Parasite body size distributions: interpreting patterns of skewness.

We examined the distributions of log body sizes of parasite species to test the prediction that the different physical constraints acting on endoparasites and ectoparasites can lead to different size distributions, and to see whether the distributions of parasite body sizes differ from those of free-living animals. We used body size distributions from 19 groups of parasitic of free-living animals, including over 7000 species and almost 2400 genera. There were differences between the body size distributions of male and female parasites, and between the body size distributions of related parasites exploiting different types of hosts. In general, parasite body size distributions were less frequently log right skewed than those of free-living animals. Ectoparasites tended to have more symmetric body size distributions than endoparasites, in which the most speciose size classes tended to be the smaller ones. Severe space constraints and a greater diversity of small niches may only allow the diversification of small endoparasites, whereas ectoparasite body sizes may face less severe limitations.