Whenever you cry, you could be giving away your location. That certainly seems to be the case for rats, as it has been discovered that mice can smell their tears.
We might have to rewind a bit here. First of all, yes, rats and mice can cry. Not only that, but male mice tears apparently turn female mice on. A few years back it was discovered that male mice produce a specific protein in their tears that act as an aphrodisiac to the females, making them more receptive to sex.
Researchers of a new study, published in Current Biology, were interested to see if male rats produced something similar, and it turns out they do. They release a novel compound called cystatin-related protein 1 (CRP1). When female rats smell this protein, it activates receptors in the vomeronasal organ, prompting the females to stop dead in their tracks.
Rats can and do predate on mice, so the researchers decided to see if the mice, which can obviously detect their own tear pheromone, could also detect the rats'. It turns out they could. In fact, they found that the rat CRP1 even activated a defense circuit in the mouse brain, causing them to stop moving and reduce their heartbeat in a bid to evade detection.
In exactly the same way that animals listen in on the calls and noises of other species, many also use their nose. This has been dubbed “olfactory eavesdropping” and is widespread across the animal kingdom. Within species, scent is an incredibly important sense, providing information to others about age, sex, and reproductive status. But this has led to other species evolving to also be able to intercept these smelly signals to give them a competitive advantage.
“Our study shows that rat CRP1 is a putative sex pheromone in rats and that mice eavesdrop the rat pheromone as a predator signal,” explained study co-author Kazushige Touhara.
This is different to just simply smelling a predator, though, because these signals are often specific pheromones within a species that can now be read by another. Foraging stingless bees, for example, may be able to eavesdrop on where other species have been feeding in order to discover where the most food is by following their scent, while the giant Asian honeybee has been found to avoid flowers that are crawling with predatory ants by sniffing them out.
This isn’t limited to insects though. Experiments have shown that possums will eavesdrop on the smell of birds’ feathers in Australia when looking for suitable nesting holes in trees, as well as eggs to snaffle, while at the same time they can discriminate between predatory dingo and domestic cat odors.
And it now seems that mice can smell the tears of rats, too, opening up the possibility that perhaps predators and prey are communicating in this way far more frequently than we had realized.
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