Maternal aggressive/protective behavior is recognized throughout mammalian species, especially during lactation. When hiking, we warn our kids not to approach bear cubs, or to get between a cub and the mother. While driving and you see a fawn, you know a doe can't be too far away and will run headlong to protect it.
The same neurohypophyseal (NH) hormone, oxytocin (OT), is responsible for both the physiological and behavioral changes, but the site of action is quite different. OT is released during parturition and in lactation not only from NH terminals into the bloodstream in order to support reproductive systems, but also within the brain, into the supraoptic nucleus (SON) and paraventricular nucleus (PVN), where it has marked behavioral impact.
OT release in the brain is involved with such reproductive events as "morphological plasticity, autoregulation of OT neuronal activity and promotion of maternal behavior, including maternal aggressive behavior to protect offspring," Inga D. Neumann of the University of Regensburg, Germany, notes. "Thus in lactating rats from a line bred for high-anxiety behavior, or HAB, OT release within both the central amygdala and the PVN was positively correlated with the level of maternal offensive behavior against an intruder." In addition, the lactating HAB dams display higher aggression and central OT release compared with a low-anxiety line.
Very importantly, along with the rise of maternal behavior during lactation, OT also reduces anxiety levels by regulating the responsiveness of the hypothalamic-pituitary-adrenal axis. "Getting aggressive is one thing, Neumann observes, "but if you need to attack a bigger animal, you also must lower your level of anxiety."
Next, Neumann and her research team tested what effect direct OT insertion into the brain of virgin female rats would have. They found that "driving up-regulation of OT receptor binding in the central amygdala, using adeno-associated viral vectors, reduced the level of anxiety and promoted social dominance behavior in virgin females."