Under the Microscope: Research with animals has shown that motherhood makes females smarter. This is also almost certain to apply to humans, writes Prof William Reville
Pregnancy and motherhood alter the structure of the female mammalian brain, making the mother more attentive to, and better able to look after, the baby. Studies on rats show that some of these maternal changes relate to behaviour such as nest building, grooming and protecting the young, and prowess at foraging for food. Other changes relate to memory and learning ability and response to stress.
The hormonal changes that occur at pregnancy, birth and lactation orchestrate neuronal changes in the mother's brain. Studies of rats have shown that these brain changes enhance the offspring's chances of survival and, furthermore, the maternal benefits persist in the mothers until they reach old age. This research is described by CH Kingsley and KG Lambert in Scientific American, January 2006.
The maternal instinct to nurture and protect the baby is the most powerfully motivated behaviour, exceeding even displays of behaviour in support of sex or appetite. Some researchers believe the gradual development of maternal behaviour was the main driver behind the evolution of the mammalian brain from the reptile brain - reptiles lay eggs and leave them whereas mammals raise their young in a nest which must be defended.
The female reproductive hormones, oestrogen and progesterone, are produced in abundance during pregnancy. Detailed animal studies since the 1940s have shown that these hormones are required for the display of maternal behaviour in animals. The hypothalamus and pituitary glands secrete the hormones oxytocin, which initiates birth contractions, prolactin, the lactation-inducing hormone, and endorphins that ease the pain of birth, and, together with prolactin and oxytocin, also stimulate maternal behaviour in female rats.
Other research has identified the brain regions that supervise maternal behaviour. A part of the hypothalamus called the medial pre-optic area (MPOA) is prominent in this regard. Several other areas are also involved and all these areas contain many receptors for hormones and other neuro-chemicals. Damaging some of these areas, eg the MPOA, eliminates maternal behaviour.
The reproductive hormones initiate the maternal response but once the offspring arrive they can independently stimulate maternal behaviour on their own. Kingsley and Lambert report that "mother rats given a choice between their newly born pups and cocaine invariably choose the pups". The brains of nursing mother rats have been studied using functional magnetic resonance imaging (fMRI) as they suckle their pups, demonstrating increased activity in a part of the brain involved with reinforcement and reward. Suckling and general pup contact may release endorphins, an opiate-like drug, in the mother's body, ensuring that it feels good for mothers to take care of their pups. fMRI studies on human mothers have also shown that brain areas regulating reward are activated when human mothers care for their children.
Mother rats must also display maternally related behaviours other than direct caring for their young, for example, they must frequently leave the nest in order to forage for food. Kingsley and Lambert hypothesised that pregnancy and birth would also enhance the foraging capacity of rat mothers and reduce the fear and anxiety that must attend their leaving the nest to gather food thereby putting themselves and their offspring at risk. These hypotheses were confirmed in an experiment where young female rats that had one or two pregnancies were compared with age-matched virgin rats. The females with experience of pregnancies could remember the location of food in a maze much better than age-matched virgin rats and they displayed lower levels of stress hormones in their blood when confronted with stress.
Studies on human mothers have also found cognitive benefits from pregnancy and motherhood. Enhanced sensory abilities allow human mothers to recognise their infants' sounds and odours. It has also been found that women who became pregnant from the age of 40 onwards are four times more likely to live to 100 than women who became pregnant before 40. This may be because the benefits of pregnancy and motherhood on the brain kick in when, otherwise, reproductive hormone levels start to decline with the beginnings of menopause.
And now some good news for fathers. Animal studies of marmoset monkeys, who are monogamous and where both parents care for offspring, show that both parents outperform non-parent monkeys on foraging tests. Studies have also shown that new fathers mimic some of the hormonal changes seen in pregnant women, although on a much lower level. For example, husbands of expectant wives have increased levels of oestrogen, the "female" hormone, as well as a surge of prolactin, the hormone associated with breast-feeding in women. It is not clear why these changes occur, but they probably help fathers to develop a caring attitude towards their offspring.
These new studies completely upend the old stereotype that having a baby temporarily sucks as much energy from your brain as it does from your body. The truth is that mother's brain becomes much more capable and, if the animal results also translate to humans, the improved brain characteristics stay with Mum for the rest of her life. Even Dad gets a little smarter. And, having borne the children, parents should heed the advice of Lady Bird Johnson: "Children are apt to live up to what you believe of them."
• William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie