Behavioural and physiological responses to food availability and predation risk

摘要

Several empirical studies have demonstrated the existence of intraspecific variation in age and size at reproductive maturity for organisms experiencing different food environments and predation risk. For some species, these changes have been shown toarise primarily through changes in foraging activity. Theoretically, changes in age and size at maturity can arise through either behavioural or physiological responses. Here we analyse two models. The first is a conventional life-history model with noexplicit recognition of the physiology of energy utilization by the organism - growth (i. e.. weight gain) is simply the difference between assimilation and respiration, and there are no physiological restrictions on the timing of maturation. The changesin age and size at maturity in response to food availability and predation risk predicted by this model are consistent with published experimental data for one particular species, the midge Chironomus tentans. Numerical calculations with parameters appropriate to this species suggest that the optimal response is purely behavioural. The second model is a general, dynamic energy budget model that takes account of the energetic costs associated with development to reproductive maturity. With that model, we prove that the optimum partitioning of energy between growth and development is independent of predation risk and food availability, thereby demonstrating the generality of the previous finding with the life-history model. On the basis of the combinedinsight from the two models, we propose that fixed allocation to growth and development, despite variation in food availability and predation risk, is optimal for a broad class of life histories. Consequently, the absence of an allocation response to experimental manipulation of food or predators should not necessarily be taken as evidence for physiological or other constraints on life-history adaptation.