Male redback spiders invite cannibalism through a ‘self-sacrificial somersault’ during sex – but why?

A new study suggests that the complex, self-sacrificial somersault performed by male Australian redback spiders during mating is governed by surprisingly simple genetics. Researchers found that the behavior, which exposes the male’s abdomen to the female’s fangs, appears to be controlled by a single gene region on the X chromosome. This finding challenges expectations that such intricate and risky behavior might require more complicated genetic mechanisms. The Australian redback spider, Latrodectus hasselti, is part of the widow spider family and shares similarities with the North American black widow. It is known for its sexual cannibalism, where females often consume their mates after mating. However, the male redback has evolved a unique strategy to increase its chances of reproductive success despite this risk. During mating, the male performs a somersault, positioning itself in a vulnerable position near the female’s mouthparts. This act allows the male to transfer sperm to the female’s two sperm storage organs, increasing the likelihood that he will sire all of her offspring rather than allowing a subsequent male to contribute. The study, published in Biology Letters, examined the genetic underpinnings of this behavior by crossbreeding redback spiders with their close relative, the New Zealand katipo spider (Latrodectus katipo). Katipo males do not perform the somersault or engage in sexual cannibalism, making them ideal for comparison. Hybridization experiments revealed that first-generation male offspring of redback and katipo parents did not exhibit the somersault behavior, suggesting it might be recessive. Further backcrossing of hybrid females with males from both species produced offspring that displayed the somersault, providing insight into the genetic inheritance pattern. Researchers analyzed the mating behaviors of 104 male offspring from these crosses. They found that the somersault was consistently associated with a specific region on the X chromosome, indicating a straightforward genetic mechanism behind this seemingly complex behavior. In contrast, the abdominal narrowing, a secondary defense mechanism used by some males to delay the female’s attack, did not follow a simple genetic pattern, suggesting it involves multiple genes or environmental factors. The study also highlighted that the somersault and abdominal narrowing behaviors are genetically distinct. Approximately half of the backcrossed males exhibited only one of the two traits, reinforcing the idea that these behaviors are encoded separately. This genetic separation implies that the evolution of these traits may have occurred independently, possibly due to different selective pressures. The findings provide new insights into the rapid evolution of complex behaviors through minimal genetic changes. According to the study’s lead author, Kardelen Özgün Uludag of the University of Hamburg, the simplicity of the genetic basis for the somersault is remarkable given its potential impact on reproductive success and survival. The research underscores how even highly specialized and risky behaviors can emerge quickly in response to evolutionary pressures. In addition to the somersault, the study explored the broader implications of hybridization between redback and katipo spiders. While the two species can interbreed, the resulting hybrids may threaten the genetic integrity of the native katipo population. Cor Vink, a spider expert at Lincoln University in New Zealand, emphasized the ecological concerns surrounding this hybridization, noting that it could lead to the loss of endemic species through genetic dilution. His research has documented instances of wild interbreeding in New Zealand, raising alarms about the need for conservation efforts to protect native spider populations. The study contributes to a growing body of research on the genetic foundations of animal behavior, particularly in arachnids. By revealing the genetic simplicity behind the redback spider’s somersault, the research opens new avenues for understanding how complex traits can arise from relatively simple genetic instructions. Future studies may explore the exact function of the identified gene region and how it interacts with other genetic and environmental factors influencing mating strategies in spiders.