Quirks & Quarks·Analysis

Feeling amorous after a romantic dinner? At least in flies, the feeling may come from the gut

A new study found that in fruit flies a meal releases a gut hormone that then triggers courtship behaviour. The connection between food and sex is a rich vein to explore in other animals too.

Scientists have investigated the relationship between the appetite for food and for sex in many animals

A new study shows male fruit flies produce a hormone after eating that stimulates courtship behaviour. (Alexlky / Shutterstock)

There's an old (sexist) chestnut that the way to a man's heart is through his stomach. A new study has shown that, at least in fruit flies, this may be quite literally true.

Scientists at the University of California San Diego have identified a chemical hormone produced in the guts of fruit flies after feeding that triggers a switch from feeding behaviour to courtship behaviour in the males. 

Among the key instincts for survival are the urge to eat and the urge to reproduce. The scientists isolated a hormone in fruit flies known as Dh31 that triggers a switch from one urge to the other in the flies.

Their study was published this month in the journal Nature.

Two fruit flies mate during an experiment. (University of Exeter)

In humans, research has shown that the pleasures of both food and sex are associated with neurotransmitters like dopamine and hormones like endorphins. These neurotransmitters make us feel good after both eating a good meal and sex.

In fact, these brain chemicals are released when we do just about anything that promotes our survival, such as the runner's high during vigorous exercise, and the rush that comes after escaping from danger. 

As evolutionary anthropologist Anna Machin told me on Quirks & Quarks, these brain chemicals are the "bribe" that evolution uses to convince us to keep eating and reproducing. 

But the fascinating connection this new research makes is connecting the brain and the gut in these fruit flies. 

While humans can playfully mix food and sex, eating and mating are not obviously connected when it comes to the animal world. It is difficult to mate while foraging for food and hard to find food while engaging in courtship.

It turns out that in these flies at least, the decision to switch from one behaviour to the other is governed by signals originating in the gut and influencing the brain, rather than just from the brain itself.

Take care in a romantic dinner. The wine may attract amorous fruit flies. (Lucky Business / Shutterstock)

The scientists verified the role of the hormone Dh31 during genetic experiments in which flies were engineered without the ability to produce it.

Those flies were only interested in feeding with no urge to mate. When the hormone was artificially activated in the guts of other flies, they quickly changed their behaviour from fancying food to finding a mate.

In fruit flies this kind of courtship behaviour includes wing shaking, body wagging, and perhaps a serenade with a courtship song.

While this research says nothing specifically about human behaviour, other research has demonstrated in mammals that the gut has much more influence on the brain and behaviour than we previously suspected, often through chemicals produced by the complex ecosystem of microorganisms in our guts called the microbiome. 

In mice, for example, it's been shown that the microbiome can affect memory, learning and anxiety. So it would certainly not be surprising to discover that there is a connection, through these organisms, between food and courtship in mammals, and even humans.

Romance is a multifaceted endeavour and food certainly enhances the pleasure. But after dinner, if you get a feeling that the time is right, it could be your gut sending you a message.

ABOUT THE AUTHOR

Bob McDonald is the host of CBC Radio's award-winning weekly science program, Quirks & Quarks. He is also a science commentator for CBC News Network and CBC-TV's The National. He has received 12 honorary degrees and is an Officer of the Order of Canada.

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