Video: Inside the world's largest gathering of snakes

Each spring in the Canadian province of Manitoba, 75,000 red-sided garter snakes (Thamnophis sirtalis infernalis) congregate inside limestone caves to form 'mating balls’ - writhing heaps of up to a hundred males tussling for a single female. 

This is the biggest gathering of snakes in the world, a phenomenon that can only happen here because of two geological factors - limestone crevices and marshes. According to an interview National Geographic did with environmental documentary photographer, Paul Colangelo, the area is perfect during summer because the snakes can feed off the wealth of frogs that live in the marshes, and in winter, those limestone crevices provide fantastic shelter deep underground. Which is just as well, because winter temperatures in Manitoba can dip down to -40 degrees Celsius. 

The crevices also provide the perfect place for the red-sided garter snakes - which are completely harmless to humans - to perform their mating balls. According to Colangelo, the males court the larger female by rubbing their chins on her head, vying for as much contact with her as possible within the mating ball.

Head to National Geographic’s website to read their interview with Colangelo about this incredible display.