People flock to Manitoba, Canada, to watch the yearly mating between the red-sided garter snakes. Why?

Explanation

Tourists flock to Manitoba, Canada each year to see the mating between red-sided garter snakes take place. When the female goes into hibernation, she releases a pheromone which attracts hundreds of male snakes. These males will form a big 'mating ball' at the den where the female hibernates. Like many other snakes, male garter snakes have two penises and will thus attempt to use the best placed penis to mate with the female. A female snake can care a male's semen around for years before she becomes pregnant.