On the marshy plains of northwest Canada, it’s springtime. Snakes slither out of the ground. It’s one of the world’s great gatherings of snakes.

Red-sided garter snakes hibernate together in groups. This allows them to stay warm in winter. When the temperatures rise, masses of these snakes break free from their dens. They migrate to their summer homes.

What guides them? Their sense of smell. The snakes have long, forked tongues.

They pick up scents from the
ground and air. These are passed to a snake’s ­mouth, where they taste the scent.

Each snake creates a scent map of its surroundings. The snakes use their scent maps to return to the same den each year.