Paternal Bonding

A new study on paternal bonding finds that when a male mouse nuzzles his offspring, new neurons are manufactured to aid his protective instincts and help him to remember his offspring later on. Mice seem to have a biological bonding mechanism that can serve them for a lifetime and it's possible that human fathers do, too.

New Neurons  

A few short years ago, Samuel Weiss, a neuroscientist with the University of Calgary in Canada found that when female mice smell the pheromones of dominant male mice, they grow new neurons. These new neurons show up in two different key areas of the brain: in the olfactory bulb and in the hippocampus. These two brain structures serve to regulate odor memories.

The neurons that are produced help the females to find and mate with dominant male mice, improving their mouse pups' survival odds. Weiss was curious whether there were any such brain changes in new mouse fathers that might change the way they behave toward their young as a means of increasing their chances for a successful life.

To that end, Weiss and Gloria Mak, one of Weiss' grad students, had mouse couples share a cage and reproduce. Some of the fathers were placed in a separate cage just after the birth of a litter of mouse pups, while other father mice were allowed to stay with mother and litter for two days to stick around and nuzzle. A third group was allowed to get close but not too close to mother and the infant pups. They were placed in a mesh enclosure just outside the cage. This allowed them to be close by so that they could observe the mother and the pups and sniff them, but not nuzzle them.

Reunion Time

After two days, the father mice of all three groups were removed from the scene and then reunited with their male offspring 6 weeks later, an age at which mice have reached full-grown adulthood. The scientists chose not to use female offspring for this part of the test, since there is a tendency for the fathers to cohabit with them and this would have muddied the research results. The researchers were able to distinguish the mice who knew their offspring because they neither sniffed nor attacked their sons.

The scientists found that they could correlate recognition to the growth of new brain neurons in the fathers' brains, according to the report that appeared in Nature Neuroscience. Weiss and Mak injected markers to tag new-formed neurons in the mice dads just after the birth of a litter. They discovered that as many as 25% more neurons grew in the olfactory bulbs and up to 40% more grew in the hippocampi of the mice who had been allowed to nuzzle their pups.

"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 from the University of Massachusetts at Amherst.