ORIGINAL PAPER Culum Brown Æ Carolyn Gardner Victoria A. Braithwaite Differential stress responses in fish from areas of high- and low-predation pressure Received: 5 January 2005 / Revised: 9 March 2005 / Accepted: 22 March 2005 / Published online: 11 May 2005 Ó Springer-Verlag 2005 Abstract We subjected fish from regions of high and low levels of predation pressure in four independent streams to a mild stressor and recorded their opercular beat rates. Fish from low-predation areas showed higher maximum, minimum and mean opercular beat fre- quencies than fish from high-predation regions. The change in opercular beat frequency (scope) was also significantly greater in fish from low- than in fish from high-predation regions. Under normal activity levels, however, low predation fish showed a reduced opercular beat frequency, which may be indicative of reduced activity levels or metabolic rate. Opercular beat fre- quency was negatively correlated with standard length as one would expect based on higher metabolic rates in smaller fish. We suggest that these contrasting stress responses are most likely the result of differential expo- sure to predators in fish from high- and low-predation areas. We argue that reduced stress responses in high- predation areas evolved to prevent excessive energy expenditure by modulating the fright response. Keywords Predators Æ Prey Æ Stress responses Æ Evolution Æ Brachyraphis episcopi Introduction Variable exposure to predation pressure has dramatic effects on virtually all aspects of an animal’s biology, encompassing physiology, behaviour, morphology and life-history traits (Endler 1995). Predators place con- siderable stress on their prey both physiologically and psychologically (Pfeiffer 1962). Recent studies have re- vealed that non-lethal costs of predation stem not only from lost foraging opportunities but also from increased energetic expenditure (Fraser and Gilliam 1992). Per- haps the best illustration of this is the fright or flight response (e.g. Barreto et al. 2003). The flight response involves all manner of correlated physiological changes in response to potential danger including rapidly increasing heart and ventilation rates (Metcalfe et al. 1987; Ho¨jesjo¨ et al. 1999). However, the repeated pro- duction of a fright response is extremely costly both in terms of time lost from other potential behaviours such as feeding, and in the sheer amount of energy consumed during the mobilisation of glycogen stores. One would expect, therefore, that such responses would be modu- lated in a manner whereby animals only show these re- sponses in appropriate situations. Vital parameters not only include an animal’s immediate response to stressful situations (i.e. their change in metabolic rate), but also how quickly they recover from them (Cooke et al. 2003). In regions where predators are relatively abundant and interactions between predators and prey are com- mon, stress responses resulting from predator distur- bance may become attenuated in prey animals through natural selection and, or, through plasticity within the lifetime of an individual. Laboratory experiments in which drosophila were reared in a stressful environment showed higher fitness when exposed to a novel stress than those reared under less-stressful conditions (Reed et al. 2003). At the other end of the scale, rat pups handled for 15 min per day over a couple of weeks had up to 50% lower glucocorticoid levels in response to moderate stressors than those that had not been handled (Caldjii et al. 2000). Evidently exposure to stresses early on in life and over evolutionary time can induce sub- stantial long-term changes in the hypothalamic-pitui- tary-adrenal axis in mammals and homologous systems in other taxa (e.g. hypothalamic-pituitary-interrenal in fishes). Communicated by G. Heldmaier C. Brown (&) Æ C. Gardner Æ V. A. Braithwaite Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, EH9 3JT, UK E-mail: Culum.Brown@ed.ac.uk Tel.: +44-131-6505553 Fax: +44-131-6506564 J Comp Physiol B (2005) 175: 305–312 DOI 10.1007/s00360-005-0486-0