Fruit flies have sex even when they’re ill, study finds 

They’re best known for buzzing round in circles searching for bananas – but overripe fruit may only be one of the fruit fly’s primary interests.

Researchers at the University of Birmingham have found that the insects continue to have sex with each other even when infected with deadly pathogens.

In the lab, infected fruit flies continued to engage in courtship and mating, regardless of whether either the male or the female fly had been infected. 

It had been thought that infections typically reduce the amount of energy available for activities like mating. 

But diseased fruit flies may be no more put off from having sex than humans are when we’ve caught a cold. 

The study focused on Drosophila melanogaster, a species commonly referred to as the fruit fly, and sometimes the vinegar fly or pomace fly. Pictured are two mating in the wild. The researchers at the University of Birmingham performed experiments in the lab (file photo)

When faced with a potential life threat, some animals respond by investing more into reproduction, likely in attempt to pass on genes to the next generation. 

This may well be what is happening with fruit flies in the conditions tested in the lab.

‘Animals have limited energy resources that need to be distributed among different activities, like fighting an infection or mating,’ said study author Dr Carolina Rezaval at the University of Birmingham. 

‘We were interested to understand how animals prioritise and balance their investment in immune defence and reproduction.’ 

The study focused on Drosophila melanogaster, a species commonly referred to as the fruit fly, and sometimes the vinegar fly or pomace fly. 

The species has been in use for over a century to study genetics and behaviour, including the courtship ritual, which is ‘composed of complex innate behaviours that culminate with copulation’, the researchers say.

In the lab, flies were infected with bacteria including Serratia marcescens and Staphylococcus aureus, both of which can seriously affect humans. 

After being infected, males and females were placed into a ‘courtship chamber’ to see whether they would mate. 

The fruit fly (Drosophila melanogaster) is a small, common fly found near ripe and rotten fruit

Mating success was measured as the percentage of flies that mated within one hour. 

Courtship and mating behaviours were similar in both infected and uninfected flies, the researchers found.  

This was also true when the flies’ immune system was artificially activated using genetic manipulation.

It was also true for both sexes – infected male flies displayed ‘high levels of courtship and mating success’, while infected female flies remained ‘highly sexually receptive’. 

Even more amazingly, uninfected flies mated equally frequently with both infected and healthy partners, suggesting that they do not avoid choosing mates who are infected.

Flies are not oblivious to infection, however – previous studies have shown that infected flies can show abnormal locomotion, sleep and feeding behaviours.

The study, published today in the journal Proceedings of the Royal Society B, suggests that courtship and mating behaviours are prioritised, even when other behaviours are altered during the development of the infection.

More work is needed to find out what is going on in the brain to maintain reproductive behaviours in the face of infection, the authors say.