Harrison's rule is an observation in

evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes such as natural selection, common descent, and speciation that produced the diversity of life on Earth. In the 1930s, the discipline of evolutionary biolo ...

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

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

body sizes tend to covary positively.

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

Launcelot Harrison, an Australian authority in

zoology Zoology ( , ) is the scientific study of animals. Its studies include the anatomy, structure, embryology, Biological classification, classification, Ethology, habits, and distribution of all animals, both living and extinction, extinct, and ...

and

parasitology Parasitology is the study of parasites, their host (biology), hosts, and the relationship between them. As a List of biology disciplines, biological discipline, the scope of parasitology is not determined by the organism or environment in questio ...

, 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 infraorder Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera was previously recognized as an order (biology), order, until a 2021 genetic stud ...

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

nematodes The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (he ...

, rhizocephalan barnacles, fleas,

lice Louse (: lice) is the common name for any member of the infraorder Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera was previously recognized as an order, until a 2021 genetic study determined th ...

, ticks, parasitic

flies Flies are insects of the Order (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwin ...

and

mites Mites are small arachnids (eight-legged arthropods) of two large orders, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari. However, most recent genetic analyses do not recover the two as eac ...

, 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 relative change in the other quantity proportional to the ...

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, Menoponidae, Philopteridae) 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 Host–parasite coevolution is a special case of coevolution, where a Host (biology), host and a parasite continually adapt to each other. This can create an evolutionary arms race between them. A more benign possibility is of an evolutionary tra ...

. 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 cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...

of parasitic infections as well.

References

{{Biological rules Animal size Parasitology Evolutionary biology