EVOLUTION WORKS IN MYSTERIOUS WAYS. Certainly, if you were setting out to design a panda from first principles, you’d go about it in a very different way to how it seems nature did. Not that there aren’t obvious respects in which today’s animal is beautifully adapted to its environment and the life that it leads, from its thick, warm fur through its hair-padded feet to its handy ‘thumb’. At the same time, though, there are aspects of the panda and its lifestyle that appear to run contrary to common sense.
DIGESTIVE ISSUES
In terms of its digestion, the panda is equipped to be a carnivore (see page 14), so how did it come to live (almost) exclusively on bamboo? Not that it’s not suited to eating bamboo in some ways, for evolution has equipped it with some of the most formidable molars in the mammal class (giving it the round face that marks it out so distinctly from other Ursidae). Nor that there wouldn’t have been a clear advantage many million years ago to being able to eat bamboo, an abundant supply of which grows in this part of the world. And, significantly, bamboo is also a static foodstuff, which means that no energy need be expended by the panda on hunting, for the stems just stand there, waving gently in the wind at most.
The most significant drawback of subsisting on bamboo is that its nutritional value is as elusive as any animal prey could be. Largely comprising as it does cellulose, bamboo might as well be wood. The digestive system of a specialist grazer – a cow or goat, for instance – would find it tough to extract nourishment from a diet of bamboo cane in life-sustaining quantities, so how is the panda, with its much simpler stomach, supposed to cope?
AN INDEFATIGABLE EATER
The panda manages to survive on bamboo in two main ways: firstly, by eating unfeasible amounts of it; and, secondly, by limiting its expenditure of energy. An adult male panda makes his way through anything up to 38kg of bamboo a day; the smaller female scrapes by on somewhat less. Both, however, end up eating in the region of 40 per cent of their body weight in food each day. In general, we associate such a ratio with much smaller creatures. Rats and mice, with their hyperactive metabolisms, only have to eat 15 per cent or so of their own weight each day. We find comparable levels to the panda in such small carnivores as weasels (while for the tiny, insect-eating shrew, the level is almost off the scale, at 95 to 100 per cent).
The panda does its best to optimise its bamboo intake by selecting the greener, less fibrous young shoots wherever possible, with the ‘thumb’ on its forepaw helping it to manipulate the springy stems. But there’s no way around the fact that it has to ingest huge quantities of bamboo in order to obtain the necessary intake of calories. Eating on such a large scale is a full-time job, easily accounting for fourteen hours daily. And despite much shorter days during the winter, the panda has to browse for even longer when it’s cold.
As much as it can, the browsing panda favours the younger, greener shoots that offer the highest possible nutritional return on the effort of eating.
A QUIET LIFE
The panda seems to retain some species memory of its carnivorous origins. It will eat meat – a small rodent, for example, should it stray within clouting distance of the panda’s forepaw; or a helpless fawn, should it should stumble upon one in a bamboo thicket; and even any carrion that it may find. But this happens so seldom that it scarcely signifies. Given the limitations of its bamboo diet, it may seem a ‘rational’ choice for the panda to take steps to eat meat more often. Yet those same limitations help to ensure that it doesn’t, for with such a meagre nutritional return on its normal regime, the panda has to do its best to conserve all the energy it can. It’s a vicious circle. Whatever form its stomach takes, and whatever sort of life its remotest ancestor led, the panda has long since lost the predator’s get-up-and-go.
This passivity seems to extend to other areas of the panda’s life, too, with behavioural patterns having emerged that economise on effort. Everything from the panda’s slow, deliberate gait to the infrequency of its social and sexual interactions may be seen as ways of reducing the unnecessary expenditure of energy. Paradoxically, though, the ultimate labour-saving strategy – hibernation – is not an option for the panda, precisely because of its dietary limitations. Playing catch-up as it does with its daily nutritional needs, the panda is never in a position to lay down the stores of fat that allow so many of its Ursidae relatives to ride out the winter months in hibernation.
Pandas simply can’t afford to eat in moderation: when their staple diet is so low in nutritional value, they have to be shameless gluttons in order to survive.
For hours and hours each day, the giant panda’s life is largely sedentary as it eats its fill. This is just as well, given the comparative nutritional poverty of its diet.
Pandas use their teeth to peel off the tough outer layer of bamboo stems to get at the softer inner tissue. They will eat more than twenty different species of bamboo, but tend to favour just a few types.
The giant panda has gigantic appetites, and not just for eating, but also for drinking. Fortunately, its mountain environment is rich in running water.