The task of understanding cetacean sexual behaviour is made harder by the fact that, unlike most mammals, males and females are almost impossible to tell apart, even when they are fully grown. There is no variation in skin colour or patterning and in nearly all cetaceans males and females show no external, physical sexual differences either (as in both males and females the genitalia are hidden). This is almost certainly because vision plays no part in their dark underwater world, compounded by the overriding need for a streamlined aqua dynamic body shape.

If you are able to closely examine their underside then you will see that in females the genital slit is quite close to the anus and in males this is almost half way between the anus and the navel. Females also have 2 barely visibly mammary slits either side of the genital opening. Females are easy to spot once they have had a calf because the youngster will be seen sticking closely to her side for several years.

Age

In toothed whales it is possible to determine the age of an individual by examining the alternate dark and light layers of dentine visible in teeth (rather like rings in a tree) inside their teeth. Broadly speaking, one complete layer is equal to one year of growth. The age of a baleen whale cannot be studied in this way as they do not have teeth. However, the horny plugs found in their ear canal sheds cells and secrets wax, creating dark and light bands that can be sectioned and counted in the same way as dentine layers in teeth.

As well as the age that they can breed and give birth, the reproductive history of female cetaceans can also be gauged from an analysis of their ovaries. With every egg produced, follicles erupt in the ovaries, and instead of disappearing like they do in most mammals, they remain for the rest of the animal's life. By counting these follicles it is possible to determine at least how many times a female as ovulated (though not the number of times this has resulted in a pregnancy).

Sexual maturity

It is believed that cetaceans reach sexual maturity before they reach physical maturity. Females tend to mature before males in many species and the age at which it is believed each species reaches sexual maturity varies also. Female bottlenose dolphins can become sexually mature as young as 5 years old, although most don't become mature until their 'teens' at around 11 or 12, and males become sexually mature between the ages of about 9 and 13 years old. Spinner dolphin females reach sexual maturity between 4 and 7 years old, and males between 7 and 10 years old and pilot whale females reach sexual maturity between 6 and 7 years old, and males at 12 years old. Common Dolphins reach sexual maturity as young as 3-4 years old, and spotted dolphins between 6 and 8 years old, whilst the killer whale does not mature until it is between 10 and 18 years old.

There may be problems during the pregnancy if a dolphin conceives when they are too young and not fully-grown, and an older female will also be more experienced and this should benefit her calf.

Breeding Seasons & Cycles

The timing and length of a species' reproductive cycle depends on both biological factors; such as the frequency of ovulation, the gestation period, and the age at which the young are weaned together with environmental factors; such as the availability of food for both the pregnant or nursing mother and her new born. In stressed populations mating takes place at an earlier age and calving at shorter intervals.

Name of Dolphin	Gestation Period	Length of nursing time
Bottlenose dolphin	12 months	12-18 months
Common dolphin	10-11 months
Spinner	10.5 months	1-2 years
Pacific whitesided dolphin	9-12 months (est.)
Atlantic whitesided dolphin	9-12 months (est.)
Risso's	13-14 months (est.)
Spotted Dolphin	11.5 months	11 months
Pilot Whale	12-15 months	22 months
Killer Whale	13-17 months

Courtship & Mating

When it is difficult even to identify the age or sex of cetaceans, the task of recognizing sexually related behaviour is extremely hard. In many respects cetaceans are no different to any other animal; in order to mate sexually mature adults must be able to find each other, confirm a readiness to breed and then obtain the opportunity to mate.Male dolphins, during the mating season, compete very vigorously with each other through showing toughness and size through a series of acts such as head butting. Some courtship behavior exists when a male finds a someone they want to mate with. Fluke slapping, high speed chases, nuzzling and body rubbing , clinging along to that female, posing for the female, stroking, mouthing, jaw clapping, and yelping.could all be part of courtship behaviour. In bottlenose dolphins such courtship behaviour, if successful, is followed by the male swimming beneath the female and mating with her, their bodies at right angles to each other. The two animals arrange belly to belly, the penis extends out of its slit and is inserted into the vagina. As in many mammals, mating is swift, the act lasts only 10-30 seconds, but is repeated numerous times, with several minutes break in between.

Dolphins don't stay with the same partner for life and, in fact, males don't play any part in helping to raise calves. Dolphins are promiscuous animals and spend a lot of their time in sexual play with almost any other member of the group.

Pregnancy & Birth

After fertilization the female's pregnancy is similar to that of other mammals. Bottlenose dolphins' gestation period (the length of time that they are pregnant) is about 11 or 12 months. Cetaceans are unusual in that the size of their offspring at birth is not determined by variations in the length of the pregnancy, but by the differential rate of foetal growth. Calving in bottlenose dolphins can take place year-round with peaks in some areas during spring and fall.

When the time comes to actually have the baby, a mother may move into certain areas, away from all the normal noisy goings-on. The initial stages of birth may be quite slow, with the flukes of the partly emerged offspring protruding for an hour or more. But once the final stage has been reached the young emerge rapidly usually within 20 to 30 minutes. This is essential as once the umbilical cord is ruptured the calf must be quickly transported to the surface to take it's first breath. A dolphin calf is born tail-first with eyes open, senses alert and enough muscular coordination to follow its mother immediately.

Cetaceans typically give birth near the surface. The lack of oxygen in the lungs of the young cetacean together with the lack of blubber means that they are not buoyant, they sink rather than float. As a result the mother, and sometimes attendant 'midwives' immediately nudges the newborn to the surface for it's first breath.

Soon after birth the calf is fed by the mother near to the surface so that it can breathe regularly. When suckling, the young approach from the rear. The mother will often turn to her side, to aid the youngster in finding the nipple. Because cetaceans do not have lips, the calf cannot suckle in the same way as terrestrial mammals. Instead, the calf grips the mother's now protruding nipple between its tongue and the roof of its mouth to make what it is believed a "tube". This speeds feeding, since the young cannot suckle for long periods due to the necessity to breathe.

According to research carried out by University of California newborn dolphins and killer whales don't sleep for a month.

For the first six months or so, a calf will feed on its mother's milk. Cetacean milk is creamy white in colour, and may be tinted pink. It contains an unusually high percentage of fat; 15 -50 percent fat, compared with 3-5 percent in cows milk and a low sugar content. This may be due to the fact that the animal must develop quite quickly. A blue whale doubles it's birth weight in only 7 days, while a cow may take 50. Then the youngster will start adding fish to its diet, which it may even start catching itself. As it gets better at catching its own food, the calf needs less milk and becomes a little less dependent on its mum.

On average bottlenose dolphin calves measure between 0.7-1.2 metres at birth and weigh around 30 kg. New-born calves are generally much paler than adults and have creases known as 'foetal folds' down their sides, probably from having been squashed up in the mother's womb. The calf is so dependent on its mum that the two must stick together like glue. When a calf is first born, its mother will whistle over and over again. The calf will learn her whistle and always be able to find her, even amongst lots of other mothers and calves.

Although the young toothed whales typically nurse for longer, all cetacean young begin to feed themselves at the age of a year or so. The young are born without teeth or baleen but these usually appear within a few months. While nursing lasts between one and a half to two years, the mother will remain with her calf for a period between three and eight years. The young live closely with learning how to catch fish and other important tasks.

Bottlenose dolphins are quite slow breeders and females usually only have a calf every three to five years, this is also the case for killer whales and pilot whales, but spinner dolphins are believed to reproduce every 2 to 3 years, and spotted dolphins every two years. Little is known about the Killer whale's breeding habits. Newborn calves have been observed throughout the year suggesting that that mating can occur at any time with no particular breeding season. Based on long-term field studies, females are believed to be reproductively active into their early 40's. The maximum age for males is unknown. Female pilot Whales have been observed to have calves as late as 35 years old, and lactate as late as 51. This evidence indicates that females may nurse their last calf until puberty (up to 10 years in males). Bottlenose dolphins also give birth to just one baby at a time; dolphin twins are very, very rare, and sadly normally die either before birth or soon after being born. The mother will not have any more calves until it has weaned the first one.

Juveniles

In general, juvenile bottlenose dolphins of both sexes will leave their maternal group somewhere between the ages of three and six. They form their own groups and properly fend for themselves for the first time, a difficult period for young dolphins. They stay in these groups until they become sexually mature. There is some variation in the age at which sexual maturity is reached, the reproduction rate and the life expectancy among the different species of dolphins.

Life-span

It is difficult to accurately determine the lifespan of cetaceans as many of them have ended artificially by being harpooned or caught in nets. Cetaceans do no live as long as had been previously thought for such large animals. While elephants regularly achieve three score years and ten, the larger baleen whales appear to live 30 or 40 years, but among the toothed whales, bottlenose dolphins seldom live longer than about 25 years, but the maximum age (which only a few will reach) is more than 40 years. An interesting fact, however, is that some female cetaceans often live beyond the menopause. Although this is common in elephants and humans, this is a relatively rare occurrence in the animal kingdom as there is little obvious value to a species for it's members to live beyond the point that they can produce offspring.

Female bottlenose dolphins, on the whole, live longer than males. They have been recorded at over 50 years of age in the wild whereas the more stressful life of the males apparently takes its toll, and they rarely live more than 35 years.