Photograph by Joel Sartore, National Geographic Photo Ark
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Domestic rats produce sex pheromones in their tears that mice can detect, a new study says.

Photograph by Joel Sartore, National Geographic Photo Ark

These Mice Spy on Their Predators—By Sniffing Their Tears

It's the first known example of prey picking up on a predator's pheromone and using it to avoid danger.

When you're prey, it pays to listen in on your predators.

A new study demonstrates that mice can "eavesdrop" on the pheromones used by rats to communicate. The study, led by Kazushige Touhara at the University of Tokyo, identifies a protein in male rats' tears that acts as a sex pheromone for rats and a warning signal for mice.

Male rats spread their tears over their body as they groom themselves, cloaking themselves in whatever chemical signals they contain. As the rodents go about their business, they leave an olfactory record of their passage. Touhara and his team were curious whether mice could pick up a signal left by rat tears. (Read why mouse tears are aphrodisiacs.)

The scientists used standard lab species: The Norway rat (Rattus novegicus) and house mouse (Mus musculus), both of which are common cohabitants in our towns and cities.

Though both are omnivorous rodents, rats have a well-documented tendency to kill and eat mice.

Scientists already knew that mice avoid rat urine, a typical prey strategy. But this paper is the first to show a prey species picking up a specific pheromone used by its predator and turning it into an early warning system.

Smelling a Rat

In the experiments, female rats spent more time investigating cotton soaked with ratCRP1 than the untreated cotton, whereas mice spent less time around it. (Watch what happens when you tickle a rat.)

Both species moved less after exposure, and further testing on the mice showed that they had lowered heart rates and body temperatures even an hour after exposure.

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House mice huddle at Point Defiance Zoo and Aquarium. The rodents are common prey for rats.

In their study, published today in Current Biology, Touhara and his colleagues propose that ratCRP1 is primarily a sex pheromone in rats, causing females to display interest and to linger in an area where a male had been.

Mice, on the other hand, also pick up the signal, but instead enter a cautious, more sedentary state to avoid attracting attention from any nearby rats.

Furry Spies

Tristram Wyatt, zoologist at the University of Oxford and expert in pheromones, says there's a strong evolutionary incentive for species to learn to listen in on their neighbors.

These Huge Rats Can Sniff Out Land Mines Oct. 7, 2015 - A group in Cambodia is using Gambian giant rats to find the nearly two million  land  mines spread out across the country. These UXOs (unexploded ordnances) injure hundreds of people every year. The rats are adept at sniffing out TNT and pointing their human companions to where the danger lies. The rodents don't set off the  mines because they're too light, and they can clear a hundred-square-meter area in under 30 minutes, much faster than a human can.

Click here to read more:
" Meet the Giant Rats That Are Sniffing out Landmines"

"As soon as you have a broadcast signal," Wyatt says, "other species can evolve a sensitivity to exploit that signal."

Touhara calls it the "game of the animal kingdom"— attempting to communicate with your allies without letting your enemies know what you’re up to. (Here's what a rat looks like when it's happy.)

Consider the auditory arms race between moths and bats. Moths have, on multiple occasions, evolved ears attuned to the frequencies bats use to echolocate, and will escape when they sense a bat's ultrasonic pulses.

Despite this being a common phenomenon, in theory, "relatively few studies have been performed of prey responding specifically to the pheromones of predators," Wyatt says.

Touhara and his team not only identified a new protein and its effects on behavior, but also mapped neural activity to show that ratCRP1 was triggering a defense circuit in mice brains, he notes.

For Touhara, he says "the next question is how this signal has evolved to be utilized as both intra- and inter-species signals.