A Canadian study has found that men's bodies undergo almost the same hormonal changes as women's during the last months of pregnancy.
Anne Storey, a psychology professor at Memorial University in St. John's, Newfoundland, said men who live with their pregnant partners show a decline in levels of testosterone and an increase in prolactin hormones -- which are linked to the production of breast milk in mothers -- as the date for delivery nears.
"We know from animal studies that these changes occur in male mice and gerbils to make them more responsive to their pups after birth," she said. "But this is the first study to show the same hormonal changes occur in men."
The study followed 34 couples in prenatal classes at a St. John's hospital by testing blood samples in the men and women throughout the mother's pregnancy.
In men, prolactin levels increased by about one-third between the middle and end of the pregnancy. While prolactin levels in men were well below those in women, Storey said the increase in prolactin, cortisol and estradiol hormones could be responsible for making men more responsive to their babies.
"How much of the reaction to babies by mothers and fathers is instinct and how much is hard-wired by hormones is hard to say," she said, "but the hormone changes do affect behavior."
This synchronizing of hormone changes between parents occurs only in couples who live together, she explained, and is similar to studies showing that menstrualcycles of women who live together tend to synchronize.
In males, high testosterone levels push them into competition for females and expanding their territory, so the decline during pregnancy could be linked to ensuring they spend more time caring for their young and less on aggressive pursuits, she said.
According to the study, the hormones that rise during the pregnancy fall off after birth, but by how much depends on whether the mother nurses the baby.
Cortisol has generally been seen as a hormone that helps people cope with stress, she explained, but the study indicates a better way of defining its effects is in providing focus. "It may help when the baby is born by preparing parents for its needs," she said.
In some animals, infants quickly learn how to survive on their own, but in humans, babies take a long time to become self-sufficient, so the hormonal changes may be a tool nature uses to ensure the survival of the species, she added.
Storey said the study didn't investigate the mechanism that causes the synchronization of hormone levels in mothers- andfathers-to-be, but some studies into the menstrual cycle phenomenon have suggested they may be linked through the sense of smell by pheromonal communication.
Pheromones are hormones that trigger reactions through the sense of smell.