The venomous bite of the funnel-web spider can kill in hours if left untreated. But why the insect-eating Australian arachnid’s venom evolved to become deadly to humans, which are neither predator nor prey of the spiders, was something of a mystery. Now, a new paper suggests that these spiders developed their potent venom to defend themselves against would-be predators while on the hunt for love, reports the Economist.
Australia’s funnel-web spiders are a group of around 40 species that spend most of their time lurking in burrows framed by their signature funnel-shaped webs. In particular, the Sydney funnel-web (Atrax robustus) is often called the deadliest spider on Earth, explained Shannon Verhagen for Australian Biographic in 2017. The 13 deaths attributed to funnel-webs are all chalked up to males of the Sydney species, but the deaths came to an abrupt halt once an effective antivenom was developed in 1981.
In the waning days of the massive fires that ravaged Australia, heavy rains in January of this year brought relief from the flames as well as a series of warnings from wildlife officials that conditions had grown ripe for a massive emergence of funnel web spiders. That’s because funnel-webs like things moist and the male funnel webs were already due to embark on their annual search for mates, which typically occurs between January and April.
The problem, as far as Homo sapiens are concerned, is that the neurotoxic venom of the male funnel-webs wandering the countryside is five times more toxic than the female’s, explains Jessie Szalay for Live Science.
The venom, which contains peptides called delta-hexatoxins, causes searing pain, convulsions, difficulty breathing and increased blood pressure in people, reports Hannah Osborne for Newsweek. Puzzlingly the venom is painful, but not deadly to non-primate vertebrates such as dogs, mice and birds.
For Inverse, Nina Pullano writes that the new study, published this week in the journal the Proceedings of the National Academy of Sciences, sequenced RNA from the venom glands of ten species of funnel-web spiders to figure out when and why the delta-hexatoxins evolved to such extremes in males.
"These toxins had originally evolved to kill insects such as cockroaches and flies,” says Bryan Fry, a venom researcher at the University of Queensland who led the study, in a statement. Fry explains that when male funnel-webs become sexually mature they embark on a dangerous journey to find a mate, during which time they seldom feed but are in considerable danger of being eaten themselves by vertebrates including rodents, birds and lizards. "The data shows that natural selection put the necessary pressure on to switch an insect-specific venom into a vertebrate-specific defensive venom,” says Fry. “And, unluckily for us, we're a vertebrate species which copped it in the process."
In the study, Fry and his co-authors say that the fact that the spiders’ neurotoxins are merely painful to non-primates and deadly to us and our close relatives is just an unfortunate evolutionary accident.
The study may offer improved understanding of how funnel-web spider venom attacks the human body, which could improve anti-venoms. In the Conversation, the researchers write that they’re also examining the insect-specific venom of the female funnel-webs in hopes of deriving improved insecticides.