Female hormone makes bad dads

The traditionally “female” hormone progesterone makes male mice aggressive towards their offspring, shows a new gene knockout study.

The traditionally “female” hormone progesterone makes male mice aggressive towards their offspring, shows a new gene knockout study.It overturns the textbook view that testosterone prompts males to threaten their pups. US researchers made the discovery after silencing the actions of progesterone in male mice. They did this by knocking out the gene which makes progesterone receptors, molecules which enable cells to respond to the hormone. This dramatically reduced aggression and enhanced paternal behaviour when infants were placed near their fathers.Three quarters of the healthy “control” mice still able to respond to the hormone committed infanticide compared with none of the knockout mice.Fatherly care “In the knockout mice we noticed something quite startling,” said Jon Levine, one of the team at Northwestern University in Chicago, Illinois. “They behaved differently, and the most obvious changes were a complete lack of aggression toward infants and the emergence of active paternal care. These animals are terrific dads.”Male mice seldom help to rear their pups, and often attack or kill them soon after birth – like adult males of many other species. This hostility had previously been attributed to testosterone, whereas progesterone is classed as the hormone controlling female reproductive behaviour.Neurobiologist Levine and his colleagues at Queen’s University, Kingston, Ontario and the Baylor College of Medicine, Houston, Texas placed each baby mouse in the cage of its father and rated paternal and aggressive behaviours. The knockout males displayed the most fatherly tendencies, often contacting pups and carrying them to the nest.Aggression switch A second experiment where a drug was used to block progesterone receptors in normal mice confirmed the team’s findings. These mice became highly paternal like the knockout mice.However, neither the knockout mice nor the control males lost their hostility towards other male mice. Disabling progesterone failed to dampen the males’ general level of aggression which stems from testosterone – so progesterone is key to infant-directed aggression in male mice, concluded the researchers.Levine told New Scientist that the team would be looking to see the effects of the same neurochemical pathway on men’s behaviour. Towards the end of pregnancy, women’s progesterone levels rapidly decrease and they produce less of the hormone receptor. “Does this happen in new fathers as well as new mothers? Does progesterone go down precipitously in men around the birth of a child – is it part of the bonding process?” he says.(Source: Proceedings of the National Academy of Sciences: New Scientist: July 2004)