Abstract
Parasites can increase their host's predation susceptibility. It is a long-standing puzzle, whether this is caused by host manipulation, an evolved strategy of the parasite, or by side effects due to, for example, the parasite consuming energy from its host thereby changing the host's trade-off between avoiding predation and foraging toward foraging. Here, we use sequential infection of three-spined sticklebacks with the cestode Schistocephalus solidus so that parasites have a conflict of interest over the direction of host manipulation. With true manipulation, the not yet infective parasite should reduce rather than enhance risk taking because predation would be fatal for its fitness; if host behavior is changed by a side effect, the 2 parasites would add their increase of predation risk because both drain energy. Our results support the latter hypothesis. In an additional experiment, we tested both infected and uninfected fish either starved or satiated. True host manipulation should act independently of the fish's hunger status and continue when energy drain is balanced through satiation. Starvation and satiation affect the risk averseness of infected sticklebacks similarly to that of uninfected starved and satiated ones. Increased energy drain rather than active host manipulation dominates behavioral changes of S. solidus-infected sticklebacks.
Highlights
Parasites have the potential to change the behavior of their hosts
We test experimentally whether host manipulation by a parasite is due to active manipulation that has evolved for this purpose or caused by a side effect of the parasite draining energy from the host, forcing it to change its trade-off between feeding and avoiding predation toward feeding (Milinski 1990; see Lefèvre et al 2008)
In the present study, the losing parasite enhances the winner’s manipulation when three-spined sticklebacks were experimentally infected by 2 S. solidus at different times. Fish infected by both an already infective and a not yet infective S. solidus show a stronger reduction in risk averseness than fish infected by either parasite alone
Summary
Parasites have the potential to change the behavior of their hosts. They can actively manipulate host behavior thereby improving their own fitness. A parasite, by definition, consumes energy from its host This can result in significant energetic costs to the host (e.g., Barber and Wright 2008; Lettini and Sukhdeo 2010; Vanacker et al 2012). An infected host could become more prone to predation without any host manipulation that would have evolved to enhance transmission, which could be exploited by parasites (Milinski 1990; Lefèvre et al 2008). If such a side effect or compensatory response achieves an optimal behavioral change from the parasite point of view, there would be no selection for an additional manipulation mechanism
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