Attractiveness and sexually-selected traits

Tags

biology, evolution, insects, Mate choice, science, sexual selection

Suppose we want to test the hypothesis that females choose particular males so they will have more attractive offspring. Verifying that hypothesis would require mate choice trials showing that particular males get chosen more often, and then repeating those trials with the offspring.  Researchers often simplify the matter by choosing some proxy of attractiveness like a particular trait — the size of an ornament, for example — and look for correlations in that trait between sires and sons. If we don’t find that sons inherit their father’s trait then can we conclude that the trait does not signal male genetic quality? What if we could show that attractive fathers tend to have attractive sons regardless of their trait sizes? This way we’re letting female insects, rather than male or female primates, tell us who’s an attractive insect.

A recent study by Fiona Ingleby from University of Exeter used fruit flies (Drosophila simulans) to address whether a particular sexual signal was reliable as an indicator of heritable male attractiveness. Several studies have shown that cuticular hydrocarbons (CHCs) affect mate choice in fruit flies. CHCs are volatile chemicals given off by the “skin” of a fly that may act as pheromones.  Ingleby and her colleagues John Hunt and David Hosken were particularly interested to see how environmental variation would affect CHCs and mate choice. They also wanted to see if there was a genotype-by-environment interaction (abbreviated “GxE“): the genotype and the environment the flies grow up in could both affect their phenotpyes (CHC production). Would males be attractive in all environments, or would they be attractive in some environments, and unattractive in others?

Ingleby captured flies in Greece, then after a few generations of laboratory domestication raised their offspring in the lab on two different types of food (oats and soy) and at two different temperatures (23C versus 25C). Her paper stresses that these four environments were not that different from each other, and not extreme, and yet they found fairly dramatic variation in phenotypes depending on the environment. Cuticular hydrocarbon (CHC) signal varied across environments, but the researchers found very strong genetic effects on attractiveness across all the environments. Sons tended to resemble their fathers in attractiveness regardless of environment. However, Ingleby, Hunt and Hosken concluded that CHCs are not a reliable indicator of male attractiveness, since the CHC phenotype changed so much across the tested environments.

The researchers considered a few alternative hypotheses to explain this apparent discrepancy: multiple traits, direct benefits and the possibility that other traits account for variation in attractiveness. Perhaps females use not just CHCs but also many other males traits and behaviors when selecting a mate. Females might be able to tell which males’ semen will be less harmful, or more beneficial. This would benefit females directly, instead of just her offspring. Also, the researchers point out, some aspects of CHC profile were reliable indicators of male genotype, and so females are probably using just some CHCs along with other traits to assess males.

The aspect of this study I find most intriguing is showing the heritability of attractiveness, instead of focusing on an arbitrarily chosen trait. Whenever a researcher hypothesizes about a trait being under selection, he hast to make a huge set of assumptions that can only be verified after painstaking data collection that may take decades. Most Ph.D. dissertations are done in less than ten years. Even if a researcher could make a pretty good guess about what traits should correlate with fitness, she would have to have really good luck in finding or creating environmental conditions that would provide good control over that trait. By the time that laboratory-level control is attained, we may have lost touch with the reality of how species live in the wild. Then an experiment might provide a good case study, but it tells us little about the actual evolution of a species. I hope to see more empirical studies that use attractiveness rather than arbitrarily selected characters.

Related articles

• Sexual selection with age-dependent mutation (lxmx.wordpress.com)
• Are humans monogamous? (theratchet.ca)
• Males’ superior spatial ability likely is not an evolutionary adaptation (esciencenews.com)

The social environment as a source of variation

Tags

evolution, insects, open access, research, science, sexual selection

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.

Remember this article is available to everyone for free, so please go read it and learn more about the authors and their other interests.

Tags

biology, climate change, insects, science

My fellow UNC graduate students Jessica Higgins and Sarah Seiter are uncovering and reporting on important effects of climate change, particularly in insects. They were recently featured in the Raleigh News and Observer as well. Check them out and learn more about their research.

Butterflies and Science

Climate change is reviving   a once rare British butterfly, according to a new article in the journal Science.   The brown argus butterfly was once scarce, but has doubled the size of its range in the last 20 years.   The study, authored by biologists at the University of York, indicates that at warmer temperatures brown argus butterflies are able to feed on more common plant species, allowing them to expand into new territory.

Brown argus caterpillars normally feed on the rockrose plant, but they occasionally use plants in the geranium family during warm summers. The rockrose usually lives on sunny, south facing slopes and brown argus caterpillars depended on these warm microclimates for survival. However, for the last 20 years summer temperatures have been on the rise and the brown argus has been shifting it’s egg laying towards geraniums.

To test whether climate caused the shift in food choice, lead author Rachel Pateman…

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