An eight-legged love tragedy may go something like this: The male spider approaches the female, who is four times his size. She scuttles away, but he creeps closer and closer. Finally, he takes hold of her with his spindly legs, climbs aboard and inserts his "penis" into her genital opening and discharges a jet of sperm. Then — quite abruptly — his legs curl underneath his body, he hangs motionless from his lover, and his heart stops beating.
The male dark fishing spider (Dolomedes tenebrosus) mates with just one female, and the act results in spontaneous death and genital disfiguration for the male, new research finds. The bride then makes a meal of her mate.
This gruesome tale isn't the first case of sexual cannibalism, in which one spider (usually the female) eats its mate after copulation. But unlike spider species in which the female kills the male,the male fishing spider appears to expire from internal causes. [Watch Video of Spider Cannibal Sex]
Study researcher Steven Schwartz, a behavioral ecology graduate student at the University of Nebraska-Lincoln, discovered the fishing spider's curious death somewhat by chance. Schwartz wanted to know whether male fishing spiders mate with only one female throughout their lifetime, a characteristic known as monogyny.
But when he took a closer look, Schwartz realized males were dying after a single mating. The females weren't killing the males; the males were dying on their own, Schwartz said.
All male spiders have two front appendages known as pedipalps. When the males are sexually mature, they ejaculate onto a sperm-web and suck up the sperm into their pedipalps, which inflate due to fluid pressure (as state known as "being charged"). During mating, the males transfer sperm to the female from one of their pedipalps, which then generally deflate.
But in the fishing spider, the pedipalp remains distended and useless after mating. The male shrivels up and dangles from the female. Within a few hours, he dies.
The cause of death appears to be linked to the pedipalp expansion, Schwartz said. In some cases, Schwartz accidentally triggered the expansion, and the spider curled up and perished.
Female fishing spiders will cannibalize the males after they die, but there are some advantages for the unfortunate fellows, at least for their genes. Eating a male can reduce the female's receptiveness to other males, increasing the chances that only the dead male will father her offspring.
"If a male can monopolize a female, other males will get smaller pieces of the reproductive pie," Schwartz said. Alternatively, serving as a "nuptial meal" may help nourish the female, leading to healthier offspring. So for the males, "not all is lost, in a sense," Schwartz said.
Monogyny and self-sacrifice occur in other spiders, such as the Australian redback. The mating strategy often arises in species with a high ratio of males to females, so males have limited opportunities to mate.
For male fishing spiders, who have just one shot at love (or mating), it's all or nothing.