Abstract
Food and parasitism can have complex effects on small mammal reproduction. In this study, we tested the effects of sex, food, and parasitism on reproductive performance of the Taiwan field mouse (Apodemus semotus). In a field experiment, we increased food availability for a portion of the mice in the population by providing sorghum seeds to a set of food stations. We reduced parasite intensity of randomly chosen mice through ivermectin treatment. We determined the number and quality of offspring for the mice using paternity analysis. We quantified seed consumption with stable carbon isotope values of mouse plasma and parasite intensity with fecal egg counts of intestinal nematodes and cestodes (FEC). In a laboratory experiment, we reduced parasite intensity of randomly chosen mice through ivermectin treatment. We quantified their immune functions by total white blood cell count, percent granulocyte count, and percent lymphocyte count through hematological analyses. We measured the FEC and energy intake of the mice. From the field experiment, the number of offspring in A. semotus increased with increasing seed consumption. Due to the trade‐off between number and quality of offspring, the offspring quality decreased with increasing seed consumption for the females. The ivermectin treatment did not affect offspring number or quality. However, the FEC was positively correlated with number of offspring. In the laboratory experiment, the percent lymphocyte/granulocyte count changed with parasite intensity at low energy intake, which was relaxed at high energy intake. This study demonstrated positive effects of food availability and neutral effects of parasitism on A. semotus reproduction. However, the benefits of food availability for the females need to take into account the offspring number–quality trade‐off, and at high infection intensity, parasitism might negatively affect offspring quality for the males. We suggest that food availability could mediate the relationships between parasite intensity and immune responses.
Highlights
Reproduction is a key component of life history and population dynamics
Food availability had positive effects on the reproductive performance of A. semotus, which is common for small mammals
We found that a higher degree of seed consumption increased the number of offspring in A. semotus
Summary
Reproduction is a key component of life history and population dynamics. In small mammals, reproductive performance is typically constrained by food availability (reviewed in Bronson, 1985; Bronson & Perrigo, 1987; Boutin, 1990; Speakman, 2008). Female voles (Microtus californicus, Ostfeld, 1986; Clethrionomys rufocanus, Ims, 1987) became spatially more aggregated than males under food supplementation, and male A. semotus became spatially more aggregated than females after they were reduced of nematode infection (Shaner et al, 2017) This is why sex-specific effects of food and parasitism on host reproduction are well documented in mammals (Díaz & Alonso, 2003; Gooderham & Schulte-Hostedde, 2011; Lo & Shaner, 2015). The goal of this study is to test the effects of sex, food availability, and parasite intensity on individual reproductive performance (i.e., number and quality of offspring) in a wild population of the Taiwan field mice (A. semotus). The mice were given parasitism reduction treatment (see “Anthelmintic treatment”) and had their tissue samples taken (see “Food addition” and “Parentage analysis”) before they were released at the trapping station where they were captured in the evening on the same day
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