‘Man flu’ may be all a matter of evolution

‘Man flu’ may be all a matter of evolution, Cambridge scientists say - Anguished complaints from men afflicted with seemingly minor ailments are often attributed to “man flu”. Now scientists say that men may have been left by evolution less well-equipped to fend off disease.

In an evolutionary twist, it appears that there is a trade-off between high testosterone levels and being able to produce a robust immune response.

The theoretical study, by University of Cambridge scientists, looked at various scenarios to test whether environmental and behavioural factors could have led men and women to evolve slightly different immune systems.

Clinical studies have found that males are more susceptible to certain infections, such as malaria, and there is some evidence for men suffering more severe symptoms and higher mortality. However, until now scientists had struggled to find an evolutionary explanation for the apparent lower level of immunity in males.

The latest study, published in the Royal Society journal Proceedings B, suggested that in some cases there could be a trade-off between developing a strong immune system and being reproductively competitive. Because males tend to compete more fiercely, this trade-off is likely to be exaggerated in men.

"If you are devoting a lot of resources to producing proteins and cells in the immune system, you may be limiting your resources for reproduction," said Olivier Restif, who led the research.

In particular, testosterone has been shown to interfere with the immune response, meaning that men with high testosterone levels are potentially at greater risk of infection. In theory, this should help females select the best men — only the strongest and healthiest would be able to afford the risk of producing lots of testosterone.

However, from time to time genetic mutations would arise that allowed males with the mutation to produce larger than normal amounts of testosterone. Although this would put carrier males at greater risk of infection, it would also make them more attractive, meaning that during times of fierce reproductive competition their genes could be passed on.

Over evolutionary timescales, the aggregate effect of such mutations could lead males to have a slightly less robust immune response than women.

“Women seem to be able to respond more robustly to immune challenges. Maybe men aren’t just playing sick, but really are more susceptible,” said Leslie Knapp, a biologist at the University of Cambridge.

There may be additional reasons why strong immune systems are selected for more strongly in women. Previous research has found that the female sex hormone oestrogen helps women fight off infection.

Kevin Maloy, an immunologist at the University of Oxford, said: “There could be a completely different explanation, like that women just have a higher pain threshold, possibly linked to pregnancy and the menstrual cycle." ( timesonline.co.uk )