Protractile toe tips in Carnivora
(writing in progress)
I think I may have spotted something original in the morphology of the Carnivora.
The topic is a mechanism, in the skeleton of the toes, whereby the last, clawed phalanx is hinged relative to the rest of the digit in such a way that the claws are protractile. What this mechanism allows, in practical terms, is that the claws can be extended to grip surfaces that they would not otherwise contact.
This is usually called ‘retractable claws) but I prefer ‘protractile toe tips’ because a) it’s not really the claws that are moving, it’s the whole terminal phalanx, and b) the normal arrangement is for the claw to be pulled up and back, and it’s only when muscles are contracted that the claws extend forward; i.e. its not so much that there’s retraction going on, it’s the protraction which is the important action.
Now, everyone knows that the classic example of protractile toe tips (= ‘retractable claws) in Carnivora – and all mammals – is the cats (Felidae), all of which have this mechanism fully developed except the cheetah, in which it has been lost. And at a slightly deeper level most zoologists will appreciate that this is a way of keeping the claws sharp given the digitigrade foot-posture of all cats. It’s obvious really: if an animal walks on its toes (which is the meaning of digitigrade) the claws risk blunting if they are not lifted off the ground.
But this superficial level of knowledge hides something more interesting and more general.
Besides the cats, protractile toe tips have evolved, by convergent evolution, in several other families: the red panda in the Ailuridae, the marten and fisher in the Mustelidae, the fossa (Cryptoprocta) in the Eupleridae, the genets, African linsangs, some civets, and some or all palm civets in the Viverridae, the Asian linsangs in the Priodontidae, one of the two species of ringtail (Bassariscus) in the Procyonidae, and probably the African palm civet in the Nandiniidae. Contrary to what one sees written in places, the wolverine does not have this mechanism.
The convergence is stronger than it may at first seem, because the above list consists of not only different families but also different suborders. Cats, viverrids and the fossa belong to the Feliformia, whereas the red panda, mustelids, and ringtail belong to the Caniformia. Within the Viverridae, several subfamilies are involved. Phylogenetic constraints don’t seem to apply much to this adaptation; it seems to have evolved at least seven times independently.
Now, the interesting thing is that, although the cats with their classic/typical protractile toe tips are digitigrade, none of the other forms mentioned above are digitigrade. So why would they have evolved this mechanism?
The answer: all of these forms are extremely arboreal, to the degree that their ankle joints are flexible enough to allow the foot to swivel backwards to grip the tree surface when the animal descends (something that some cats such as the ocelot and probably also the clouded leopard also seem able to do despite being digitigrade). As the animal hangs its weight from the hindlegs, which must grip the tree trunk firmly, it maintains its grip by extending its sharp hind claws (protracting its toe tips, at least on the hind limbs).
So what is original about this observation? It is that the various arboreal forms mentioned above (except for the arboreal cats) are essentially plantigrade, both on the ground and in the trees. They have evolved protractile toe tips despite being plantigrade, and because their extreme application of plantigrade locomotion to descend from trees means that the sole of the hindfoot does not provide enough friction unless the sharp claws enhance traction.
If it’s true that protractile toe tips are generally an adaptation to plantigrade arboreality in Carnivora, then where does that leave the cats? Well, I now need to establish whether cats use the sole of the hind foot (which is furred, not bare) to provide traction on tree stems. I suspect that this is not the case, and that cats are digitigrade when climbing, as they are on the ground, with a few exceptions such as the ocelot. If I’m right, then the truly odd members of the Carnivora turn out to be terrestrial cats, such as the lion, serval, and possibly Pallas cat and clouded leopard, which have protractile toe tips in combination with an inability to climb well.
One form that I still need to find out about is the otter civet (Cynogale), which does not seem to climb well but has this mechanism.
Although everyone takes it quite for granted that all felids other than the cheetah have protractile toe-tips (usually called ‘rectractable claws’), a survey of such claws throughout the Carnivora casts the presence of this feature on the hind limbs in a puzzling light in the case of large species which do not climb trees.
There’s no puzzle in the case of the forelimbs, because it makes perfect sense that the foreclaws are used in securing prey and they need to be kept sharp in digitigrade animals such as cats.
However, in the case of the hindlimbs, the function of sharp claws is somewhat ambivalent. In the case of the larger felids, which do not climb to any significant extent, the presence of protractile toe-tips on the hindfeet is perhaps surprising.
Virtually all species of felids other than the cheetah can climb vertically up tree trunks, partly by means of the sharp hind claws. However, as you know, the problem is coming down again. Even a cat as small as the domestic cat can get stranded up a tree for the simple reason that this combination, of digitigrade foot-form and sharp claws, is useless for descending headfirst. The cat has to descend tail-first, or else jump down to the ground. Above a certain height, above a certain body mass, and beyond a certain trunk-smoothness, the cat concerned can find itself painted into a corner, as it were, after quite easily climbing a tree. This may help to explain, for example, why the tiger seldom climbs despite being perfectly capable of climbing.
In the case of Smilodon, which was more massive even than the tiger, it seems unlikely that the adults climbed trees at all. They had no predators to escape from because they were the top predators in their ecosystems. And at any height above a few metres, the sabretooth would risk injury in its descent.
If so, why have protractile toe-tips on the hindlimbs at all?
The photos below confirm that the hind toes were similar to the fore toes in the sheaths that protect the ‘retractile claws’ in Smilodon, although there is a puzzling inconsistency in these skeletal reconstructions in the relative sizes of fore and hind toes. At any rate it seems out of the question that Smilodon lacked this mechanism on its hindlimbs while having it on its forelimbs. It was much like other felids in having both fore- and hindlimbs similarly adapted.
Now, all the non-felid Carnivora with protractile toe-tips on the hindlimbs use the sharp hindclaws for descending trees, which is possible in those diverse lineages because the ankle is flexible enough to bend so far backwards that the carnivore can hang from its hind feet while descending the tree. In at least three species of felids (margay and clouded leopards), the ankle joint is similarly flexible and these cats are unusual in being able to descend trees headfirst, as do Carnivora as diverse as fossa, Bassariscus, various viverrids including the binturong and the semi-amphibious otter civet (Cynogale), and Asian linsangs.
So this means that the felids that are too big to climb trees are exceptional among Carnivora with protractile toe-tips in at least two respects: firstly then do not really use the hind claws to climb trees, and secondly they do not have sufficiently flexible ankles to use the hind claws to descend trees. This means that the only likely use for their eversharp hind claws is for offense, e.g. securing prey – in contrast to most Carnivora with protractile toe-tips on the hind limbs, which don’t seem to use the sharp hind claws to secure prey at all.
I think these distinctions have been overlooked previously.
What seems to emerge from this: sabretooth cats and other large felids are unusual among all Carnivora, in having exclusively OFFENSIVE function for the protractile toe-tips on the hind limbs. When most people look at a picture of a sabretooth cat, they notice the exceptional form of the canine teeth, but they overlook the fact that the ‘retractile claws’ on the hind feet are, in their own way, almost as odd. Although this applies to the lion and tiger (and probably snow leopard) almost as much as to Smilodon, the difference is that in Smilodon the forelimbs are disproportionately large, even more so than in the tiger (which has somewhat wasted-looking hindlimbs compared with lion and most other big cats). Because the massive forelimbs were probably the main limbs used in securing prey in the case of Smilodon, the fact that the hind toes retained the ‘retractile claws’ is noteworthy.
http://upload.wikimedia.org/wikipedia/commons/7/7a/Smilodon_californicus.jpg
http://messybeast.com/images/smilodon4.jpg
http://www.angelfire.com/mi/dinosaurs/images/casts/smilodon_fatalis_tarpit.jpg
http://upload.wikimedia.org/wikipedia/commons/c/c9/Smilodon_populator_Dientes_de_Sable.jpg
(writing in progress)
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