Nutritional Ecology of Microtine Rodents: A Simulation Model of Mineral Nutrition for Brown Lemmings

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Abstract
A model of mineral nutrition was used to test the hypothesis that brown lemmings receive adequate amounts of macronutrients (N, P and Ca) if there is sufficient forage to meet their energetic requirements. In the model energy needed for maintenance, growth and reproduction is treated as a driving variable that determines rates of ingestion of plant material by individual lemmings. The availability of N, P and Ca to each animal depends on the amount of forage eaten, the concentrations of these nutrients in the forage and the proportion assimilated. The model determines the requirements for each mineral and for energy as a function of the lemmings'' size, metabolic rate, growth rate and reproductive output. Maintenance requirements for nutrients (but not energy) depend on fecal losses associated with passage of food through the gut and on urinary losses. P and Ca interact during the deposition and mobilization of skeletal material, and absorption efficiency of P depends on Ca status. Nutrient status, represented by proportion of normal pool size, varied considerably in simulations for animals of several age classes and reproductive conditions under different environmental conditions. Low concentrations of nutrients observed in natural forage during some years produced severe nutrient depletions in adult females with large litters (7 or more young) during late summer and winter. There appeared to be little leeway for conditions to vary without producing some nutrient depletion. The hypothesis that nutrient requirements of lemmings will be met if energetic requirements are met was not supported. Inefficient use of energy in forage by lemmings can be explained as a mechanism to improve nutrient intake. The inclusion of mosses in the diet may be important primarily because of their low digestibility. When the proportion of mosses in the diet increases more food must be ingested to obtain sufficient energy, and more nutrients are ingested. Simulation results suggested several critical experiments on the reproduction and growth of lemmings when consuming different natural diets.