The Australian redback spider has one of nature's more unsettling rituals: the male redback spider flips himself toward the female's fangs when they mate, as though inviting her to make a meal of him.
A study published Aug. 12 indicates that both the somersault and the brief abdominal tightening that can buy some males extra time may trace back to a relatively simple genetic setup.
Scientists reported in Biology Letters that the difference between Australia's redback spiders and New Zealand's katipō spiders, which do not perform this sacrificial flip, may be shaped by more than one genetic factor.
According to Smithsonian Magazine, the evidence points to at least two separate influences rather than a single tightly linked bundle.
Researchers explored that possibility by crossbreeding redbacks and katipōs and then tracking 104 successful matings involving hybrid males.
Because the somersault is followed by an abdomen-constricting move that can delay being eaten, the team had expected the two behaviors to be inherited together.
That was not what the observations showed: Roughly half of the hybrid males displayed both behaviors, while the others showed only one or the other.
That outcome suggests the two traits are passed down independently.
As evolutionary biologist Michael Kasumovic of the University of New South Wales said: "Given how important both traits are to survival, we would guess that they should be strongly linked together. But they aren't, and that suggests they evolved independently. And that's really cool."
Native to Australia, redbacks are small venomous spiders, and their males are unusual even within the widow spider family.
Among the 31 known widow species, this self-sacrificial mating behavior has been documented in only two: redbacks and brown widows.
Lead author Kardelen Özgün Uludağ, a behavioral biologist at the University of Hamburg, said: "What makes this system so intriguing is the fact that we are dealing with two species here, which are very closely related. This is very uncommon in animals."
Redbacks and katipōs are thought to have split only about 100,000 years ago, making the contrast between them especially notable.
The findings challenge assumptions about how complicated behaviors evolve. Researchers think the redback's somersault may be tied to a gene or supergene at a single location on the X chromosome, while abdominal constriction appears more complex and may involve multiple genes, along with possible signals from females.
Beyond the mating mystery, the research may also matter for conservation. In New Zealand, invasive redbacks are hybridizing with native katipō spiders, putting local populations at risk.
A better understanding of how interbreeding works and which traits are shared between the species could help researchers sharpen efforts to protect katipōs.
In this case, genetics may offer practical clues for managing an invasive species before it pushes a native one closer to collapse.
As ecologist Cor Vink of Lincoln University, who helped collect spiders for the study, said: "The more we know about the interbreeding, the more we know about what we can do to combat redbacks and katipō interbreeding in New Zealand."
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