Kate Umbers was hiking through Australia’s Snowy Mountains in the autumn of 2008, when she saw her first mountain katydid—a thumb-sized insect with the colour and texture of a dead leaf. “I recognised it from the guide books and picked it up excitedly,” she says. “It immediately vomited and flashed its bright colours.”
Emphasis on bright. The insect’s dull brown wing casings flew apart to reveal vivid bands of red, black, and electric blue. The inconspicuous leaf suddenly transformed into a garish Christmas bauble.
Many animals do something similar. When a threat gets close, they flash bright colours, show off distracting eyespots, strike aggressive poses, and spray off-putting chemicals. They hiss, rattle, puff, and arch. These spectacles are called deimatic displays and they are supposedly meant to distract or intimate predators. Bright colours, in particular, are often messages that scream: “I AM TOXIC; DO NOT EAT ME.” For some animals, these claims are bluffs. For the mountain katydid, they are genuine warnings—this insect is full of foul-tasting chemicals.
But Umbers noticed something unusual about its displays: the katydid only flashed its colours after an attack.
“I was struck by how easy it was to catch them and how little resistance they put up,” she says. “They waited until they had been grabbed before revealing any defences.”
Together, with Johanna Mappes, whose work I’ve written about before, Umbers carefully tested 40 captive katydids. She found that they almost never used their displays when she tapped a pen near their heads, blew gently on them, or waved a book overhead to simulate a passing bird. They only flashed upon contact, when she poked them or tried to pick them up.
That made no sense. Animals are meant to use deimatic displays to avoid attacks. It’s no use screaming, “I’m dangerous,” when beaks are already grabbing you or teeth are already sinking into your flanks. “Not only does this seem like a pretty bad strategy, it is counter to current thinking on how deimatic displays work,” says Umbers. “There is no theory to allow for such an adaptation, and yet there it is.”
“It’s a neat study, which suggests that we might have misread some kinds of animal signals, and misunderstood the different uses that startle defences can have,” says Mike Speed from the University of Liverpool.
Umbers suspects that the insect might prioritise stealth over shock. It blends into its surroundings, and if it revealed its colours, it would instantly break its own camouflage. That would be worthwhile if the colours worked as intended. But if predators don’t encounter these katydids very often, they might not know what the warnings mean and attack anyway. And while many animals use deimatic displays as distractions, to give themselves time to escape, the katydid is terrible at fleeing. It lacks the powerful jumping legs of its relatives, and the females can’t even fly. It does, however, have a very tough shell.
So, the katydid’s strategy seems to be: hide for as long as possible, rely on a tough shell to withstand a first strike; and hope that a mouthful of foul chemicals will deter a second one. “The species is combining the best of both worlds by walking softly and carrying a big stick,” says Tom Sherratt from Carleton University
But why the colours? Umbers suggests that they might reinforce the off-putting nature of the insect’s foul taste. Sherratt agrees, and notes that other animals couple chemicals and signals in this way. Some caterpillars, for example, vomit noxious substances and make clicking noises, when touched.
Umbers is now trying to work out what actually eats the katydids—the most likely candidates are ravens and magpies—and how they react to different aspects of the insects’ defence.
“Startle displays seem quite common in nature but are very much understudied,” says Martin Stevens from the University of Exeter. “What makes them effective—unexpectedness, novelty, anomaly, conspicuous colours, and so on—isn’t clear. The current study is therefore a nice start in understanding this.”
Reference: Umbers & Mappes. 2014. Postattack deimatic display in the mountain katydid, Acripeza reticulata. Animal Behaviour http://dx.doi.org/10.1016/j.anbehav.2014.11.009