Pleioxenous host-restriction in fleas

A review of the pleioxenous genera of fleas, those genera whose species are confined to hosts of a single family, indicates that such a restriction results from one of several situations. Genera of 10 or more taxa (species and subspecies) were considered, so as to exclude very small genera that might be relicts. Polyxenous genera, whose species occur regularly on two or more families (and frequently on two or more orders) of hosts are far more numerous than pleioxenous genera.

Bat fleas (Ischnopsyllidae) are unique in that all genera are pleioxenous. This seems to reflect the fact that most bat fleas occur on cave-dwelling bats, and that the cave provides for larval development of bat fleas. Five pleioxenous genera occur on rodents in deserts or semiarid regions. The extreme climatic changes that have been concomitant with desertification from the Mid-Cenozoic, and especially in the Pleistocene, are paralleled by the proliferation of the mammalian hosts themselves in deserts. Three pleioxenous genera are found on fossorial rodents (Geomyidae). These rodents appear to have been isolated and rejoined during climatic changes in western North America; and rapid speciation of their fleas followed speciation of the hosts. One pleioxenous genus, and others that are nearly pleioxenous, parasitize cricetine mice in Central America. It is suggested that the Cenozoic archipelago which occupied this region provided an environment for rapid speciation of both hosts and their fleas. Two pleioxenous genera are found on tree squirrels in the tropics. Although fleas of mice occasionally make permanent transfers to squirrels, as the former occupy squirrel nests, reverse transfers (from tree squirrels to mice) are rare. Two genera of fleas are restricted to murid mice in Africa south of the Sahara; this is a region in which murid mice usually outnumber species of other rodent families. One pleioxenous genus occurs on Arvicolidae, possibly a result of rapid speciation of hosts during the extreme climatic changes in boreal habitats during late Cenozoic and Pleistocene.

There is a tendency for the apparently more ancient families of fleas (e.g. Stephanocircidae, Pygiopsyllidae and Hystrichopsyllidae) to have very few pleioxenous genera but many polyxenous genera. The more modern families of fleas (e.g. Leptopsyllidae and Ceratophyllidae) contain a higher percentage of pleioxenous genera.