Caution and Courage
If Liz devoted all her attention to her infant, however, both mother and child would perish. A mother rat that stays safely in the nest with its offspring also dooms them to death from hunger and thirst. Mothers of both species must find ways to resolve the competing demands on their time. In other words, women are not the only members of the animal kingdom who find themselves juggling the duties of a working mom.
To allow a rat mother to toggle between caring for its young and heading out to find food, an area of the midbrain called the periaqueductal gray (PAG) acts as a circuit breaker. In 2010 researchers at the University of São Paulo proposed that the PAG weighs the balance between eating and acting maternally by evaluating input from the brain's limbic system, a set of structures that governs survival-type behaviors. No exact parallel to the PAG's toggle function in rats has been identified in humans yet, but much has been made of a mother's superhuman ability to multitask, perhaps reflecting a similar adaptation.
When a mother ventures into the world, she puts her vulnerable baby at risk. But she may be more attuned to potential threats, perhaps even exaggerating them, suggests research at the Health Sciences Federal University of Porto Alegre in Brazil. Researchers there have shown significant alterations in the architecture of dendrites in the medial nucleus of the amygdala, which in addition to its important role in the olfactory system also controls defensiveness and avoidance behavior. Indeed, when Liz shops she scans the stores for risks to her baby, avoiding the creepy guy by the magazines or the sketchy teens by the vending machines.
Although overall Liz is more cautious, she is also probably much bolder in the face of a threat than she was before becoming a mother. Psychologist Jennifer Wartella, now at Virginia Commonwealth University, has found that, compared with virgins, mother rats exposed to a stressful open-field maze were less likely to freeze in place, explored more readily and appeared to experience less fear (that is, Wartella saw fewer switched-on neurons in the amygdala). With its fear response in check, a rat mom may be able to forage more efficiently and return to its nest and vulnerable offspring more quickly.
Helping a mother navigate the world is her improved ability to decipher the clues in the environment. Recently our student Kelly Rafferty and her colleagues at our lab have been investigating a mother's ability to plan ahead. They allowed mother and virgin rats to forage in an unfamiliar maze that contained water. The rats were then returned to their home cages, some of which contained a water bottle and some of which did not. Subsequently they were moved back to the maze containing water. The mother rats assigned to the waterless home cage spent more time near the maze's water sources and drank more water, as compared with both mothers with full access to water and virgin females. After accounting for potential differences in the rats' thirst, the neuroscientists concluded that the mothers appear to anticipate a future environment and plan for it.
As the previous experiments demonstrated, mother rats seem to excel at tasks that require enhanced attention. Behavioral neuroscientist Kelly Lambert of Randolph-Macon College and her colleagues have collected other evidence of sharp-witted mothers. In 2009 they showed that when it comes to identifying which cue among several signals food, mother rats perform best. And work by Amy Au and Tommy Bilinski, then working in our lab, has begun to identify the rats' strengthened ability to deduce the meanings of symbols. The researchers designed experiments where a rat in an environment learns to associate, say, a triangle or a set of wavy lines with a food reward. After being moved to a new environment, lactating females transferred their knowledge from the old setting to the new one better than virgin females did, again suggesting a heightened attention to detail.
A human mother's brain undergoes a striking structural metamorphosis, too. In 2010 using magnetic resonance imaging studies, neuroscientist Pilyoung Kim, now at the National Institute of Mental Health, and her colleagues found significant increases in gray matter in mothers' brains in the weeks and months after they give birth. Gray matter, which got its name from the color of unmyelinated axons, is a layer of tissue packed with neurons. The growth the scientists saw was particularly visible in the midbrain, parietal lobes and prefrontal cortex—all areas involved in infant care. The mothers with the biggest increase in gray matter volume also reported the more positive perception of their babies.