When baby mice are exposed to the powerful, pungent smell of cat pee, they don’t run and hide from feline predators later on in life. The findings, presented at the annual Society for Experimental Biology meeting in Prague last week, might explain how both cats and mice can hang out around humans, living off our food or the food we leave behind. 

"Because the young mice (less than 2 weeks-old) are being fed milk while being exposed to the odor, they experience positive reinforcement," Vera Voznessenskaya of Russia’s A. N. Severtsov Institute of Ecology and Evolution explains in a statement. "So they don't escape the cats when exposed to cat odor later on." Baby house mice (Mus musculus) subjected to cat pee during that critical period – from about two weeks to 28 days after birth – simply don’t feel the need to avoid that same odor when confronted with it as adults.

The team identified the molecule in cat urine responsible for these effects: L-Felinine, a unique sulfur-containing amino acid. Previous work revealed how this compound can block mouse pregnancies and reduce the size of litters. When neurons in the mouse brain pick up the scent, BBC reports, they trigger a series of reactions, which includes elevating the levels of stress hormones. "It's something that has existed in cats and mice for thousands of years," Voznessenskaya says. 

While early exposure alters their behavioral response to cat odor, the mice still experience physiological and hormonal responses. “In fact, mice that had experienced the odor showed stress response (elevated corticosterone) to cat odors in the same way as controls,” Voznessenskaya adds. Heightening levels of corticosterone is an innate response. That means early olfactory experiences with cat pee seem to produce a dissociation in response to predator scents at the behavioral level and the hormonal level later in life.