December 2012

Where's the evolution?

Many animals use sound to communicate, especially when it comes to wooing the opposite sex. But what’s a male frog to do when his sexy croaks are drowned out by passing semis? Past research has shown that some frogs and other vertebrates are able to change their calls in ways that help compensate for traffic noise. Biologists from the University of Bielefeld in Germany wanted to find out if the same is true of insects. The results of their study were published last month.

The researchers focused on the grasshopper species Chorthippus biguttulus. To attract females, male grasshoppers rub their hind legs over a vein on their wings, producing a buzzing call that females use to locate and select their mates. In C. biguttulus, the male’s song is a two-part harmony composed of some low pitches and some high ones. These harmonics are important: songs with only one set of pitches are not as attractive to females as are songs with both. Unfortunately for roadside Romeos, the low pitches in their calls overlap with the pitches at which traffic noise is quite loud, raising the possibility that highway noise is scrambling a key line of communication in this species — and putting a real damper on the pick-up scene!

To find out if the grasshoppers have changed their tunes due to a noisy environment, the researchers collected C. biguttulus individuals living at eight sites along German autobahns, spots so noisy that the traffic drowned out their calls and the researchers had to locate the insects by sight alone. The biologists also collected C. biguttulus individuals living in quiet habitats a few kilometers away from each of these sites. Back in the lab, under quiet conditions, the songs of all the males were recorded and compared to see if they had different characteristics.

Most aspects of the songs of males from noisy and quiet environments were the same: the structures of both sets of songs were similar, and they were produced at about the same degree of loudness. But there was one key difference. Grasshoppers collected from roadside populations produced songs with low pitches that were slightly higher than the low pitches produced by grasshoppers from quiet fields. The biologists hypothesize that this shift enables roadside females to distinguish the low pitches in the males’ calls from ambient noise.

This research raises the key question of what caused this shift in song harmonics. There are a few possible explanations:

1. The shift in song harmonics represents evolutionary change brought about by sexual selection. Perhaps grasshopper populations normally include individuals with slight variations in their song pitches. If roadside females are better able to hear and find males with a slightly higher low register, these males may get to reproduce more. And if the song variation is a genetically-based trait, then we would expect the roadside population to evolve a higher low register over the course of many generations.
2. The shift in song harmonics came about through phenotypic plasticity. Phenotypically plastic traits are those that may be expressed differently based on an organisms’ current or past environment. Changes caused by phenotypic plasticity may occur in a single generation. Perhaps there is something about maturing in a noisy environment that causes grasshoppers to produce songs with higher low registers.
3. The shift in harmonics is caused by maternal effects. Perhaps female grasshoppers in noisy environments somehow change the resources they allocate to their eggs in a way that results in males that produce songs with higher low registers.
4. The shift in harmonics is caused by epigenetic effects. Epigenetic traits are those that are heritable, but are not passed down through the sequence of DNA. Perhaps there is something about being in a noisy environment that causes males or females to produce gametes with heritable chemical changes that lead to male grasshoppers singing with higher low registers.

Which of these possibilities is the most likely explanation? There are a few reasons to think that the shift in song harmonics was caused by sexual selection and represents true evolutionary change.

• First, it’s important to keep in mind how a song is actually produced. For humans, birds, and other animals with complex vocal organs, changing pitch is simply a matter of tightening a few muscles and can easily be accomplished behaviorally without any genetic changes. However, a grasshoppers’ song is produced mechanically by rubbing ridges on their legs over a wing vein. Thus, the pitch of a grasshopper’s song is affected by the physical properties of this system — the size, shape, and resonant properties of the leg ridges and wings. It seems more likely that changes in these physical properties would be caused by a genetic change than by phenotypic plasticity.
• Second, grasshopper song harmonics is a trait that we would expect to be highly susceptible to sexual selection. The population is likely to have genetic variation affecting this trait, and this trait is critical to reproduction. It’s easy to imagine that a male grasshopper with a song that is unattractive because a critical component is masked by traffic noise would be much less likely to reproduce (and hence have much lower fitness) than a grasshopper with an audible harmony.
• Finally, the researchers discovered another line of evidence consistent with the idea of evolutionary change. Grasshoppers collected near ‘older’ highways — those that had been built earlier — tended to have shifted their song harmonics more than grasshoppers collected near ‘younger’ highways. In other words, more time exposed to the hypothesized mechanism of selection (traffic noise) is associated with more change. This finding is more consistent with the idea of sexual selection (which acts incrementally over generations) than it is with any of the other explanations (which can lead to large changes in a generation or two).

Of course, more studies will be needed before we can confidently say which explanation is the best. When new research on this topic is published, we’ll update this story. To learn more, stay tuned. The grasshoppers certainly are!