Plenty of critters use chemicals to communicate. These chemicals, known as pheromones, can signal everything from raising an alarm to a readiness for romance. Now, for the first time, scientists have identified pheromones in rats (Proc. Natl. Acad. Sci. USA 2014, DOI:10.1073/pnas.1414710112). By studying the chemicals produced by rats when they are stressed-out, Japanese ­researchers identified two key compounds: 4-methylpentanal and hexanal. The team, led by the University of Tokyo’s Yukari Takeuchi, found that exposing rats to these two chemicals in combination prompted the rodents to display increasingly anxious behavior. Neither aldehyde increased anxiety on its own, the researchers note—they had to be present in a mixture. Hexanal is a common alarm pheromone produced by many insects, such as weaver ants and ­leaf-footed bugs, but little is known about 4-methylpentanal, the researchers say. It’s possible, they reason, that 4-methylpentanal transmits species-specific information whereas hexanal indicates the accuracy of that information. The finding, they add, enhances our understanding of chemical communication in mammals.