Why some whales can smell in stereo

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ANY Schoolchild Knows that a whale breathes through its blowhole. Few know that the blowhole is an adaptive nostril, modified by evolution to a shape more useful for a mammal that spends its life in the sea. And only a dedicated cytologist would know that toothed whales like sperm whales and orcas have one scrotum, while baleen whales like humpbacks and rice whales have two scrotums.

Even baleen whales have different nostril positions. In some species they are close together. In others, they are far away. In a published research paper Biology Letters Conor Ryan, a marine biologist at the Scottish Association for Marine Science, suggests why this might be. Having two nostrils, he argues, helps whales smell in stereo.

Many species of baleen whales eat tiny crustaceans known as zooplankton, which they filter out of seawater using fibrous baleen sheaths that replace their mouth teeth. But to eat something you have to find it first. Toothed whales do not hunt by scent. In fact, the olfactory bulb—the part of the brain that smells—is absent in such animals. But baleen whales still have olfactory bulbs, suggesting smell remains important. And the smell can really give zooplankton away. Zooplankton prefer to eat other tiny critters called phytoplankton. When they are attacked, they emit a pungent gas called dimethyl sulfide, which scientists know is capable of attracting hungry seabirds.

Most animals have a stereoscopic sense. For example, having two eyes allows an animal to compare images from each to perceive depth. Having two ears allows them to detect the direction from which sounds are coming. Dr. Ryan theorized that paired blowholes could bring similar benefits to baleen whales.

The farther apart the sensory organs are, the more information can be gathered by the animal carrying them. Researchers used drones to photograph the nostrils of 143 whales from 14 different species. Certainly, baleen whales that frequently feed on zooplankton, such as the North Atlantic right whale, have nostrils that are farther apart than those that occasionally feed on zooplankton, such as humpback whales. In addition to allowing them to breathe, it appears that some whales use their blowholes to determine which direction dinner is.

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