The Healthy Male Wins the Mate
Male-Male Competition Causes Parasite-Mediated Sexual Selection for Local Adaptation (2020) Gómez-Llano et al., The American Naturalist, https://doi.org/10.5061/dryad.cjsxksn35
The natural world changes constantly: temperatures fluctuate, predators and parasites enter into the ecosystem, and the landscape itself could change (looking at you, Yellowstone). These changes mean that organisms are under a constant pressure to adapt to local conditions. Due to this pressure, one of the biggest questions for conservation biology is if species are able to adapt fast enough to keep up with environmental changes. Sexual selection is thought to promote rapid adaptation to such environmental changes, but most of the evidence comes from laboratory studies.
Our study looked at adaptation to one of nature’s ubiquitous pressures: parasitism. We were interested in the strength of selection by parasites and if there was subsequent adaptation by the host in a wild population.
What We Did
We used a long-term monitored population to quantify how parasites affected mating success and female fecundity in a damselfly. This involved using data from 15 years of mating and parasitism data for the common bluetail damselfly (Ischnura elegans). Doing this allowed us to relate mating success to parasite load (e.g. if you’re a damselfly that tends to have more parasites, does that affect how often you mate?)
Then, we conducted competition experiments in which we paired females with either a single male (parasitized or non-parasitized) or two males (one with parasites, one without). This allowed us to identify if sexual selection was driven mainly by females choosing males or by the males competing for females.
Finally, we estimated how the parasite affected male and female fitness over the 15 year study. If the host is progressively less affected by the parasite over time (e.g. males with parasites mated more and females with parasites laid more eggs), it would be evidence for local adaptation.
Did You Know: Male vs. Female Fitness
Males and females from species that reproduce sexually both increases their fitness by passing their genes on to the next generation, but they do it in different ways. Females increases fitness by laying more eggs/having more offspring. Any egg that a female lays is hers, so they all increases her fitness.
Males on the other hand have to ensure that they are the ones to fertilize the eggs that are laid, so they increase their fitness by mating as much as possible. Male damselflies also scrape out the sperm from previous matings, just to make doubly-sure that they are the ones who father the offspring the female lays.
What They Found
Being parasitized reduced mating success of both sexes, but it had a stronger effect on males (being parasitized was more detrimental to mating for males than females). Interestingly, there was not an effect of parasites on fecundity.
The experiments show that competition between males was the main driver of sexual selection. These results suggest that males with parasites have less energy and cannot compete with healthy males for access to females. Therefore, there is sexual selection and it seems to be caused by competition between males.
Across the 15 years of data, the number of parasites has not changed, but the number of damselflies with parasites has decreased. Therefore, resistance to being parasitized has increased. And the effect of parasites in fecundity has also changed, showing that individuals with parasites are not as strongly affected as before, which is evidence of parasite tolerance. Together, our results show evidence that adaptation to parasites has evolved due to sexual selection.
The experimental design did not allow us to completely tease apart the influences of competition between males and female choosiness. Therefore, although the main mechanism is male competition, it was impossible to measure the relative importance of each of these behaviors individually.
This study is a great (if I say so myself) example of local adaptation, measuring the strength of selection in males and females in two fitness components. We were able to identify the mechanism behind such selection, in addition to being able to track the change in the effect of parasites through time. Awesome!
Miguel Gomez-Llano is a community ecologist at the University of Arkansas who loves climbing but hates biking up hills. You can read more about Miguel and his research here, and follow him on Twitter @mgomezllano.