Why Male Redback Spiders Flip Onto Their Mate's Fangs—and Die

For the male redback spider, mating is a

For the male redback spider, mating is a spectacularly fatal finale. Midway through copulation, the male performs an acrobatic backward somersault, placing his abdomen directly onto the female’s waiting fangs. She promptly bites him, and his death becomes part of the reproductive process—his body serves as a meal that may boost her egg production. But why would a male voluntarily sacrifice himself in such a dramatic way? New research suggests the answer lies deep in his genetic code.

Scientists had long suspected that this suicidal somersault was a fixed behavior, but they weren’t sure whether it was learned, triggered by the female’s aggression, or hardwired into the male’s biology. To find out, a team of researchers at the University of Toronto Scarborough turned to an unlikely experiment: crossbreeding Australian redbacks with their less suicidal cousins from New Zealand.

The New Zealand redback—a close relative—rarely performs the somersault. When the researchers mated Australian males with New Zealand females and vice versa, they observed the offspring’s behavior. The results were striking: the sons of Australian mothers and New Zealand fathers still carried out the fatal flip, while those with New Zealand mothers and Australian fathers did so far less often. This pointed to a genetic basis, likely passed down through the mother’s side.

Further analysis revealed that the behavior is controlle…

Further analysis revealed that the behavior is controlled by a small set of genes, possibly linked to the male’s nervous system or muscle control. The researchers discovered that even when the female was sedated and unable to bite, the male still performed the somersault—proving the act is not a response to her cues but an automatic, genetically driven movement.

This finding challenges the idea that the male’s sacrifice is purely a strategic choice. Instead, it appears to be a dramatic example of how evolution can shape behavior for the benefit of the species, even at the individual’s expense. The male’s death not only provides the female with nutrients but also increases his own chances of fathering more offspring, since the female is more likely to use his sperm if she is feeding on him.

The study, published in the journal *Animal Behaviour*, offers a rare glimpse into the genetic underpinnings of a bizarre and fatal mating ritual. It also raises new questions about how such extreme behaviors evolve—and why some populations develop them while others do not. For now, the redback’s somersault remains one of nature’s most striking examples of the lengths to which genes will go to ensure their own survival.