American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Male-male competition causes parasite-mediated sexual selection for local adaptation”

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Miguel Gómez-Llano, Aaditya Narasimhan, and Erik Svensson (Sep 2020)

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A marked <i>I. elegans</i> male used in this study. Males were marked with colour powder which is transferred to females during mating. This method allowed the authors to identify which males have mated and which have not.<br/>(Credit: Aaditya Narasimhan)
A marked I. elegans male used in this study. Males were marked with colour powder which is transferred to females during mating. This method allowed the authors to identify which males have mated and which have not.
(Credit: Aaditya Narasimhan)

Parasites can affect species in a myriad of ways, one of these is by reducing their reproductive success. In this study, the authors found that male mating success was reduced when they were infected with parasites, but interestingly female fecundity was not. Further experiments showed that males with parasites were less able to compete with healthy males for access to females. Because mating success of infected males was reduced, local adaptation to become more resistant to parasites evolved, as shown by a long-term monitoring program.

While working in a system of ponds in Southern Sweden, the authors of this study noticed that in a population of the common blue tail damselfly (Ischnura elegans), particularly heavily infested with parasite mites, males with parasites were usually found as singles while males with no parasites were, seemingly, more likely to be found mating. With this idea in mind, the authors design a series of experiments to test if this idea. They not only show that this was true and parasitized males were less likely to reproduce, they also show that the main mechanism behind this was competition between males. Because parasites reduce male condition, infected males are less likely to succeed when competing with other males for access to females.

This area of Sweden has been thoroughly studied and a database of the population exists from the last 15 years. This data allowed the authors to test if parasite resistance (individuals are less likely to be infected) and tolerance (individuals get less affected by parasites) has evolve as a response to the reduced reproductive success of parasitized males. Their results show evidence for adaptation due to a reduced reproductive success of parasitized males in a wild insect population.


Sexual selection has been suggested to accelerate local adaptation and promote evolutionary rescue through several ecological and genetic mechanisms. Condition-dependent sexual selection has mainly been studied in laboratory settings while data from natural populations are lacking. One ecological factor that can cause condition-dependent sexual selection is parasitism. Here, we quantified ectoparasite load (Arrenurus water mites) in a natural population of the common bluetail damselfly (Ischnura elegans) over 15 years. We quantified the strength of sexual selection against parasite load in both sexes and experimentally investigated the mechanisms behind such selection. Then, we investigated how parasite resistance and tolerance changed over time to understand how they might influence population density. Parasites reduced mating success in both sexes, and sexual selection was stronger in males than in females. Experiments show that male-male competition is a strong force causing precopulatory sexual selection against parasite load. Although parasite resistance and male parasite tolerance increased over time, suggestive of increasing local adaptation against parasites, no signal of evolutionary rescue could be found. We suggest that condition-dependent sexual selection facilitates local adaptation against parasites and discuss its effects in evolutionary rescue.

La competencia entre machos causa selección sexual mediada por parásitos, promoviendo la adaptación

Se ha sugerido que la selección sexual puede acelerar la adaptación y facilitar el rescate evolutivo a través de diversos mecanismos ecológicos y genéticos. La selección sexual sobre la condición ha sido estudiada principalmente en el laboratorio, mientras existe una escasez de datos de poblaciones silvestres. Un factor ecológico que puede causar selección sexual sobre la condición es el parasitismo. En este estudio cuantificamos la carga de ectoparásitos (ácaros acuáticos Arrenurus) en una población silvestre de una especie de odonatos (Ischnura elegans) por 15 años. Cuantificamos la fuerza de la selección sexual en contra de individuos parasitados en ambos sexos e investigamos los mecanismos detrás de dicha selección. Posteriormente investigamos como la resistencia y tolerancia al parásito ha cambiado a lo largo del tiempo y como puede influenciar la densidad poblacional. Los parásitos redujeron el éxito reproductivo de ambos sexos, y la selección sexual fue más fuerte en machos que en hembras. Los experimentos sugieren que la selección sexual fue mediada principalmente por competencia entre machos, con poco o ningún efecto de elección de pareja. Aunque la resistencia y tolerancia al parásito incrementaron con el tiempo, sugiriendo adaptación al parásito, no se encontraron señales de rescate evolutivo. Sugerimos que la selección sexual sobre la condición facilita la adaptación al parásito y discutimos sus efectos en el rescate evolutivo.