Parasite load

Parasite load is a measure of the number and virulence of the parasites that a host organism harbours. Quantitative parasitology deals with measures to quantify parasite loads in samples of hosts and to make statistical comparisons of parasitism across host samples.

In evolutionary biology, parasite load has important implications for sexual selection and the evolution of sex, as well as openness to experience.

A single parasite species usually has an aggregated distribution across host individuals, which means that most hosts harbor few parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology: use of parametric statistics should be avoided. Log-transformation of data before the application of parametric test, or the use of non-parametric statistics is often recommended. However, this can give rise to further problems. Therefore, modern day quantitative parasitology is based on more advanced biostatistical methods.

In vertebrates, males frequently carry higher parasite loads than females. Differences in movement patterns, habitat choice, diet, body size, and ornamentation are all thought to contribute to this sex bias observed in parasite loads. Often males have larger habitat ranges and thus are likely to encounter more parasite-dense areas than female conspecifics. Whenever sexual dimorphism is exhibited in species, the larger sex is thought to tolerate higher parasite loads.

In insects, susceptibility to parasite load has been linked to genetic variation in the insect colony. In colonies of Hymenoptera (ants, bees and wasps), colonies with high genetic variation that were exposed to parasites experienced lesser parasite loads than colonies that are more genetically similar.

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