One of the World's Deadliest Spiders Evolved Its Killer Venom in Hunt for Sex

One of the world's most deadly spiders may have developed its extremely toxic venom as a way of protecting itself while on its annual sex march.

The venom of Australian funnel-web spiders contains delta-hexatoxins, which are the peptides that makes the venom dangerous for humans and primates. There have been 13 recorded deaths, though none since antivenoms became available in the 1980s. It is estimated 30 to 40 people are bitten every year.

The venom attacks the nervous system, blocking nerve impulses to the muscles and causing paralysis of the entire nervous system. This leads to a range of symptoms including muscular twitching, breathing difficulty, fast pulse and increased blood pressure, among others.

Scientists know that males of the Australian funnel-web spider are far more toxic than females. They also know their venom does not affect other mammals in the same way it does humans and primates but why this is the case has remained unknown.

In a study published in PNAS, researchers led by Bryan Fry of the University of Queensland have examined the venom from 10 different funnel-web spider species to better understand why the bites from males are so deadly to humans.

"Delta-hexatoxins exert fatal neurotoxic effects in humans by keeping nerves turned on, so that they keep firing over and over again," Fry said in a statement. "It has puzzled scientists why these toxins are so deadly to humans, when they and other primates haven't featured as either prey or predator during the spider's evolution. And we couldn't understand why most human deaths were being caused by male funnel web spiders, which seemingly had much deadlier venom than females."

Through their genetic analysis, the team was able to show the evolutionary pathway that led to the primate-centric venom. Findings showed a "remarkable sequence of conservation" of the delta-hexatoxins that suggests the venom developed to perform a defensive role.

During mating season in the summer months, male funnel-web spiders leave their nests in search of mates. This is a dangerous venture for the males, Fry said. "[They] wander quite considerable distances in search of females. This can be quite treacherous, and these male funnel web spiders started to encounter dangerous vertebrate predators, such as the dunnart, a small nocturnal mouse-like marsupial."

The findings showed the funnel-web spider venom originally evolved to target insects, including flies and cockroaches. However, it appears natural selection led it to change to become a vertebrate-specific defensive venom. "The toxicity to humans, the team said, was just an "evolutionary coincidence."

"Unluckily for us, we're a vertebrate species which copped it in the process," Fry said.