Harrison's rule is an observation in

evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life fo ...

by Launcelot Harrison which states that in comparisons across closely related species, host and

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

body sizes tend to covary positively.

Parasite species' body size increases with host species' body size

Launcelot Harrison, an Australian authority in

zoology Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the Animal, animal kingdom, including the anatomy, structure, embryology, evolution, Biological clas ...

and

parasitology Parasitology is the study of parasites, their hosts, and the relationship between them. As a biological discipline, the scope of parasitology is not determined by the organism or environment in question but by their way of life. This means it fo ...

, published a study in 1915 concluding that host and parasite body sizes tend to covary positively, a covariation later dubbed as 'Harrison's rule'. Harrison himself originally proposed it to interpret the variability of congeneric

louse Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera has variously been recognized as an order, infraorder, or a parvorder, as a result o ...

species. However, subsequent authors verified it for a wide variety of parasitic organisms including nematodes, rhizocephalan barnacles,

fleas Flea, the common name for the order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds. Fleas live by ingesting the blood of their hosts. Adult fleas grow to about long, ar ...

lice Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera has variously been recognized as an order, infraorder, or a parvorder, as a result o ...

, ticks, parasitic flies and

mites Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear evid ...

, as well as herbivorous insects associated with specific host plants.

The variability of parasite species' body size increases with host species' body size

Robert Poulin observed that in comparisons across species, the variability of parasite body size also increases with host body size. It is self-evident that we expect greater variation coming together with greater mean body sizes due to an allometric

power law In statistics, a power law is a Function (mathematics), functional relationship between two quantities, where a Relative change and difference, relative change in one quantity results in a proportional relative change in the other quantity, inde ...

scaling effect. However, Poulin referred to parasites' increasing body size variability due to biological reasons, thus we expect an increase greater than that caused by a scaling effect. Recently, Harnos et al. applied phylogenetically controlled statistical methods to test Harrison's rule and Poulin's s Increasing Variance Hypothesis in avian lice. Their results indicate that the three major families of avian lice (

Ricinidae The Ricinidae are a family of a larger group Amblycera of the chewing lice. Most commonly they are ectoparasites of birds. The family includes the genus ''Ricinus ''Ricinus communis'', the castor bean or castor oil plant, is a species of p ...

Menoponidae Menoponidae is a monophyletic family of lice in the superfamily of chewing lice, Amblycera, often referred to as the chicken body louse family. They are ectoparasites of a wide range of birds including chickens, which makes them important to und ...

Philopteridae The Philopteridae are a family of Ischnocera, chewing lice mostly parasitic on birds. The taxonomy and systematics of the group are in need of revision; the Philopteridae are almost certainly paraphyletic. Genera Some notable species are also l ...

) follow Harrison's rule, and two of them (Menoponidae, Philopteridae) also follow Poulin's supplement to it.

Implications

The allometry between host and parasite body sizes constitutes an evident aspect of host–parasite coevolution. The slope of this relationship is a taxon-specific character. Parasites' body size is known to covary positively with fecundity and thus it likely affects the

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...

of parasitic infections as well.

References

{{Biological rules Animal size Parasites Lice Evolutionary biology