Abstract
The “sicker sex” idea summarizes our knowledge of sex biases in parasite burden and immune ability whereby males fare worse than females. The theoretical basis of this is that because males invest more on mating effort than females, the former pay the costs by having a weaker immune system and thus being more susceptible to parasites. Females, conversely, have a greater parental investment. Here we tested the following: a) whether both sexes differ in their ability to defend against parasites using a natural host-parasite system; b) the differences in resource allocation conflict between mating effort and parental investment traits between sexes; and, c) effect of parasitism on survival for both sexes. We used a number of insect damselfly species as study subjects. For (a), we quantified gregarine and mite parasites, and experimentally manipulated gregarine levels in both sexes during adult ontogeny. For (b), first, we manipulated food during adult ontogeny and recorded thoracic fat gain (a proxy of mating effort) and abdominal weight (a proxy of parental investment) in both sexes. Secondly for (b), we manipulated food and gregarine levels in both sexes when adults were about to become sexually mature, and recorded gregarine number. For (c), we infected male and female adults of different ages and measured their survival. Males consistently showed more parasites than females apparently due to an increased resource allocation to fat production in males. Conversely, females invested more on abdominal weight. These differences were independent of how much food/infecting parasites were provided. The cost of this was that males had more parasites and reduced survival than females. Our results provide a resource allocation mechanism for understanding sexual differences in parasite defense as well as survival consequences for each sex.
Highlights
The host-parasite relationship is one of the most widespread and important in nature [1]
The rationality for why sex difference is as follows: Given that investment in parasite defense is energetically costly (e.g. [9],[10]), mating effort should be traded off against parasite defense but given that in most mating systems males invest more in mating effort it would stand to reason that there would be a male sex-bias in parasitism ([5],[6],[7],[11])
The sex bias difference in the observational data set was corroborated by our experimental study when animals were infected with gregarines (Table 3): experimental infection increased the number of parasites, but males ended up being more parasitized than females and the difference clearly increased as age increased (Fig. 4)
Summary
The host-parasite relationship is one of the most widespread and important in nature [1]. A great deal of research in recent decades has been devoted to understanding how the host’s life history traits have been shaped by parasitic defense mechanism investment (reviewed, for example by, [2],[3]). One particular research focus has tried to explain the sex-specific difference in parasite burden [4], which relies on the fact that males are less immunocompetent than females The rationality for why sex difference is as follows: Given that investment in parasite defense is energetically costly An example of mating effort is competition for resources that males use to attract females ([8],[12]). Theories of sexual selection and resource allocation explain why males are the ‘‘sicker’’ sex [4]
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