December 15, 2008

Scientists Say Whale Teeth Aid In Mating

Research suggests that the bizarre teeth of male beaked whales have evolved to help females choose their mates.

Apparently, the males use the two teeth on the outside of their jaws not for eating, but for scratching each other.

DNA analysis shows the teeth probably evolved as secondary sexual traits to help females select males of the right species, scientists said.

Beaked whales make up the least known group of whales or dolphins. They spend most of their time deep in the ocean foraging for food, surfacing rarely and briefly.

But some species are only known from dead whales washed up on shore and have never been seen alive.

Scott Baker, associate director of the Marine Mammal Institute at Oregon State University said beaked whales are among the least known, least understood and, frankly, most bizarre whales in the ocean.

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

He said 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.

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

DNA samples were taken from 14 beaked whale species and used it to construct a family tree depicting how the various species had developed.

One theory is that different groups of beaked whales emerged in ocean canyons that were more or less isolated from the wider oceans, and that this pattern of evolution was responsible for different shapes of tooth.

However, the genetic work suggests otherwise.

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

"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."

The 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.

It 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.

This is the first time that secondary sexual selection has been shown to shape the evolution of any marine mammal, researchers said.

Natacha Aguilar, who has been studying beaked whales in the Canary Islands for a decade, agreed that the theory made sense.

"Little is known about the social structure of beaked whales, but at least some species live in harem-like groups where one adult male accompanies a group with females and juveniles," she said.

"In this context, the male will need to fight with other males for access to a female group, and to be attractive to the females for them to choose him.

"These parameters all favor the hypothesis of sexual selection as a force in shaping the most striking sexual dimorphism characteristic in beaked whales, the tusks."

Still, much about these elusive animals remains a mystery, including how many there are in the seas, where they live, and exactly how many species exist.


Image Caption: The males of most beaked whale species sport two unusual teeth on the outside of their mouths. Each species' teeth are a different shape. They appear to play no role in eating. The males use the teeth to scratch each other, leaving patterns of scars that can be used to identify individual whales.


On the Net: