Scientists Flip Neural Switch in Male Mice from Cuddle to Kill

October 7, 2015 | Sarah Tse

Baby mice in a nest
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Scientists have discovered the brain regions in male mammals that control the Jekyll/Hyde-like transition from killing offspring to caring for them.

You’ve probably seen the documentaries showing male lions taking over a pride and ruthlessly eliminating all the existing cubs. It’s not just senseless violence or an intimidation tactic; like everything else in nature, it comes down to sex. Many mammalian species tend to lose their mojo right after they have offspring because lactation uses up so much energy that there’s little left for ovulation. Of course, males are too impatient to wait for the babies to wean off their mothers’ milk, especially since the babies don’t even belong to them — thus the rampant infanticide.

Once a female loses her offspring, she becomes fertile again and the male gets his chance to father his own kids. That’s when he switches from brutal baby-killer to super-dad. Instead of dispatching every baby he encounters, he takes care of them — even if they’re not his.

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The evolutionary reasoning for this infanticidal behavior is pretty straightforward: kill babies that carry some other guy’s genes so you can sow your own wild oats. But scientists at the RIKEN Brain Science Institute in Japan were interested in the specific neural mechanisms behind this behavioral transition, so they decided to investigate brain activation patterns in mice.

They exposed adult male mice to pups, while recording any correlations between the males’ behavior and their levels of neuron activity in nine different brain regions. It turns out that a brain region called cMPOA (caudal medial preoptic area) expresses a neural activity protein called c-Fos in association with paternal behavior. On the other hand, aggressive behavior towards pups occurred in a different region called the rhomboid nucleus of the BsTrh (Bed nuclei of the stria terminalis rhomboid nucleus) which was also associated with c-Fos expression. Looking at the c-Fos expression patterns in each region even enabled the scientists to determine whether the mice exhibited paternal or infanticidal behavior in the past.

Next, the research team wanted to see if these patterns were simply an artifact resulting from the behavior, or if the brain regions were actually causing it. When they added lesions to the BsTrh region in male mice who had never before mated, the mice stopped targeting pups. Likewise, adding lesions to the cMPOA region in mouse fathers caused them to regain the murderous impulses of their bachelor days. Even when the male mice were separated from newborn pups by wire mesh, the activation of each brain region successfully predicted the mice’s attempted behaviors.

Upon analyzing the connections between each brain region, the scientists found that the cMPOA has an inhibitory effect on the BsTrh. Since c-Fos levels in the cMPOA also increased during mating, the researchers suspect that cMPOA awakening lowers the baseline activity in the BsTrh, causing the behavioral switch.

To test this theory, they used light to optogenetically stimulate the cMPOA neurons that act on the BsTrh in virgin male mice, whose cMPOA regions should have normally remained dormant. This experiment allowed them to calm the mice’s infanticidal urges, although the optogenetic stimulation took several days to have its effect.

The researchers now want to test their theory in primates to see if fiddling with the same brain regions can stop monkey fathers from attacking helpless young. These discoveries may someday even yield insights into how we can predict and prevent aggressive behavior in humans, particularly in those who seem immune to our natural affinity for adorable babies.

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