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Narwhal tusk size plays role in sexual selection, researchers say

The long single tusk of the narwhal whale is for sexual selection, Arizona State University researchers concluded in a study released on Wednesday. Photo courtesy of Harvard University Gazette
The long single tusk of the narwhal whale is for sexual selection, Arizona State University researchers concluded in a study released on Wednesday. Photo courtesy of Harvard University Gazette

March 18 (UPI) -- The unicorn-like tusk of the narwhal whale is an element in sexual selection, and size matters, a study published on Wednesday indicates.

The male narwhal, which typically grows to 4,200 pounds and a length of 17 feet, grows a single tusk from the front of its head. Regarded as a modified tooth, some grow to eight feet in length. But since the species spends much of its time beneath the ice of the Arctic Ocean, research is rare and scientists have had to speculate on the tusk's purpose.

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A study, published Wednesday in the journal Biology Letters offers evidence that the tusk is there for sexual selection, like plumage on a peacock or an elk's antlers.

"Broadly, I'm interested in sexual selection, which is responsible for creating some of the craziest traits in biology," Zackary Graham of Arizona State University, the lead author of the study, said in a press release. "As an evolutionary biologist, I try to understand why some animals have these bizarre traits, and why some don't."

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Data on the morphology, or structure, of 245 adult male narwhal skulls, collected over 35 years, were analyzed for the study. The length of the tusks varied from 1.5 to 8.2 feet in length, in narwhals of otherwise similar body size. They are used as weapons in battles between two males, and researchers concluded that the tusk is a sexual signal for female narwhals.

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Graham said evidence of "tusking," or scarring when two male narwhals rub tusks in battle, and examples of tusks impaled in skulls during aggressive behavior, were found.

"Based on the disproportional growth and large variation in tusk length we found, we provide the best evidence to date that narwhal tusks are indeed sexually selected," researchers wrote in the study. "By combining our results on tusk scaling with known material properties of the tusk, we suggest that the narwhal tusk is a sexually selected signal that is used during male-male contests."

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The narwhal is regarded as an endangered species, and is largely found in Canada.

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