Abstract
We investigated a causal chain of relationships between habitat specialization and parasite species richness in rodent communities in Southeast Asia, and the consequences for variation in immune investment (using spleen size), the degree of sexual competition (using testes) and sexual size dimorphism (SSD). We used data gathered on rodents, their habitat specialization and their parasites (macro- and micro-parasites) in Southeast Asian landscapes. The results supported the hypotheses that parasite diversity drives the evolution of host life-traits and sexual selection. Firstly host habitat specialization explained the variation in parasite species richness. Secondly high parasite species richness was linked to host immune investment, using the relative spleen size of rodents. Thirdly according to the potential costs associated with immune investment, the relative spleen size was found to be negatively correlated with the relative size of testes among rodents. Fourthly, a positive relationship between male-biased SSD and parasite species richness was observed supporting the role of parasitism in sexual selection. Finally, the variation in SSD was positively associated with the degree of habitat specialization. Highest values of female-biased SSD were associated with habitat specialization, whereas highest values of male-biased SSD concerned synanthropic or generalist rodent species. These results, also correlative, will help to facilitate selection of the species that should be thoroughly investigated at the population level to better understand the selective effects of parasites on rodent life-history and behavior.
Highlights
The search for factors that explain the diversity of parasites has been the goal of numerous studies, whilst the effect of parasite diversity on the evolution of host life-history traits has attracted less, though growing, attention (Kamiya et al, 2014; Morand et al, 2015c)
Approval notices for trapping and investigation of rodents were given by the Ministry of Health Council of Medical Sciences, National Ethics Committee for Health Research (NHCHR) Lao PDR, number 51/NECHR, and by the Ethical Committee of Mahidol University, Bangkok, Thailand, number 0517.1116/661
Impacts of Parasites and Parasite Risk on Spleen Size and Investment in Immunity The results confirmed the hypothesis that high parasite species richness affects the strength of host immune investment (Morand, 2015), potentially due to the high energetic costs associated with most types of defenses (Brock et al, 2014). We found that both parasite species richness and the risk of being infected, estimated by the central position of a rodent host in habitat sharing, were statistically correlated with the relative variation of spleen size
Summary
The search for factors that explain the diversity of parasites has been the goal of numerous studies, whilst the effect of parasite diversity on the evolution of host life-history traits has attracted less, though growing, attention (Kamiya et al, 2014; Morand et al, 2015c). Comparative analyses investigating parasite species richness in mammals, and in rodents, have consistently shown that host density (or host population size) and host geographic range are the main determinants explaining parasite richness among host species (Kamiya et al, 2014; Morand, 2015). Among other potential determinants, rodent behavioral traits (with the exception of host sociality or group size) have received less attention, compared to comparative studies on other mammals such as bats or ungulates, (for interspecific studies see Bordes et al, 2007; for intraspecific studies see Hillegass et al, 2008; Viljoen et al, 2011)
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