Arthritis in rhinoceros How the rhino got his walking stick

The Economist Oct 31st 2012, 17:41 by M.K. | RALEIGH

IF, AS may soon sadly be the case, rhinoceroses were extinct and only their fossils could be examined, what could be found out about them? That, mutatis mutandis, is the problem facing palaeontologists who wish, as most do, to go beyond the mere description of bones and fill in details of the lives that the animals they study once led. To do so, they often draw analogies with the behaviour and ecology of existing species. But Kelsey Stilson, a student palaeontologist at the University of Oregon who really does study rhinos, has reversed this method. As she told this year’s meeting of the Society of Vertebrate Palaeontology, held in Raleigh, North Carolina, on October 17th, the grand sweep of fossil history helps to illuminate one of the minor mysteries of the animal kingdom: why rhinoceroses suffer so much from arthritis.

Pretty-well every adult rhino alive today is arthritic. The beasts have damage to their ankles and also to the points where the long bones of their limbs, the humerus and the femur, meet their sockets. One symptom of this is the formation of cysts in the bone, caused by uneven growth. Such cysts fossilise well, and Ms Stilson thus thought they could be used to look at how rhinos have arrived at their plight.

She and her colleagues looked at 2,700 specimens drawn from the past 50m years. What they saw was a remarkable pattern of ever increasing arthritis. Between 50m and 40m years ago, roughly 35% of modern rhinos’ ancestors had joints which sported cysts. By 30m years ago that had increased dramatically, to 60%. It then remained unchanged for several million years, before rising to 80% 10m years ago. This was followed by a steady increase in prevalence that led ultimately to the figure of 100% found today. It looks, counterintuitively, as though evolution is somehow favouring the spread of a chronic disease.

Which it probably is, but not because arthritis is somehow good for rhinos. What it is actually favouring is the evolution of a body plan that can get away with being arthritic in the way that, say, an antelope could not.

If you are a herbivore, as rhinos are, and thus not at the top of the food chain, evolution can take you in one of several directions. Nocturnality and crypsis, in their different ways, make you difficult to see. Or, like the antelope and the rabbit, you may develop speed. But another approach is to become big, ornery and armoured—and that is what rhinos have done. Their arthritis is both caused by this strategy, and a mark of its success. If every rhino now lives to a ripe-old arthritic age, it is because no predator dares try to kill him.

http://www.economist.com/blogs/babbage/2012/10/arthritis-rhinoceros