-- MAIN LINKS --

-- EVOLUTION & ADAPTATION -- BALEEN WHALE SPECIES --
-- CLASSIFICATION -- CHARACTERISTICS OF BALEEN WHALES --
-- TOOTHED WHALE SPECIES -- DOLPHINS -- SEALS --
-- ENDANGERED SPECIES -- THREATS -- GAMES --

DOLPHINS: Reproduction and calf development

--- Background

Most of what we now know about dolphin reproduction has been discovered in controlled environments like oceanaria, because it is very difficult to study this in the wild in detail. The knowledge gained from these captive studies have facilitated studies of wild dolphins. All the information in this section deals with bottlenose dolphins, the species we have the most information about.

--- Maturity

Bottlenose dolphins become physically mature at a length of approximately 245 cm, when they are about 13 years old. Females become reproductively mature at about 10 years of age. A female is
considered reproductively mature if she ovulates regularly. Sexual maturity (the onset of ovulation) occurs at about 5-7 years. Males become sexually mature at a later age, between 10 and 15 years.

There are some indications that females become reproductively senile (go through a menopause), but there are records of females in their late thirties still successfully reproducing. As in cattle, female dolphins are usually called cows and male dolphins are called bulls. A dolphin baby is called a calf.

--- Periodicity and seasonality

Dolphin births can occur year round, but are mostly concentrated in certain times of the year. This peak breeding season varies per geographical area. In Florida, the peak season is in spring and early
summer, with a secondary peak in early fall. What exactly influences this seasonality is unknown, although probably photo-period (length of daylight time), food availability and water temperature play a role. These factors can influence the physiology and the hormonal balance.

When a female gives birth, she usually does not become pregnant during the lactation period. On average, there is a gap of about half a year between the end of lactation and a new pregnancy. This means that dolphin females usually can give birth successfully once every 3 or 4 years.

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--- Reproductive behaviour

In dolphin social behaviour, sexual interactions play an important role. Not only between males and females, but also among males. This behaviour seems to play a role in the determination of
dominance in the group. Dominant males are often seen mounting submissive males and females. These interactions are usually brief and can look fairly violent. They are not even necessarily restricted to animals of the same species.

Prior to mating there is often a lot of physical contact between the male and the female. This includes touching the pectoral and dorsal fins and the tail flukes as well as snout-to-genital contact. Since these behaviours are also part of normal affiliative behaviour, it is often difficult to tell if this is regular social behaviour or mating behaviour. This pre-mating behaviour can also include mild tooth raking (which can result in so-called rake marks: parallel superficial scars on the skin). The actual mating is usually brief.

Dolphins are polygamous animals. There is no permanent bond between a male and a female. In the wild, male pairs often roam around, looking for female groups with susceptible females. These
females are then separated from the group, mating occurs and the males leave again.

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--- Pregnancy

Dolphin pregnancy lasts about 12 months. The success of a pregnancy depends on a number of factors. Young females (5-10 years of age) can get pregnant, but the chances of a successful birth
are slim. The calves of young females, if not stillborn, usually do not survive. In older and more experienced females, the chances of a successful pregnancy are a lot higher. Still, the chances of the calf surviving its first year are not really high, compared to adults. Whereas the adult survival rate is 95-96% the calf survival rate is lower than 80% in the wild. This estimate does not take into account calves that die within a few days after birth and are never seen by researchers. The number of stillbirths and failed pregnancies in the wild is unknown. Overall about 14% of the mature females produce a calf that survives its first year every year.

In most oceanaria, the dolphins are examined regularly and pregnancies can be detected early by measuring the progesterone level in the blood. When a pregnancy is suspected or detected, this can be followed up by regular ultrasound examinations. Many dolphins are trained to lay still in the water and cooperate with such an examination so it can be performed often. (Note that the sounds produced by ultrasound machines are not audible to dolphins. Although dolphins used a form of ultrasound for echolocation, the dolphins' echolocation signals are in the 60-100 kHz range. The medical ultrasound machine uses signals that have a much higher frequency (2-5 Mhz), which is beyond the dolphins' hearing range. These examinations therefor do not disturb the dolphins).

Dolphins in oceanaria that participate in performances can continue to do so when they are pregnant. The trainers will avoid asking the pregnant females high energy behaviours such as high jumps and slide outs. For the rest it is left up to the animals themselves. If they do not want to participate, they are free to refuse. As can all performing dolphins. Participation in a performance is always voluntary and the trainers have no means to force the animals to perform anyway.

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--- Birth

Normally a female gives birth to a single calf at a time. Twins are very rare and usually do not survive. The calf is born under water. Usually, the calf is born tail first. (Although usually calves are born tail first, head first births also occur and can be successful.) At the onset of birth, the female can be seen flexing her body in contractions. During these contractions, milk can be ejected from her nipples, which are located left and right of the genital opening annex birth canal. Next the calf's tail becomes visible. The mother may swim around with the tiny tail protruding for some time. Next the rest of calf's body emerges and when the calf emerges completely, the umbilical cord is severed. The calf then swims to the surface to takes its first breath, after which it is picked up by its mother and starts swimming in her wake.

If the calf does not swim to the surface immediately, the mother will push it to the surface. This often is a sign that the calf is weak. Occasionally, dolphins will also push stillborn calves to the surface and carry it around for some time. This has been observed both in oceanaria and in the wild.

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--- Calf development

When the calf is born, its dorsal fin and tail flukes are soft and floppy. In a short time, these fins will straighten and stiffen. They need to be soft at birth, otherwise the birth process would be very difficult. When inside the womb, the calf is curled up. This can still be seen for quite some time after birth, because the calf will have some light stripes on its body, called fetal folds. As the calf ages, these folds become darker and in time will become invisible. At birth, calves are 90-130 cm long and weigh up to 30 kg. The dolphins from Florida are somewhat smaller and at birth they measure 115-120 cm and weigh around 20 kg.

The calf can swim right away after birth. However, most of the time, the calf will ride the mother's wake, so that it can follow the mother without spending a lot of energy trying to keep up. That is why you will usually see the calf slightly behind and above the mother. This is called the echelon position. In addition, by riding the mother's wake the calf does not need to pay a lot of attention to where it is going. At first, its coordination is not too good and it can easily bump into things. Its echolocation system does not work yet. It may take a couple of months before this system is fully functional. The calf stays in touch with its mother by touching her with its flippers and by sound. The mother whistles a lot. The calf will gradually develop its own whistle repertoire and in time its own unique signature whistle.

The calf grows fast on its diet of high fat milk. Dolphin milk contains 33% fat. Leevi, the calf born to Näsi at the Delfinaario, was at birth about 1 m long and weighed about 20 kg. One year later, he was nearly 2 m long and weighed close to 100 kg.

Dolphin calves begin taking solid food at 4-11 months of age. For some time they will consume both solid food and mother's milk. They usually are fully weaned at 18-20 months, although there has been one case of a 38 month old calf that was still not weaned.

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--- Calf care

There are individual differences in the way mothers take care of their calves. Studies in oceanaria have shown a number of different strategies to child rearing in dolphins. In general, older and more
experienced mothers are better equipped for the task than first time mothers, especially young, not fully mature mothers. The mother and calf are quite close during the calf's first year. Sometimes, another female in the group will help in taking care of the calf, occasionally taking the calf along, giving the mother a break.

Often only one female does this. This female is often referred to as an aunt, although this female is not necessarily related to the mother or the calf. This aunt sometimes even assists during and immediately after the birth. Usually, the aunt is an older and more experienced female, without a calf of her own. There have been a few, where the aunt completely took over the calf and would not let the mother near it. These have been rare, though.

Who takes on the role of aunt differs per group. At the dolphinarium in Kolmården, Sweden, this role it taken by an old female, Vicky, who herself had numerous calves. However, at the Särkänniemi Delfinaario, the aunt role was taken by Niki, the youngest female with no experience in calf care herself. The males play no role in the care of the calf and in the wild are even kept away (or stay away) from the calves.

Especially in the first weeks, the mother keeps the calf close to her. The calf is usually positioned slightly behind the dorsal fin and a bit above the mother. This position is called the echelon position. In this position, the calf experiences the maximum force of the wake the mother generates while swimming and is thus pushed along by its mother. The calf then does not need to spend much energy for swimming: it gets a free ride from its mother (see above)

The mother's nipples are located next to the genital slit. There is one on each side. The nipples are hidden in small mammary slits. If you look closely at the picture, you can see the mammary slits. When nursing, the calf leaves the echelon position and goes toward the nipples. The female will turn slightly on her side to allow better access to the area. When the calf nudges the nipple, the mother will actively squirt the milk in the calf's mouth. This speeds up the nursing process.

After birth, the calf will start nursing within a couple of hours. At first the calf will nurse a couple of times per hour. The nursing bouts are very short, lasting only a few seconds. Once the calf gets older and more experienced, the number of nursing bouts decreases and and the length of the nursing bouts increases.

Dolphin milk is very rich. It contains 33% fat and about 7% protein. The milk contains about 1% lactose. There are some variations among different species. Harbour porpoise milk contain 45% fat and 11% protein, blue whale milk 42% fat and 11% protein and spotted dolphin milk 17-25% fat and 7-9.6% protein. On this rich diet, a calf grows fast and quickly develops a thick insulating blubber layer.

During the first weeks, the mother keeps the calf close to her. Once the calf becomes older it will leave her side more and more often. The mother will however keep an eye on the calf and retrieve it when it strays too far or displays inappropriate behaviour. When a calf misbehaves, the mother will discipline it by pushing against the bottom, pushing it or blasting it with loud sounds. In this way, the calf learns its place in the group and learns proper social behaviour. The calf also learns by observation. This is quite obvious in oceanarium settings. If the mother participates in a performance, the calf will usually start performing the same behaviours as its mother, without any encouragement from the trainers.

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--- Different sexes

The genitals in both females and males are concealed, but all that is needed is a glimpse of the belly. Both sexes have a navel and a genital slit.

Female genitals

In females, the genital slit lies closer to the anus than to the navel. The genital slit is longer than in males. In addition, the female has one mammary slit on both sides of her genital slit. The teats are concealed in the mammary slits, and become more visible only when the female is pregnant or nursing her calf.

In males, the genital slit lies a little bit closer to the navel than in females. In addition to these differences, the sexual dimorphism in dolphins can be seen in the external appearance and the body build: the males are usually more robust and bigger in size than the females.

Male genitals

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