"They are the only cetacean species with tusks, and scientists have long wondered why, since their diet primarily is squid."

Scientists have now used DNA analysis to show the teeth probably evolved as secondary sexual traits to help females select males of the right species.

The shape of the teeth, or tusks, varies markedly between different species.

In some, they actually appear to hinder feeding, as they wrap over the upper jaw, preventing it from opening fully.

Females do not show teeth; and this difference between the sexes, or sexual dimorphism, is virtually the only way to tell them apart.

Ornamental tusks

"It turns out that tusks are largely an ornamental trait that became a driver in species separation," said Dr Baker, whose research is reported in the journal Systematic Biology.

"The tusks help females identify males within their species, which could otherwise be difficult as these species are quite similar to each other in shape and coloration."

So females use the shape of the teeth to select males of the right species to mate with. They may also choose mates based on the size or shape of the individual's teeth or of the scars they bear.

In turn, this also means that the more successful males are the ones with the shape of teeth most characteristic of that particular species, ensuring that the shapes are preserved and perhaps enhanced over evolutionary time - a secondary sexual characteristic.