Male redback's bizarre death somersault during sex is in its genes

Male redback spider's dramatic death somersault during sex coded in its DNA In short: Researchers have spotted a genetic source for the bizarre sexual cannibalism behaviour of the redback spider. Male spiders, which somersault onto their partners' mouths to be eaten while mating, appear to inherit this trait via sex chromosomes. What's next? Genetic data and courtship information could help to conserve a related threatened New Zealand spider, the katipō. Male redback spiders live fast and die young, ending their lives in a gory ritual of sexual cannibalism. Female spiders eat their male partners while they mate, and males appear to encourage this behaviour, with their final act being a dramatic somersault onto the female's jaws. A new study, published in Biology Letters, has pointed to a genetic source for this behaviour. The researchers compared the activity of the Australian redback to its close, but non-cannibalistic cousin, New Zealand's katipō. By breeding the two species together, the researchers, who are based at the University of Hamburg in Germany, learned more about the DNA behind the self-sacrificing somersault. Bruno Alves Buzatto, a behavioural ecologist at Flinders University who wasn't involved in the research, said the findings were "very striking". "It supports the idea that being killed during copulation and cannibalised can actually lead to the male's advantage,"Dr Buzatto said. The self-sacrificial redback backflip Loading...Redback spiders (Latrodectus hasseltii) are "monogynous": meaning male spiders mate with only one female over their lifetime. This mating ritual is quite intricate, featuring a "somersault" so that the males' large abdomens are right next to the females' fangs for ease of biting. Then, the male redbacks constrict their abdomens, which makes them look like they're being squeezed, so they can cheat death for long as possible. The secret sex lives of redback spiders have been documented for decades, including research by scientists at the University of Toronto seen in the video from ABC TV's Catalyst program in 2010: Loading...Dr Buzatto said the self-sacrificing death dance gave males an evolutionary advantage: it improved their chances of offspring. "The process of being cannibalised increases the time that they mate with the female. Being able to transfer more sperm to the female's spermatheca, which is the organ that the females use to store sperm, provides an advantage." Female redbacks can store sperm and use it to lay eggs for up to two years after mating. The katipō (Latrodectus hasseltii), a venomous spider native to New Zealand, is very closely related to the redback. But katipō spiders don't engage in this cannibalistic mating ritual. Instead, males and females tend to mate with each other multiple times. Cor Vink, an entomologist at Lincoln University in New Zealand who assisted with the study, said katipō evolved from redback spiders about 100,000–200,000 years ago: a blink of an eye in evolutionary terms. "In that small amount of time, that whole behaviour is lost," Dr Vink said. "It's really interesting when behaviours like that can be lost. We don't know enough about how behaviours are coded in the DNA of invertebrates." Cross-breeding spiders to test genetics The researchers bred male redbacks and female katipō together to see how mating behaviours were displayed in their offspring. Dr Vink collected both species of spider from different sites in New Zealand, where the redback spider is an invasive species. "We collected adult redbacks and then fed them up, took them back, grabbed their egg sacks and then they got sent to Germany. We did the same thing with the katipō as well," he said. Both redbacks and katipō bites can require antivenom, but there haven’t been any recorded deaths in decades (for redbacks) or centuries (for katipō). Dr Vink said both spider species were "shy and retiring", but he and his fellow collectors wore gloves as an extra precaution. "When they bite you, you won't die, you'll just feel like you're going to,"Dr Vink said. "It's a pretty horrible experience from what I've been told and I'm not willing to find out what it's like." Once the spider eggs had made it to Germany, the researchers bred male redbacks and female katipō together and reared male hybrids to maturity. Then, they added the hybrids to webs built by, and housing, female katipō, filming their behaviour with infrared cameras. They analysed the footage from 104 mating rituals, looking for signs of both the somersault trick and the abdominal constriction. Roughly half the hybrids performed the life-ending somersault while mating. Because of the way they'd bred male redbacks with female katipō, this 50/50 split allowed the researchers to conclude the somersault trait was inherited on a spider sex chromosome. But the other behaviour — the abdominal wiggle — wasn't displayed as clearly, suggesting the genetics for this trait were more complicated. This surprised the researchers, who were expecting both traits to have a common genetic source. Michael Kasumovic, an evolutionary biologist at the University of New South Wales who wasn't involved in the research, said this was a neat finding. "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," he said. "This study highlights that maybe we need to think about how different species interact and how that can lead to new traits evolving and new species appearing." Dr Buzatto said he wasn't surprised by the study's findings. "We know that a lot of very complex behaviours can be encoded by some fairly simple genetic basis." But he said spider sex chromosomes were very complicated, and differed from species to species, meaning there was still much to learn. "I feel like the more we look, the more weird and surprising things we'll find with spiders," Dr Buzatto said. "We've probably only described and documented a quarter of the spiders that exist, maybe a third, we're not too sure … That really opens people's minds to why we find all these crazy, unusual, surprising behaviours and life histories in these animals." Genetic research to conserve NZ's spiders Dr Vink said this genetic research could help to protect the katipō, which is in decline in New Zealand — in part because of the redback spider. In New Zealand, redback spiders are predating on and threatening several native species like skinks and beetles. But the biggest threat to the rare katipō is that the two spiders can interbreed and create hybrids. "In New Zealand, we have a very real danger of losing a species through hybridisation,"Dr Vink said. In his own research, Dr Vink is studying redback spider pheromones to see if they can be used to control populations. "The idea is that if we can attract enough males using the pheromone, we can reduce the male population, which will crash the overall population because males can only mate once," he said. Research into courtship and genetics can help with this conservation technique, alongside giving researchers more data to track katipō and redback populations.