Metabolic theories in ecology interpret ecological patterns at different levels (individuals, populations, communities) through the lens of metabolism, often applying allometric scaling with rates of energy use described as power functions of body mass. However, the application of metabolic theory at higher levels requires a sound theory for metabolism at the individual level.

In this thesis, metabolic theory has been developed and applied in three different contexts; 1) growth of individual organisms under food limitation, 2) life-history theory for age and size at maturity for individual organisms, and 3) population growth of marine mammals exposed to bioaccumulative toxicants through their diet.

In the first context, a new mechanistic model for individual growth was developed, based on an energy balance that expresses growth as the net result of energy assimilation from food and various metabolic costs. The model can account for effects of body composition and cellular-level growth patterns, but foremost it considers a potential trade-off between regulated maintenance and growth under food limitation. The model was successfully calibrated and validated against empirical data for an insect (house cricket) under both unlimited and limited food conditions. Interestingly, the empirical calibration indicated that the energy per unit body mass that an organism allocates to maintenance of body structures may increase as the organism grows and may also be upregulated under food limitation.

In Paper I, the maintenance-growth model (MGM), is presented, derived and demonstrated via numerical simulations and comparisons with available growth data. In Paper II and III, MGM is calibrated and evaluated against collected data for house crickets growing under unlimited and restricted food supply, respectively.

In the second context (Paper IV), it was investigated how models for individual growth and mortality can be combined with life-history theory to model plastic responses in age and size at maturity under varying resource conditions. The new growth model (MGM) was implemented to account for the trade-off between somatic maintenance and growth. It was also investigated how life-history models that predict the occurrence of maturity are affected by the presence of an overhead threshold, a minimum size that organisms must reach in order to mature and exceed in order to reproduce. It was found that the existence of an overhead threshold, that previously has been considered to be a crucial assumption for predicting realistic reaction norms for age and size at maturity, may not be crucial after all.

In the final context (Paper V), a model was developed for bioaccumulation of toxicants and their effects on survival rates, fertilities, age structure and population growth in marine mammals. Allometric scaling of biological rates were applied in the parametrisation of the model. The model was successfully calibrated and validated against empirical data for Baltic grey seals affected by PCB. The model could demonstrate that decreased female fertility (caused by a toxicant) may considerably increase bioaccumulation of the toxicant due to decreased offload from females to offspring.