Do cetaceans have milk teeth?

In most mammals, humans included, the dentition develops in two stages: first the milk teeth, which come through in early childhood and fall out at around six to twelve years of age, then the permanent teeth that replace them and stay with the animal for the rest of its life. This strategy is called diphyodonty (“two generations of teeth”). Cetaceans, although all mammals, do not follow this rule. To understand their particular dentition, one first has to distinguish the two large groups that make up the cetacean order.

Odontocetes: a single set of teeth for life

Odontocetes, dolphins, killer whales, sperm whales, porpoises, belugas, narwhals, are the toothed whales. They form a very clear exception among mammals: they are monophyodont, which is to say that they have only one single generation of teeth throughout their existence. A first set comes through during early infancy (the biological equivalent of our milk teeth), but that set never falls out. And the second set, which in an ordinary mammal would normally replace it, remains a bud in the jaw and never develops.

This peculiarity is explained by the very function of their teeth. In odontocetes, the teeth are not used for chewing. Their prey, fish, squid, sometimes small marine mammals, are swallowed whole, sometimes after simply being immobilised by a quick bite. The teeth therefore serve mainly for gripping, not for cutting. For that function, simple conical teeth, all identical (they are said to be homodont), are amply sufficient, and their moderate wear calls for no replacement.

Striking variation from one species to another

While all the teeth of a given individual look alike, the number of teeth varies enormously from one species to another. The Ganges river dolphin can have more than 130. The spinner dolphins of the genus Stenella have up to 250, the absolute record among cetaceans. At the other extreme, some beaked whale species have only one or two pairs of functional teeth, sometimes only in the males.

The narwhal is the most singular case: its normal teeth remain as buds in the jaw and never emerge, except one, the male’s left upper canine, which grows in a spiral until it reaches three metres in length and passes through the upper lip. It is probably the most extraordinary tooth in the animal kingdom.

Mysticetes: no teeth at all

The second large group, the mysticetes, the baleen whales (blue whale, humpback whale, grey whale, rorquals and so on), has no teeth at all in adulthood. Over the course of evolution they were replaced by baleen, the keratin plates hanging from the upper jaw that filter krill and small fish.

Remarkably, mysticete embryos develop tooth buds in the mother’s uterus! These buds even begin to form, before regressing and disappearing completely before birth. It is a powerful evolutionary vestige, direct evidence of the common ancestry of baleen whales and toothed whales. About 34 million years ago, at the end of the Eocene, the two lineages separated and the ancestors of the mysticetes progressively lost their teeth in favour of the filtering system. But the genetic programme for the dentition has remained encoded in their DNA, switching on only briefly during embryonic development.

A strategy that raises questions

Why this absence of renewal in odontocetes? The leading hypothesis is that the food, slippery and easy to swallow, places little mechanical demand on the teeth. A single well maintained set of teeth can therefore last a whole lifetime, and a cetacean lifetime can be long: 60 to 80 years in the killer whale, more than 200 years in the bowhead whale. This evolutionary strategy is so remarkable that it has inspired palaeontologists and biostratigraphers: examining the growth layers in the teeth now makes it possible to date an individual’s age precisely, as one would read the rings of a tree.

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