Some of the most interesting studies on life-histories and mate choice have been done with laboratory populations of insects. Crickets are especially well-suited to these studies because their most salient characteristic, their call, has qualities that are affected by different evolutionary forces. Calls have a frequency (pitch), but males also exert effort calling. Several researchers have picked up on the different meaning of these two variables for sexual selection in crickets. The “ears” of female crickets are probably tuned to a particular frequency, just as (American) human ears seem tuned to 440Hz. This should lead to stabilizing selection: males whose calls are too high-pitched will not attract mates or attract the wrong ones (e.g. from another species); the same should apply to males whose calls are too low-pitched. However, frequency should be mostly independent of the amount of time a male spends calling: this should depend on his health (how much time can he spend?), the predators and parasites that might tune in to his call, and the density of other males, i.e. competitors. The overall effect should be directional selection, i.e. favoring males who call more when they can (condition-dependence).
Michael Kasumovic and his colleagues Matt Hall and Robert Brooks recently raised male Australian Black Field Crickets to determine if their juvenile environment affected their calling effort and their pitch. The researchers ingeniously hypothesized that perceived density of competitors would affect male song in different ways: perceiving more competitors should not affect pitch, whereas competitor density should definitely affect calling effort. If males perceive that they have more competitors, they will have to call more to find mates. This produces directional selection on calling effort. However, females will still tune in to the same frequency, regardless of how many males are around. The effect of juvenile social environment therefore coincided with whether directional selection or stabilizing selection was the dominant force on a particular trait. They fooled the crickets into believing there were more competitors by broadcasting calls to developing males in the lab. Since they also knew that female choice was affected by the females’ perception of male density, they also raised females under similar conditions.
I find this study really interesting because it shows how many possible sources of phenotypic variation there are. When we consider the lek paradox, a big problem is that the conditions under which we expect to lose genetic variation are very narrow: we have to suppose that the genes act in a certain way, that there is no mutation, and so on. We might as well deal with friction-less pulleys and billiard balls. My research focuses on phenotypic variation due to age, and Kasumovic and his colleagues have focused on the social environment. This study reminds us that the complexity of life means that there is a huge number of reasons we should expect lots of phenotypic variation. That phenotypic variation should lead to plenty of ways that we can maintain genetic variation in populations. However, science proceeds in baby steps of understanding: each potential idea has to be tried out and tested to death. We can think of these sources of variation a lot faster than we can do experiments, or even produce worthwhile theory (that is, find out if the ideas really make sense). The lek paradox will therefore be with us for a while.
Another really interesting thing about this article is that it’s published in the new Open Access journal Ecology and Evolution. That means everybody interested can go and read it: while you’re there check out all the other primary research articles you have access to free of charge. This is primary research, i.e. the research papers written by the people who performed the experiments.
Here’s the abstract:
The juvenile environment provides numerous cues of the intensity of competition and the availability of mates in the near environment. As research demonstrates that the developing individuals can use these cues to alter their developmental trajectories, and therefore, adult phenotypes, we examined whether social cues available during development can affect the expression and the preference of sexually selected traits. To examine this, we used the Australian black field cricket (Teleogryllus commodus), a species where condition at maturity is known to affect both male calling effort and female choice. We mimicked different social environments by rearing juveniles in two different densities crossed with three different calling environments. We demonstrate that the social environment affected female response speed but not preference, and male age-specific calling effort (especially the rate of senescence in calling effort) but not the structural/temporal parameters of calls. These results demonstrate that the social environment can introduce variation in sexually selected traits by modifying the behavioral components of male production and female choice, suggesting that the social environment may be an overlooked source of phenotypic variation. We discuss the plasticity of trait expression and preference in reference to estimations of male quality and the concept of condition dependence.