Aug. 2, 2004 MADISON - Everyone knows not to get between a mother and her offspring. What makes these females unafraid when it comes to protecting their young may be low levels of a peptide, or small piece of protein, released in the brain that normally activates fear and anxiety, according to new research published in the August issue of Behavioral Neuroscience.
"We see this fierce protection of offspring is so many animals," says Stephen Gammie, a University of Wisconsin-Madison assistant professor of zoology and lead author of the recent paper. "There are stories of cats rescuing their kittens from burning buildings and birds swooping down at people when their chicks are on the ground."
In terms of biology, it makes sense that mothers would lay down their own lives to protect their offspring, especially if it means the parents' genes will be passed down to the next generation, says Gammie. But he adds that despite all the observations and the theories explaining why mothers display this behavior - commonly known as maternal aggression - very little research has investigated the biological mechanisms that turn on this trait in new mothers.
"We've known for a long time that fear and anxiety decrease with lactation," explains Gammie. "Maybe it's this decrease that allows mothers to attack during a situation that normally would evoke a fear response."
Testing this hypothesis, the Wisconsin professor and his colleagues studied the link between maternal aggression in mice and levels of corticotropin-releasing hormone (CRH), a peptide that acts on the brain to control behavior.
About six days after a group of mice gave birth, the new mothers received injections containing either one of three doses of CRH or a saline solution with no amount of the peptide. Following each injection, which was given once a day for four consecutive days, the researchers returned the mother mice to their pups. Twenty-eight minutes later, the researchers removed the pups and introduced a male intruder.
Under normal conditions, female rodents will fiercely attack the males, says Gammie, noting that the males sometimes eat pups and that "the best defense for the mom is the offense."
For the study, only the mice that received either no dose or a low dose of the peptide displayed the expected behavior. As the levels of CRH increased, the number of attacks and the duration of them dramatically decreased.
The results show, for example, that while the mice with the lowest levels of CRH attacked more than 20 times for the duration of about 45 seconds, the mice with moderate levels of the peptide attacked about six times over about eight seconds. Mice with the highest levels of CRH didn't attack at all.
"When we put the male in the cage, some moms would just sit there. They weren't protective at all. If anything they were skittish. They showed a fear response," says Gammie.
The researchers note that altering the levels of the peptide appeared to affect only maternal aggression; normal maternal behaviors, such as nursing, were observed in all mothers both before and after the encounters with male mice.
Based on the results, Gammie says, "Low CRH levels appear to be a necessary part of maternal aggression. If you don't keep them low, you won't see this fiercely protective behavior."
He adds that this finding - some of the first evidence suggesting a biological mechanism that enables parents, regardless of the potential danger, to defend their offspring - may also begin to explain why mothers occasionally neglect or harm their offspring.
"Postpartum depression in some individuals has been linked to higher levels of CRH release and an overly active stress response," explains Gammie. "If CRH needs to be low to see maternal protection of offspring, as our work suggests, then it explains why moms with high postpartum depression and high CRH not only may neglect, but also may abuse, their children."
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