Risk-Taking Behaviour and the Timing of Life History Events: Consequences of Body Size and Season

Abstract

When faced with behavioural options differing in energetic gain and mortality risk due to predation, an individual's best compromise to the conflicting demands of growth and survival will depend upon both its current energetic state and the future opportunity for growth. Such state- and time-dependent tradeoffs are often investigated using dynamic programming. By specifying the relationship between fitness and the state variable of interest at the time of some relevant life history event. fitness-maximizing solutions for all state and time combinations can be found. To date, however, no dynamic programming model has considered the possibility that animals may be capable of delaying life history events beyond the time period modelled. In such cases, in addition to being influenced by future life history events. short term behavioural responses to foraging-predation risk tradeoffs may also indirectly affect the timing of those events. I use dynamic programming (1) to investigate the effects of body size and time of year on patterns of risk-taking behaviour in animals capable of postponing life history events, and (2) to explore the outcome of such individual decisions on the subsequent timing of life history events and the states of individuals undergoing those events. In doing so. I relax the basic dynamic programming assumption of a finite time horizon and allow individuals to postpone initiating the life history event until some future favourable period of time. Such delays are frequently observed in anadromous fishes, including coho salmon, Oncorhynchus kisutch; hence, I use the relevant features of their biology to develop the model and illustrate the general problem of interest.