Body mass and sex-biased parasitism in wood mice Apodemus sylvaticus

Abstract

Male sex‐biased parasitism (SBP) occurs across a range of mammalian taxa and two contrasting sets of hypotheses have been suggested for its establishment. The first invokes body size per se and suggests that larger individuals are either a larger target for parasites, trade off growth at the expense of immunity or cope better with parasitism than smaller individuals. The second suggests a sex‐specific handicap whereby males have reduced immunocompetence compared to females due to the immunodepressive effects of testosterone. The current study investigated whether sex‐biased parasitism is driven by host ‘body size’ or ‘sex’ using a rodent–tick (Apodemus sylvaticus–Ixodes ricinus) system. Moreover, the presence or absence of large mammals at study sites were used to control the presence of immature ticks infesting wood mice, allowing the impacts of parasitism on host body mass and female reproduction to be assessed. As expected, male mice had greater tick loads than females and analyses suggested this sex‐bias was driven by body mass as opposed to sex. It is therefore likely that larger individuals are a larger target for parasites, trade off growth at the expense of immunity or adapt behavioural responses to parasitism based on their body size. Parasite load had no effect on host body mass or female reproductive output suggesting individuals may alter behaviour or life history strategies to compensate for costs incurred through parasitism. Overall, this study lends support to the ‘body size’ hypothesis for the formation of sex‐biased parasitism.

The wild life of tick-borne pathogens

“…Our results regarding intrinsic factors are consistent with previous studies, which showed higher tick burdens on males compared to females 215 , an increase in tick burden with increasing body mass of the host 95,215 and higher tick burdens on Apodemus mice compared to Bank voles 89,95 . These differences have been attributed both to differences in day range between host individuals 215 and differences in resistance against or tolerance for parasites between host species 13,23 .…”

“…These differences have been attributed both to differences in day range between host individuals 215 and differences in resistance against or tolerance for parasites between host species 13,23 . Our results regarding extrinsic factors (deer and predator abundance, and rodent density) are also consistent with previous studies, which Determinants of Tick Burden on Rodents | 89 found an increase in larval burden on rodents with increasing deer density 32 and a decreasing larval burden with increasing rodent density 95,227 .…”

“…We measured the tick burden on three rodent species, Bank vole (Myodes glareolus), Field vole (Microtus agrestis), and Wood mouse (Apodemus sylvaticus) in 20 forest plots in the Netherlands that differed strongly in their vertebrate assemblage composition, and related these to both intrinsic and extrinsic factors. The intrinsic factors were species, sex and body mass, which are known determinants of tick burden on small mammals 32,89,95,215 . The extrinsic factors were the relative abundance of deer, the relative abundance of predators and rodent density.…”

“…The likelihood of a host individual encountering a parasite thus depends on intrinsic factors (individual characteristics of the host; e.g., species, size and sex) as well as extrinsic factors (abundance of other animals in the community). While the relationship between parasite burden and intrinsic factors has been studied in many systems [215][216][217] , the relationship with extrinsic factors is less well understood 218 , let alone their relative importance.…”

Ectoparasites and fitness of female Columbian ground squirrels

“…[22,[65][66][67][68]), there is increasing evidence from studies that show few or no effects (rodents: [55][56]69], bats: [24,70], birds: [71,72]). In female Columbian ground squirrels, contradictory results for adult females and their offspring were obtained for populations in the same geographical area, though during different times (this study versus [27]).…”