Unlike the large whales, dolphins and porpoises appear to have no special mating and calving grounds to which they travel in winter. Researchers have noticed that with some species, such as orcas and bottlenose dolphins, new calves appear at certain times of the year. This suggests that mating is somewhat seasonal. By monitoring captive dolphins, scientists have determined the exact length of pregnancy (gestation period) in some species. For bottlenose dolphins, it is 12 months; for orcas, 17 1/2 months. But the gestation period is unknown for most dolphin species.
Are dolphins and porpoises monogamous (mating with a single individual for life) or promiscuous (mating with many others)? Scientists have strong evidence that most dolphins are promiscuous. Only river dolphins and some porpoise species stay in small enough groups that they might be monogamous. but small groups alone, of course, do not tell the whole story, since the groups may change over time. the key to learning more about reproductive habits, as with other social behavior, is the identification and sexing of individuals and long-term monitoring.
For years, scientists have suspected that some of the interactions they see routinely are probably courtship, or preludes to mating behavior. Recent research on bottlenose dolphins has provided the first detailed insights. The breakthrough came in Shark Bay at Monkey Mia, Australia, where researchers have been able to witness dolphin behavior under ideal field conditions. the bottlenose dolphins there associate close to shore in fairly calm water and are particularly accessible to human visitors in a small boat. Some 300 dolphins - each named by researchers - have been closely watched for two decades, first by Elizabeth Gawain and, for the past 10 years, by Rachel Smolker, Richard Conner and Andrew Richards. In recent years, the researchers have focused on the exuberant young males, whose behavior had been called "playful" by most observers. What were the dolphins really doing?
As adolescents, most male bottlenose dolphins form tight, long- term bonds with one or two other males. These buddies swim, bow- ride, fish and play together. From time to time, they will herd a female, trying to get her to travel with them; eventually, they attempt to mate with her.
When there are few potential mates, however, male competition can become fierce. Two groups of buddies gang up and sometimes steal a female away from another group. Only one group of buddies stays with the female, the role of the others having been simply to help out. But days later, the favor may be forgotten, and the gang that helped may join up with a third group of buddies to steal the female again.
Females, the researchers found, are more varied in their associations. Some stay alone, some travel with a few other females, and some move from subgroup to subgroup. Off Florida, where the females have been studied extensively, they stayed with other related females but sometimes went visiting.
Dolphins and porpoises mate belly to belly, like whales. Most females do not conceive every year. Bottlenose dolphins give birth to calves every four to five years on average, as do orcas. the babies are usually born tail first. An assisting female may help the calf to the surface for its first breath of air. In some species, such as orcas, this job may be performed by a male - or the calf will surface by itself. After mating, dolphin and porpoise fathers do not appear to take an active role in caring for their offspring. But several species, such as pilot whales, Atlantic spotted dolphins and bottlenose dolphins, have other females that act as baby-sitters.
Bottlenose dolphin calves nurse for 18 to 20 months but may remain under their mother's care and tutelage for three to six years. A few other dolphin species stay with their mothers to the age of maturity and longer. A young female orca leaves her mother's side only when her first calf is born, at about age 15. Even then, she will probably remain in her mother's subgroup for life. Like few other animals, orcas share with their close relatives the key events of their lives - from birth to death.
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