October 23, 2008

Female Spiders Devour Mates For Better Offspring

Spanish researchers said Friday that female spiders who consume their male suitors produce stronger, larger and more numerous offspring than spiders with more mundane diets. 

The scientists also found that the merciless mother spiders preferred waiting until mating was complete before consuming their suitors, ensuring they would hatch their spiderlings.

The work was the first "natural" study to validate the legend about spiders exhibiting such behavior, and shows why it may be beneficial.

"Now we know that, at least in one species, sexual cannibalism benefiting females occurs in nature," Dr. Jordi Moya-Larano of the Estacion Experimental de Zonas Aridas in Spain, the study's leader, said in a statement.

The scientists found that the Mediterranean tarantulas used in the study did not eat their mates, but instead consumed males prior to courtship, typically after they had already mated with another male spider.

Previous studies have suggested that males may sacrifice themselves for the sake of their offspring, but the current research showed that at least with Mediterranean tarantulas, males are simply unlucky victims and only the spiderlings benefit.

Other studies had also suggested that analyzing spiders in a lab environment produced altered results, perhaps because the spiders were under duress or could not obtain all their required nutrients. 

To account for this, the researchers used a field experiment in which they observed the spiders in the wild, often snatching males from the jaws of females before they were eaten.

"At natural rates of encounter with males, approximately a third of L. tarantula females cannibalized the male,"  wrote the researchers in a report about the study.  

"The rate of sexual cannibalism increased with male availability, and females were more likely to kill and consume an approaching male if they had previously mated with another male," they said.

"We show that females benefit from feeding on a male by breeding earlier, producing 30 percent more offspring per egg sac, and producing progeny of higher body condition. Offspring of sexually cannibalistic females dispersed earlier and were larger later in the season than spiderlings of non-cannibalistic females."

One theory maintained that females who ate males were merely more aggressive and better hunters.  However, when the males were saved just in time, those females did not produce better offspring, indicating that the nutrients obtained by consuming the males were important.

The study was published in the Public Library of Science journal PLoS ONE.  The full report can be viewed at