Sex is an expensive and risky business. It steals time and drains precious nutrient resources. And each act of reproduction runs the risk of messing up carefully crafted genetic blueprints. So why do we do it?

The answer might seem obvious to you. But it's not so clear to biologists who consider that despite a logical alternative -- asexual reproduction by simple cloning without the help of a partner -- sex is preferred in the wild.

Asexuality, which came first, is seen in plants that send out underground runners and flatworms which, when cut in half, grow a new head on one half and a new tail on the other. Some microbes and fungi lean this way, too, and have since life began.

Scientists don't know how sex even got started. But they have long suspected that organisms prefer sex specifically because of the risk. The slight shuffling of genes produced through sexual reproduction may help organisms adapt more easily to a stressful or changing environment, the thinking goes.

Belief is not the basis of conclusions, however.

A new study using genetically modified yeast helps to settle the question: Sex is indeed beneficial.

The experiment pitted a strain of yeast that reproduces sexually against a modified, asexual version of the same strain. Each grew and reproduced at the same rate, said Matthew Goddard of the University of Auckland.

Then Goddard and his colleagues raised the stakes, providing less food to the little critters. Under these conditions, those engaging in the ultimate act still managed a growth rate of 94 percent whereas the asexual strain only reached 80 percent.

Sexual organisms seem fit to survive.

The research is detailed in the March 31 issue of Nature.

There are shortcomings in the study. For one, it does not reveal why sexual organisms are hardier in stressful conditions. And it fails to explain why the burden of sexual reproduction differs so much between males and females. Not only do females spend more time and energy developing and raising offspring, in most cases, but their sex cells are significantly more expensive to make compared to a male's.

"We are sill far from a definitive answer to the question of why sexual reproduction is so common," Rolf Hoekstra of Wageningen University wrote in a review of the study also published in the journal.

Sexual Leftovers