Male amphipods increase their mating effort before behavioural manipulation by trematodes

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Abstract
Certain parasites appear to alter the behaviour of intermediate hosts, making them more susceptible to predation by final hosts (adaptive parasite manipulation). In some cases, however, hosts are expected to respond by increasing their reproductive effort when first parasitized (adaptive host response). We tested predictions of these two hypotheses for males of the amphipod Corophium volutator (Pallas). Consistent with adaptive parasite manipulation, males infected by the trematode Gynaecotyla adunca (Linton) were likely to crawl on the surface of a mud flat, but only when visual predation by the final host, semipalmated sandpipers, Calidris pusilla L., was likely (during the day), and after trematodes had developed to their infective stage. Males appeared to compensate for parasitism by being more likely to mate, and perhaps by increasing ejaculate size. However, parasitized males that mated when first infected were less likely to mate again once their parasites reached their infective stage, despite their increased crawling at that time, which is associated with mate searching. We did not find that trematodes reduced host survival, apart from expected increases in predation rate on amphipods; in fact, highly infected amphipods actually lived longer than lightly infected ones. Taken together, our results suggest that adaptive host responses can occur before parasite manipulation is realized.

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