August 8, 2004
Well, that's not all of the story. It turns out that the fierce behavior that compels a mother to risk her own life in defense of her young may depend as much on chemistry and biology as on a mother's love.
Researchers at the University of Wisconsin-Madison have found evidence that a mother's protective behavior is regulated somewhat by the presence of a peptide hormone that is present in nearly all animals, including humans and mice.
Give mom a shot of corticotropin-releasing hormone (CRH), a peptide that acts on the brain to control behavior, and she will cower in the corner when a menacing male approaches her kids. Take it away and she'll kick his butt.
But that shouldn't take anything away from mom, says Stephen Gammie, assistant professor of zoology at the university.
"It doesn't make me feel any different" about something as romantic as a mother's love, Gammie says, "if I understand how it works."
Gammie and his colleagues, who published their research in the current issue of Behavioral Neuroscience, began tackling the biological basis of what is known as "maternal aggression" at a common starting point. Scientists have known for many years that fear and anxiety decrease in a mother during the period when she is lactating, or nursing her young.
The Wisconsin group wanted to know if that loss of fear accounted for the fierce behavior that most mothers can exhibit in the face of danger. Can it be that they will attack a threatening male primarily because they no longer fear him?
They suspected that CRH might be the substance regulating maternal aggression because other studies have shown that it "plays a big role in making you feel fearful and anxious," Gammie says.
So they recruited several mice for their experiment. Six days after the mice had given birth, they began injecting them with either a dose of the peptide, or a saline solution that did not contain the peptide.
The injections were given once a day for four consecutive days. A few minutes after the injection each mother was separated from her pups and a male intruder was introduced. The changes in behavior were dramatic.
The mice that received the placebo acted like mothers who don't know the meaning of fear. They attacked the male more than 20 times in just 45 seconds, the researchers found. The more CRH the mice received, the less combative they became.
"At the highest dosages we saw no aggression at all," Gammie says. "The female just kind of cowered in the corner. So that suggests that as we elevate this stuff it's having a very specific effect on protective behavior."
The results strongly suggest that the peptide plays a crucial role in maternal aggression, at least for mice, and probably for humans. It wouldn't be the first time that some naturally occurring substance has a similar effect on humans and other animals.
Several studies have shown that the aggression that males often exhibit against other males, for example, is influenced by the presence of another natural compound, serotonin.
"Low serotonin means high aggression," Gammie says. "You see that in humans, you see that in monkeys, you see that in rats."
But of course there are many factors that influence animal behavior, including a mother's willingness to risk her own life to protect her young. The desire to pass along your genes is a powerful force, and the Wisconsin research, which still needs to be verified by others, is only part of the picture.
"This is part of the puzzle," Gammie says. "But I don't think we have all of the pieces together."
There's one thing he's sure of, though. It's not a good idea to mess with a new mother, regardless of the species.
When he was living in Seattle a few years ago he was dive-bombed by a crow. The crow screeched as it dove repeatedly toward his head, and so naturally he kept looking up. A glance around his feet told him why the crow was being such a devil. Her chicks were near him on the ground.
"There's only a brief period each year when the chicks are on the ground," he says, and the black bombers had the right idea.
"Their behavior is really effective because it gets your attention away from the chicks," he says.
By the way, a few weeks ago I was walking my border collie when a ptarmigan bounded out from under a bush, making a terrible scene. It flapped its wings furiously, puffed itself up, and charged at the dog.
A few days later, I found out why. While walking along the same road I spied the bird, accompanied by four chicks.
My dog was unhurt, although a bit mortified over getting beaten up by a bird.
But at least it was a mother bird.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.