Dolphin Web

Sexual behaviour in bottlenose dolphins is an important component of social relations between individuals. Adults as well as sexually immature juveniles are often seen exhibiting various behavioural signs associated with courtship. Much of this activity may be used in the greeting and bonding of individuals after a period of separation and may not always be directed towards reproduction. It is also believed that a great deal of apparent sexual activity is merely play. Nevertheless, dolphins have been identified to be quite promiscuous and form no permanent social bonds with their mates.

The genitals of dolphins are internal as a result of evolutionary streamlining of their bodies. Because of this, it is difficult to distinguish between the sexes by superficial observation. Close examination of the underside (ventral surface) of the dolphin reveals sexual differences. The penis of the male is retained within a prepuce and appears externally as a slit similar to the genital slit of the female. The only conspicuous differences between the sexes is the distance between the anus and the genital slit. In the female, the genital slit appears to be continuous with the anal opening, whereas in the male it is spaced farther apart. The female has two additional mammary slits on either side of the genital slit which retains the nipples.

Bottlenose dolphins become sexually mature at variable ages. Females reach sexually maturity from about 8 to 10 years of age. Sexual receptiveness is generally communicated through behavioural means and chemical secretions, and is usually initiated by the female. Signs considered to be associated with courtship include chasing, nuzzling, rubbing, stroking, and erection. Such “foreplay” may continue over an extended period of time, but intromission lasts only 10 to 30 seconds. When mating occurs the male usually swims underneath the female. Quite commonly during courtship and mating, dolphins scratch each other with their teeth, leaving characteristic “rake marks” which usually heal shortly afterwards.

Bottlenose dolphins have a gestation period of twelve months. Birth can be difficult because the new-born calf is expelled from the uterus directly into the water in which it can drown or lose body heat at an excessive rate. Birth occurs rapidly and calves are born tail first. The calf’s dorsal and pectoral fins are quite soft and flaccid allowing it to pass more easily through the birth canal. The umbilical cord snaps during or soon after delivery, and the calf either instinctively swims or is guided to the surface to take its first breath. Often the mother is aided by an assisting dolphin referred to as an “aunty” or “midwife” dolphin. This “aunty” dolphin may sometimes help the calf to the surface to breathe if it is having difficulty doing this.

Calves weigh between 11 and 18 kilograms at birth and are usually from 107 to 122 centimetres long. They are generally darker than adults in appearance and may have light stripes on their sides as a result of their skin being “folded” while the calf is constricted in the womb. The rostrum is covered by fine hairs which disappear after several weeks. The calf is suckled underwater from nipples concealed in the mother’s mammary slits. The milk is extremely rich and is actively squirted into the mouth of the calf by muscular action, increasing the speed of transfer so as not to dilute it. The calf nurses two or three times per hour for less than one minute at each instance. Because the milk is high in fat and protein, it promotes the rapid growth of a thick, insulating layer of blubber.

Calves nurses for 12 to 18 months and begin to take fish when they reach a size of 160 to 170 centimetres (usually around six months of age). The calf remains extremely close to its mother and establishes a very tight bond. The mother tends to direct the calf’s movements and it is carried along in the mother’s “slip stream”, a form of hydrodynamic wake that develops as a result of the mother’s swimming. This helps the calf swim and enables the mother to keep up with the other member of the pod. Female calves often remain with their mothers throughout their lives, while male calves may wander soon after weaning.