ScienceShot: How to Tell if a Spider's Been Sleeping Around

Courtesy of Stefanie Zimmer

ScienceShot: How to Tell if a Spider's Been Sleeping Around

It may be an awkward question, but male St. Andrew's Cross spiders (Argiope keyserlingi) need to know how many partners their potential mate has already had sex with. Females have two vaginalike receptacles, placed side by side, in which males deposit sperm using two penislike palps. To prevent others from copulating with a former love, males will break off a palp after copulation and permanently seal the female’s genital orifice. When a male loses a palp, he can have sex again only if his last sex organ aligns with a partner’s unfilled genital opening—a left-sided phallus couples with a left-sided “vagina,” for example. If a male (smaller spider shown above) approaches an incompatible female or one that’s been plugged twice, he not only wastes time, but he also runs the risk of being cannibalized by a pregnant and hungry lady spider. But males aren’t clueless to a beloved’s former promiscuity, according to a report published this week in Behavioral Ecology. A suitor was placed on a wooden peg linked by silk strands to the webs of two prospective paramours. Males used pheromones on the silk to detect how many previous partners the females had, and chose those with only one former lover 75% to 90% of the time over females that had mated twice. When deciding between two females that had mated once, however, males couldn’t distinguish which genital opening was available based on pheromones alone, meaning their “spidey sense” had its limits. In a third experiment, virgin males with two palps were presented with a virgin female, but interestingly, they never mated twice, opting to save a palp for a future mistress. These findings suggest that A. keyserlingi males have adapted to be polyamorous, the researchers say, unlike other Argiope species, which tend toward monogamy.

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