"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.
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].)
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