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Last Updated:  Tuesday, 25 February, 2003, 00:06 GMT
Secret of paternal bond
Lab mice
Mice are usually very poor fathers
A hormone more usually associated with pregnancy and the maternal bond may also control the way men react towards their children.

In experiments on mice scientists have shown that the hormone, progesterone, plays a key role in regulating male aggression towards infants.

It had been thought that this type of behaviour was controlled by the male sex hormone testosterone.

Paternal behaviour may be based in the same biology as maternal behaviour
Dr Jon Levine
Scientists from Northwestern University in Illinois found that a lack of progesterone reduced aggressive behaviour and stimulated male mice to act in a fatherly way towards their offspring.

Lead researcher Dr Jon Levine said: "We discovered that the hormone progesterone and its receptor are important in males, not just females.

"Paternal behaviour may be based in the same biology as maternal behaviour."

Like adult males of many other species, male mice rarely contribute to parental care and often attack or kill infants soon after birth.

Although this hostile behaviour had previously been attributed to testosterone, no link has been conclusively established.

The scientists focused on mice with a gene that blocked progesterone receptors in normal mice.

Dr Levine said: "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."

None of the mice whose progesterone was knocked out killed their offspring, compared to a rate of 74% in a control group who had normal progesterone levels.

In a separate experiment, the researchers used a drug to block the progesterone receptors in normal mice and found that they too became highly paternal.

Lack of progesterone had no effect in reducing aggression towards other adult mice.

Human link

Dr Levine said the same mechanism may be important in other mammals - including humans.

Dr Pierre Bouloux, a neuroendocrinologist at the Royal Free Hospital in London, told BBC News Online that progesterone was not thought to play a significant role in humans.

In fact, it was only produced in small quantity during the synthesis of other more important chemicals.

He said it was possible that exposure to high levels of progesterone while in the womb might have some impact on later behaviour.

Dr Bouloux said: "The results are interesting, but it is extremely tenuous to suggest that they can be extrapolated to male behaviour and paternal care in humans."

The research is published in the journal Proceedings of the National Academy of Sciences.

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