A male's seminal fluid can influence the size of a female's future offspring even when he's not the father, a study of neriid flies has found.

Key points: Female neriid flies mate with multiple partners even when they are not fertile.

Female neriid flies mate with multiple partners even when they are not fertile. Semen collected from bigger males may nuture future offspring sired by smaller males.

Semen collected from bigger males may nuture future offspring sired by smaller males. Study suggests semen is useful for more than genes.

In response, females may have evolved to seek out and benefit from these seminal resources when they're not fertile.

The research, published today in Trends in Ecology and Evolution, was led by evolutionary biologist Dr Angela Crean of the University of New South Wales.

She observed that when a large male neriid fly (Telostylinus angusticollis) mated with a female before she was fertile he would pass along his seminal fluid, while her eggs remained unfertilised.

But when the female mated with a second, smaller male two weeks later and laid eggs fertilised with his sperm, Dr Crean was stunned to find the offspring were closer in size to the first male.

"An abundance of recent research has shown that ejaculate, even in the tiny quantities exchanged by flies, contains more than just genetic material," she explained, adding that seminal fluid contains proteins, peptides and non-coding genetic material.

"It's well known that chemicals in the ejaculate can influence a female's behaviour.

"In Drosophila [fruit flies], for example, sex peptides transferred in seminal fluids have a range of effects, from getting females to lay more eggs and change her feeding behaviour to stopping her from wanting to mate with other males and making her more aggressive.

"In this case we found that the seminal fluid has an impact on future offspring as well: previous mating partners were able to influence the size of subsequent offspring, even when they didn't sire them."

Mating behaviour influenced by resources in semen

Other animals, such as birds and some primates, have evolved to choose males that will provide food, resources and parental care, even if they are not yet ready to mate.

Dr Crean proposed that female mating behaviour may be similarly influenced by resources in the seminal fluid, even when males only provide a tiny ejaculate.

"Some insects like crickets give huge sperm packets, known as spermatophores, which the female snacks on in what is known as a nuptial gift," she said.

"In this case mating with multiple partners, even if the female's eggs are not ready, makes sense because she still benefits: while the female neriid flies don't eat the seminal fluid, they are still able to absorb its contents.

"This is actually one of the most efficient routes of resource delivery for a female. Internal fertilisation gives males a channel to inject other chemicals into the female."

Dr Crean said at the evolutionary level, "females may be able to distinguish between genetic and non-genetic benefits, and mate before their eggs are ready to obtain good seminal fluid".

"They could then switch preferences in their fertile phase to get the best genes," she said.

She said it is an active area of debate in humans as well, with research showing women may show different preferences for partners depending on the stage of their menstrual cycle.

"Of course in humans, males help with raising the offspring so the female is looking for other attributes in the father of her children," Dr Crean said.

"But for animals where only sperm is transferred, we're trying to break through the myth that the only thing females get from males is genes.

"We want to change the way we think about what drives female choice and female mating patterns."