spotted hyena
The social spotted hyena tends to have a wider variety of odor-producing bacteria in its scent pouch than the solitary striped hyena, scientists say. Wikipedia Commons/Ikiwaner

Human communication through scent is pretty unsophisticated, limited to recognizing things like “the baby needs to be changed” or “that strong cologne means I must avoid him.” But for hyenas, a wealth of information can be exchanged in a sniff. Now, a new study suggests that some of the most important cogs in the hyena’s odorous mail service may be some tiny little helpers: bacteria.

Hyenas leave scented messages for each other through a behavior known as “pasting.” A hyena wipes its bottom on grass stalks, smearing the plants with a waxy yellow secretion called “paste.” This paste is produced in small scent glands located near the hyena’s anus, and it packs a pungent punch.

“To me, it smells like fermenting mulch,” says Michigan State University microbiologist Kevin Theis. “Other people say cheap soap.”

Theis and other scientists have long suspected that certain bacteria in the hyena’s body may play a symbiotic role in the animal’s life. They also suspected that some of the most important bacteria would be those bacteria that respire and eat and excrete through fermentation (the same process that’s key to baking bread and brewing beer).

But it wasn’t until recently, with the advent of faster genetic sequencing technology, that they could really put the theory to the test. For a new study, published in the Proceedings of the National Academy of Sciences on Monday, Theis and his team collected paste from the scent pouches of 40 spotted hyenas and 33 striped hyenas in Kenya. They used genetic sequencing techniques to map out the various bacterial communities residing in the paste from each hyena.

The researchers found that spotted and striped hyenas had very different communities of bacteria in their pastes. In both species, the microbial landscape was dominated by fermentative bacteria, which are known to create odorous chemicals. Within a single hyena social network, there were also distinct differences. Theis and colleagues think that a wealth of information is passed through the scent of paste: a hyena’s sex, age and reproductive status, among other factors.

But how do the two different species maintain such distinct bacterial communities, when they live so close to each other? There may be some physiological or genetic factor that makes the scent pouches of striped hyenas more hospitable to a certain kind of bacteria than spotted hyenas, and vice-versa. Theis isn’t sure yet; that’s the subject for his next study.

Hyena social groups may also be reinforcing their bacterial ties through what’s known as scent over-marking. Oftentimes, when one hyena sees another pasting, it’ll walk over and paste on top of the freshly laid mark.

“Theoretically, that could promote cross-infection,” Theis says. “You’d have members of the same social group infecting each other.”

Theis is intrigued that the team found a greater variety of bacteria in the pastes of the spotted hyenas. Spotted hyenas are much more social animals than the striped hyena, which is more likely to live alone or, at most, in a pair.

“Maybe the striped hyenas don’t have as much to say to each other,” Theis says.

SOURCE: Theis et al. “Symbiotic bacteria appear to mediate hyena social odors.” Proceedings of the National Academy of Sciences published online 11 November 2013.