By Dr. Karen Shaw Becker
Many birds love to feast on caterpillars, which, being slow-moving and often brightly colored, make an easy meal when they’re plucked from leaves. However, certain caterpillars have a trick up their proverbial sleeves to ward off would-be predators: they mimic birds’ distress calls in order to scare them away.
Caterpillars’ “acoustic signals,” including clicking, squeaking and crackling, have been documented for more than 100 years.1 In 2011, it was further revealed the walnut sphinx caterpillar, a moth larva, makes squeaking noises by compressing its body cavity, forcing air out of holes in its sides known as spiracles.
In a study published in the Journal of Experimental Biology, researchers documented that the sphinx caterpillars whistled upon being attacked by yellow warblers, which then caused the birds to fly away.2
“Birds responded to whistling by hesitating, jumping back or diving away from the sound source,” the researchers noted, concluding that the whistling diverted predators by startling them. However, a recent study presented at the July 2017 International Symposium on Acoustic Communication by Animals took it a step further to determine whether the whistles worked via another mechanism: mimicking the birds’ alarm calls.
Caterpillars Mimic Birds’ ‘Seet’ Calls
Many songbirds produce seet calls, which are distinctive warnings that alert others when predators are nearby. Researchers wondered whether the caterpillars’ whistles could actually be similar to the birds’ seet calls, so they set up an experiment to find out, playing recordings of the whistles and various birds’ seet calls to birds feeding at backyard bird feeders.
As expected, the birds fled when they heard the seet calls (while continuing feeding when a finch song was played). However, they also fled when they heard the caterpillar whistles. Study author Jessica Lindsay, a graduate student at the University of Washington, told The Scientist:3
“For both the genuine alarm call and the caterpillar whistle, birds responded by fleeing the bird feeder and taking a long time to return or freezing in place … If nuthatches were present, we would see them flicking their wings, which is a sign of distress … This is the first instance of deceptive alarm calling between an insect and a bird, and it’s a novel defense form for an insect … I think that’s pretty wild.”
Caterpillars Have a Variety of Defenses
Caterpillars have a variety of defenses, including chemical ones. Swallowtail butterfly larva, for instance, have osmeteria glands behind their head that release a foul smelling and tasting compound if it’s attacked. Other caterpillars, like those of the geometrid moth, use protective coloration to camouflage themselves into their surroundings, mimicking objects such as twigs.
Caterpillars are even capable of changing their color to more closely match the twigs in their environment. According to research published in Peer J, “[P]eppered moth (Biston betularia) larvae respond to color and luminance of the twigs they rest on, and exhibit a continuous reaction norm” that increases camouflage against predatory birds.4 Slug caterpillars, meanwhile, use a form of defense called mimesis, which is the mimicry of natural objects.5
In the slug caterpillar’s case, they look much like a furry mammal or hair ball. Other caterpillars may have body hairs in the silk of their cocoon, which act as a mechanical deterrent against predators. The saddleback caterpillar even has spiky brightly colored barbed hairs that release poisonous venom. The bright colors and spikes warn predators to stay away. Still other caterpillars may have scales and eye spots, and puff out their heads when threatened, making them appear much like a snake.6
Caterpillars living in tropical forests have also developed a complex, symbiotic relationship with ants, which protect them from wasps that would otherwise eat them. The caterpillars produce amino acids that ants feed on, and in return the ants will bite wasps that threaten the caterpillars. To call the ants to action, caterpillars make a rhythmic call as well as secrete a chemical to put them on high alert.7
What Other Creatures Use Acoustical Mimicry?
Although the featured study is the first to suggest caterpillars take advantage of acoustic mimicry, this defense tactic is thought to be used by a variety of other creatures, although less often than visual mimicry. For instance, burrowing owls are known to make a buzzing sound to imitate the warning rattle of a rattlesnake.8 Certain caterpillars of predatory and cuckoo butterflies may also use acoustical mimicry of queen ant calls to work their way into a higher position in ants’ social hierarchy.9
Certain moths also use acoustic mimicry to survive, mimicking the sounds of foul-tasting insects, which discourages bats from eating them. “Given these results and the widespread presence of tiger moth species and other sound-producing insects that respond with ultrasonic clicks to bat attack, acoustic mimicry complexes are likely common components of the acoustic landscape,” the researchers noted.10
It’s quite fascinating how many different defense tactics insects, birds and other animals use to ward off predators, with many of them still unknown to humans. In the case of caterpillars’ “crying wolf” with their made-up seet calls, these are actually loud enough to be heard by humans, and if you’re curious what they sound like, watch the video below.