A large, flamboyant bird has given biologists an insight into the relationship between sex and ageing.
The male Houbara bustard has striking ornamental feathers that it displays while running around and "booming" to attract a mate.
As scientists report in Ecology Letters, birds that indulge in more of these sexual displays age faster.
The more "showy" males experienced earlier age-related declines in the quality of their sperm.
The team used 10-years-worth of data on the sexual behaviour and fertility of more than 1,700 North African Houbara bustards that were bred by conservationists in Missour, Morocco.
"The birds are a threatened species, and the data was collected as part of an ongoing conservation programme aimed at increasing their numbers in the wild," explained lead researcher Brian Preston, a scientist based at the University of Burgundy, France.
The scientists measured how much time each male spent carrying out its elaborate display, and compared this to changes in its fertility that are associated with ageing.
By examining the birds' sperm, the researchers discovered that all males experienced a "dramatic decline" in their fertility as they aged.
"Over the age of six years," said Dr Preston. "They began to produce much smaller ejaculates with immobile and frequently abnormal sperm."
"But the key finding was that males that had invested most effort displaying to females in their earlier years experienced the onset of this age-related decline in fertility at a younger age.
"They effectively seem to 'burn themselves out' sooner."
Evolutionary puzzle
Bustards are particularly interesting to scientists trying to understand the biology of ageing, because they are such long-lived birds. The male bustards studied here were between one and 24 years of age
"We might expect that genes that would cause animal to deteriorate with age would be 'weeded out' by natural selection," said Dr Preston. "So it's something of a problem for evolutionary biologists."
One of the theories to explain this evolutionary anomaly is that animals might "overspend" on activities in their early life, by, for example, expending lots of energy on gaudy sexual displays. This might be at the cost of maintaining their body properly in the longer term.
"This is precisely the kind of relationship we have found," said Dr Preston.
"Life is risky," he continued. "Predators, parasites and diseases are likely to prevent animals from living for long periods anyway. So it might be better for them to spend now and not worry about later."
John Burnside, a zoologist from the University of Bath said the findings were "very interesting".
"These results show what may be a classic trade-off - that males invest more in breeding displays when they are young lose sperm viability when they're older," he told BBC Nature.
Studies like this, he added, help "fill in the jigsaw of the complex feedback systems that can occur during natural selection".
By Victoria Gill
Science reporter, BBC Nature
http://www.bbc.co.uk/nature/14388541
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