Dogs, most of us think, have the best noses on the planet. But a new study reveals that this honor actually goes to elephants. The power of a mammal's sniffer hinges on the number and type of its olfactory receptor genes. These genes are expressed in sensory cells that line the nasal cavity and are specialized for detecting odor molecules. When triggered, they set off a cascade of signals from the nose to the brain, enabling an animal to identify a particular smell. In the new study, scientists identified and examined olfactory receptor genes from 13 mammalian species. The researchers found that every species has a highly unique variety of such genes: Of the 10,000 functioning olfactory receptor genes the team studied, only three are shared among the 13 species. Perhaps not surprisingly, given the length of its trunk, the African elephant has the largest number of such genes—nearly 2000, the scientists report online today in the Genome Research. In contrast, dogs have only 1000, and humans and chimpanzees, less than 400—possibly because higher primates rely more on their vision and less on their sense of smell. The discovery fits with another recent study showing that Asian elephants are as good as mice (which have nearly 1300 olfactory receptor genes) at discriminating between odors; dogs and elephants have yet to be put to a nose-to-trunk sniffer test. Other research has also shown just how important a superior sense of smell is to the behemoths. A slight whiff is all that's necessary, for instance, for elephants, such as those in the photo above, to distinguish between two Kenyan ethnic groups—the Maasai, who sometimes spear them, and the Kamba, who rarely pose a threat. They can also recognize as many as 30 different family members from cues in their urine.