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Monday, Jan. 15, 2001

NATURAL SELECTIONS

The effect of hormones on fatherhood


It is usually thought that men share only symbolically, if at all, in the experience of pregnancy, but recent studies have shown that paternal males undergo changes in the same hormones as maternal females. The work promises to biologically verify the experiences of new fathers.

Djungarian hamster males act as midwives during birth.

Among mammals, males typically do not provide direct paternal care. In some species, such as rats, males will attack and consume newborn young at sight. But in species whose males do provide paternal care, there is evidence that hormones mediate the onset and the maintenance of paternal behavior.

This is most remarkably shown in Djungarian hamsters (Phodopus campbelli), whose males act as midwives during birth. They pull pups from the females, lick off birth membranes, open the pups' airways and clean up the amniotic fluid and the placenta. The males even clean and rebuild the nest area, and baby-sit when the female goes off to feed.

The hormones prolactin and cortisol, which have known influences on maternal behavior, have also been measured in the blood of paternal hamster males. It is believed that changes in these hormones stimulate paternal behavior.

"Males and females have the same DNA, except for a small number of genes on the male's Y chromosome," said Katherine Wynne-Edwards and Catharine Reburn, of the Department of Biology at Queen's University, Ontario. This means that paternal males are likely to experience hormonal effects along the same brain pathways as females.

Writing in the biology journal Trends in Ecology and Evolution, they suggest that this hormonal effect also occurs in humans. In recent studies some men have reported pregnancy symptoms, and in tests report strong emotional responses to babies. These men show, like women, high levels of prolactin and cortisol hormones. They also show low levels of the sex steroids such as testosterone.

The studies are correlational (scientists prefer a more rigorous experimental approach), but, according to Wynne-Edwards and Reburn, they promise to illuminate both maternal and paternal behavior during the course of pregnancy. Subjects were men attending prenatal classes in Canada with their pregnant partners. They agreed to give blood samples and undergo testing, and as such should be regarded as a homogenous sample of highly motivated fathers.

Blood was sampled twice from the men. Between the samples, the men held a soft doll wrapped in a soiled blanket from the neonatal nursery. At the same time, the couple listened to the prerecorded cries of unconsoled babies, and watched a video about breast-feeding a newborn baby. Each adult then completed a questionnaire about male pregnancy symptoms and their emotional responses to the test stimuli.

Men who reported strong emotional responses to the standardized stimuli had significantly higher prolactin concentrations than men who did not respond to the stimuli. Both men and women also had higher prolactin and cortisol concentrations immediately before the birth, and lower sex steroid concentrations immediately after the birth.

Men reporting pregnancy symptoms experienced significantly larger decreases in testosterone between the two blood samples than did men without the symptoms, but since such "snapshot" samples of testosterone concentrations have a poor record of predicting behavior, the authors were cautious about the conclusions. "It is possible that lower testosterone concentrations subtly alter 'priorities,' but there is no experimental test of that hypothesis yet," said Wynne-Edwards.

In males as well as females, prolactin accesses key areas of the brain involved in maternal behavior. Mutations in the prolactin receptors in the brain have been shown to impair maternal behavior, and in naturally paternal species, prolactin levels correlate with the expression of paternal behavior.

Chemical signals from the pregnant female are the likely cues that trigger paternal behavior. Human cultures that show extensive paternal care also have well developed pair-bond relationships, and within a pair-bond, hormone concentrations between couples are correlated. The next place to look will be in cultures with different expectations of paternal involvement. In Tibet, for example, some women have more than one husband. And in some Islamic cultures men have more than one wife. Testing the hypothesis of hormonal involvement in paternal behavior in these situations will be logistically easier, says Wynne-Edwards, than trying to recruit Canadian men who are "uninvolved": those isolated from their pregnant partner.

What should be concluded from this? Recent work indicates that naturally occurring paternal behavior is functionally equivalent to maternal behavior. A wide variety of hormones are involved in maternal behavior, and these same hormones will also influence paternal behavior.

"Hormones do not 'make' any person do anything," warned Wynne-Edwards. "It is easiest to think of hormones as altering thresholds, or probabilities, for a behavior to occur, but not as causing them." Hormonal changes during a woman's ovulatory cycle don't "make" her cry or get angry, but may subtly alter the thresholds for evoking stronger responses to environmental stimuli (in other words, the insensitive thing you said). Similarly, lower testosterone levels in men may alter "priorities" in male behavior.

What this work promises to do though, is show how hormones and behavior interact in a social context, and to provide biological verification for the experiences of involved fathers.

E-mail Rowan Hooper at: rowhoop@nies.go.jp


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