The males of many animals compete with each other for females. This can be through direct fighting, as in the case of crickets and fruitflies. However, males also compete for female attention through courtship displays. One common way males compete for females is through song. Song allows females to assess males through how good they think their song is, and choose the best male accordingly. We usually think of male birds singing to attract females, but lots of insects sing for this purpose too.
Bark beetles are perhaps best known for being pests of conifers in North America and coffee plantations around the world. However, despite their destructiveness these little guys have some pretty interesting behaviour. Female beetles construct a ‘gallery’, trenches she bores out into wood where she will reside and wait for a male to mate with. Later she also lays eggs in her gallery.
While building the gallery, the female bark beetle covers it in pheromones that act to attract males to it. She then waits for males to find her. When a male finds a gallery, he will approach, singing as he does through a series of chirps. However, only particular males are permitted access into her gallery; others are barred access at the entrance, and even pushed out completely by the female.
A new study by Lindeman and colleagues from Carleton University in Canada has shown that males differ in the songs that they sing when they approach females. Some males made more chirps, some made faster chirps and some mixed up the type of chirps they made more than others. What’s more, the type of song a male sung was correlated with his body size. As females prefer to mate with larger males, this meant that they could use the song of the visiting male to guess how big he might be.
This is indeed what the females did. One specific aspect of the male’s song: how many uninterrupted chirps he made, was very important to females in whether they decided to let the male into their gallery to mate or not.
The authors describe the males’ songs as acting as a kind of ‘password’ to gain access to the female’s gallery.
Southern pine beetle damage: hspauldi
A bark beetle Dendroctonus valens: Joseph Berger, Bugwood.org
Bark beetle galleries: Ilona L
Lindeman, A. A., & Yack, J. E. (2015). What is the password? Female bark beetles (Scolytinae) grant males access to their galleries based on courtship song. Behavioural processes. 115, 123-131.