enough concept, one that is now widely recognized as true, but if we move beyond that statement, the light that it casts on behavior fades quickly: the world of parasites, hosts and behavior is shadowy, and boundaries are ill-defined. For instance, at first glance, the growing list of altered behaviors tells us very little about how those alterations happen, much less how they evolved. Some cases of parasite-induced behavioral change are truly manipulative, with the parasite standing to benefit from the changed behavior. In other cases, the altered behavior has an almost curative, if not prophylactic, effect; in those cases, the host benefits. This paper will provide an overview of the conflicting (and coinciding) demands on parasite and host, using examples from a wide range of taxa and posing questions for the future. In particular, what does the larger world of animal behavior tell us about how to go about seeking insights – or at least, what not to do? By asking questions about the sensory–perceptual world of hosts, we can identify those associations that hold the greatest promise for neuroethological studies of parasite-induced behavioral alterations, and those studies can, in turn, help guide our understanding of how parasite-induced alterations evolved, and how they are maintained.
Parasite manipulation of host personality and behavioural syndromes
The past decades have seen mounting evidence that parasites alter their host’s behaviour in ways that benefit transmission, based on differences in the expression of behavioural traits between infected and control individuals, or on significant correlations between trait expression and infection levels. The multidimensional nature of host manipulation has only recently been recognised: parasites do not target single host traits, but instead suites of interrelated traits. Here, I use recent research on animal personality (behavioural differences among individuals consistent across time and situations) and behavioural syndromes (correlations at the population level among distinct behavioural traits, or between the same trait expressed in different contexts) to provide a framework from which simple testable patterns of host behavioural changes can be predicted. Following infection, a manipulative parasite could (i) change the temporal consistency of its host’s behavioural responses, (ii) change the slope of a host reaction norm, i.e. the way host behavioural traits are expressed as a function of an environmental gradient, or (iii) decouple two or more host behavioural traits and/or change the way in which they correlate with each other. Two case studies involving trematode parasites and their freshwater hosts are used to provide empirical illustrations of the above scenarios. These clearly illustrate the full richness of behavioural alterations induced by parasites, and how these effects would go unnoticed using the classical trait-by-trait comparisons of mean values between parasitised and non-parasitised individuals. However, the power of animal personality and behavioural syndromes to inform research on host manipulation by parasites will only be fully realised when underlying mechanisms are elucidated and linked to their phenotypic impacts.
Multidimensionality in parasite-induced phenotypic alterations: ultimate versus proximate aspects
In most cases, parasites alter more than one dimension in their host phenotype. Although multidimensionality in parasite-induced phenotypic alterations (PIPAs) [FULL TEXT] seems to be the rule, it has started to be addressed only recently. Here, we critically review some of the problems associated with the definition, quantification and interpretation of multidimensionality in PIPAs. In particular, we confront ultimate and proximate accounts, and evaluate their own limitations. We end up by introducing several suggestions for the development of future research, including some practical guidelines for the quantitative analysis of multidimensionality in PIPAs.
Diversity and evolution of bodyguard manipulation
Among the different strategie