4 The bear necessities
4.1 The grizzly bear
There are three activities in Section 4, asking you to summarise information in the form of lists. In the first two, the answers are given but in the third, about the diet of the giant panda, they are not. You are asked to tick off the points in your list as you read on through the section. As you gain more study experience, you should feel less dependent on the answers always being provided in full. Aim to take more responsibility for your own learning and to judge for yourself whether you have answered the questions appropriately. When you have completed Section 4, check whether you spotted all the points that I have included. You may even be pleasantly surprised to find that you've included some that I've missed!
As DA points out [p. 159], most members of the bear family are omnivorous. The grizzly bear is a particularly magnificent member of that family.
Watch the sequence from 23.02-30.49 in 'The Opportunists' and make a list of the different sorts of food that the grizzly bear utilises during the course of a year.
You should have compiled an impressive list: fresh-caught salmon, berries, sedges, grasses, caterpillars and clams. Grizzlies probably eat many other sorts of food not covered in the programme. Indeed, LoM also mentions horsetails, skunk cabbage, lily roots, elderberries, cranberries, mice, squirrels and marmots [p. 159].
Not only do grizzly bears eat a great diversity of foods, in certain seasons they also eat prodigious quantities. During late summer - as they lay down reserves of fat that enable them to survive the impending winter - they may eat for 20 hours a day. One female grizzly in Alaska was observed eating nothing but blueberries for 14 hours, with only 30 minutes off for a midday rest; the TV programme gave a figure of 200 000 berries per day. The digestive system that bears have inherited from their carnivorous ancestors lacks the special stomach that enables herbivores to break down plant material so efficiently. Their inability to extract much of the energy it contains helps explain why bears harvest vast amounts of plant material, a proportion of which passes through their guts unaltered. Indeed, grizzlies are believed to play a very significant ecological role, simply through the distribution of undigested seeds in their dung.
DA mentions [p. 159] the 'delicacy' that the grizzly bears of Yellowstone Park enjoy in summer. The fact that the highly digestible bodies of tiny cutworm moths comprise 18% protein and 35% fat in mid-August makes it less surprising that grizzly bears are prepared to climb to 10 000 feet (3000 m) in Yellowstone and elsewhere to feed on them. It is possible that the bears see birds, such as ravens, Clark's nutcrackers and grey-crowned rosy finches, flying up to the high country and know that it is time for them to go there as well. But how did they discover that the birds were going there to feed on the thousands of moths hiding under rocks during the day so that they were in position to feed on alpine flowers at night?
The TV programme (27.57-30.18) emphasises the importance of laying down 'energy reserves' during summer feeding, mainly as fat - you'll recall the process of (in the words of the commentary of the programme) 'piling on the calories' during the summer bonanza. The mobilisation of these reserves is what sustains the grizzly over the 5- to 6-month period of overwintering in its den, during this unique type of hibernation.
Question: In what respects is this type of hibernation in the grizzly bear different from the examples of hibernation given in course S182_2, for example in the European hedgehog?
In the hedgehog, body temperature drops as low as 5 °C (S182_2 Figure 8); DA talks of a body temperature drop of only 'several degrees' in the grizzly (authorities quote a drop from 38 °C to 34 °C). But the heart rate of the grizzly falls substantially 'to about 10 beats per minute' according to the commentary, which is not a great deal different from that of the hedgehog. Hedgehogs arouse spontaneously from hibernation, though the purpose of these periods of being warm remains largely unknown; other small hibernators use these periods to drink, eat stored food and eliminate waste. Most authorities take the view that an overwintering grizzly does none of these things, even during the occasional moments of 'stirring' from its prolonged sleep referred to in the TV commentary at 24.07.
Indeed, overwintering in bears is such a very different physiological phenomenon from the true hibernation discussed in S182_2, that many biologists prefer the term winter dormancy (or winter lethargy) to describe it. Since the term hibernation is used in LoM (and the TV programme) we shall use it here, but as DA points out [p. 159] winter sleep in bears is different from 'true' hibernation, and indeed, an ability to survive this degree of suspension of bodily functions for up to six months is unique amongst mammals. It involves a high metabolic price too - maintaining a body temperature (Tb) of 34 °C for this period helps explain why the demands on the bear's energy reserves are so severe. You'll recall calories being counted down in the programme, with the burning up of 'a million calories' according to the commentary. All the more intriguing to puzzle, therefore, why this expensive physiological trick evolved in bears.
Question: Can you think of any advantages of a form of hibernation where a high Tb is maintained? Thinking about the size of the bears as well as their reproductive habits might help.
Bears remain responsive and alert to danger if Tb is maintained. Perhaps the risks of discovery during this type of hibernation - by wolves or possibly other bears yet to establish a den - are very great. The size of bears is likely to be a factor too - mammals this big are not able to hide! A substantial drop in Tb during hibernation will necessitate a substantial energetic investment when hibernation ends - and perhaps during periodic spontaneous arousal. This is manageable for small mammals, where the amount of heat required to warm the body mass is relatively modest; but for animals as large as a grizzly, reheating on this scale may be physiologically impossible. Finally, for the female, it may that a high Tb is necessary to support the development of the cubs that, as DA mentions, are born as the mother 'dozes' [p. 162]. Bear cubs are very small at birth relative to adult size, and need to grow quickly if they are to withstand the rigours of the following winter. Lactation requires a high level of metabolism.
Whatever the reason, or combination of reasons, there is much to learn from the grizzly about this type of hibernation. But you'll appreciate that it takes a particularly intrepid biologist to enter the den of a responsive, overwintering grizzly, armed only with a determination to further scientific knowledge. Little wonder that so much more is known about hibernation in the hedgehog than in the grizzly.