People: John Hutchinson, Andrew Pitsillides

In 1706, Mr Patrick Blair, a Scottish doctor, was asked to lead a dissection of an elephant that had died near Dundee, and he subsequently published a lengthy 1710 account that was the first detailed anatomical description of an elephant. Blair’s study noted that the elephant had six toes, at least on the front foot, and possibly on both feet. Later descriptions corrected this - elephants have five (not six) toes. A strange toe-like “cartilaginous rod” called the prepollex or prehallux (the “predigits”) was occasionally noted in the fore and hind feet and seems to be the "sixth finger" that Blair saw. What, then, are these predigits?

Blair’s Osteographica Elephantina
Image from Blair’s Osteographica Elephantina (1710) showing 6-toed elephant.

Almost exactly 300 years later, scientists have solved this mystery. A team lead by Professor John R. Hutchinson of The Royal Veterinary College in London, United Kingdom, has confirmed that the enigmatic predigits are not real toes, but even more interesting than that. Rather, the team found that the predigits are enlarged, elongate versions of small, rounded, tendon-anchoring bones present in many mammals, called the radial or tibial sesamoids. Elephants have turned these humble little bones into massive strut-like structures that rival the size of the true toes. The results of this study were published in the journal Science on Friday, 23 December 2011 (www.sciencemag.org; paper on Learn more! tab).

Fascinatingly, Hutchinson’s team found that the giant predigits of elephants form in a very strange pattern, first developing as the cartilaginous rod that previous studies had often dismissed as a minor curiosity, but late in life beginning to turn into bone that formed in haphazardly-positioned patches around the cartilage. Some elephants they studied even lacked bony predigits at adulthood. This is not so unusual, as sesamoid bones in other species tend to be highly variable.

Cartilage surrounding the prepollex
Cartilage (translucent green) surrounding the prepollex (yellow) of an adult elephant, from a CT scan. (copyright John Hutchinson, 2011)

Not fingers, but playing finger-like roles

Giant pandas also have false extra “thumbs” that are famous examples of evolutionary “exaptation,” or the co-option of old structures for new functions, and correspond to the same sesamoid bones in elephants. Earlier this year a Swiss team noted that moles, which have the same structures, have taken the molecular mechanisms used to develop true digits and applied those mechanisms to turn their sesamoid bones into false sixth fingers as well. But pandas use their predigits for grasping and moles use them for digging. So what do these odd structures do in elephants? This is a tricky question because the predigits are concealed deep in the foot tissue.

Hutchinson’s team, recognizing that the predigits point backwards into the fatty “heel” pad of elephant feet and thus are uniquely well positioned to control it, took cadaveric elephant feet and compressed them under life-like loads in a CT scanner. They found that that elephants’ predigits act to stiffen the fat pad and even (in the hindfoot) have an internal joint like a true finger that moves to reorient the prehallux. By doing this, elephants transfer some of the loads from the sole of their foot up to their wrist and ankle bones, partly bypassing the upright toes, and so the predigits act to make elephants more “flat-footed”, perhaps avoiding concentration of too much force onto the toe bones.

Elephant left fore foot in rear view, showing the joint where the prepollex connects
Elephant left fore foot in rear view, showing the joint where the prepollex (not shown) connects (circled; bottom right side) onto the first metacarpal bone. (copyright John Hutchinson, 2011)

Fossils show how elephant feet evolved

Hutchinson noticed that the predigits all had joints attaching them to the other foot bones, and so wondered if any fossil elephants might have had similar joints— in other words, when did these unusual predigits evolve? The team saw this was an important question, because of how elephant predigits support the foot in a “tip-toed” posture while also making it mechanically more flat-footed. Could this reveal how elephants evolved their strange foot posture and foot pad itself?

Gradually, Hutchinson and colleagues gathered information from the best fossils of elephant predecessors around the world, including early, smaller, more amphibious animals like Numidotherium and Barytherium, as well as the multi-tonne later giant elephant relatives such as Deinotherium, mastodons, and mammoths. They found a very consistent pattern. The earlier elephants had feet that seemed to have been positioned in a much more flat-footed, less vertical posture than living elephants. Unfortunately the feet were not well enough preserved to show if the predigits were there in any form, but the horizontal foot bones left little room beneath the foot for a fat pad or enlarged predigits.

Evolutionary diagram showing how elephant foot form and posture evolved
Evolutionary diagram showing how elephant foot form and posture evolved (copyright Julia Molnar, 2011)

In contrast, all the later, giant, more land-adapted elephant kin 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 quite similar to that of living elephants. This indicated that elephant ancestors must 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.

Hutchinson et al's new study shows 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. Like the clumsy false thumbs of pandas, 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. So, Merry Christmas and Happy Holidays to elephants everywhere; keep enjoying your “sixth fingers” — they are still fascinating scientists after three centuries.

We thank the BBSRC for funding!

Our joint research was published in the journal Science, and was mainly funded by the Biosciences and Biotechnology Research Council (BSBRC) in the UK.

MOST GRAPHICS/MOVIES WERE GENERATED BY CO-AUTHOR JULIA MOLNAR. TO INQUIRE ABOUT USE AND DISTRIBUTION, please contact John Hutchinson

elephant-feet
6 pictures of elephant feet from our research, including gait/pressure pad analyses (work in progress/in press), and motion capture analyses of foot motion (bottom image) as well as elephants across their ontogeny. Copyright John R. Hutchinson, 2011.
elephant-prepollex-cleaned-up-for-histology
Elephant prepollex cleaned up for histology (left) and then cut open to show internal structure (pink area on right is bone). Copyright Alan Boyde, 2011.
elephant-predigit
Elephant predigit sectioned for histology (pink areas are bone; white are cartilage). Copyright Alan Boyde, 2011.
Anaglyph (colour 3D image) of a cavity inside elephant predigit bone. Copyright Alan Boyde, 2011.
 
Cross-sectional images of elephant before (left) and after (right) preparation for electron micrographs to show bone density patterns and structure. Copyright Alan Boyde, 2011.

Elephant prehallux reconstructed from miroCT scans 

(Yellow material is bone; cartilage has been made transparent. Copyright John Hutchinson, 2011)

Elephant fore and hind foot (manus, pes) structures labelled

(Copyright John Hutchinson/Julia Molnar, 2011)

The evolution of proboscidean (elephants and kin) fore/hind foot (manus, pes) on left/right respectively, with structures labelled. Early slides show earlier animals. Copyright John Hutchinson/Julia Molnar, 2011.

Panda's handBarytherium hand

 On left: The right fore foot (manus) of the very early elephant relative Barytherium, shown from above. On right: The left fore foot of a giant panda, shown in palmar view, displaying the predigit (left side; black arrow). Copyright Cyrille Delmer, Julia Molnar and John Hutchinson, 2011.
Barytherium
very early elephant relative Barytherium

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