Crossing Experiments Pin Genetic Basis of Redback Spider's Self-Sacrificial Somersault
Research published in Biology Letters has identified a likely sex-chromosome-linked genetic basis for the "self-sacrificial somersault" performed by male redback spiders (Latrodectus hasselti) during copulation, a behavior absent in their close relative, the New Zealand katipō (Latrodectus katipo). The study, whose first author is Kardelen Özgün Uludag of the University of Hamburg, used crossing experiments between the two species to determine how the dramatic mating behavior is inherited (The Guardian, 2026-08-12).
Male redbacks are eaten by their sole reproductive partner after sex. During copulation, a male uses his pedipalps to transfer sperm to the larger female's two separate sperm storage organs. To sire all of the offspring she produces, he must inseminate and plug both. During this process, the male somersaults into the female's jaws, an act described by Cor Vink, an associate professor and spider expert at Lincoln University in New Zealand. The redback is one of only two monogynous widow spider species, meaning males typically mate with a single female. Of the 31 species of widow spider, male sacrificial behavior is usually observed only in redbacks and brown widows.
The self-sacrificial flip is thought to help maximize reproductive success. Cannibalized redback males copulate longer than noncannibalized competitors and can double their paternity relative to rivals, a finding first reported by M.C.B. Andrade in 1996 in Science and cited in support of the adaptive self-sacrifice theory of sexual cannibalism (Andrade, 1996 via Wiley, 2008-02-12). Andrade's subsequent work documented that males are cannibalized in roughly 65% of matings after positioning their abdomen above the female's fangs (PMC). But the strategy carries a clear risk: males may be consumed before they can transfer sperm to the female's second storage organ.
The New Zealand katipō diverged genetically from redbacks approximately 100,000 years ago. Unlike redbacks, katipō males do not engage in sexual cannibalism; they inseminate the female and run off the web. This behavioral divergence between two closely related species provided the natural experimental setup. German researchers bred female katipō with male redbacks and tracked how many offspring inherited the somersaulting behavior. Close to half of the third-generation offspring also somersaulted, an inheritance pattern consistent with sex-chromosome linkage rather than a more complex polygenic mechanism (ABC News, 2026-08-11; The Guardian, 2026-08-12).
The broader context here connects to the longer-standing theoretical framework for understanding male complicity in terminal mating. According to sexual cannibalism theory, male complicity can be adaptive when the male's future reproductive value is low. In monogynous species, where a male's entire reproductive output hinges on a single mating, the calculus shifts: the potential gain from prolonging copulation and deterring rival males through sacrifice can outweigh the cost of death. J.A. Miller's 2007 paper in Evolution, cited over 100 times, examined the repeated evolution of male sacrifice behavior across spider lineages, identifying the Australian redback as the most thoroughly studied example.
Not all male redbacks follow the sacrificial path uniformly. Andrade's lab previously found that some males evade cannibalism by mating with immature females too young to consume them, a tactic reported in 2016 and explored in a follow-up study in 2017 (The Guardian, 2016-09-20; Phys.org, 2017-12-13). The existence of this alternative tactic raises questions about the conditional expression of the somersault behavior and whether the newly identified genetic mechanism is subject to environmental or physiological modulation.
The venomous redback spider is widespread throughout Australia and has been unintentionally introduced to countries including New Zealand, Japan, and the United Arab Emirates. The crossing-experiment findings offer a genetic anchor for a behavior that has, until now, been studied primarily through its adaptive payoff rather than its hereditary mechanism. By demonstrating that the somersault can be inherited across species lines in a pattern consistent with sex-chromosome transmission, the study narrows the search for the molecular basis of a behavior that has evolved repeatedly and independently across widow spider lineages.
