主催: Primate Society of Japan
Parasites constrain host fitness and are theoretically capable of regulating host abundance and population dynamics. However, empirical demonstrations of regulation are still rare in wildlife populations, and nothing is yet known about whether parasites can regulate wild primate population dynamics. In this study, we used experimental parasite removal in a simple model system comprised of Japanese macaques (Macaca fuscata fuscata) and their gastrointestinal helminth parasites to test the prediction that parasitism constrains host body mass and reproductive success and are thus capable of regulating host populations. We collected data on female macaque (N=20) body mass, breeding success and gastrointestinal nematode infection between January 2012 and March 2015 on the island of Koshima in southern Japan. Parasites were experimentally removed from a subset of females (N=11) by repeatedly administering broad-spectrum anthelmintics (Stromectol® or Drontal® Plus) on seven occasions during this time. Fecal egg count reduction tests demonstrate high efficacy of drug treatment against one of the two most common parasites, Oesophagostomum aculeatum, but only moderate efficacy against the other, Trichuris sp. A general linear mixed-effects model reveals a significant interactive effect between treatment, dominance rank and season, which suggests that the slope of the relationship between dominance rank and body mass is different for treated and control females depending on which season the measurements were made. Specifically, high-ranking females seem to receive the most benefit from parasite removal, and this is particularly true in summer and autumn. Finally, multiple analyses show that reproductive success over this two-year period was significantly higher in treated females, an effect which remained after controlling for age and dominance status, whereas no a priori differences in birth rates were detected in the decade preceding this experiment. Together, these results suggest that nematode parasites are capable of regulating Japanese macaque populations via condition-dependent effects on female reproduction. We thus provide the first empirical support for host regulation by gastrointestinal parasites in primates.