Why Do Male Redback Spiders Backflip Into Death During Sex?
New research has found that a gene linked to sex chromosomes probably explains why male redback spiders do a backflip straight into the female's jaws during mating — getting eaten in the process. The study, led by Kardelen Özgün Uludag of the University of Hamburg, crossbred redbacks with a close cousin species that does not do the flip, to trace how the behavior is passed down (The Guardian, 2026-08-12).
Male redbacks are eaten by their single mating partner after sex. During mating, a male uses a pair of small appendages near his mouth to deliver sperm to the female's two separate sperm storage organs. To father all of her offspring, he has to fill and plug both. While doing this, he flips his body into her fangs. Cor Vink, a spider expert at Lincoln University in New Zealand, described the act. The redback is one of only two widow spider species where males mate with just one female. Across 31 widow spider species, this sacrificial behavior shows up mainly in redbacks and brown widows.
The flip seems to boost the male's reproductive success. Redback males that get eaten mate longer and can father twice as many offspring as rivals who escape, a finding first reported by M.C.B. Andrade in 1996 in Science (Andrade, 1996 via Wiley, 2008-02-12). Andrade's later work showed that males are eaten in about 65% of matings after they position their body above the female's fangs (PMC). But there's a risk: the female may consume the male before he can deliver sperm to her second storage organ.
The New Zealand katipō, a close cousin of the redback, split off genetically about 100,000 years ago. Unlike redbacks, katipō males do not sacrifice themselves — they deliver sperm and leave the web. This difference between two related species gave researchers a natural experiment. They bred female katipō with male redbacks and watched how many descendants inherited the somersault. Nearly half of the third generation also flipped, a pattern that points to a gene on a sex chromosome rather than many genes working together (ABC News, 2026-08-11; The Guardian, 2026-08-12).
The broader context here is about why a male would willingly die for a single mating. In species where a male mates only once, his entire reproductive future depends on that one encounter. The potential gain from mating longer and blocking rival males through sacrifice can outweigh the cost of death. J.A. Miller's 2007 paper in Evolution, cited over 100 times, looked at how male sacrifice has evolved repeatedly across different spider lineages, calling the Australian redback the most thoroughly studied example.
Not all males follow the sacrificial path. Andrade's lab found that some males avoid being eaten by mating with females that are too young to consume them, a tactic reported in 2016 and studied further in 2017 (The Guardian, 2016-09-20; Phys.org, 2017-12-13). This escape route raises questions about whether the somersault is always triggered or whether conditions like the female's age can switch it on or off.
The venomous redback spider is common across Australia and has been accidentally introduced to New Zealand, Japan, and the United Arab Emirates. Until now, scientists have studied the somersault mostly through its evolutionary payoff — what the male gains from dying. By showing that the flip can be inherited across species lines in a pattern consistent with sex-chromosome transmission, this study points researchers toward the specific genetic mechanism behind a behavior that has evolved multiple times, independently, in widow spiders.
