Powerful urine is mind-altering
Alpha-male pheromones cause females to make brain cells.
Female mice make new brain cells when they detect a dominant male's urine, researchers have found. The discovery gives a clue as to how the chemical messages shape their receiver's taste in mates.
Urine is rich in the sex pheromones that many animals use to recognize and choose their mates. But how they work is unclear. So Samuel Weiss from the University of Calgary in Alberta, Canada, and his colleagues looked at their effects on the brain.
The team housed adult female mice with soiled litter for a week. Animals exposed to urine from dominant males showed around a 25% increase in new neurons in two brain regions. Those exposed to clean bedding, or urine from females or subordinate males showed no such increase.
The results, published in Nature Neuroscience1, suggest that pheromones from dominant males stimulate the female brain to make new neurons.
Female mice prefer dominant males, but females given a chemical that blocks neuron production became indifferent to status. "Adult neurogenesis may be involved in female mate selection," says Weiss.
Select by smell
Neurons grew in the hippocampus, a brain region involved in learning and memory, and the olfactory bulb, which is involved in smell. Both regions make new neurons throughout life; events such as running, learning and mating trigger increases in one area.
This is the first study to find both regions responding to the same stimulus. "Seeing an increase in both areas at once was surprising," says Weiss, "but then mate selection is an intrinsically important behaviour."
Weiss thinks that the pheromones bind to specialized receptor proteins, which then signal to another brain region called the hypothalamus, triggering the release of hormones that cause the birth of new neurons.
"This mechanism is likely to be just one piece of the puzzle," says pheromone researcher Barry Keverne from the University of Cambridge, UK. Keverne has shown that male mouse urine can also trigger the production of new neurons inside a female's vomeronasal organ, a sense organ thought to aid pheromone detection in some mammals.
We don't know whether pheromones trigger neuron formation in humans, although we do have receptors similar to those found in mice. It's possible that some types of human sexual behaviour could be affected by pheromones, says Zhengui Xia from the University of Washington in Seattle, who studies neuron production.
But whether a subconscious whiff of an alpha-male's urine could turn a woman's head is still a matter of speculation. "Olfaction is a subtle and underappreciated sense," says Weiss. "Maybe we've underestimated its importance."
- Mak, G. K. et al. Nature Neurosci. doi:10.1038/nn1928 (2006).