Reproduction, Development, and Social Behavior
Gestation and Birth
A single calf is born after a gestation of about 12 months. The calf associates closely with its mother for as long as six years and sometimes longer. A female typically produces six to eight calves in her lifetime and can reproduce even in old age. Females in the Sarasota Bay population may live well into the 50s and males into the 40s. Currently in Sarasota Bay, a 48-year-old female has a newborn calf!
Growth and Development
Sarasota Bay’s adult bottlenose dolphins are sexually dimorphic in size, with adult males weighing as much as 25 percent more than adult females. At birth, however, males and females are about the same size—about 20 kilograms (45 pounds) and about 100 centimeters (40 inches) long. Neonates have a few hairs on the rostrum, no teeth, and slightly floppy dorsal fins and flukes. The dorsal fin and flukes stiffen up to adult strength by about two weeks of age, and hairs disappear by about two weeks to a month of age. The first teeth appear at about three months, and a full set is in place by about five months.
During the early stages of growth, females tend to grow faster than males and are larger until about six years of age. Females reach sexual maturity at five to 12 years of age and attain full body mass and length—about 160 kilograms (355 pounds) and about 240 centimeters (95 inches) in length—at about ten years. At age ten, males are at only about 70 percent of their potential maximum body size and by the age of sexual maturity at between ten and 13 years reach about 180 kilograms (400 pounds) and about 270 centimeters (105 inches) in length—about 25 percent heavier and longer than adult females. There is a 2:1 ratio of females to males in the Sarasota Bay population due to higher juvenile mortality among males.
Males and females mate belly to belly, and both sexes may have multiple partners over their lifetime. In Sarasota Bay, it is common for a female’s sequential young to have different fathers.
Genetic and behavioral data gathered from many studies over the years suggest that the bottlenose dolphin has a polygynous mating system in which males compete with one another for access to females, and a relatively small number of dominating males may sire most of the young.
Competition between males may be through direct physical contact, such as by blows of the peduncle and fluke of one animal to the body of another. Competition can also be indirect, such as when a male protectively herds a group of females in a way that excludes access by other males. The moderate expression of size-related sexual dimorphism in this species supports the idea that males compete with one another for dispersed females rather than monopolizing access to aggregations females. If the latter were the case, we would expect males to be much larger than females.
Scientists have carefully documented the daytime activities of bottlenose dolphins in Sarasota Bay. On average, they travel 67 percent of their daytime hours (moving from one area to another and searching for prey), feed 13 percent of the time, socialize four percent, sleep two percent, and mill around without obvious purpose 14 percent. Because of the obvious difficulty of studying an animal underwater in the dark, very little is known about the nighttime activities of bottlenose dolphins.
|Female bottlenose dolphin and young swimming together. Photo courtesy of Bud and Jo Krames.|
Bottlenose dolphins often form sub-groups within the larger group, which may number 20 or more individuals. Common sub-groupings (usually of three to five individuals, not including calves) include all-female bands; nursery groups of mothers and calves; mixed-sex all sub-adult groups; and small all-male groups. Mixed-sex, sub-adult groups are among the most active in Sarasota Bay. Animals in these groups do lots of socializing, chasing, and leaping.
At the onset of sexual maturity, males may form coalitions of two to three individuals that may stay together for several decades. Nursery groups in Sarasota Bay consist of several mothers and calves. Researchers believe that these associations may facilitate social development among calves by creating “play pens”—areas where the young animals interact while their mothers swim protectively nearby.
Females invest a great deal of time and effort in caring for their calves and may maintain social bonds with them for many years. Females also care for calves that are not their own, and it is common for younger, non-reproductive females to baby-sit other females’ calves. This behavior may give the young females “babysitter” experience practicing maternal skills before having young of their own.
Besides habitat destruction, pollution, and over-fishing, bottlenose dolphins are threatened by a few natural predators, most notably sharks. Recent studies show that several species of sharks, including the great white, tiger, and mako, attack and prey on bottlenose dolphins. In Western Australia, where extensive studies of dolphin-shark interactions have been conducted, as many as 70 percent of dolphins bear shark-attack scars, and an estimated 15 percent of the population may be attacked each year. The exact mortality rate from shark attack is unknown. The risk of shark predation may have been an important factor leading to the evolution of group living (for protection) in bottlenose dolphins.More than 30 percent of Sarasota Bay’s bottlenose dolphins have visible shark attack scars, and seasonal population movements correlate with the abundance of sharks: Dolphins tend to move out of areas where sharks are abundant. In the presence of a shark, dolphin anti-predator behavior varies with the circumstances. Some simply swim away from the shark, others ram or bite it, and yet others launch coordinated group attacks to drive the predators away.