff spine of the fossils suggests that Nuralagus didn’t hop much, if at all, say Quintana Cardona and colleagues. They describe its pace as  “low-gear walking.”

Also reflecting the island effect, two of the rabbit’s neighbors grew to considerable size: a Cheirogaster land tortoise and a Muscardinus species that was a relative giant among dormice.

Island evolution without predators to watch out for may explain some of the other odd features of Nuralagus. Its skull suggests a smaller brain relative to its body size. The space for eyes also looks skimpy, and it probably didn’t have tall, Easter-bunny ears.

What the rabbit king of Minorca did have were paws adapted for digging, a help in finding food to sustain its regal size. So far, though, there’s no sign of giant carrots.