Before attempting to copulate, male St. Andrew's Cross spiders (Argiope keyserlingi) want to know exactly how many others their potential mates have already had sex with. May not be your typical first date question, but these spiders have long dispensed with the pleasantries -- females are conspicuously bigger, sexual cannibalism is known in this genus, and he only gets to mate twice, so he needs to be sure.
Females of this species have two (vaginalike) sperm-storage receptacles, which are right next to each other. During sex, males deposit sperm using one of their two (penislike) pedipalps. In an attempt to prevent others from copulating with the female after they do, males will break off a palp and use it to permanently seal off the female’s opening. Scientists call these “genital plugs.” Once the male loses one of his palps, he can only have sex again if his remaining phallus aligns with a female’s unfilled genital opening (left to left, right to right, for example). Oh the woes of fixed ipsilateral insemination.
If a male approaches an incompatible female or one who has already been plugged twice, he wastes time competing with other males’ sperm. After all, once the sperm is deposited into her sperm-storage organ, he doesn’t actually know that his sperm will be used to fertilize her eggs. This is referred to as cryptic female choice: she wants the highest quality genes for her spiderlings. Not to mention, any male approaching a female risks being eaten if she’s pregnant and hungry. All this might explain why these males prefer virgins, researchers say. So, exactly how clueless are males of their prospective mates’ promiscuity? Do they really need to get up close and personal before knowing?
To see if males can detect the strength of “sperm competition” and the presence of other males’ mating plugs, a team led by Stefanie Zimmer from the University of Hamburg, Germany, placed males on a wooden peg that was linked by silk strands to the webs of two females. And the team watched who, and how, he chooses.
They found that males use pheromones on the silk to detect how many previous partners the females have had. They chose females with only one former partner, over females who have mated twice, 75 to 90 percent of the time. However, if both females have mated only once, the males -- using just pheromones alone -- couldn’t tell which of them would be compatible. Specifically, he didn't know who would have an available (virgin) genital opening that matches his.
Lastly, in experiments with a virgin male and a virgin female, the male would not mate with her twice. He would save a palp for another female later, even if he was surrounded by females who have already mated twice and he didn’t know if he’d encounter a compatible female. The researchers reasoned that males of this species have adapted to polyamory -- unlike others in the orb-web genus Argiope, which are monogamous. These males would rather search for another female, than monopolize one.
The work was published in Behavioral Ecology this week.