Researchers in Sweden have discovered what changes in the brain of mice that unlock the so-called maternal aggression. 

Female mice are normally not aggressive but become suddenly aggressive during pregnancy and after giving birth. Researchers at Stockholm University and EARA member Karolinska Institute have discovered that a group of neurons previously known to be responsible for aggressiveness in males is dormant in females but becomes active during this period.  

They observed that females that had recently given birth, when socialising with other mice, had behaviours such as chasing or biting, which did not happen with female mice that had never been pregnant. In the mothers, the researchers observed that the dormant brain region was activated.  

Using light to turn these neurons on and off — optogenetics — in females who were mothers, the team discovered that these neurons are responsible for maternal aggression. By adding the maternal hormones to postmortem brain tissue, the team also found that these hormones make it easier to turn on these neurons after they first become active.  

These findings suggests that mammals’ brains could be more flexible than previously thought, turning on and off behaviours according to the requirements of specific life periods. 

“This is a study on laboratory mice, and we do not currently know whether the results can be transferred to humans. But the mechanism that we identify here… may reflect a principle of brain flexibility with relevance beyond maternal aggression,” said Christian Broberger, leader of the study published in Nature Communications.