New research reveals that motherhood triggers a “sleeping” aggression circuit in a new mom’s brain. It’s a switch women already share with men, but it normally lies dormant. Until motherhood.
Earlier studies have shown that while male mice fight to assert dominance, for example, female mice remain (for the most part) peaceful. Until they become mothers. Lactation appears to launch a profound behavioral shift. New mothers fiercely defend their pups against intruders.
This sudden (while reversible) transformation has puzzled neuroscientists for years. And the question lingers. How does the brain facilitate such an on-demand behavioral rewrite?
A new Nature Communications study shows that maternal aggression emerges when a dormant, hormone-sensitive circuit in the hypothalamus “switches on.” Karolinska Institutet and Stockholm University researchers found that, specifically, neurons in the ventral premammillary nucleus (PMvDAT) — previously linked to male aggression — suddenly become hyperactive during nursing. The discovery provides a cellular explanation for how maternal hormones temporarily unlock an otherwise dormant behavioral program.
Hormones as Behavioral Switches
The team found that two hallmark maternal hormones, prolactin and oxytocin, excite PMvDAT neurons. In brain-slice recordings, adding either hormone triggered electrical depolarization, rapid firing, and heightened responsiveness — all signs of increased neural excitability. The hormones also boosted synaptic communication, essentially “priming” the circuit for action.
The researchers found that, during lactation, prolactin levels surged to 10 times higher than in non-mothers, while markers of hormone receptor activation spiked in the same neural region.
Together, these changes transformed once-quiescent PMvDAT neurons into a hyperexcitable network poised for defensive aggression.
“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 – how a behaviour that is normally outside an individual’s repertoire can become available for a limited period of its life – may reflect a principle of brain flexibility with relevance beyond maternal aggression, ” Stockholm University professor of neurochemistry and co-author Christian Broberger added in a statement.
Flipping the Switch
To test causality, the researchers manipulated these neurons directly. Using optogenetics, team members showed that stimulating PMvDAT neurons in lactating mothers triggered immediate attacks on intruding mice. On the flip side, silencing these neurons curb aggression dramatically.
Among the non-mothers, however, the same neuron manipulation produced a greater sense of curiosity rather than combat. This suggests that hormonal and physiological changes during motherhood “unlock” the circuit, lowering the threshold for aggression only when defense becomes adaptive.
The study also revealed an interesting competition between care and combat. When the researchers activated the PMvDAT circuit during nursing, mothers stopped retrieving their pups — abandoning caregiving behaviors even without a visible threat. This, they suggest, indicates that the same neural system that drives defense suppresses nurturing when activated, forcing the brain to prioritize caregiving or protection.
In non-mothers, chronic infusion of prolactin and oxytocin into the PMv failed to provoke aggression. But it did blunt social interest and caregiving instincts. Hormones alone weren’t enough; pregnancy and lactation likely remodel broader brain circuits to support this temporary behavioral metamorphosis.
The findings challenge the long-held assumption that male and female aggression arise from fundamentally different brain wiring. Instead, both sexes appear to share a common neural architecture, flexibly activated under different biological states. In the male mice, PMvDAT neurons are chronically active and support dominance-related aggression. In the female mice, the same cells remain silent until maternal hormones reactivate them.
Evolution’s Neural Shortcut
From an evolutionary standpoint, it makes sense. Rather than forging an entirely new circuit for maternal defense, evolution appears to have simply repurposed an existing aggression pathway — toggling it on when hormones signal that offspring are vulnerable.
The research paints a more nuanced picture of motherhood as a delicate balancing act, driven by hormonal control over overlapping brain networks. Prolactin and oxytocin — long celebrated for nurturing and bonding — also energize aggression when circumstances call for it.
Further Reading
Postpartum Hormonal Contraceptives Linked to Higher Depression Risk
