For a father to truly bond with his children, he needs to grow some new gray matter. At least that seems to be the case in mice. A new study shows that when a mouse father nuzzles his pups, he develops new neurons that help him remember—and protect—those offspring later in life. The results suggest that in mice, and perhaps in humans, young babies and dads bond biologically in ways that can last a lifetime.
A few years ago, neuroscientist Samuel Weiss of the University of Calgary in Canada discovered that female mice grow new neurons when they smell pheromones from dominant male mice. The neurons appear in two key structures of the brain: the olfactory bulb and the hippocampus, which together coordinate the memory of odors. That helps the females identify and mate with the dominant males, thereby giving their offspring better odds at survival. Weiss wondered if bonding with their pups would cause similar changes in the brains of new mouse fathers, altering their behavior toward their offspring in ways that might give the pups better odds of success in life.
To find out, Weiss and one of his graduate students, Gloria Mak, let mouse couples cohabit in a cage, mate, and produce pups. They removed some of the fathers to another cage as soon as the pups were born, let others hang around for a couple of days and nuzzle with mom and baby, and let a third group hang around, watching and sniffing—but not nuzzling—from outside of a mesh tent. Then the researchers removed all of the dads and reunited them with their sons 6 weeks later when they were fully grown. (They avoided testing daughters because father mice tend to mate with them, making the study hard to interpret.) Dads who did not attack their sons and who avoided sniffing them were seen as recognizing their offspring.
That recognition correlates with new neuron growth in dad's brain, the team reports online today in Nature Neuroscience. When Mak and Weiss injected a marker that tags newly formed neurons into the fathers just after their pups were born, they found up to 25% and 40% more new neurons, respectively, in the olfactory bulbs and hippocampi of mice that had nuzzled their pups.
The researchers probed deeper to uncover the specific signals that regulated the birth of these new neurons. They knew that the hormone prolactin causes mothers to produce milk and also helps both mothers and fathers to develop parental behaviors. So they tested whether mice dads with a mutation that kept their brain cells from responding to prolactin would grow new neurons as they nuzzled their newborn pups. They didn't. What's more, these dads treated their offspring like strangers after they were full-grown, indicating that they didn't remember them.
To clinch the case, the researchers injected the mice that could not respond to prolactin with a different hormone that also stimulates parental nurturing and neuron formation. The treatment restored new neuron growth in the two brain structures controlling smell and memory, and it restored the mouse dads' ability to recognize their pups when they were full-grown. "The results speak of the general importance of social interactions," particularly those early in development, to develop positive memories that last, Weiss says.
"What is quite new here is that they point at a clear physiological mechanism, a neural system that plays a role in recognizing offspring," says neuroscientist Geert de Vries of the University of Massachusetts, Amherst. Although others have documented changes in the maternal and paternal brain as the animals care for their offspring, says de Vries, "I have not seen as nice and comprehensive study as this."