Marathon season has started and runners - fast and not-so-fast - have begun battling it out in 26-mile races around the world.
But another competition is also taking place on runners' online fundraising pages, where the attentions of the fundraiser - not just the donor - are being fought over, and donations are more than just a selfless act.
It is perhaps unsurprising that when people donate to friends' fundraising web pages, they are influenced by the size of donations made by others.
The larger the donations they see pledged, the more they are likely to give.
But men appear to behave very differently to women when it comes to donating to someone of the opposite sex.
When men see large donations to "attractive" female fundraisers, they up the stakes considerably, giving up to four times more than they would have given otherwise.
According to a study published in Current Biology, which looked at nearly 700 fundraising pages from the 2014 London Marathon, what male donors are doing is competing for female attention.
Researchers from University College London and the University of Bristol, UK, said men were engaging in "competitive donating" and their actions could have an evolutionary function.
Dr Nichola Raihani, an evolutionary biologist from UCL, said: "It's unlikely men are aware of doing it, but they are programmed to improve their success with females compared to other males."
Donating generously can signal wealth and social status, as well as kindness, to potential partners.
Women were not found to act in the same way in the study, however, which makes sense in evolutionary terms, Dr Raihani said.
"Women place more emphasis on status and wealth in the opposite sex, whereas men look for youth and fertility in potential mates," she said."
For both men and women, fundraisers who smile on their pages are perceived to be more attractive than those who don't - and they receive more donations.
Online fundraising pages have become a very popular way of collecting donations for a chosen charity before an event like a marathon.
And the big six marathon races around the world - Tokyo, London, Boston, Berlin, Chicago, New York - raise millions for charities.
The London Marathon raised £46.5m for good causes in 2007, making it the largest single annual fundraising event in the world.
It has broken that record every year since and last year the race raised £53.2m.
Cancer Research UK, as London's official charity this year, knows how important online fundraising is.
The charity has 2,000 runners in the race and a target to make £2.5m from donations.
Study co-author Prof Sarah Smith from the University of Bristol, had some practical advice for the charity to pass on to its fundraisers.
"I would say get your generous friends to donate early and make sure you put a good picture up, preferably one in which you are smiling," she said.
"Not everyone can be among the most attractive fundraisers, but our results also show that a picture in which the fundraiser is smiling can be just as effective, boosting donations by more than 10%."
Dr Hanna Zagefka, reader in social psychology at Royal Holloway, University of London, says attractive people have always had an easier ride.
"Attractive people are treated better, are more popular and generally get better jobs," she said.
"It is also assumed they are more intelligent."
As a result, pictures of attractive, smiling female fundraisers put donors in a better mood and encourage generous behaviour.
Mum-of-two Rebecca Couper is running the London marathon for Hospice UK, because they looked after her father and supported her family after he died.
She has a picture of her running shoes on her fundraising page, rather than one of her face.
But she says it doesn't seem to have made a difference.
Female friends and family members have been her biggest donors and she's raised nearly £1,300 so far.
"I didn't want to plaster my face up there," she explained.
"I'd rather put up views of where I'm running and how much training I've doing, instead of pictures of me, red-faced, that aren't very flattering."
Charles Stewart, a doctor living in London, is raising money for the Neuro Foundation and has reached £1,400 in donations.
He chose a picture on his fundraising page which he felt looked like him, in running gear.
"Smiling wasn't a conscious choice. An action shot would have been better but they are usually taken from far away," he said.
"The picture would have had to suit my vanity I suppose, but I'm amazed anyone sponsored me at all."
He believes men and women aren't that different when it comes to competing for attention from the opposite sex.
So is there no such thing as an altruistic donation on a fundraising page?
Dr Raihani says people can be very generous and their reasons for giving to charity are well-meaning, but that doesn't mean their motives have not evolved to benefit them in some way.
She uses the act of eating as an example.
"Our primary drive is to dispel the feeling of hunger, which is pleasurable, but the evolutionary purpose is to make sure we don't starve and die," she pointed out.
Explaining why people give generously, when there is no obvious benefit for them doing so, has kept scientists and psychologists busy for many years.
Research suggests giving can create a warm glow, which lasts much longer than spending money on oneself.
And Dr Zagefka says this means that helping others has a prolonged effect on well-being - something which people often don't realise.