From flat foot to fat foot: RVC-led research reveals nature of elephants' “sixth toes”
23 December 2011
After 300 years of speculation, a study published today (Friday 23 December 2011 ) in the journal Science by a team of researchers led by staff at the Royal Veterinary College (RVC) has revealed how what was once thought of as a sixth toe in elephants is actually a remarkable example of evolution.
In 1710 Mr Patrick Blair - a Scottish doctor - published the first detailed anatomical description of an elephant, which had died near Dundee. Blair noted that the elephant had six toes, sparking three centuries of discussion around the strange toe-like structure occasionally noted in elephant's fore and hind feet.
Now, a team lead by Professor John R. Hutchinson of the RVC has confirmed that the enigmatic structures - called "predigits" - are not real toes. They are in fact enlarged, elongated versions of small, rounded, tendon-anchoring bones - called the radial or tibial sesamoids - present in many mammals. Elephants have evolved these humble little bones into massive strut-like structures that rival the size of their actual toes. Previous studies assumed they were just crude cartilaginous rods, not even bones, but Professor Hutchinson's team showed that elephants use a remarkably unique, patchy pattern of mineralization to turn the cartilage into bone as elephants grow. The predigits point backwards into the fatty 'heel' pad of elephant feet and thus are uniquely well positioned to control it. Stiffening the fat pad allows elephants to transfer some of the loads from the sole of their foot up to their wrist and ankle bones, partly bypassing the upright toes and perhaps avoiding concentration of too much force onto the real toe bones.
The research was funded by the Biotechnology and Biological Sciences Research Council (BBSRC). Foot disorders such as arthritis and injuries including bone fractures are a major health problem for many captive and domestic animals and this is a drain on the national economy as well as seriously reducing welfare. Improving our understanding of the anatomy and mechanics of animals' feet will help diagnose and prevent these painful and costly disorders.
Professor Hutchinson and colleagues gathered information from the best fossils of elephant predecessors around the world, including early, smaller, more amphibious animals and the multi-tonne giant elephant relatives such as mastodons and mammoths. They found the earlier elephants had feet that seemed to have been positioned in a much more flat-footed position with the bones more horizontal than living elephants. In contrast, all the later, more land-adapted giants had clear joint surfaces on their foot bones for what must have been large, fairly modern predigits in all four feet, and the other foot bones connected in an upright posture similar to that of living elephants.
Professor Hutchinson said: "Elephant ancestors seem to have evolved the enlarged predigits, upright feet, and fat pads about 40 million years ago as they became more terrestrial and larger. As far as we know, elephants are the only animals to use enlarged sesamoid bones in this new supportive role. Other large land mammals have lost them and correspondingly never developed a large fatty foot pad or other features unique to elephant feet. However, strange groups of mammals such as pandas and moles have evolved similar structures for other functions such as grasping bamboo or digging. These seemingly insignificant little bones thus can be a marvellous ingredient for evolution."
"Our new study exemplifies how evolution can take strange turns by tinkering with old structures to gradually use them more for new functions, rather than re-inventing or suddenly manifesting complex novel features for new functions. Elephant 'sixth toes' are not as complicated or flexible as true fingers, but are good enough for what elephants use them for, and that's all that matters in evolution."
Notes to editors
Professor Hutchinson's research was funded by the Biotechnology and Biological Sciences Research Council (BBSRC).
Journalists are welcome to contact Professor Hutchinson and the RVC ahead of the embargo date, but are respectfully asked not to publish any material until after 19:00 (GMT) on Thursday 22 December 2011.
John R. Hutchinson
Professor of Evolutionary Biomechanics
Mobile: +44 (0)7843 629 162
Please contact the Science journal press package team at firstname.lastname@example.org to receive an official version of the paper.
RVC Press Office contact before December 23, 2011
Owen Morris: Communications Management
Tel: +44 (0)1727 733887
After December 23, 2011 - please contact Professor Hutchinson directly via +44 (0)7843 629 162 (Note that Professor Hutchinson will be on holiday in the USA, but plans to be as accessible to journalists as possible; email or mobile phone will be the best way to reach him directly.)
About the Royal Veterinary College
The Royal Veterinary College is the UK's first and largest veterinary school and a constituent College of the University of London. It also provides support for veterinary and related professions through its three referral hospitals, diagnostic services and continuing professional development courses. www.rvc.ac.uk
BBSRC invests in world-class bioscience research and training on behalf of the UK public. Our aim is to further scientific knowledge, to promote economic growth, wealth and job creation and to improve quality of life in the UK and beyond.
Funded by Government, and with an annual budget of around £445M, we support research and training in universities and strategically funded institutes. BBSRC research and the people we fund are helping society to meet major challenges, including food security, green energy and healthier, longer lives. Our investments underpin important UK economic sectors, such as farming, food, industrial biotechnology and pharmaceuticals.