Birds sing, frogs call, crickets chirp—these are just a few examples of animals using sound to communicate. While we humans may get a kick out of hearing a melodious song in the morning or an eerie howl at night, for the animals, these sounds can mean the difference between finding a meal or becoming one. The serious consequences of singing, calling and chirping mean that strong evolutionary pressures often shape the traits associated with producing these sounds.
Woodpeckers are a diverse group of bird species known for their “drumming” behavior, in which they perch vertically on trees and slam their beaks into the trunks, boring holes in the bark. The birds forage this way, and they carve out cavities for nesting and caching food. But the audible drumming is not just a by-product of making holes—it has also evolved as a way for the birds to communicate. Different species have different drumming patterns that have diverged over time, varying in features such as rhythmic structure and timing. Recent research exploring the processes that shape woodpecker drumming has provided new insights into the complexity of this bizarre and specialized head banging.
What drives the evolution of species-specific, multifunction tapping signals? In a courtship context, males evolve drums to entice females to mate—the sexier, the better. In a territorial context, drumming is about staking claim to an area and scaring off potential intruders—the more threatening, the better. Drums encode species identity, so they evolve to ensure birds can recognize their own species—the more distinct, the better. A sound that travels far and cuts through background noise will reach more neighboring birds—the louder and clearer, the better.
But there is a downside to conspicuousness: drums that are loud and clear are also more likely to be intercepted by eavesdropping predators. And amid this confluence of selection pressures on drum structure, choosing the right tree for drumming matters. Trunk diameter, bark density and tree condition all affect the sound. Physical traits such as beaks and necks, along with the trees themselves, comprise the percussive instruments woodpeckers use to produce their communication signals, so these are also implicated in the evolutionary tug-of-war that shapes woodpecker drums.
As an evolutionary biologist and behavioral ecologist, I am passionate about uncovering the intricate evolutionary stories behind even the simplest-seeming behaviors we observe around us. There is almost always more than meets the eye (or ear). Although my own research focuses on singing behavior in songbirds, the woodpecker drumming system shares many similarities with birdsong, and some exciting findings have recently emerged.
A study led by Meredith C. Miles, now at Brown University, explored the interplay between physical constraints and sexual selection, two key drivers in the evolution of woodpecker drums across species. Drumming is an acoustic display that lacks pitch, so drum patterns vary in two main ways: length (number of taps) and tempo (speed of taps). Across woodpecker species, there is substantially more variation when it comes to length—but why? Apparently, tempo is more physically constrained. There are only certain tapping speeds that a bird is able to achieve, mechanically.
Miles and her co-authors found that tempo was correlated with body size but length was not, implicating body size as a potential limiting factor in the speed at which birds can head bang. They also found an interesting pattern for the less constrained drum trait: length. In species with stronger sexual selection—more competition between males for mates—drums were longer in duration, but there was no correlation with tempo. These relationships together suggest that when one component of a signal is constrained (tempo), sexual selection might drive another, more variable trait toward elaboration (length). This would mean that different signal components within a drum can evolve independently, further complicating the drum evolution story.
Why would elaboration make a drum more attractive to females? One likely explanation is that displays that are physiologically challenging to produce (that is, fast, long or loud) inherently reveal a drummer’s condition. Faster drums require quick muscular control, longer drums require stamina, and so on. Individuals capable of more elaborate drumming are likely to be healthy and strong, perhaps because of top-notch genes or because they are efficient at acquiring food—both useful traits in a mate.
Potential mates can quickly assess a drummer by listening to his performance. Then they can make an informed decision to pursue or avoid the flirtation. Such discernment may be why Miles and her colleagues found that strong sexual selection drives longer drums.