The Sicker Sex: Understanding Male Biases in Parasitic Infection, Resource Allocation and Fitness

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.

Are sexes equally parasitized in damselflies and dragonflies?

“…Age group 2 (mature) used for this study. See Córdoba‐Aguilar and Munguía‐Steyer () for further details.…”

“…On the other hand, the vast majority of host species do not have a sex bias and the ones that do, represent both sexes at an almost equal number. An exception, and a study worth noticing, was done by Córdoba‐Aguilar and Munguía‐Steyer (). They studied damselflies with gregarine and water mite parasites both in the field and in a laboratory experiment and found that males harbor more parasites than females.…”

“…digesting body fluids, causing wounds and reducing overall agility (Åbro ). Gregarines (Apicomplexa: Eugregarinorida) are endoparasites, which infect the host, either as an oocyst attached to a vector (Åbro ) or through ingestion of infected water (Córdoba‐Aguilar and Munguía‐Steyer ). Gregarines reduce host's fitness by e.g.…”

“…There are studies on invertebrates, conducted mainly with a few species of either damselflies or dragonflies, which have found that males have more parasites (McKee et al , Lajeunesse et al , Córdoba‐ Aguilar and Munguía‐Steyer 2013), in contrast there are also studies implicating a female bias (Bunker et al ) and further studies which have found no difference between sexes in their parasitism levels (Nagel et al , Forbes et al , Mlynarek et al ). Higher parasitism rates in males have been explained with differential habitat use between males and females (Lajeunesse et al ), longer persistence of infestation in males (McKee et al ) and increased resource allocation to fat production in males (Córdoba‐Aguilar and Munguía‐Steyer ). Females have experienced higher rates of parasitism due to behavioral differences between the two sexes (Bunker et al ).…”

The demographic pattern of infection with chronic wasting disease in reindeer at an early epidemic stage

“…A common pattern among mammals is higher infection levels in males compared to females (Schalk and Forbes 1997, C ordoba-Aguilar and Mungu ıa-Steyer 2013, Metcalf and Graham 2018 and in the young and senescent compared to prime-aged individuals (Hayward et al 2011, Abolins et al 2018, Benton et al 2018. Physiological and behavioral differences are expected to be major drivers of the skewed appearance of many diseases (Guerra-Silveira and Abad-Franch 2013).…”