The causes of individual and seasonal variation in the metabolic rate of Knot Calidris canutus
Basal metabolic rate (BMR), an individual bird's minimum rate of energy expenditure, was followed in adult and juvenile captive Knot throughout their annual cycle, in conjunction with measurements of total body mass (BM) and body composition (lean mass and fat mass, as predicted using Total Body Electrical Conductivity). Adult captive Knot increased significantly in BM during spring, primarily due to fat deposition. Most juvenile Knot did not display fat deposition in their first spring in captivity. A seasonal peak in BMR, often double the seasonal minimum, occurred during spring but typically took place, on average, 5,11 and 4 days (respectively) after the seasonal peaks in BM, lean mass and fat mass. Little of the variation in BMR seen within or amongst captive Knot, irrespective of physiological state, was explained by variation in a single parameter (BM, lean mass or fat mass). As variation in BMR was not simply a consequence of variation in total lean mass, the average metabolic output per gram of the lean tissues must also have altered seasonally. During fat deposition in spring, Knot exhibited a significant increase in liver mass and a significant elevation (approximately 50% higher) in the activity of succinate dehydrogenase (SDH, an indicator of metabolic activity) in the small intestine. Such adaptations may have assisted an increase in fat deposition rate. SDH activity decreased by approximately 60% in the pectoral muscle of Knot during this period. Such a reduction in SDH may also aid fat deposition as it lowered an individual’s overall BMR. As Knot BM decreased after the spring peak, then BMR decreased in parallel with a decrease in SDH activity in their pectoral muscles. The spring peak in overall BMR may indicate an increase in the maximal sustainable metabolic rate (MMR) of an individual during migratory flight. If a relationship exists between BMR and MMR, then variation in metabolic activity rather than variation in the mass of various lean tissues (e.g. pectoral muscle) will increase metabolic scope without increasing the energetic costs of flight.