Repost: Night of the Nuralagus rex

[This post was originally published on March 24, 2011]

Giant bunnies are not scary. MGM learned that the hard way with the 1972 schlock film Night of the Lepus. No amount of fake blood or artificial saliva could turn those rabbits into real monsters, and the brief moments when actors in bunny suits attacked their co-stars looked like some kind of “Meet the Easter Bunny” mall event gone horribly awry. (“Here comes Peter cottontail, and he’s headed right for us! Run!”)

The stilted acting, the harebrained plot, the hordes of rabbits smeared with goo – Night of the Lepus was simply awful, but there is one subtle aspect of the film that, as a natural history nerd, really bugs me. The giant rabbits look too much like little rabbits. Sure, the best way to make a movie about enormous, bloodthirsty bunnies is to put a few normal-sized ones on miniature sets, but this little trick steps around the fact that there are constraints that cause organisms to change in shape as they change in size. A real-life giant rabbit recently discovered in the fossil deposits of Minorca, Spain might provide us with a better model of what an enormous “Lepus” would look like.

Sitting smack in the middle of the Mediterranean just south of Spain, the island of Minorca was once home to a unique assemblage of animals that evolved in splendid isolation. Minorca was a kind of a natural, evolutionary experiment station – just as Madagascar was for lemurs, Hațeg Island was for dwarf dinosaurs, and Flores was for tiny humans and ground-dwelling storks – and so the assemblage of animals that once lived upon it was a mish-mash menagerie whose ancestors had fortuitously arrived there. Between three and five million years ago, Minorca hosted a varied group of organisms mostly made up of birds and small reptiles, with only two very large animals – the giant tortoise Cheirogaster gymnesica, and the huge, newly-described rabbit Nuralagus rex.

What we know of the titano-bunny comes from assorted bits and pieces locked within hardened slurries of bone and rock that filled fissures on Minorca like a kind of fossil concrete. To extract the bones, paleontologists Josep Quintana, Meike Köhler, and Salvador Moyà-Solà dissolved the red limestone surrounding the bones using acetic acid, and they wound up with elements representing almost the entire skeleton of Nuralagus rex. The creature was certainly a rabbit, perhaps a descendant of the species Alilepus turolensis from prehistoric Spain, but quite different from your garden-variety leporids.

Using the available bits and pieces from multiple individuals, Quintana and co-authors estimated Nuralagus to average about twenty six pounds. This would make Nuralagus about six times more massive than its living cousin, the common European rabbit, but the difference was not in size alone. A rabbit can’t grow to be twenty six pounds or more without a few tweaks to its anatomy.

Nuralagus was a relatively inflexible and bulky animal. For one thing, the giant probably couldn’t hop like smaller rabbits do. The lumbar region of its back – the part of the spine behind the ribs, before the hips – was short and relatively stiff for support, which the describers of Nuralagus write “reflects a low-gear mode of locomotion and reduced leaping capabilities.” This is consistent with the rabbit’s limb anatomy.

Compared to a European rabbit, Nuralagus had splayed fore- and hindfeet. This arrangement spread out the rabbit’s bulk over a wider area and reduced the amount of shock to its joints. Likewise, the place where one of the lower arm bones – the ulna – articulated with the wrist was positioned in such a way that the rabbit’s whole forefoot would have contacted the ground, rather than just the tips of the toes as in other rabbits. Lacking the leaping abilities of other species, Quintana and colleagues suggest that Nuralagus used its spread, curved fingers and robust arms to dig for roots and other underground foods.

Clues as to how such a large rabbit could have evolved at all can be seen in the skull. For a giant rabbit, Nuralagus had a relatively small head with small eyes, and the ear holes on the outside of the skull are no larger than they are in European rabbits. Small brain, small eyes, small ears, and a husky body – this was not a rabbit that was well-suited to quickly detecting and escaping danger.

Nuralagus didn’t have to. Based upon the fossils found so far, there were no large predators on the Minorca during the time Nuralagus was alive. Fossils of barn owls have been found in the same deposits as the big rabbit, but by then the huge rabbits were far too large for raptors to fly off with. A virtually predator-free island provided a setting in which the adaptations so typical of rabbits could be modified, and this has happened to rabbits more than once.

On some of Japan’s Ryu-Kyu Islands, there lives a leporid called the Amami rabbit. The living descendant of a rabbit subgroup that radiated through Asia a little before Nuralagus evolved on Minorca, the Amami rabbit also evolved on islands that lacked major predators and it shows some surprising similarities to Nuralagus. Though not as large, the Amami rabbit is a stoutly-built species with small ears and relatively splayed toes. The bigger size of Nuralagus required some differences in how it supported itself, but the general parallels between it and the Amami rabbit are remarkable.

Why two distantly-related rabbits became adapted in similar ways remains a mystery. Even though they both evolved from typical rabbit ancestors – shallow convergence, rather than deep convergence as when two drastically different forms converge in anatomy – the reason for this kind of parallelism is an unresolved question in evolutionary biology. What is it about isolated island environments that causes repeated instances of convergent evolution? As 19th century naturalists such as Charles Darwin and Alfred Russell Wallace so beautifully demonstrated, life on islands can tell us much about how evolution works, and perhaps the study of elephant birds, dwarf mammoths, giant rabbits, and other seemingly aberrant island creatures will yield further secrets about evolution’s grand pattern.

References:

Quintanaa, J.; Köhler, M.; Moyà-Solà, S. (2011). Nuralagus rex, gen. et sp. nov., an endemic insular giant rabbit from the Neogene of Minorca (Balearic Islands, Spain) Journal of Vertebrate Paleontology, 31 (2), 231-240 : 10.1080/02724634.2011.550367