Water flux and estimated metabolism of free-ranging gentoo and macaroni penguins at South Georgia
Water turnover rates were measured in gentoo and macaroni penguins breeding sympatrically on South Georgia Island. At the time of this study, adult male macaronis were attending the nest while female macaronis and both sexes of adult gentoos were making regular foraging trips to sea and returning to feed their chicks. Both species feed principally on krill, Euphausia superba, although gentoos also feed on fish. The average water turnover rate in 2 fasting male macaronis was 12.5 ml·kg-1·day-1 with a half-time for water turnover of 36 days. The mean water flux rate in feeding birds was 155 ml·kg-1·day-1 in gentoos and 184 ml·kg-1·day-1 in macaronis. The half-times for water turnover were 2.8 days, and 2.6 days, respectively. The average metabolic rate of fasting macaronis calculated from water turnover rates was 5.6 W·kg-1 or 1.8 x the standard metabolic rate (SMR). In order to calculate prey consumption and average daily metabolic rate (ADMR) from water flux rates in feeding birds, it was assumed that a) the only sources of water are from metabolism and performed water in the diet and b) the composition of the diet is known. Based on the type of prey consumed, the calculated ADMR was 7.1 W·kg-1 or 2.6×SMR (n=5) for gentoos and 9.1 W·kg-1 or 2.9×SMR (n=3) for macaronis. The ADMR of female macaronis making regular trips to sea was 1.6 x greater than the fasting metabolism of males brooding the chick.