Abstract

As one of the elements for a model on the food requirements of Humboldt penguins Spheniscus humboldti we determined, via gas respirometry, metabolic rates while swimming and resting in water. During rest in water at 19 degrees C Humboldt penguins (mean body mass 3.6 kg) required 5.95 W kg(-1). This corresponds to a thermal conductance in water of 0.2975 W (kg degrees C)(-1) (at T-a 19 degrees C and assuming a Tb Of 39 degrees C). When swimming in a 20 m long channel, metabolism rose from 8 W kg(-1) at a speed of 0.6 ms(-1) to 23.1 W kg(-1) at 2.2 m s(-1). Transport costs (the cost to move 1 kg of body mass over a distance of 1 m) reached a minimum at 1.4 ms(-1) with 8.1 J (kg m)(-1), which corresponds to 0.89 J (Nm)(-1). We corrected for acceleration and deceleration in the channel to determine transport costs of free-ranging Humboldt penguins travelling at sea, which were calculated as 7 J (kg m)(-1) (0.71 J [Nm](-1)), at 1.7 m s(-1). Birds feeding chicks need to balance the costs of either (1) returning to the breeding island for the night and travelling back to the feeding grounds in the morning or (2) incurring increased thermoregulatory costs associated with resting at sea overnight. Simple calculations show that at water temperatures of 19 degrees C we expect Humboldt penguins to show a tendency to remain at sea overnight if foraging areas are >4 km from their island. In colder waters (12 degrees C), this distance increases to >9 km. Using previously published data on at-sea activity of Humboldt penguins, we found that foraging costs during chick rearing amount to 340 g anchovies d(-1). Finally, we present a general model to convert Humboldt penguin activity data at sea to food requirements.