For newborn mice to suckle for the very first time and survive, they depend on a signature blend of scents that is unique to their mothers. The findings, published online on October 4 in Current Biology, a Cell Press publication, reveal that mom's natural perfume consists of odors emitted from the amniotic fluid, which served to nourish and protect those young mice before they were born.
That's not what researchers led by Lisa Stowers of The Scripps Research Institute expected to find. Earlier studies in rabbits showed that suckling depends not on a complex odor brew but on a pheromone, a singular chemical factor that triggers a characteristic behavioral response.
"Surprisingly, unlike the rabbit, we found no evidence of a classic pheromone in the mice," Stowers says. "Instead, we found that the pup 'learns' the individual scent blend of the mom. Every mom is likely to have a different signature odor." It can't be isolated and purified in the way a pheromone can.
The discovery reveals that even innate behaviors like suckling aren't necessarily hard-wired. Nursing mouse pups must learn the smell of mom. The behavior is primed by the unavoidable experience of the amniotic fluid before birth and the secondary experience of that odor blend at the mother's nipple immediately after birth.
The fact that newborn rabbits initiate suckling in a different way has evolutionary implications. "These results suggest that mammalian species have evolved multiple strategies to ensure the onset of this critical behavior," the researchers write.
The findings further suggest that other behaviors that appear to be innate might involve some hidden learning too. And, what does it all mean for us humans?
"Human pheromones have not been conclusively identified and their existence remains controversial," Stowers says, although humans can pick up on signature scents. "If the mouse can effectively use this mechanism to release such an important behavior, it is possible that signature odors underlie the release of other apparently innate behaviors; in mouse or even humans."
When it comes to breastfeeding, on the other hand, "our knowledge that mouse and rabbit each use a different mechanism to initiate suckling leaves plenty of room for humans to use an entirely different mechanism as well."
Logan et al.: "Learned recognition of maternal signature odors mediates the first suckling episode in mice."