When food access is challenging: evidence of wood mice ability to balance energy budget under predation risk and physiological stress reactions
Prey species must balance the energetic cost of locomotion, foraging effort, and predation risk. In our work, we assessed how the wood mice (Apodemus sylvaticus) balance these costs by manipulating food access difficulty and predation risk. Live trapping was conducted in Madrid (Spain) where 80 Sherman traps were set in four different plots. To assess how wood mice manage their energy budget, all traps were first subjected to a control period followed by four treatments of 3-day duration in which food access was experimentally manipulated (free access, straw balls, straw balls wrapped in metal wire, opened plastic bottles, closed bottles). Predation risk was simulated by exposing half of the traps to fox feces. To quantify food intake, we weighed the remnants of food left by each captured mouse. Furthermore, we collected mouse fecal samples from traps to evaluate the physiological stress response by quantifying fecal corticosterone metabolites (FCM). Results showed that despite mice generally avoiding traps treated with fox feces, predation risk did not modulate food intake or FCM levels. By contrast, the experimental manipulation of food access determined the amount eaten and increased FCM levels, probably owing to the different degrees of difficulty and the energy required to obtain the food. Moreover, recaptured individuals ate more, indicating that experience critically determines mice ability to reduce the costs of accessing food. By analyzing the joint variation between mice intake and FCM levels depending on food restriction treatments, we found that mice were able to modulate their energetic expenditure depending on food access difficulty probably in order to achieve a positive marginal value of energy.
For wild animals, to optimally balance the energetic budget is critical to maximize fitness. However, foraging decisions are influenced by numerous internal and external factors which are poorly understood. We studied free-ranging mice behavioral and physiological stress response to different food restriction treatments, testing also the influence of predation risk and experience. We found that predation risk modulated mice capturability but not the feeding behavior or the stress response. We also discovered that mice seem to be able to adjust energy allocation when a novel resource appear, and that energy budget optimization may be mediated via glucocorticoid release and individuals experience.
KeywordsFood access difficulty Food restriction Risk of predation Small mammals Glucocorticoids
The authors wish to thank José España, technician director of the Cañada Real Open Center, for providing us the red fox fecal material needed to perform the experiments. We also would like to express our thanks to the Autonomous Community of Madrid (Spain) for providing the permits required to conduct this study and to the Autonomous University of Madrid (Spain) for allowing us to carry out this project. Also, a special thanks to Juan Malo and Cristina Mata for lending us the Sherman live-traps.
The dataset analyzed during the current study are available from the corresponding author on reasonable request.
Compliance with ethical standards
This research complies with the regulations on the protection of animals used for scientific purposes (Directive 2010/63/EU of the European Parliament and of the Council of 22 September 2010 and the Spanish legislation (Royal Decree 53/2013)). The study had the approval of the Autonomous Community of Madrid (reference number 10/240775.9.16) and favorable reports from the Ethics Committee of the Autonomous University of Madrid (CEI 73-1330).
Conflict of Interest
The authors declare that they have no conflict of interest.
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