Physiology of prolonged fasting in greater snow geese (Chen caerulescens atlantica)

-Greater Snow Geese (Chen caerulescens atlantica) may face periods of complete food deprivation during incubation in the High Arctic. We studied the ability of these birds to withstand a prolonged fast. We measured rate of mass loss, patterns of carbohydrate, lipid and protein utilization, and changes in resting metabolic rate in 11 captive geese subjected to complete food deprivation. Geese lost 44% of their initial body mass during fasting periods that ranged from 19 to 42 days (mean 34 days). The rate of mass loss reflected the three phases commonly associated with the changing rates of lipid and protein catabolism in homeotherms. Phase I (11 days) was characterized by a decreasing rate of body-mass loss and a rapid increase in plasma f-hydroxybutyrate level, indicating body-lipid mobilization. During phase II (22 days), the rate of body-mass loss stabilized at a low level. Plasma concentration of 3-hydroxybutyrate peaked in the first half of phase II and then decreased. Plasma uric-acid level, an index of protein catabolism, increased during the second half. Resting metabolic rate of fasting geese was reduced by 38% below the pre-fast value, more than predicted from body-mass decrease alone. Phase III (3 to 4 days) was characterized by an increase in the daily rate of mass loss. Plasma concentration of uric acid increased markedly, while I-hydroxybutyrate values remained low and stable. Glycemia and total plasma protein level also started to fall. Greater Snow Geese did not tolerate fasting as long as larger birds, like penguins, probably because they were less efficient in sparing their protein reserves during phase II. Greater Snow Geese endurance of fasting may be limited by their use of flight, which ultimately limits their capacity to carry the large fat reserves required for prolonged fasting. Received 30 April 1991, accepted 13 January 1992. IN WILD BIRDS, fluctuations in food availability may impose fasting for more or less predictable periods of time. Energy requirements of small passerines allow them to tolerate fasting in winter for only 10 to 37 h before dying (Kendeigh 1945, Ketterson and King 1977), but large birds of prey may be deprived of food for 11 to 13 days and remain in good health (Hatch 1970, Garcia-Rodriguez et al. 1987). Even in the presence of food, some species may fast because feeding conflicts with other important activities. Red Jungle Fowl (Gallus gallus; Mrosovsky and Sherry 1980), most penguins and petrels (Croxall 1982, Cherel et al. 1988b), and many Arcticand temperate-nesting anatids (Korschgen 1977, Krapu 1981, Mainguy and Thomas 1985, Thompson and Raveling 1987, Ankney and Afton 1988, Parker and Holm 1980) greatly reduce their food consumption during egg laying and incubation. In these birds, only one parent incubates and it is not fed by its mate. Fasting endurance may have an adaptive value and be a key determinant of reproductive success in species facing a negative energy balance during nesting. The reproductive success of nesting geese depends largely on the quality of nest attendance by females during incubation (Murphy and Boag 1989). To reduce risks of predation and intraspecific nest parasitism, females stay on the eggs and feed little from laying to hatching (Ryder 1970, Ankney and MacInnes 1978, Raveling 1979, Aldrich and Raveling 1983, Prop et al. 1984, Mainguy and Thomas 1985, Thompson and Raveling 1987). Males do not incubate, but assume most of the territorial defense and also reduce their food intake (Inglis 1977, Stroud 1982). They lose weight, though to a much lesser extent than females (Ryder 1975, Ankney 1977, Owen and Wells 1979). In Lesser Snow Geese (Chen caerulescens caerulescens), females feed very little after completing laying. Without sufficient energy reserves to endure a long fast, some females do not complete incubation and may even die before its termination (Harvey 1971, Ankney and MacInnes 1978). Greater Snow Geese (Chen caerulescens atlantica) presumably also become anorexic during their 24-day incubation period (Lemieux 1959), although their nesting behavior is largely unknown. Despite the obvious importance of fasting in