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Most animals I see around seem to have two nostrils - humans, snakes, birds, fish .. and so on. From reading online I see that 2 nostrils provide a stereo olfactory effect. This stereo effect is caused by the difference in flow of air through the nostrils.

Does this stereo effect apply only to humans? Are there any animals that have a different number of nostrils? Say, more than 2, or less.

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up vote 1 down vote accepted

Moles and rats also smell in stereo (Catania 2013 and Rajan et al 2006), as do desert ants (link). The latter is rather interesting, given that ants "smell" using their antennae, which suggests multiple sensory structures, not necessarily nostrils, are required for stereo olfactory ability.

EDIT: Hammerhead sharks have two nares and also stereo olfactory ability (here). Dolphins have one spiracle, as do a number of other cetaceans.

It's actually not clear whether humans have stereo olfactory ability, (see edit below) as the "stereo" aspect derives from having nostrils that are sampling from different spatial locations. However, the research in the eastern mole suggests that this is not so far-fetched an idea, as their nostrils are not so far apart.

EDIT of EDIT: I take that back about humans: we do appear to have stereo olfaction.

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