The evolutionary purpose of kissing has long eluded scientists. Smooching is risky, given things like pointy teeth, and inherently gross, given an estimated 80 million bacteria are transferred in a 10 second kiss. And yet, from polar bears to humans, albatrosses and prairie dogs, many animals kiss. So, what gives? Evolutionary biologist Matilda Brindle tells us the sordid details driving this behavior, what distinguishes different kinds of kissing and whether culture has anything to do with why people kiss. Interested in more of the science behind love and connection? Email us your question at shortwave@npr.org. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.