Optimal seasonal routines across animals and latitudes

Most animals face seasonal fluctuations in food availability and need to develop an annual routine that maximizes their lifetime reproductive success. Two particularly common strategies are reducing energy expenditure and building storage to sustain the animal in meager periods (winters). Here, we pose a simple and generic model for an animal that can decide, at each time during the season, on its level of foraging effort and on building energy stores. Using dynamic optimization, we identify the optimal annual routines that maximize the trade-off between energy and mortality over a life-long horizon. We investigate how the optimal strategies depend on the body size and longevity of the animal, and upon the seasonal variability in the environment. We find that with large fluctuations, the optimal annual routine for small animals is to develop a surviving egg/spore stage rather than to attempt to survive the winter. Medium sized animals invest heavily in reserves to allow long hibernation, while larger animals only need smaller reserves and a shorter hibernation period. In environments with smaller fluctuations, organisms do not need energy stores or hibernation but reduce foraging activities during spring and summer where their fitness is highest. Our optimization model can be used as a null hypothesis to explain the annual routines of animals of all body sizes across the globe.