Again and again across the animal kingdom, males die younger than females — a consistent, puzzling pattern of premature expiration that new research suggests may be the unavoidable biological cost of impressing the ladies.

An emblematic example of the trade-off is seen among houbara bustards, a large Middle Eastern bird with exuberant male courtship displays. Males age faster than females, and biologists found that exceptionally exuberant males age fastest.

The measure of aging was, appropriately, declining sperm quality.

“It is the males that invest most effort into extravagant sexual display that experience this spermatogenic ‘burn-out’ at an earlier age,” wrote researchers led by biologist Brian Preston of France’s University of Burgundy in an August 1 Ecology Letters paper.

Background to the study is the phenomenon of aging and why it happens. After all, it might make sense for natural selection to eliminate those genes and mutations responsible for physical decline.

But that perspective doesn’t account for the near-inevitability of predation or disease or plain bad luck. Animal life is likely to be short, and so evolution might nurture a tendency to invest in activities with short-term reproductive benefits and long-term costs.

“It might be better on balance to spend now and not worry about later,” said Preston.

The need to spend, however, is not equally divided among the sexes. As a general rule, it’s easier for males to reproduce than females; male reproduction is limited only by the number of possible mates, while females are limited by the requirements of pregnancy. Therefore females must be choosy.

Out of that dynamic comes the panoply of displays used by males to convey reproductive desirability. It makes sense that males investing heavily in courtship would burn out young, their energies expended on loud calls and bright colors and elaborate dances.

Enter the houbara bustard, with a name redolent of leather-jacketed young men gunning their motorcycles and a courtship display to match. As Preston and colleagues describe, hopeful males “erect an ornamental ‘shield’ of long white feathers in front of them as they begin to run at high speed, often circling a rock or bush,” then emit “several subsonic ‘booming’ calls.” They do this for up to18 hours a day, for months on end.

Male bustard enthusiasms are such that, fortunately for Preston’s group, they’ll happily copulate with female bustard mannequins, depositing semen in conveniently positioned petri dishes. Thus the researchers could assemble a decade-long record of bustard-semen quality, cross-referenced with individuals’ courtship displays.

The resulting pattern was clear and consistent. Male bustards who boomed loudest and danced longest produced the most, best sperm, but only for a little while.

Their sperm quality — considered representative of general health and fitness — peaked when they were just 4 years old. By 6 years old, it was in sharp decline. Like late-middle-aged men with sports cars, alligator-skin shoes and erectile dysfunction, however, male bustard courtship displays continued with youthful zeal even as their sperm withered.

According to Preston, the researchers haven’t yet measured the lifespans of their birds, which can reach beyond 20 years in captivity. But “a general decline in physiology leading to a shorter lifespan would be the expectation,” he said.

While the findings do support an evolutionary trend toward sacrificing long-term health for short-term benefit, “Another possibility is that there’s more than one way to have a successful reproductive life, by playing either the short or the long game,” said Preston. “It’s possible that less gaudy males enjoy more limited reproductive success but do so over a longer period of time.”

Might reproductive imperatives and metabolic consequences follow the same rules in Houbara bustards as in humans, among whom female life expectancy tends to be significantly longer?

“Humans are not under the same intensity of sexual selection that Houbara males are,” said Preston, “but it’s a tantalizing possibility that similar patterns may occur.”

Images: 1) Houbara bustard. (Hanne & Jens Eriksen/Birds Oman) 2) Porsche. (Eli Christman/Flickr)

Video: Yves Hingrat

See Also:

“Sexually extravagant males age more rapidly.” By Brian T. Preston, Michel Saint Jalme, Yves Hingrat, Frederic Lacroix and Gabriele Sorci. Ecology Letters, August 1, 2011.