"Sexy" Tusks Led to New Whale Species?

Tasha Eichenseher
National Geographic News

December 19, 2008

The two curled, tusklike teeth of the male beaked whale evolved to attract females as well as to battle other males, according to new research.

Female beaked whales' apparent attraction to the tusks may have spurred the development of new species.

The unusual tusks—found on the outside of the male's mouth—have baffled scientists because they are not used for capturing prey, said study co-author Scott Baker, associate director of the Marine Mammal Institute at Oregon State University.

"Up until now, the purpose of the beaked whale tusks has been mysterious," he added.

Beaked whales and the narwhal are the only cetacean species with tusks, according to Baker. Cetaceans are marine mammals such as whales and dolphins. In the narwhal the single unicorn-like tusk is a modified upper tooth, whereas the paired tusks of the beaked whales are modified lower teeth.

After using DNA samples to reconstruct the evolutionary relationships of 13 of the 14 known beaked whale species, Baker and lead author Merel Dalebout of the University of New South Wales, Australia, said the tusks may help females identify male members of their own species to mate with.

The findings are published in the December issue of the journal Systematic Biology

Family Dynamics

Existing beaked whale populations are found in nearly every ocean. Dalebout and Baker have discovering new species as recently as 2002.

These rare whales are mysterious, because they spend much of their time at great depths searching for food—primarily squid, which the whales suck up like a vacuum cleaners.

"There are several beaked whale species that are still known from only a handful of specimens (strandings) and some have never been seen alive," Dalebout said via email.

Robert Pitman, a marine biologist with NOAA, was not part of the tusk research but recently studied what may be a new beaked-whale species near Palmyra Island in the Pacific Ocean.

"For people interested in cetacean evolution the most perplexing problem has always been: Why are there so many species of [beaked whale]?" he said. For example, there are only three known right whale species.

Baker and Dalebout, using whale DNA samples, set out to answer that question by piecing together how divergent, and often geographically isolated, species are related.

"When populations become isolated, we expect them to adapt to different niches and diverge—geographic isolation creates speciation," said Baker, who received partial funding for his study from the National Geographic Society's Committee for Research and Exploration. (The National Geographic Society owns National Geographic News.)

But in the ocean, these geographic barriers are absent, and other forces, like sexual selection, could be involved in the formation of new species.

In the case of the beaked whale, Baker's team noticed that tusks often differed between closely related species, particularly when they were known to overlap in their distribution.

The researchers hypothesize that the large whale teeth evolved over time to help females distinguish males of one species from males of another.

"[Male beaked whales] of different species are rather similar in size and appearance," Baker said. "In a way these teeth are a kind of ornament or signal for the females."

NOAA's Pitman, who received funding for his work from the National Geographic Waitt Grants Program adds, via email: "Sexual selection can be an important spur for evolutionary innovation—I think Dalebout et al. are on to something here."

However, he adds, he doesn't think the animals are using visual cues to find one another.

"These are acoustic animals that spend perhaps 99 percent of their time in pitch black," Pitman said.

Sparring Whales

The males also use the tusks for sparring, giving each other crisscrossed patterns of scars. This could be another form of sexual selection, Baker explained, but directed toward other competing males, rather than toward attracting females.

"No one has ever actually seen one of these battles," Pitman said. "Presumably it all happens in the abyss."

"Unfortunately these rare whales face an increasing number of threats," Dalebout said. From ingesting plastic and being caught on longlines and other fishing gear to falling victim to noise pollution, the beaked whale has a tough road ahead, according to experts.

(Related: "U.S. Navy Sonar May Harm Killer Whales, Expert Says" [March 31, 2004].)