Jerker Vinterstare, Kaj Hulthen, P. Anders Nilsson, Helen Nilsson Sköld, and Christer Brönmark
Most animals constitute potential prey and must respond appropriately to predator-mediated stress in order to survive. Numerous prey also adaptively tailor their response to the prevailing level of risk and stress imposed by their natural enemies, i.e. they adopt an inducible defence strategy. Predator exposure may activate the stress axis, and drive the expression of anti-predator traits that facilitate survival in a high-risk environment (the predation-stress-hypothesis). Here, we quantified two key morphological anti-predator traits, body morphology and colouration, in crucian carp following exposure to presence or absence of a predator (pike) as well as to experimental manipulation of physiological stress via implants containing either cortisol or a cortisol inhibitor. We found that predator-exposed fish expressed a deeper-bodied phenotype and darker body colouration as compared to non-exposed individuals. Skin analyses revealed that an increase in the amount of melanophores caused the dramatic colour change in predator-exposed fish. Increased melanisation is costly, and the darker body colouration may act as an inducible defence against predation, via a conspicuous signal of the morphological defence or by crypsis towards dark environments and a nocturnal lifestyle. By contrast, the phenotype of individuals carrying cortisol implants did not mirror the phenotype of predator-exposed fish but instead exhibited opposite trajectories of trait change; a shallow-bodied morphology with a lighter body colouration as compared to sham-treated fish. The cortisol inhibitor did not influence the phenotype of fish reared in predator presence i.e. neither body depth nor body colouration differed between this group and predator-exposed fish having a sham implant. However, our results illuminate a potential link between stress physiology and morphological defence expression.