BODY-COMPOSITION, FAT RESERVES, AND FASTING CAPABILITY OF CAPE GANNET CHICKS

Age-related changes in lipid, water, and non-lipid content and energy density of four major body parts (integument, viscera, leg and pectoral muscles, and carcass) of Cape Gannet chicks, juveniles, and adults were studied. The proportion of water was typically high for neonates and decreased during development to near adult levels at fledging. The percentage of lean dry matter remained almost constant for muscles and carcass but decreased in viscera and integument due to fat deposited during growth. Energy density was linearly related to age (kJ/g = 4.46 + 0.076 Age (days), r2 = 0.845, P < 0.001). Fasting capability (FC) of chicks (survival time when using only their lipid reserves, calculated from lipid mass and field metabolic rate) increased exponentially up to 70 days of age (FC = 0.6e0.05 Age, r2 = 0. 815, P < 0.001). Newly fledged birds have a fasting capability between 8-16 days and adults only 1-2 days. Time between consecutive feeds, from field observations at the nest, for chicks 21-40, 41-60 and 61-80 days old were 11.9 +/- 0.7 h, N = 74; 16 +/- 1.1 h, N = 50; and 20 +/- 0.6 h, N = 35, respectively. The results are contrary to the hypothesis that fat deposits are an insurance against an unpredictable fast, although they may serve for this purpose. Fat deposits may be the physiological consequence of the temporal pattern of feeding by adults, or they may serve as energy sinks to increase energy requirements relative to the nutrient and protein requirements of chicks.