(1) Some persons say that love makes the world go round. Others of a less romantic and more practical turn of mind say that it isn’t love; it’s money. But the truth is that it is energy that makes the world go round. Energy is the currency of the ecological system and life becomes possible only when food is converted into energy, which in turn is used to seek more food to grow, to reproduce and to survive. On this cycle all life depends. (2) It is fairly well known that wild animals survive from year to year by eating as much as they can during times of plenty, summer and fall, storing the excess, usually in the form of fat, and then using these reserves of fat to survive during the hard times in winter when food is scarce. But it is probably less well known that even with their stored fat, wild animals spend less energy to live in winter than in summer. (3) A good case in point is the whiter-tailed deer. Like most wildlife, deer reproduce, grow, and store fat in summer and fall when there is plenty of nutritious food available. A physically mature female deer in good condition who has conceived in November and given birth to two fawns during the end of May or first part of June, must search for food for the necessary energy not only to meet her body’s needs but also to produce milk for her fawns. The best milk production occurs at the same time that new plant growth is available. This is good timing, because milk production is an energy consuming process—it requires a lot of food. The cost cannot be met unless the region has ample food resources. (4) As the summer progresses and the fawns grow, they become less dependent on their mother’s milk and more dependent on growing plants as food sources. The adult males spend the summer growing antlers and getting fat. Both males and females continue to eat high quality food in the fall in order to deposit body fat for the winter. In the case of does and fawns, a great deal of energy is expended either in milk production or in growing, and fat is not accumulated as quickly as it is in full grown males. Fat reserves are like bank accounts to be drawn on in the winter when food supplies are limited and sometimes difficult to reach because of deep snow. (5) As fall turns into winter, other changes take place. Fawns lose their spotted coat. Hair on all the deer becomes darker and thicker. The change in the hair coats is usually complete by September and maximum hair depths are reached by November or December when the weather becomes cold. (6) But in addition, nature provides a further safeguard to help deer survive the winter—an internal physiological response which lowers their metabolism, or rate of bodily functioning, and hence slows down their expenditure of energy. The deer become somewhat slow and drowsy. The heart rate drops. Animals that hibernate practice energy conservation to a greater extreme than deer do. Although deer don’t hibernate, they do the same thing with their seasonal rhythms in metabolism. Deer spend more energy and store fat in the summer and fall when food is abundant, and spend less energy and use stored fat in the winter when food is less available. (7) When the "energy crisis" first came in 1973-1974, I was living with my family in a cabin on the edge of an area where deer spend the winter in northern Minnesota, observing the deer as their behavior changed from more activity in summer and fall to less as winter progressed, followed by an increase again in the spring as the snow melted. It was interesting and rather amusing to listen to the advice given on the radio: "Drive only when necessary, " we were told. "Put on more clothes to stay warm, and turn the thermostat on your furnace down. " Meanwhile we watched the deer reduce their activity, grow a winter coat of hair, and reduce their metabolism as they have for thousands of years. It is biologically reasonable for deer to reduce their cost of living to increase their chance of surviving in winter. (8) Not every winter is critical for deer of course. If the winter has light snow, survival and productivity next spring will be high. But if deep snows come and the weather remains cold for several weeks, then the deer must spend more energy to move about, food will be harder to find, and they must then depend more on their fat reserves to pull them through. If such conditions go on for too long some will die, and only the largest and strongest are likely to survive. That is a fundamental rule of life for wild, free wandering animal such as deer. (9) Yes, life-and death, too-is a cycle that goes round and round, and when animals die their bodies become food for other life forms to use by converting them into energy. (10) And the cycle continues.According to the author, what makes the world go around is (). A. love B. truth C. money D. energy