The seasonality hypothesis states that climates characterized by
large annual cycles select for large body sizes. In order to study
the effects of seasonality on the evolution of body size, we use a
model that is based on physiological rules and first principles. At
the ecological time scale, our model results show that both larger
productivity and seasonality may lead to larger body sizes. Our
model is the first dynamic and process-based model to support the
seasonality hypothesis and hence demonstrates the importance of
basing models on physiological processes.
We focus not only on variability at the ecological time scale, but
also on the temporal variations in seasonality existing at
geological time scales. A particularly strong forcing of seasonality
exists on the scale of 20,000-400,000 years, the scale of
Milankovitch cycles. Therefore, we simulated the evolutionary
response of body size to a Milankovitch-type of forcing of climate
and food density. Results illustrate that for a given level of
investment in reserves body size may track climatic cycles, and
that below a certain seasonality threshold the body size will
decrease rapidly, leading to extinction.