Male redback spiders perform a self-sacrificial somersault directly into the jaws of females during mating, a complex behavior that new research published in Biology Letters on Wednesday shows has a remarkably simple genetic basis linked to the spider’s X chromosome, according to researchers led by Kardelen Özgün Uludag of the University of Hamburg.
The Mechanics of Redback Spider Cannibalism and Mating
The redback, also known as the Australian black widow, stands as one of two monogynous widow spider species where males typically mate with a single female. During copulation, a male redback utilizes two arm-like appendages known as pedipalps to transfer sperm to the larger female’s two sperm storage organs.
To survive the initial transfer, the male executes a distinct acrobatic maneuver. “He’s pumping sperm, he flips his abdomen right over into her mouthparts and then he constricts his abdomen … to make it hard for the female to get a purchase on his abdomen. That lets him survive the first mating,” said Cor Vink, an associate professor and spider expert at Lincoln University in New Zealand.
Genetic Basis of the Self-Sacrificial Somersault
Out of the 31 known widow spider species, male self-sacrifice is typically restricted to brown widows and redbacks.
By breeding female katipō with male redbacks, the research team tracked how many offspring inherited the somersaulting behaviour. Roughly half of the third-generation descendants also performed the somersault, leading the research team to determine that the trait connects to a particular spot on the X chromosome of the spider.
“It is quite remarkable that a highly complex phenotype such as somersaulting, which may lead to partial reproductive isolation, can have such a simple genetic basis,” the researchers noted.
“Our work shows that this highly complex cannibalistic behaviour can evolve rapidly, with very few genetic changes,” said Kardelen Özgün Uludag.
Did You Know?
In order to sidestep a lethal meal following mating, earlier studies discovered that certain males have evolved a tactic where they mate with immature females who lack prior experience in consuming their mates.
Reproductive Success and the Risk of Interbreeding
Ideally, a male redback needs to mate twice. “Ideally, males should mate with the same female two times because females have two separate sperm storage organs,” said Kardelen Özgün Uludag.
If the male dies during the first mating sequence before reaching the second storage organ, another male can step in. “If a male dies after the first mating, another male can mate with the same female and then half of the female’s offspring would be from the second male,” Uludag said.
At the same time, the venomous redback spider has established a wide presence across Australia and has found its way accidentally into other nations, such as Japan, New Zealand, and the United Arab Emirates. Vink has found evidence that interbreeding between native katipō and invasive redbacks occurs in the wild. The katipō is thought to have diverged genetically from redbacks about 100,000 years ago.
“This is a concern because we could lose our endemic protected species just through hybridisation,” Vink said, describing the work as an “elegant paper” that shed light on the redback’s “weird mating behaviour”.
Frequently Asked Questions
Why do male redback spiders somersault during mating?
Male redbacks somersault into the jaws of the female during copulation while pumping sperm, which lets him survive the first mating, with the behavior linked to a specific location on the spider’s X chromosome.
Do all widow spider species practice sexual cannibalism?
No. Out of the 31 known widow spider species, male self-sacrifice is typically restricted to brown widows and redbacks.
How many times do male redbacks need to mate?
Males ideally should mate with the same female two times because females have two separate sperm storage organs, which a male needs to inseminate and plug to sire all of the offspring she produces.
Join the Conversation
What are your thoughts on this evolutionary trait? Share your perspective in the comments below, or subscribe to our newsletter for more breaking wildlife updates.
Keep reading