Behavioural Processes 68 (2005) 237–240 Commentary Do all parasites manipulate their hosts? Sam P. Brown a,b a Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK b Section of Integrative Biology, University of Texas, 1 University Station, Austin, TX 78712, USA Received 31 August 2004; accepted 31 August 2004 Thomas et al. (2005) offer a very stimulating review of both the achievements of and prospects for research into parasitic manipulation. Perhaps in keeping with the maturity of this research field, the notion of parasitic manipulation received only a brief passing definition in their review, before current controversies took cen- tre stage. At the risk of pedantry, my aim in this com- mentary is to focus closely on definitions of the related concepts of host manipulation and virulence. I argue that an inspection of these concepts and their biolog- ical underpinnings leads to the conclusion that while parasites are not necessarily manipulative, a majority of parasites can be seen to be manipulators. Perhaps the only unambiguous cases of non-manipulation are restricted to socially-defective microparasites. To begin, consider the opening definition offered by Thomas et al. (2005). ... host manipulation, which occurs when a parasite en- hances its own transmission by altering host behaviour [p. 2, lines 4–5] Throughout their review, and indeed throughout the host-manipulation literature, we see numerous cases that fit exactly with this definition—typically the par- E-mail address: sam@biosci.utexas.edu. asite is a helminth worm, and the enhanced transmis- sion is between an intermediate and definitive host in a complex life cycle. However, the review also drops hints towards a larger scope for the host manipulation concept. At times, the authors talk of parasites altering the host phenotype—a more inclusive term than be- haviour. Elsewhere, they allude to parasites enhancing aspects of fitness other than transmission (for instance their persistence in the face of a host immune response). Putting these two generalisations together, we can ex- pand the above definition to ... host manipulation, which occurs when a parasite enhances its fitness by altering its host’s phenotype This broader definition is consistent with the full range of examples cited in Thomas et al. (2005), and mirrors more inclusive previous definitions (e.g. Brown, 1999). It is also remains consistent with the interpretation of host-manipulation as an example of Dawkins’ notion of the extended phenotype (Dawkins, 1982, 1990; Thomas et al., 2005). By broadening the definition of host-manipulation beyond its historical roots in macroparasites, do we run the risk of creating a definition that is merely tautological with parasitism? Consider two mutually- exclusive classes of parasite: manipulators and non- 0376-6357/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.beproc.2004.08.015