Fluid balance of the reindeer shows marked seasonal changes that are closely related to the availability and quality of feed. In summer the amount and nitrogen content of the feed are high in comparison to low-protein, low-mineral lichen that is the main feed during winter. Thus water intake, which is related to feed intake, is high in summer. High water intake results in high water turnover, because kidney of the reindeer has poor ability to concentrate urine. In summer water is also lost through evaporation, although the number of functional sweat glands is low, and furthermore, even insects may cause a marked decrease in plasma volume. In winter no liquid water is available and melting the snow is energetically costly. Voluntarily decrease in feed intake decreases the need of water and production of metabolic water is increased due to mobilization and oxidation of fat. Body fat and total body water are inversely related and total body can even be taken as an indicator of animal's body condition. Because both water and electrolytes are available in limited amounts during winter, it would be expected that concentrations of aldosterone would be high. However, the opposite has been detected, high concentrations in the summer and low in winter, but no explanation to this is available. In addition to these seasonal changes that are part of the adaptation of reindeer to its surroundings, reindeer also show changes in fluid balance due to rapid changes in ambient temperature.