Identification of individuals is a prerequisite in many behavioural studies. Visible Implant Elastomer (VIE) colour tags are a well-established way to mark animals. VIE tagging does not seem to affect individual growth or survival. However, studies verifying their neutrality during social interactions are less common. Here, individual male and female zebrafish Danio rerio were simultaneously given the choice between two shoals, each consisting of six fish. Members of one shoal were uniformly marked with one VIE tag of a given colour, whereas the other shoal was sham-tagged. In total, 10 different colours were used. Test fish spent significantly more time near the tagged shoal (56%) than near the sham-tagged shoal (44%). Tag colour did not significantly influence the preferences. The results highlight the importance of confirming the neutrality of colour tags before using them in behavioural studies. Especially, our study advises caution when using marked and unmarked fish simultaneously.
High predation risk during development induces phenotypic changes in animals. However, little is known about how these plastic responses affect signalling and competitiveness during contests. Herein, we have studied the consequences of anti-predator plasticity during the intra-sexual competition of Pelvicachromis taeniatus, a cichlid fish with mutual mate choice. We staged contests between adult size-matched siblings of the same sex derived from different environments: one fish was regularly exposed to conspecific alarm cues since the larval stage (simulating predator presence), the other fish to control conditions. Rearing environment did not affect the winner of contests or total aggression within a fight. However, contest behaviour differed between treatments. The effects were especially pronounced in alarm cue-exposed fish that lost a contest: they generally displayed lower aggression than winners but also lower aggression than losers of the control treatment. Thus, perceived predator presence modulates intra-sexual competition behaviour by increasing the costs associated with fighting.
Parasites with complex life cycles often alter the phenotypic appearance of their intermediate hosts in order to facilitate ingestion by the final host. However, such manipulation can be costly as it might increase ingestion by less suitable or dead-end hosts as well. Species-specific parasitic manipulation is a way to enhance the transmission to suitable final hosts. Here, we experimentally show that the altered body colouration of the intermediate host Gammarus pulex caused by its acanthocephalan parasite Pomphorhynchus laevis differently affects predation by different fish species (barbel, perch, ruffe, brown trout and two populations of three-spined stickleback) depending on their suitability to act as final host. Species that were responsive to colour manipulation in a predation experiment were more susceptible to infection with P. laevis than unresponsive species. Furthermore, three-spined stickleback from different populations responded to parasite manipulation in opposite directions. Such increased ingestion of the intermediate host by preferred and suitable hosts suggests fine-tuned adaptive parasitic manipulation and sheds light on the ongoing evolutionary arms race between hosts and manipulative parasites.