Sword and dagger arachnid fights may explain weapon evolution
Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Earth 26 November 2015
By Michael Slezak
Species: Pantopsalis cheliferoides
Habitat: The wet forests of northern New Zealand
Choose your weapon… now fight! Harvestmen are the first animals found to have different types of weapon in a single species. Setting up fights between them could help explain why the weird and wonderful world of animal weaponry got so diverse.
In the animal world – especially among arthropods like insects and arachnids – evolution has produced a wide array of weaponry, including giant horns and long snouts.
But why evolution should produce this diversity, seen even within same groups of insects, is puzzling.
"One thing that people have been fascinated with is why there are so many different types of weapons in closely related species," says Christina Painting from the University of Auckland in New Zealand. "We don't know what the benefits are to evolving different weapons at all."
Cage fights
Painting's unique find, a species with two weapons – and two different fighting tactics – promises to pry the question open. That will mean setting up some cage fights.
Harvestmen are arachnids. They look like spiders but are more closely related to scorpions, though they lack venom. They generally have large mouthparts for scavenging food and, in males, for fighting.
But Pantopsalis cheliferoides, which lives in the wet forests of the north Island of New Zealand, is very unusual.
Many animals in which the males have weapons have two different ways of developing: small males develop small or no weapons, and big ones develop big weapons. Males with big weapons will tend to fight, while the small, unarmed males will often either sneak around or pretend to be females.
Small P. cheliferoides males have small jaws, which don't work well as weapons and so they sneak around rather than fight.
But large ones can have one of two jaw types, and two fighting techniques.
Some big males have long, slender jaws (pictured below). They use those like swords, swinging them around wildly. Other large males have short, broader jaws, which they use like a dagger – jabbing viciously at competing males (top picture).
Tom Saunders
The harvestmen with dagger jaws seem to be more aggressive and potentially able do more damage, Painting says. "It's just anecdotal evidence at this stage but I imagine they could clip each others' legs off," she says.
Now that she's found this diversity in a single species, Painting plans to set up fights between the two types and see which wins. But it's not about sport.
It's a unique opportunity to explore the question of weapon evolution, Painting says. Since two species of dung beetle, say, won't generally interact, it's impossible to compare the costs and benefits of different weapons. But harvestmen of the same species must naturally encounter one another.
"It's like watching evolution in action," Painting says.
She and her colleague Greg Howell now have funding to study the costs and benefits of each weapon.
Bruno Buzatto from the University of Western Australia in Perth studies this question too and is excited by the find. He is keen to find out what underlies the evolution of these different weapons.
"I think it's a good opportunity to study the genetics behind this – if it's purely genetic and one fights much better than the other, why doesn't it drive the other to extinction? Maybe the different males do better under different habitats or different conditions," Buzatto says.
Journal reference: Scientific Reports, DOI: 10.1038/srep16368
Read more: "Why insects are the real rulers of the world";
"Weird warriors: The animal world's dirtiest fighters"
(Images: Christina Painting)
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