US scientists have discovered a major new class of pheromones that may affect mating behaviour in mammals.

Researchers at Washington University in St. Louis have shown that sulfated steroids, excreted in the urine of female mice, excite a sensory organ in the noses of male mice and other animals. They say the chemicals are important in social communication and may help male mice choose a mate.

Timothy Holy and colleagues used fractionation techniques and mass spectrometry to identify the compounds, which activate nerve cells in an organ called the vomeronasal organ. They found that sulfated steroids triggered a substantial amount of sensory activity in the organ. 'There have been quite a few reports of other compounds that excite these cells,' says Holy. 'But here, for the first time, I think we have a large family of compounds that collectively excite a large number of cells.'

In mammals, steroid hormones are deactivated before excretion by the addition of sulfate groups. Sulfated steroids are known to be used by fish for signalling, but have never been recognised as active pheromones in mammals.

Holy says the compounds could be involved in stress signalling and social communication across a broad spectrum of mammals. The team have already shown that stressed mice produce much higher levels of one particular steroid sulphate that is structurally similar to corticosterone, a stress regulator in rodents. 

Peter Brennan, who studies the vomeronasal system at the University of Bristol, UK, says that male mice may use the steroid hormones to assess the reproductive status of potential mates. 'The ability to sense these compounds could allow mice to sense different stages of the reproductive cycle of females. This is maybe one reason why you don't get these compounds produced in the males, as they don't cycle.' However, he says the compounds could have many different effects on behaviour and warns against making speculative claims about their role without behavioural evidence.

Holy thinks the research could help to alter our perceptions about the roles of pheromones, which have been commonly regarded as chemical alarm bells. 'Maybe it's much more of a case that the recipient of the signal is evaluating the sender, rather than the sender transmitting some magical signal that causes the recipient to go crazy,' he says.

Hayley Birch

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