Costly plastic morphological responses to predator specific odour cues in three-spined sticklebacks (Gasterosteus aculeatus)

Predation risk is one of the major forces affecting phenotypic variation among and within animal populations. While fixed anti-predator morphologies are favoured when predation level is consistently high, plastic morphological responses are advantageous when predation risk is changing temporarily, spatially, or qualitatively. Three-spined sticklebacks (Gasterosteus aculeatus) are well known for their substantial variability in morphology, including defensive traits. Part of this variation might be due to phenotypic plasticity. However, little is known about sticklebacks' plastic ability to react morphologically to changing risks of predation and about the proximate cues involved. Using a split-clutch design we show that odour of a predatory fish induces morphological changes in sticklebacks. Under predation risk, i.e., when exposed to odour of a predator, fish grew faster and developed a different morphology, compared to fish reared under low predation risk, i.e., exposed to odour of a non-predatory fish, or in a fish-free environment. However, fast growing comes at cost of increased body asymmetries suggesting developmental constraints. The results indicate that sticklebacks are able to distinguish between predatory and non-predatory fishes by olfactory cues alone. As fishes were fed on invertebrates, this reaction was not induced by chemical cues of digested conspecifics, but rather by predator cues themselves. Further, the results show that variation in body morphology in sticklebacks has not only a strong genetical component, but is also based on plastic responses to different environments, in our case different predation pressures, thus opening new questions for this model species in ecology and evolution.