How are whale calves born?

Giving birth in the open sea poses an extraordinary challenge: the calf has to breathe from the moment it is born, unaided, without drowning, in a medium where it could suffocate in a matter of seconds. In cetaceans, evolution has shaped a mode of birth and of suckling unique among mammals. No zoologist remains unmoved by the beauty of these adaptive solutions: a calf born at sea is probably one of the most vulnerable and the best prepared newborns in the animal kingdom.

Gestation: long or very long

Gestation is long in cetaceans. Longer, as a rule, than in land mammals of comparable size. The harbour porpoise, the smallest of them, carries its young for about 10 to 11 months. The common bottlenose dolphin for about 12 months. The fin whale and the blue whale, despite their gigantic size, have a comparable gestation, 11 to 12 months, which is explained by the seasonal migratory cycle that requires annual synchronisation between birth and the warm tropical zone. The killer whale is intermediate at 15 to 18 months. And the record belongs to the sperm whale, which carries its young for 14 to 16 months.

At birth, the size of the calf also varies enormously. The harbour porpoise gives birth to a calf of 70 to 90 cm. The blue whale, a calf of 6 to 7 metres and 2.5 tonnes, already larger than an elephant. The sperm whale weighs about a tonne for 3.5 to 4.5 metres at birth.

Delivery: the “tail first” solution

In land mammals, the newborn usually emerges head first. This is efficient: the head is the hardest part and opens the way. The problem, for a cetacean, is that the passage takes time, and that during those minutes or hours the baby is partly out and partly still engaged in the canal. If the head came out into the water first, the calf could inhale water before its whole body was out and it could rise to breathe.

Evolution has selected an elegant solution: the “tail-first” presentation. The calf emerges tail first. This posture is rare among mammals but consistent in cetaceans. It has two advantages: the head stays inside the canal, and so away from the water, until the last moment, and when the head finally emerges the calf is already almost entirely in the water and can be guided to the surface at once.

Delivery lasts from a few minutes to several hours. The umbilical cord breaks without being tied off; it is simply stretched until it tears. Once born, the calf is pushed to the surface immediately by its mother, or sometimes by another female in the group (allocare is documented in sperm whales, in dolphins, and in some killer whale populations). Its first breath comes within the seconds that follow. This is a critical moment: if the breathing reflex does not fire, the calf dies.

Suckling underwater

How is a calf to suckle underwater without swallowing salt water? Evolution has solved this problem too, and elegantly. The mother’s teats do not hang as they do in land mammals. They are concealed in mammary slits on either side of the genital slit, half retracted into the skin. When the calf wants to suckle, it presses its rostrum against the slit, which opens.

And the mother actively ejects her milk by muscular contraction of the peri-mammary muscles. The calf has no need to suck or to draw: it receives the milk straight into its mouth, like a syringe injecting. This active ejection allows the milk to be transferred quickly (the calf can suckle in a matter of seconds before going back up to breathe), and avoids letting seawater into the mouth.

The milk itself is extraordinary. It contains between 30% and 50% fat, against just 4% for human milk. Its consistency is that of toothpaste or mayonnaise. This richness allows spectacular growth. A blue whale calf gains about 90 kilograms a day. It can double its weight in five months. It is this rapid accumulation of fat that will let it set off on migration at a few months old and survive in colder waters.

The role of the group

In several species, birth is a collective event. In sperm whales, several females of a social unit attend the delivery and then take part in the collective protection of the calf. While the mother dives deep to hunt, another female (a “babysitter“) stays at the surface with the young animal. This division of tasks is essential: the calf cannot dive deep with its mother, but neither can it stay alone at the surface, where it would be vulnerable to predators.

Allocare is also documented in some resident killer whale populations and in common bottlenose dolphins. See our page on how long the mother-calf bond lasts.

Neonatal mortality

Mortality in the first year is high in several species. In the common bottlenose dolphin it is estimated at between 30% and 50% depending on the wild population. The main causes: predation by sharks (in warm waters), maternal abandonment (sometimes linked to stress), undernutrition, and chemical contaminants transferred through the milk, which is extremely rich in lipids, see our page on PCBs and microplastics. In humpback whales migrating between the tropics and the poles, neonatal mortality is lower (perhaps 10% to 15%) thanks to the choice of calving grounds in sheltered warm waters.

Sources

• Slijper, E.J. (1962). Whales. Hutchinson & Co., London.
• Mann, J., Connor, R.C., Tyack, P.L. & Whitehead, H. (eds.) (2000). Cetacean Societies: Field Studies of Dolphins and Whales. University of Chicago Press.
• Whitehead, H. (2003). Sperm Whales: Social Evolution in the Ocean. University of Chicago Press.
• Würsig, B., Thewissen, J.G.M. & Kovacs, K.M. (2018). Encyclopedia of Marine Mammals, 3rd ed. Academic Press.
• Oftedal, O.T. (1997). “Lactation in whales and dolphins: evidence of divergence between baleen- and toothed-species”. Journal of Mammary Gland Biology and Neoplasia, 2.

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Further reading