Host resources govern the specificity of swiftlet lice: size matters

1. An important component of parasite diversity is the specificity for particular host taxa shown by many parasites. Specificity is often assumed to imply adaptive specialization by the parasite to its host, such that parasites are incapable of surviving and reproducing on 'foreign' hosts. 2. Specificity, however, need not be due to adaptation to particular hosts. Some parasites may be specific simply because they are incapable of dispersing among host taxa. For example, 'permanent' parasites like chewing lice spend their entire lifecycle on the body of the host and require direct contact between hosts for dispersal. 3. The role of adaptive constraints in parasite host-specificity has seldom been tested in natural populations. We conducted such a test by comparing the relative fitness of host-specific lice experimentally transferred among closely related species of cave swiftlets in northern Borneo. 4. The survival of lice in most of these transfers was significantly reduced in proportion to the mean difference in feather barb size between the donor and recipient species of hosts. Thus, adaptation to a particular resource on the body of the host does appear to govern the specificity of swiftlet lice. 5. In transfers where lice survived, microhabitat shifting on the body of the host was observed, whereby the mean barb diameter of the feathers on which the lice occurred was held 'constant'.