It’s common knowledge that a mother and father's DNA help form their offspring's genetic make-up – but now scientists have thrown something else into the mix.
Researchers studying fruit flies found their progeny's size could be influenced by the length of a previous mate.
This may be because chemicals in the original male's seminal fluid can have a lingering effect, and scientists have even suggested that something similar could occur in humans.
The theory is known as 'telegony' and was first proposed by Aristotle in ancient Greece.
It was one of the reasons kings were banned fr om marrying divorcees.
The theory was discredited thanks to the advent of genetics. But now, writing in the journal Ecology and Evolution, scientists have suggested flings can influence the offspring of future mates.
Professor Russell Bonduriansky, of the University of New South Wales in Australia, said: 'Traditionally, the idea is when this type of mating takes place, there is no resource transfer and there is no paternal care.
'Males contribute DNA to fertilise an egg, but we believe there is something more complex going on.'
His researchers began thinking about evolutionary preferences for seminal fluid in 2014 while studying the offspring of female fruit flies that mated with males of varying sizes.
They found if a male, either large or small, mated with a female fly before she was fertile, he would pass his sperm along, but the immature eggs would not be fertilized.
The surprise came two weeks later when it was discovered the size of the young was determined by the size of the first male the mother mated with, rather than the second male that sired the offspring.
The lingering effects appeared to stem fr om chemicals in the first male's seminal fluid, even though he was not the father.
Prof Bonduriansky said if seminal fluid plays a critical role for future offspring regardless of the father's actual identity, then females may have evolved to exploit the benefits.
On top of situations wh ere semen is just passing through, some female animals can store semen from multiple males before allowing any of it to fertilize their eggs, and this system could have advantages beyond holding out for the best DNA.
Prof Bonduriansky said: 'Females might be choosy even when they do not have eggs ready to be fertilised.
'They might be getting something for future offspring that will be fertilized later on, or they might be getting something for themselves.
' This idea has been around for decades when thinking about pairings based on obvious resources.
Female gibbons and hawks, for example, have evolved to choose males that provide food, territory, or the promise of parental care, even if they are not ready to have offspring.
But Prof Bonduriansky said that reasoning hasn't been applied to systems wh ere there's nothing but a small ejaculate being transferred.
And size might not matter as much as previously thought. Seminal fluid is chemically complex, with proteins and tic messengers called RNA floating in the liquid outside of the sperm.
So even the effects of a small ejaculate could be significant, giving females a largely unexplored bonus from sex.
Prof Bonduriansky said: 'It is pretty clear now seminal fluid is packed with paternal RNA. And this would be in humans, mice, fruit flies and nematode worms at least.'
He added: 'In some systems, mostly nematodes and mice, there is evidence these RNAs can play a role in early embryonic development.
'But the jury is still out on exactly what effects these molecules have.'
Prof Bonduriansky said researchers typically think of seminal fluid in a small ejaculate as playing different roles, but not as a resource that females purposefully seek out. He added: 'And that might not be the case.'
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Researchers studying fruit flies found their progeny's size could be influenced by the length of a previous mate.
This may be because chemicals in the original male's seminal fluid can have a lingering effect, and scientists have even suggested that something similar could occur in humans.
The theory is known as 'telegony' and was first proposed by Aristotle in ancient Greece.
It was one of the reasons kings were banned fr om marrying divorcees.
The theory was discredited thanks to the advent of genetics. But now, writing in the journal Ecology and Evolution, scientists have suggested flings can influence the offspring of future mates.
Professor Russell Bonduriansky, of the University of New South Wales in Australia, said: 'Traditionally, the idea is when this type of mating takes place, there is no resource transfer and there is no paternal care.
'Males contribute DNA to fertilise an egg, but we believe there is something more complex going on.'
His researchers began thinking about evolutionary preferences for seminal fluid in 2014 while studying the offspring of female fruit flies that mated with males of varying sizes.
They found if a male, either large or small, mated with a female fly before she was fertile, he would pass his sperm along, but the immature eggs would not be fertilized.
The surprise came two weeks later when it was discovered the size of the young was determined by the size of the first male the mother mated with, rather than the second male that sired the offspring.
The lingering effects appeared to stem fr om chemicals in the first male's seminal fluid, even though he was not the father.
Prof Bonduriansky said if seminal fluid plays a critical role for future offspring regardless of the father's actual identity, then females may have evolved to exploit the benefits.
On top of situations wh ere semen is just passing through, some female animals can store semen from multiple males before allowing any of it to fertilize their eggs, and this system could have advantages beyond holding out for the best DNA.
Prof Bonduriansky said: 'Females might be choosy even when they do not have eggs ready to be fertilised.
'They might be getting something for future offspring that will be fertilized later on, or they might be getting something for themselves.
' This idea has been around for decades when thinking about pairings based on obvious resources.
Female gibbons and hawks, for example, have evolved to choose males that provide food, territory, or the promise of parental care, even if they are not ready to have offspring.
But Prof Bonduriansky said that reasoning hasn't been applied to systems wh ere there's nothing but a small ejaculate being transferred.
And size might not matter as much as previously thought. Seminal fluid is chemically complex, with proteins and tic messengers called RNA floating in the liquid outside of the sperm.
So even the effects of a small ejaculate could be significant, giving females a largely unexplored bonus from sex.
Prof Bonduriansky said: 'It is pretty clear now seminal fluid is packed with paternal RNA. And this would be in humans, mice, fruit flies and nematode worms at least.'
He added: 'In some systems, mostly nematodes and mice, there is evidence these RNAs can play a role in early embryonic development.
'But the jury is still out on exactly what effects these molecules have.'
Prof Bonduriansky said researchers typically think of seminal fluid in a small ejaculate as playing different roles, but not as a resource that females purposefully seek out. He added: 'And that might not be the case.'
www.dailymail.co.uk