It’s an autumn morning in the Australian Alps. Dark brown grasshoppers are climbing out of the alpine heath and descending to the earth. Each one has a smaller grasshopper riding piggyback. As the temperature rises, the smaller ones change colour to an intense, gorgeous turquoise. Then the fights break out.
The females—the larger ones—bury their abdomens in the soil and try to focus on egg-laying while rival turquoise males attempt to knock off the guy on her back. There’s biting, kicking, mounting, grappling and mandible-flaring—where the defending male arches his back, shakes his head and exposes his black, fang-like mandibles. In the chaos, females get injured. Males collect scars and lose legs or wings.
These two-centimetre gladiators are thermocolour skyhoppers, the only type of grasshopper known to do battle. “We were like, that’s pretty weird—we’ll study it,” says Kate Umbers, an evolutionary biologist at the University of Western Sydney, who in 2012 was the first scientist to document these fights. “They just go to town on each other.”
These grasshoppers are endemic to the highest snowy mountains of the Australian Alps, bouncing from the alpine vegetation “like blue popcorn” and surprising passing hikers. “They just ping in every direction when you walk through the shrubs,” says Umbers. They hatch from the soil in January and die when the snows arrive in May.
Since the 70s, we’ve known how the skyhoppers change colour. The cells just beneath their exoskeleton contain two types of granules: large brown ones and tiny transparent ones. When temperatures are lower than 10°C, the granules are haphazardly mixed up inside the cells, and the skyhopper looks brown or black. As the day warms, the clear crystals rise to the surface of the cell, forming a reflective layer. They scatter blue light while the brown granules underneath absorb all the other colours, so the insect appears turquoise. The skyhopper reaches peak brightness above 25°C.
It’s a purely mechanical response, requiring no decision-making, hormones or brainpower on the part of the skyhopper. It even happens for a while after they’re dead. So what’s the point of being blue? Researchers thought the colour-change might help the grasshoppers regulate their body temperature—the way chameleons go white when they want to cool down. But if that’s true, why would only male skyhoppers do it?
Umbers wondered if skyhopper colour-change might be linked to fighting. To test this theory, she and her colleagues captured the grasshoppers at Dead Horse Gap in southern New South Wales. They tied pieces of cotton around the males’ bodies, keeping them on short leashes so that they could walk around but not attack each other.
Then they played skyhopper Bachelorette: each female was given a choice of mates of different shades of turquoise and black. But the ladies didn’t seem to care how bright the gents were.
In the next experiment, Umbers put five male skyhoppers into an “arena” and unleashed them. They milled around peacefully until Umbers added a female. One male began mating with her, and the hostilities quickly escalated. Umbers found that males chose to fight those of a similar colour to themselves, suggesting the turquoise colour provides competitors with clues about their rival. A pale male might assume he’ll lose to a bright one, and not bother challenging him. Only when the adversaries are closely matched—if they’re both saying to themselves, “I reckon I can take him”—will they fight it out.
That’s the hypothesis for now. Plenty of other questions remain—like, if brightness is such an important cue, why isn’t it always switched on?
“We don’t know why you would ever want to be not turquoise,” says Umbers.
Ninety species of arthropods—stick insects, beetles, crabs, spiders and dragonflies—can reversibly change colour like the skyhoppers. And they’ve all evolved different ways of doing so.
The tortoise beetle Chariodella egregia, for instance, shunts water between different layers in its carapace, causing it to change from shiny gold to cherry red. “They play with light, if you like,” says Stuart-Fox.
Evolution suggests there’s a purpose to all this dressing up—but so far, for most arthropods, we have no idea what that purpose is.
Still, learning more about how exactly animals transform themselves could have human applications in fashion, robotics, paints, computer screens, and the design of new sensors and materials. Meanwhile, new cameras, spectrometers, and other devices are helping us to get closer to what each creature sees.
“We’re only at the tip of the iceberg,” says Stuart-Fox, “of trying to understand how animals use and control all of those colours.”