Heat production studies on normal and oil-covered jackass penguins (Spheniscus demersus) in air and water

Heat production and the importance of intact feathers for insulation were studied in jackass penguins Spheniscus demersus at various temperatures (5-25°C) by comparing their oxygen consumption in air and water with that of birds whose feather insulation was impaired when they were accidentally covered in partially weathered crude oil. Special attention was given to employing an experimental procedure that could relate oxygen consumption to level of activity. The mean ± SD metabolic rate recorded for normal inactive birds (188 ± 24 kJ kg-1 day-1) was the same in air and water but only 80% of the mass specific rates predicted for non-passerine birds. Wetting a bird increased the inactive rate in air 1,33 times while slow swimming in water above 10°C caused a 1,76-fold increase. At low air temperatures (5°C) postural changes and feather fluffing were associated with a very low rate of 122 ± 29 kJ kg-1 day-1, which we interpret as behavioural control of penguins over energy expenditure. Jackass penguins in sea water at 10°C and above were within their thermal neutral range thus indicating that the water phase of their life cycle is not energetically more expensive than the terrestrial phase. Activity level of the birds was an important influence on oxygen consumption; we suggest it is the main reason for the variations in metabolic rates within closely related species reported in the literature