If you are a small animal, it is useful to know whether there is anything around that might want to eat you. Stephen Liberles from Harvard Medical School in Cambridge, Massachusetts, and his colleagues have analysed urine samples from a variety of zoo inhabitants, including lions and bears, and discovered how rodents can use smell to do just that. In a research published today in the Proceedings of the National Academy of Science, the team identifies a chemical found in high concentrations in the urine of carnivores that makes mice and rats run for cover1. Chemicals have already been identified that allow prey to recognize a known predator. But this is the first example of a generic clue that allows an animal to detect any potential predator, irrespective of whether the two species have ever come into contact.
The researchers started by analysing an engimatic group of olfactory receptors discovered in 2001 called trace amine-associated receptors (TAARs)2. They are found in most vertebrates, in varying numbers. Mice, for example, have 15, rats 17 and humans have just 6. Very little is known about what chemicals bind to them. Liberles and his colleagues found that one member of the receptor family, TAAR4, is strongly activated by bobcat urine, which is sold online and used by gardeners to keep rodents and rabbits away. They managed to extract the molecule responsible for activating the receptor, called 2-phenylethylamine. They then wondered whether the molecule was specific to the bobcat. But the urine from other animals cannot always be bought as easily. “Also, commercial products may be contaminated, whereas we wanted to be sure we were studying only natural substances,” says David Ferrero, a graduate student in Liberles's lab and first author of the study. So the researchers collected urine samples from a range of sources, including zoos in New England and South Dakota. Their collection covered 38 species from predators such as lions, snow leopards and servals to herbivores including cows, giraffes and zebra. They also tested humans, cats and various rodents. The operation was not trivial. A giraffe had to be trained to urinate in a cup, and Ferrero had a nose-to-nose encounter with an uncooperative jaguar when the animal jumped against the bars as he approached its cage.