Even the most useless males in the animal world contribute to the long-term evolutionary fitness of a species, researchers report, by weeding out the genetic flaws that would otherwise doom the females to extinction.
A study on the subject, published online Monday by the journal Nature, provides an experimentally supported answer to the age-old question, "Why do males exist?"
The question isn't as frivolous, or as man-hating, as it sounds: For decades, evolutionary biologists have puzzled over the pluses and minuses of sexual reproduction. The arrangement comes at a heavy cost: Only half of the species is capable of producing offspring.
"We wanted to understand how Darwinian selection can allow this widespread and seemingly wasteful reproductive system to persist, when a system where all individuals produce offspring without sex — as in all-female asexual reproduction — would be a far more effective route to reproduce greater numbers of offspring," Matt Gage, a biologist at the University of East Anglia in Britain, said in a news release about the study.
Several explanations have been put forward over the years: For example, when environmental conditions turn stressful, mixing up male and female genes can lead to new combinations of traits that are better-suited to cope with the change.
Another argument goes all the way back to Charles Darwin, the biologist who pioneered evolutionary theory. He proposed that sexual selection — in which males compete for the favors of females — played a role alongside other forms of natural selection in ensuring the survival of the fittest.
"The theory is well-developed, but it's very difficult to test it," Ricardo Azevedo, a biologist at the University of Houston who has studied the issue, told NBC News.
Meet the beetles
Gage and his colleagues tested Darwin's hypothesis over the course of 10 years, by fiddling with the sex lives of Tribolium flour beetles. The bugs are pests that feed off wheat and other grains, including the food in your pantry. If you've ever come across mealworms in a bag of flour that's gone bad, you know how much of a pain the beetles can be.
The beetles are also notable for their sex roles: The males contribute nothing to the upbringing of offspring, other than their genes. That makes them well-suited for an experiment aimed at figuring out why the females are better off even if they have to deal with useless males.
The research team bred several groups of beetles under different conditions — ranging from a scenario where 90 males competed to mate with just 10 females, to a scenario where the females were assigned their mates with no choice in the matter.
The beetles were bred under these conditions for seven years, representing about 50 generations' worth of bugs. Then each community of bugs was inbred, to reveal whether or not harmful mutations had built up in the beetles' genomes.
Some of the beetle populations that had been involved in lusty competition were still going strong even after 20 generations of brother-sister pairings. In contrast, all of the populations that experienced little or no sexual selection to begin with went extinct within eight generations of inbreeding.
Out with the bad genes
The results suggest that when females aren't allowed to choose fitter mates, the "mutation load" gradually builds up in the genomes of a given population — with disastrous results when the population is put under stress. Gage told NBC News that sexual selection "helps to purge out 'bad genes' from a population."
Azevedo said the experiment is "very exciting, because it's a very nice demonstration of what other researchers had predicted."
Gage said it's valid to generalize from beetles to other species, but he stressed that most males are good for more than their genes.
"The main exception to our model system is in making comparisons with species where males provide important direct benefits to offspring production, such as through care or feeding," he said in an email. "Trends towards monogamy are necessary for ecology to drive those mating patterns to persist (otherwise males will be investing in the offspring that they are not the father of)."
Thus, it's not quite right to say the study fully explains why men exist. Rather, it shows why even good-for-nothing males exist. One would hope there's a difference.
Could super-intelligent females find a way to do without males? Gage noted that such a scenario was outlined in a 1915 novel titled "Herland." The book describes a utopia where women reproduce through parthenogenesis, or "virgin birth." Parthenogenesis occurs in species ranging from sharks to snakes to Komodo dragons, but it has not been documented among mammals in the wild.
The newly published study shows why it's probably better that way, Gage said.
"Our findings show that, while populations like 'Herland' get short-term benefits from asexual reproduction and an absence of sexual conflict, they are vulnerable to ... mutation accumulation over evolutionary timescales," he wrote. "Without sexual selection, mutations are less effectively purged from the population, putting it at higher eventual risk of extinction."
In addition to Gage, the authors of "Sexual Selection Protects Against Extinction" include Alyson Lumley, Łukasz Michalczyk, James Kitson, Lewis Spurgin, Catriona Morrison, Joanne Godwin, Matthew Dickinson, Oliver Martin, Brent Emerson and Tracey Chapman.