Study Explains Why Presence of Males Shorten Female Lifespans in Some Species

Roundworm males secrete signaling molecules that shorten the lifespan of the opposite sex by more than 20 percent, according to researchers from the Stanford University School of Medicine.

For several years, researchers have known that the presence of some male worms and flies can result in the early death of their female or hermaphroditic counterparts. Though not always entirely clear why this is, some hypothesized it had to do with the physical stress of mating.

Published in the journal Science Express, the new study suggests a far more sinister reason.

The researchers observed the common laboratory roundworm, known as Caenorhabditis elegans, which generally live about 20 days. Roundworm populations consist on average of 0.01-0.1 percent males, with the remainder consisting of hermaphrodites, which can self-fertilize but produce more offspring when they mate with a male.

Not only did the presence of males trigger the expression of a wide number of genes associated with signaling and sensation in hermaphrodites, but the researchers found that the by putting the hermaphrodites on plates where males had been was enough to cause their early death.

According to Anne Brunet, associate professor of genetics, these results suggest "the possibility that the male-induced demise is not just due to the physical stress of copulation but instead involves some degree of active signaling."

One possible reason for this is that by killing off their counterparts, the males are able to conserve resources for their offspring and decrease mating options for other males.

"In worms, once the male has mated and eggs are produced, the hermaphrodite mother can be discarded," Brunet said. "The C. elegans mother is not needed to care for the baby worms. Why should it be allowed to stay around and eat? Also, if she dies, no other male can get to her and thus introduce his genes into the gene pool."

The scientists found the same trend in species of roundworm with distinctive males and females in equal amounts, suggesting the phenomenon is not exclusive to hermaphroditic populations with low male populations.

"The observation that this male-induced demise is present in several species of worms and has also been shown in flies suggests that it could have some adaptive benefits," Brunet said. "It will be interesting, of course, to determine whether males also affect the lifespan of females in other species, particularly mammals."