The Evolution of Rhino Arthritis in the Cenozoic

Arthritis is one of the most common skeletal pathologies, occurring in one-sixth of humans. Rhinocerotids provide a natural system for understanding the evolutionary underpinnings of arthritis. The severity and prevalence of arthritis in Rhinocerotidae increased substantially from 50 million years ago to the present. All five living species of rhinoceros develop arthritis before they reach maturity. Fossil rhinoceros relatives from 50 million years ago (Ma) show a dramatically different pattern of arthritic development. What changed from 50 Ma to today? Rhinos became graviportal, evolving from Hyrachyus, which was about the size and shape of a large dog, to the one-ton, stout-limbed animals of today. Despite this order of magnitude increase in size, rhinos also consistently display cursoriality (the habit of running) through time. These competing factors of increasing size and cursoriality provide a possible driver for the prevalence of arthritis. This study traces the history of arthritic development in the rhino lineage, finding that the distribution of arthritis is related to increasing body size, but that there are also clearly evolutionary effects determining its prevalence. This study is especially important because it examines an apparent pathology that persisted and even worsened despite millions of years of evolution that should have selected against it.