How does a dog know where a smell is coming from?

A dog on a walk stops dead. Its nose drops to the pavement, it sniffs in quick bursts, then it veers left, then right.

It isn't just noticing a smell. It is collecting information, very fast.

A dog can sniff up to seven times a second.

When researchers measured seven dogs, from 6.8 kg to 52.9 kg (15 to 117 lb), every one sniffed at between four and seven times a second, whatever its size. Human sniffing, by comparison, runs at well under once a second.

These aren't ordinary breaths. Sniffs are short, sharp pulls of air that keep fresh samples moving through the nose.

Each nostril takes its own sample.

The same team built a computer model of airflow through a dog's nose, reconstructed from scans. At the peak of a sniff, each nostril drew in air from only about 1 cm (0.4 in) in front of it. That is less than the gap between the nostrils, so each one samples a separate patch of air.

The researchers suggest the two samples may be compared to help locate the source. It is something like smelling in stereo, a phrase borrowed from a 2006 study in which rats needed both nostrils to pinpoint a smell. For dogs, the evidence comes mainly from the airflow model.

In an airflow model of a sniffing dog, each nostril drew air from its own small patch, about 1 cm ahead, and exhaled air left down and to the sides. Researchers suggest the separate samples may help a dog work out where a smell is coming from. Simplified, not to scale.

Then comes the clever part: breathing out.

The shape of a dog's nose sends exhaled air down and out to the sides, not straight ahead. That has also been seen in high-speed flow experiments. Near the ground, the outgoing air stirs up scent beside the nose. In the model, swirls within it draw that scent towards the nostrils for the next sniff.

A dog breathes out to the sides, which limits how much it blows away the scent directly ahead of its nose.

Inside, the routes split. In the model, about 12–13% of inhaled air went to a dedicated smelling chamber at the back of the nose, and exhaled air bypasses that chamber, leaving the scent there for longer.

The next time a dog stops dead and starts sniffing, it isn't just catching a smell. It is taking several samples a second, two at a time, and breathing out in a way that helps keep the next one ready.