Especially Bears

Within China, the giant panda often is called daxiongmao by local people, literally 'large bear-cat' in Chinese (Schaller et al, 1985). Its scientific name Ailuropoda melanoleuca actually means black and white cat-footed bear. The black and white colouration of the panda allows it to blend in with its high mountain forest surroundings, which often are blanketed with thick snow. When threatened, pandas climb the nearest tree, where this coloration renders them almost undetectable.

The giant panda is indeed a type of bear, of the subfamily Ailuro-podinae in the family Ursidae. During evolution, it diverged from the main bear lineage (comprised of seven other species) 15 to 25 million years ago (Lumpkin & Seidensticker, 2002). Interestingly, the giant panda's nearest relative (genetically speaking) is the spectacled bear (Tremarctos ornatus) which inhabits the mountainous regions of South America. Unlike its ursid counterparts, which are principally omnivores, the giant panda is a 'grass-eating' bear with 99% of its diet as bamboo. This, in part, explains some of its unique morphology, including the skull's expanded zygomatic arches and the associated powerful muscles for mastication (Nowak & Paradiso, 1983). The giant panda's dentition is also different from, for example, a similarly sized black bear because of broad, flattened premolars and molars designed to break and grind bamboo. As a 'hypo carnivore', the giant panda is also partial to meat, with its teeth specially suited to crush bones (Lumpkin & Seidensticker, 2002). Its forefoot, with the modified and well-described sixth toe (Gould, 1982) or wrist bone, is considered unique among species, allowing it to grasp its food more securely.

During winter, the giant panda's survival in its cold, wet mountainous habitat is enhanced by the superb insulation provided by short, thick fur. It has no tolerance for heat, in part because of its lack of capacity for passive heat loss or evaporative cooling (Lumpkin & Seidensticker, 2002). Unlike its bear counterparts, the giant panda does not hibernate, probably because of the need to forage throughout the year for its low-energy diet of bamboo. One of its most unique features is its adaptation from carnivory to herbivory while amazingly retaining the digestive system of the former. The result is the need to spend 14 hours of each day searching, selecting and consuming bamboo (Lumpkin & Seidensticker, 2002).

Perhaps most interesting (and one of the incentives for the work in this text) is the overall low reproductive rate of the giant panda. Note the use of the word 'low' and not 'poor'. There has been much embellishment by the popular press about 'poor reproduction' in this species. This misperception is derived from the well-known challenges of breeding pandas in artificial conditions in captivity. However, there has never been any systematic study of reproductive efficiency in wild giant pandas in nature. Obviously, it cannot be true that the giant panda is normally poor at reproduction or it would never have evolved or survived to modern times. Nonetheless, the species has developed some fascinating and rather illogical characteristics that are less than ideal for ensuring reproductive success. It is a seasonal breeder with the female entering oestrus (heat) in late winter/early spring. This trait in itself is nothing special, and the environmental stimulus inducing oestrus is likely to be increasing day length, although no one is sure. However, unlike other bears, the giant panda is monoestrus, displaying sexual receptivity once per year for only 2 to 3 consecutive days. In turn, the male produces prodigious numbers of motile spermatozoa, probably because of the need to ensure conception if given the chance to mate with a female, who normally is sexually 'turned off for more than 360 days per year. Further evidence for the physiological reproductive prowess of the male giant panda includes the species' comparatively short and repeated copulations, each 1 to 8 minutes in length (Zhang et al., 2004), unlike in other bears. Resulting embryos are free-floating in the uterine horns for an undetermined interval (a phenomenon called delayed implantation), which is common in bears, as is the eventual production of one or two small, comparatively immature cubs. Enigmatically, however, in the case of giant panda twins, one offspring is usually rejected by the dam and dies soon after birth. The giant panda cub is relatively slow-growing, although the species as a whole achieves sexual maturity at a time comparable to other bear species.

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