Is there anything a caterpillar can do to cause a bird, one of its biggest predators, to duck for cover? If your answer is no, think again and read on. (If your answer was yes, congratulations you are very, very smart, but read on anyway).
Caterpillars are a great example of defense mechanisms at work. These slow moving, relatively vulnerable creatures have a wide variety of defense strategies. Primary defenses are aimed at preventing a predator from ever becoming aware of the caterpillar’s presence. These include things like camouflage, limiting foraging times, staying on the underside of leaves or physically removing evidence of their presences by cutting leaves. When their primary defenses fail, caterpillars have an arsenal of secondary defenses. These can be chemical (tasting bad), physical (stinging spines and hairs) or behavioral (thrashing, mimicking other creatures, retracting portions of their bodies to appear larger).
Now, according to new research, we can add whistling to the list of secondary defenses, at least for the North American walnut sphinx caterpillar (Amorpha juglandis; 1). Although defensive sounds are hardly unusual in the insect world, we usually associate them with hard-bodied insects that have many mechanisms for producing sound. The study described by Bura et al. found that the sounds produced by the walnut sphinx caterpillar were done so using a method of sound production never before described in caterpillars; the forceful expulsion of air through the spiracles. In other words, the caterpillars whistled through their spiracles when threatened. The researchers used small dots of latex to obstruct all the spiracles and then systematically removed it until they could identify which sets of spiracles were involved in sound production. They found that as long and the 8th abdominal spiracles were clear, the caterpillars could whistle.
Next, the researchers placed the caterpillars in the cages of three yellow warblers (Dendroica petechia) who were accustom to being fed live food including caterpillars. The caterpillars didn’t make any sounds until they were attacked, at which time they whistled. The birds responded to the sounds by flinching, cocking their heads or diving away from the caterpillar. Each of the birds attacked the caterpillar more than once, and in each case the caterpillar responded with whistling.
All of the caterpillars survived their time in the warblers’ cages with no obvious injuries. The caterpillars remained in the cages an average of 8 minutes and 59 seconds after the final attack. The tests were ended when the birds showed no more interest in the caterpillars.
So the answer to the question: What can a caterpillar do to make a bird duck for cover? Whistle. And how do you whistle? Well if you are a walnut sphinx caterpillar, you open your 8th spiracles and blow.
- Bura, V., Rohwer, V., Martin, P., & Yack, J. (2010). Whistling in caterpillars (Amorpha juglandis, Bombycoidea): sound-producing mechanism and function Journal of Experimental Biology, 214 (1), 30-37 DOI: 10.1242/jeb.046805