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.

Empirical evidence has shown that historical exposure of polychlorinated biphenyls (PCBs) to Baltic grey seals not only severely affected individual fitness, but also population growth rates and most likely caused the retarded recovery rate of the depleted population for decades. We constructed a new model which we term a toxicokinetic–toxicodynamic (TKTD) population model to quantify these effects. The toxicokinetic sub-model describes in detail the bioaccumulation, elimination and vertical transfer from mother to offspring of PCBs and is linked to a toxicodynamic model for estimation of PCB-related damage, hazard and stress impacts on fertility and survival rates. Both sub-models were linked to a Leslie matrix population model to calculate changes in population growth rate and age structure, given different rates of PCB exposure. Toxicodynamic model parameters related to reproductive organ lesions were calibrated using published historical data on observed pregnancy rates in Baltic grey seal females. Compared to empirical data, the TKTD population model described well the age-specific bioaccumulation pattern of PCBs in Baltic grey seals, and thus, the toxicokinetic parameters, deduced from the literature, are believed to be reliable. The model also captured well the general effects of PCBs on historical population growth rates. The model showed that reduced fertility due to increased PCB exposure causes decreased vertical transfer from mother to offspring and in turn increased biomagnification in non-breeding females. The developed TKTD model can be used to perform population viability analyses of Baltic grey seals with multiple stressors, also including by-catches and different hunting regimes. The model can also be extended to other marine mammals and other contaminants by adjustments of model parameters and thus provides a test bed in silico for new substances.

Metabolic theories in ecology interpret ecological patterns at different levels through the lens of metabolism, typically applying allometric scaling to describe energy use. This requires a sound theory for individual metabolism. Common mechanistic growth models, such as 'von Bertalanffy', 'dynamic energy budgets' and the 'ontogenetic growth model' lack some potentially important aspects, especially regarding regulation of somatic maintenance. We develop a model for ontogenetic growth of animals, applicable to ad libitum and food limited conditions, based on an energy balance that expresses growth as the net result of assimilation and metabolic costs for maintenance, feeding and food processing. The most important contribution is the division of maintenance into a 'non-negotiable' and a 'negotiable' part, potentially resulting in hyperallometric scaling of maintenance and downregulated maintenance under food restriction. The model can also account for effects of body composition and type of growth at the cellular level. Common mechanistic growth models often fail to fully capture growth of insects. However, our model was able to capture empirical growth patterns observed in house crickets.

A growing animal ingests food from the environment and distributes the assimilated energy between chemical energy stored in synthesized biomass and energy spent on metabolic processes, including food processing, maintenance, activity and overhead costs for growth. Under food restriction, the growth rate is usually decreased. However, the extent of this reduction may be influenced by a potential trade -off with maintenance metabolism. The latter seems to be down-regulated under food restriction in some animals and up-regulated in others. Recently, the Maintenance-Growth Model (MGM) was developed for ontogenetic and post-mature growth, including several aspects not considered by common mechanistic growth models, most importantly the division of maintenance costs into non-negotiable and negotiable parts, where the latter can be up- or downregulated under food restriction. Using empirical data, MGM has been calibrated and successfully applied to an insect growing under ad libitum conditions. Here, the model is further calibrated to newly collected individual data for the same species growing under two different regimes of food restriction, complemented with previously collected data for food-limited cohorts. We find that two alternative model scenarios of MGM are able to generate rather good predictions of observed growth under food restriction, assuming either upregulated maintenance or decreased effective assimilation (assimilation minus energy spent on processing and searching food). We find the latter scenario least plausible, implying that the current study provides the first indication for the occurrence of upregulated maintenance in an insect species when food is scarce, an unexpected result that requires further investigation. The inclusion of maintenance regulation in MGM enables the new growth model to be used in the modelling of life-history dependent trade-offs between maintenance, growth and maturation for various other species.