Why do dogs sniff each other's butts?

When two dogs meet, they often go straight for a whiff of each other's behind. And anyone living in a multicat household will tell you that cats regularly put their nose under their fellow felines' tails.

So why do dogs sniff each other's butts? It turns out, there's a good reason behind this bottom-sniffing obsession.

"Dogs can tell [another dog's] Health and reproductive status, identify what they have eaten, and generally catch up on the 'news' from smelling each other's rear ends," said Ellen Furlong, an associate professor of psychology who specializes in animal cognition at Transylvania University in Lexington, Kentucky.

Mating also plays a role. "Male (particularly unaltered) dogs may also spend more time sniffing females for information about their reproductive status," Furlong told Live Science in an email. 

But how is it possible for dogs to glean all of this information from a simple butt sniff? One of the first clues for unraveling this mystery came in 1976, when a study on dogs and coyotes was published. The study found that two small pouches, one on each side of a dog's anus — known as anal sacs — release a cocktail of smelly chemicals, including fishy-smelling trimethylamine and pungent propionic acid and butyric acid. These anal sac secretions probably serve as chemical communication signals between dogs, according to a study published in 2021.

Related: Why do cats' claws retract but dogs' claws don't?

Each chemical released by a dog is likely to relay specific information to another dog, but scientists have yet to crack this scent code. A study published in 2023 made some headway in decoding this chemical language by comparing the anal sac secretions of male and female dogs. It found that certain chemicals were released only by females and others only by males.