In many species, mothers aggressively defend their offspring from danger. But a mother may abandon her protective instinct and instead cower in fear if the level of a specific hormone in her central nervous system is too high, according to a study published in the August issue of Behavioral Neuroscience.
The hormone, corticotropin-releasing hormone, is a fear-inducing protein that influences behavior and has been linked to depression, particularly in lactating women. Under normal conditions a lactating female's anxiety recedes as her CRH levels decline.
\They [become] what you might call fearless,"" said Stephen Gammie, UW-Madison assistant professor of zoology and lead author of the study.
Lactating females can be capable of heroic feats when they are defending their offspring, Gammie said, but this is the first time maternal aggression has been definitively linked to CRH levels.
""It needs to be low in order for [the mother] to show this protective behavior, and that's what had not been shown before,"" Gammie said.
In the study, Gammie's team tested CRH levels and maternal aggression in mice. Mice with six-day-old pups were separated from their offspring and then placed with male mice. Typically lactating mice fiercely attack male mice, who sometimes eat pups. In the study, however, female mice that received CRH injections did not exhibit the usual response.
""With the CRH injections we get the reverse,"" said assistant researcher Alejandro Negron, the second author of the study. ""[The female] would be more anxious and stressed. So that would impair her from attacking the male intruder.""
The results suggest human mothers who do not relate to their children the way they normally would, as in postpartum depression, or who neglect or abuse their children, may suffer from high levels of CRH, Gammie said.
However, CRH's influence on a lactating rodent may differ from its influence on a woman, Vaishali Bakshi, UW-Madison professor of psychiatry, said.
""It would be an interesting way to study Steve's hypothesis, clinically, to examine [the CRH levels of] women with postpartum depression while they have the symptoms and later on after they've recovered,"" Bakshi said. Nonetheless, she added, Gammie's study is a valuable contribution to literature on CRH and maternal aggression.
""It's a new role that's being proposed for the CRH system ... People have looked at it in a sort of general way, but not in the level of detail that he did,"" Bakshi said.
