Some males of the sexually cannibalistic nursery web spider wrap the female’s legs with silk before and during copulation. And according to a new Biology Letters study, bigger males with longer front legs are more likely to mate and less likely to be eaten prior to copulation.
The reproductive success of most males depends on their ability to procure a mate and successfully fertilize her eggs. When sperm competition is high, males might prevent their mates from re-mating by producing sperm plugs and seminal toxins, for example.
Males from animal groups where the females are sexually cannibalistic have an extra challenge: survive the sexual encounter. Across various groups of cannibalistic spiders, sexual cannibalism is often linked to sexual size dimorphism, or a difference between male and female sizes. Males that are relatively larger or have relatively longer legs seem to be better at avoiding female cannibalistic attempts. In species without sexual size dimorphism, males have been known to use strategies like feigning death or using sedative pheromones to render the females unconscious.
Alissa Anderson and Eileen Hebets from the University of Nebraska-Lincoln focused on one particular mating behavior: the wrapping of females in silk by male nursery web spiders (Pisaurina mira). Females have wider carapaces (that is, larger body sizes), but mature males have elongated forelegs, and their unusual behavior is thought to reduce female mobility.
The duo hypothesized that size dimorphism influences the male’s ability to initially engage a female in copulation, while silk wrapping allows him to acquire additional sperm transfer opportunities without being cannibalized. To test this, they randomly paired females with males that are capable of silk wrapping or manipulated males who would still go through the wrapping motions, but no silk would come out.
They found that male nursery web spiders were more likely to copulate and less likely to be cannibalized when their legs were longer than those of the female and when the male-to-female carapace width ratio was larger. Regardless of size, males that couldn’t wrap females achieved fewer insertions for sperm transfer than males that could wrap, and they were more likely to be cannibalized after insemination too.
For a female, cannibalizing the male after receiving sperm is quite beneficial. “She can use those resources for her developing offspring,” Anderson explains to IFLScience over email. But for the male, if he survives, he gets to look for additional females to mate with. “This conflict is what can lead to the evolution of unusual and extreme behaviors such as copulatory silk wrapping,” Anderson adds.
So size and copulatory silk wrapping are sexually selected traits that benefit male reproductive success, but whether their behavior benefits or is costly for the females is unresolved. During the mating trials, females tried to free themselves from their silk bindings (presumably to partake in their post-mating meal), but virgin females often passively allowed the wrapping to take place.
Image in the text: Male Pisaurina mira wrapping female with silk prior to copulation. Alissa Anderson