# In 1945 the animal physiologist samuel brody reported

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that grizzly bears need considerably less than three times more energy than humans. In 1945, the animal physiologist Samuel Brody reported a relationship between a mam- mal’s size and its minimal daily energy needs in the form of an equation, where E is the energy needed for one day and W is the animal’s weight. Plug in the average human’s weight (60 kg),and E is nearly 1500 kcal—not a bad approximation for our average minimal daily energy needs. Plug in the average Denali Park grizzly bear’s weight (200 kg), and the energy required is about 4000 kcals per day. The average Arctic ground squirrel weighs in at around 2400 g, nearly three-fourths of which is water. In late summer, when they are preparing for a long winter’s hiberna- tion, half of the remaining grams are pure fat while the rest are mainly proteins and car- bohydrates. Each gram of fat contains 9 kcal of energy. Each gram of protein and carbohydrate has about 5 kcal of energy. Run these figures through a calculator to see that the average Arctic ground squirrel is packed with about 4200 kcals of energy. All the average grizzly bear needs to do is catch one Arctic ground squirrel each day and its basic, daily energy needs are filled. Still, one caribou weighing 130 kg could provide a grizzly bear’s energy needs for several days. Predators often have a choice of prey. Usually the environment offers a number of potential prey species. Ecologists evaluate how predators choose prey in terms of energy efficiencies. What is the ratio, for example, between energy expended catching a caribou compared with the energy payoff once it is captured? While it is true that a caribou would supply the bear’s energy requirements for several days, caribou are not easy to kill. If a digging bear even glances up, a nearby caribou can easily bound away. To chase a caribou, assuming that it is not sick, wounded, or very young, is to expend a lot of energy on an effort that probably will not be successful. Hunting Arctic ground squirrels, it turns out, is much more efficient. Burrows have only one entrance. If the bear just keeps digging it will eventually be successful.At the end of the burrow is a 4200 kcal tidbit. Now we have a plausible explanation for what at first appeared to be bizarre behavior in grizzly bears. Ecology is science . Notice that, until we got quantitative and started thinking in terms of energy needs, efficiencies, and balances, we had little more than an interesting story. Modern ecology, like other modern sciences, is quantitative. Ecological research often uses the same methods, steps, and processes as other branches of biology. It usu- ally starts with observations gathered in the field, but moves quickly from description to experimentation. Testable hypotheses are proposed. At its best, ecology takes mea- surements for testing hypotheses. Measuring specimens is easy enough—length, weight, degree of coloration. But how does one measure an organism’s surroundings? Doing so is the heart of ecology.Data are collected,analyzed,and related to what is already known from previous studies. Ecology strives to get to the point where predictions can be reli-

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