Cockroaches love the smell of their own poop. Their feces contains a chemical signal that tells other cockroaches to come on over and party. As a new study in the journal PNAS finds, that signal comes from the cockroach’s gut microbiome.

The research team, led by North Carolina State University entomologists, first verified that German cockroaches (Blattella germanica, one of the species that inhabit restaurants and homes worldwide) are indeed attracted to the smell of poop. When given the choice between walking toward water infused with extracts of adult female cockroach feces and plain water, they would head for the poop.

To figure out why, they raised some cockroaches in completely sterile conditions in the lab, with sterilized feces, food, and water, ensuring that those roaches lacked the bacteria usually present in the cockroach gut microbiome. Other cockroaches were raised normally, without sterilizing their food, water, or poop. When a group of young cockroaches were exposed to the feces of these two experimental groups, they consistently gravitated toward the poop laced with normal cockroach gut bacteria over the sterile feces, and responded to it faster. However, they tended to prefer the smell of poop from members of their own colony, rather than from other colonies. 

The researchers concluded that the bacteria in their friends' poop are the source of the attraction for cockroaches. This, they hypothesize, might be a way of sussing out which roaches are members of the group and which are strangers. Because the variety of bacteria in the microbiome can vary depending on the environment, young cockroaches might be able to tell the difference between a spot where members of its colony congregate and where an unfamiliar (and perhaps hostile) colony of cockroaches hang out based on the attractive familiarity of the poop smell, as determined by local gut bacteria. 

This research could have implications for future pest control methods, as determining which chemical signals roaches are attracted to could help lure them toward poisons and traps. 

[h/t: Smithsonian]