Male redback spiders in Australia perform one of the most extreme mating rituals in the animal kingdom. They flip themselves directly into a female's fangs during sex, get partially eaten alive, and still manage to finish transferring their sperm. Then they break off their own reproductive organs to block other males. Now, scientists think they know why, and the answer is surprisingly simple.
According to Live Science, a new study has found that this spectacular and lethal behavior has a genetic explanation rooted in the spider's X chromosome. The redback, known scientifically as Latrodectus hasselti, is Australia's version of the black widow. Its close relative, the katipō of New Zealand, does none of this. The katipō mates quietly and parts ways without any acrobatics or cannibalism.
The two species may have split less than 100,000 years ago, which makes the difference in behavior all the more striking. "What makes this system so intriguing is the fact that we are dealing with two species here, which are very closely related," said Kardelen Özgün Uludağ, a doctoral student of behavioral biology at the University of Hamburg in Germany. These two species may have separated less than 100,000 years ago, but they still show entirely different behavior and observable traits. "This is very uncommon in animals," she said.
To investigate the genetic roots of the behavior, Uludağ and her colleagues needed a way to compare the two species at the genetic level. They started by exposing katipō females to redback males, producing hybrid offspring. Those hybrids were then crossbred with either pure redback or pure katipō males to create males with mixed genetic backgrounds. The researchers could not run the experiment the other way around because redback females eat katipō males rather than mate with them.
The hybrid males were then paired with katipō females so researchers could observe their behavior without the males being killed. The team tracked how often each hybrid male performed the backflip and whether they also constricted their abdomens, a maneuver that appears to slow blood loss and extend the time they can survive while being consumed.
The results were unexpected. The two behaviors did not appear together as a package. Hybrid males performed both the backflip and the abdominal constriction only about half the time. The other times, they did one or the other but not both. This suggested the two actions are controlled by separate genetic pathways, not a single behavioral switch.
More surprising still, the backflipping behavior showed a pattern tied to the X chromosome. Male and female spiders both carry two X chromosomes, but females carry two full sets of genetic material on theirs while males carry only one. The somersaulting followed a dominant-recessive inheritance pattern, which is unusual for a complex behavior. Most behaviors tied to survival or reproduction tend to involve many genes working together, making simple inheritance patterns rare.
The katipō equivalent of this behavior does not exist, which gave the researchers a clean comparison point. The question of how such a dramatic behavioral difference evolved so quickly between two nearly identical species remains open. The study does not identify the specific gene or genes responsible, but it narrows the search to a region of the X chromosome and establishes the inheritance pattern, which gives future researchers a clear starting point.
The findings were reported in a study that Uludağ conducted as part of her doctoral research at the University of Hamburg.
