Host manipulation by Ligula intestinalis: accident or adaptation?

Numerous studies have demonstrated that parasites with complex life-cycles can cause phenotypic modifications in their hosts that lead to an increased rate of transmission, and suggest that these modifications are the result of parasitic adaptations to manipulate the host. Little attention is paid, however, to separating the possibility of adaptive host manipulation from i ncidental (if fortuitous) side-effects of infection. In this study we combi ne statistical and analytical tools to interpret the impact of the macropar asite Ligula intestinalis L. (Cestoda, Pseudophyllidea) on the behaviour of its intermediate fish host (the roach, Rutilus rutilus L.), using field da ta on a natural system. Two distinct sets of generalized linear models agre e that both the presence and the intensity of infection contribute to a mod ified behavioural response in the host. This was illustrated by a preferenc e for the lake-edge in infected fish during autumn. Furthermore, the effect of parasites upon their host is heterogeneous with respect to parasite siz e, with larger parasite individuals having a disproportionate impact. A ser ies of game-theoretic models of adaptive host manipulation illustrate a pot ential rationale for a size-dependent manipulation strategy in parasites. T hese findings illustrate the potential complexity and functionality of the impact of L. intestinalis upon its fish host, which together reduce the par simony of the alternative 'incidental effect' hypothesis.