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Abstract

No living organism seems left untroubled by parasites. Parasitologists study with increasing enthusiasm when and how parasites alter host behaviour. Elucidating the neurological, endocrinological and molecular mechanisms mediating possible changes in the host’s behaviour is unlikely to explain how parasites induce forms of “bizarre”, “odd” or “drastically new and strange behaviours”. We argue that parasites make use of behaviour programs that already existed in their hosts rather than creating host behaviour de novo. From an evolutionary and/or ontogenetic point of view, parasites might ‘fake’ an everyday life signal, activate a silent sub-repertoire, or even free dormant (i.e. phylogenetically old) behaviours. We illustrate by means of a few, well-known phenomena how a thorough ethological approach will be essential in determining the origin of the shown host behaviour and to differentiate between superficially similar but separate behaviour syndromes. Parasites may even merit the label of scouts in behaviour research, unmasking behavioural capabilities not at hand in the host’s standard repertoire, and occasionally they might be used as tools to draw attention to relevant nervous control areas.