题目内容

Herman Melville`s description of the social arrangement of sperm whales in Moby-Dick is remarkably similar to others` description of more familiar mammals, where males dominate a group of females, fight off interloping males, and breed with the females in the harem. This arrangement is found in a variety of mammals, including such diverse species as elephant seals, elk, lions, and even chimpanzees, but it does not apply to sperm whales. Would you really expect sperm whales, different in almost every respect from all other mammals, to conform to some sort of behavioral norm? Of course not, and they don`t, but there is a surprising similarity to another huge mammal. In what they call "A Colossal Convergence," Weilgart, Whitehead, and Payne describe the remarkable similarities between the sperm whale, the largest toothed whale, and the largest terrestrial mammal, the African elephant (Loxodonta africana). Just as sperm whales are able to communicate over long distances, so too can elephants. Females announce their availability for breeding by sending out a series of low-pitched calls, and reproductively receptive males converge from all directions. Elephant calls, far below the range of human hearing, also serve to attract distant family members to the scene of excitement, distress, or separation. Elephants and sperm whales have unusual but remarkably similar life history parameters, wide-ranging behavior, and ecological success, as well as the largest brains, respectively, on land and in the ocean. Their societies are based on matrilineal groups of about ten related females that often form temporary associations, of a few days or so, with other female groups. After leaving their mother`s group when they are about six years old, male sperm whales become increasingly solitary and range to higher latitudes as they grow to about one and a half times the length and three times the weight of females-the most extreme case of sexual size dimorphism among cetaceans. Another striking parallel with elephants is in the delayed age of effective breeding by males: although male elephants and sperm whales become sexually mature during their late teens, they do not seem to take a significant role in breeding until their late twenties. In the case of elephants, this is because younger males do not enter the behaviorally dominant but physiologically demanding state of musth [a periodic condition characterized by heightened aggression] in the prime breeding season. Male sperm whales in the same age range usually remain in productive high-latitude waters, away from the tropical breeding grounds of the females. In these highly sexually dimorphic species, it probably pays younger males to concentrate on growth rather than competing with their much larger elders for the few breeding opportunities presented by slowly reproducing females, who might bear one young every four or five years.