Live fast, age fast—at least if you're a male houbara bustard.
That's because male bustards that perform longer courtship displays lose sperm quality faster than males that do not put on elaborate seduction shows, a new study suggests.
The sheer energy required to keep up marathon performances eventually takes its toll on the sperm production of the flashy males, which actually start out with healthier, more robust sperm than their humdrum rivals.
"In nature, life is very risky, so you need to balance the benefit that certain behaviors can give you at present, and the costs that these same behaviors can incur later on," said study senior author Gabriele Sorci, an evolutionary biologist at the University of Burgundy in France.
In the case of the bustards, Sorci thinks the showy males get the balance right, in terms of survival. In part because of the high chances of dying young in the wild, he said, "it's always the best [reproduction] strategy to have early benefits and eventually pay a later cost."
The new study represents the first time that such a tradeoff has been linked to declines in male fertility in any species, Sorci added.
Showy Birds Firing Blanks
In their North African habitat, male houbara bustards perform long displays to attract females up to six months out of the year. Some males keep at it for several hours a day and perform more often, while others invest less of their resources to luring mates.
After strutting for a while, a male erects an ornamental "shield" of long white feathers and then runs at high speed, often circling a rock or a bush, according to the study, published recently in the journal Ecology Letters.
The show climaxes in a flash of black and white feathers and several booming calls so deep they're almost out of range of human hearing, according to Sorci.
Females will often select males that run more laps while taking fewer and shorter breaks.
For the study, the team used ten years of data taken from more than 1,700 male bustards—ranging from 1 to 24 years old—at captive-breeding facilities in Morocco.
Each day, workers would observe the males' courtship behavior, which is roughly the same as in the wild. The scientists then added up the number of days the males were seen displaying, and for how long. The result was an index of "male sexual-display effort" for every year of each bird's life.
Also daily, a team would place a dummy female under each male bird to initiate mating and then capture his ejaculate in a petri dish, the study says. Scientists recorded the quality of the bird's semen—how many sperm were in each ejaculate, how well they swam, etc.
The results showed that the most avid performers during youth released smaller quantities of semen, with more dead and abnormal sperm, at older ages. The data also uncovered a still unresolved mystery: Though these flashy males had passed their reproductive prime, the show still went on, Sorci noted.
Fabrice Helfenstein, an evolutionary ecologist at the University of Bern in Switzerland, said via email that the study "is sound—it is based on a lot of animals and uses properly new statistical tools."
For instance, the captive birds proved useful: "Such aging processes are usually hard to reveal in wild animals," which are generally thought to die too young to suffer the effects of aging, said Helfenstein, who was not involved in the new study.
Casanovas, Take Note
In general, the idea that investing in sexually attractive traits early in life racks up costs later could possibly be applied to other species, including humans, study co-author Sorci said.
For instance, showy male bustards may be the "bird equivalent of the posers who strut their stuff in bars and nightclubs every weekend," study leader Brian Preston, also of the University of Burgundy, said in a statement.
"If the bustard is anything to go by, these same guys will be reaching for their toupees sooner than they'd like."