The musteloid superfamily, however, also contains one other living species that does not fit into any of the three main families: the red panda (Ailurus fulgens). Six years ago, I took a look at the history of Western knowledge of this animal, and of how it relates to other mammal families. The upshot of that, you may recall, is that there is broad agreement that the red panda is the only living species in its family, distinct from raccoons, skunks, and weasels although quite how was unclear. Since I wrote that, a further study has come out supporting the evolutionary tree I described as "the current best bet", but I'd be remiss if I didn't point out that at least one contradictory study has also been published. It's perhaps fair to say that the four families of musteloids all appeared fairly suddenly at around the same time (likely around the Grande Coupure) and the exact sequence of events is difficult to disentangle.
The red panda was first scientifically described in 1825, over forty years before the (not closely related) giant panda. At the time, it was placed in the then-recently named raccoon subfamily of bears. In 1843, John Edward Gray separated them out from the raccoons, creating the red panda family for that one species alone. Initially, he considered them, like the raccoons, to be a subfamily of bears, because they walk on the soles of their feet as bears do, but, of course, that particular scheme no longer holds.
Red pandas do, superficially, look a lot like raccoons. They are about the same size and have a bushy red tail with red-and-white stripes that look remarkably raccoon-like. They are generally reddish in colour, but with black fur on the legs and white markings on the face that, again, resemble the masks on many raccoon species. Look closer, however, and there are some significant differences.
The teeth of red pandas follow the same general pattern of other musteloids, with a row of clipping incisor teeth followed by a single canine and then five or six cheek teeth in each side of each jaw. However, the canines are unusually small for a carnivoran and the cheek teeth are much broader and flatter than in any of their relatives, forming a wide and relatively complex grinding surface that's better suited to chewing up tough plant matter than for slicing meat. These entirely replace the long carnassial blade that carnivorans normally have and are an indication of their specialised diet.
Another unusual adaptation is the elongation of the radial sesamoid bone. This is not a bone found in regular human anatomy, although it is found in some other mammal species, notably including carnivorans. It is essentially an extra wrist bone that forms within a tendon just below the base of the thumb, and probably does little more than help support body weight in other animals. Even in the red panda, it is not especially large (just larger than it normally would be) but it does articulate with the rest of the wrist and is attached to the musculature, allowing it some degree of movement.
It's not quite the fully-developed "second thumb" of giant pandas, but it does at least allow the animal an improved ability to grip bamboo shoots. And this, of course, relates again to the diet of red pandas, 80 to 90% of which consists of bamboo. The remainder is formed of fruits, buds, lichen, and other easily digestible plant matter, and occasionally things like bird eggs. Bamboo is, however, not very nutritious, and red pandas lack the anatomical adaptations that hoofed mammals do to fully digest tough plant matter. They partly get around this by carefully selecting the best species of bamboo, and feeding primarily on tender new shoots and leaves, but even so they need to eat pretty much their own body weight in food each day.
This dietary specialisation goes a long way to explaining why red pandas also have a narrow preference for habitat. They inhabit densely forested terrain from 1500-4500 metres (5,000-15,000 feet) in elevation, preferring places that not only have plentiful bamboo but a thick tree canopy and lots of fallen logs and low-lying shrub that make it easier to access their preferred food. They also prefer south-facing slopes, presumably for the warmth and are only common on slopes of over 45% in places where smoother terrain is occupied by giant pandas.
This restricts them to the southern side of the Himalayas and associated mountain ranges. Two subspecies are recognised, and there has been some debate recently as to whether they should be considered entirely different species. The more widespread of the two lives from Nepal in the west along the mountain range as far as China, passing through Bhutan, northeast India and the extreme north of Myanmar on the way. The second is restricted to China, in the Hengduan Mountains of western Sechuan and Yunnan provinces.
Red pandas sleep during the full light of the day, curling up on a high tree branch or stretching out, cat-like, if the weather is too hot. They are active during the twilight hours as well as during the night proper, and are mostly solitary, scent marking their territory using middens to defecate in as well as using the glands on their paws and at the base of the tail. They are not, however, aggressive towards one another, presumably staying well away when they find another red panda has scent-marked an area.
The mating season runs from January to early March, so that the young are born between June and July. Litters are small, with just two young being most common, although they can vary from single births up to four. As in many other carnivores, the young are born blind and helpless, opening their eyes at around day 18. They reach sexual maturity in their second year.
The amount of forest loss across their range, and their dislike of agricultural land even when it includes bamboo, makes it obvious that their overall population must be in decline, but accurate population estimates are lacking, so we don't know quite how fast this is happening. They are also hunted, as much for the pet trade as for their fur or meat and suffer high infant mortality where humans and dogs are common. As a consequence of this, and because of their distinctive nature as the only living member of their family, they are internationally listed as an endangered species as a precautionary measure.
However, while the family may contain only one living species, it does have more of a fossil record than that might lead one to believe. Several fossil species are known, although most of them from only very fragmentary remains - in some cases, a single tooth. The closest known relative of the living species is probably Parailurus, which is known from places as far afield as Slovakia and Japan, taking central Asia along the way. It seems to have been very similar to the modern species, except for being about 50% larger, and probably more omnivorous, and must also have been more adaptable given that such a wide area cannot have been entirely covered in high-altitude stands of bamboo.
Parailurus lived not long before the Ice Ages, alongside many other species of the family, showing that, while they were never exactly diverse, they were at least once more so than they are today. The best-preserved fossil of the family is not much older, at around 5 million years, and was discovered in, of all places, Tennessee. This is Pristinailurus, and there are sufficient remains to determine that it was only more omnivorous than even Parailurus, but also that it would have been able to climb trees without difficulty, perhaps feeding on their leaves as well as seeking shelter.
This would, however, have lived alongside the much larger Simocyon, which is best known from Spain, but has also been discovered in both China and North America. This was a puma-sized animal with teeth that indicate a diet that was still almost entirely carnivorous. Significantly, it already had the expanded sesamoid bone that modern red pandas use to grip onto bamboo, showing that this must have originally evolved for a different purpose. The most likely explanation is that it helped the animal to climb, perhaps initially to escape larger predators and then as more of a lifestyle choice, before it became co-opted for feeding.
The oldest known fossil species currently assigned to the family are those belonging to Amphictis, which lived across much of the Northern Hemisphere in various different forms, and are thought to have been fairly typical small carnivores for their time. The oldest ones lived around 30 million years ago, not long after molecular estimates suggest that the family first originated in a rapid burst of diversification that split from the first skunks, raccoons, and weasels.
[Photo by Mathias Appel, from Wikimedia Commons.]
Do we have any idea why the family has apparently declined pretty precipitously in the geologically recent past? They're rather too small to have been victims of the post-Ice Age megafaunal die off.
ReplyDeleteI haven't any specific theories, although the decline does coincide with the Ice Ages, so changing climate for an animal with a relatively restrictive diet may be an issue. Arguably, a carnivoran with a herbivorous diet is going to be at risk of being outcompeted by herbivores that can at least ferment their food and extract more nutrition from it.
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