74 Behavioral and Physiological Significance of Minimum Resting Metabolic Rate in King Penguins * Corresponding author; e-mail: l.g.halsey@bham.ac.uk. Physiological and Biochemical Zoology 81(1):74–86. 2008.  2007 by The University of Chicago. All rights reserved. 1522-2152/2008/8101-61712$15.00 DOI: 10.1086/523318 L. G. Halsey 1, * P. J. Butler 1 A. Fahlman 2 A. J. Woakes 1 Y. Handrich 3 1 Centre for Ornithology, School of Biosciences, University of Birmingham, Edgbaston B15 2TT, United Kingdom; 2 North Pacific Universities Marine Mammal Research Consortium, University of British Columbia Marine Mammal Research Unit, 2202 Main Hall, Vancouver, British Columbia V6T 1Z4, Canada; 3 Institut Pluridisciplinaire Hubert Curien, Centre National de la Recherche Scientifique–Universite ´ Louis Pasteur Unite ´ Mixte de Recherche 7178, De ´partement Ecologie, Physiologie et Ethologie, 23 rue Becquerel, F-67087 Strasbourg Cedex 02, France Accepted 6/14/2007; Electronically Published 10/30/2007 ABSTRACT Because fasting king penguins (Aptenodytes patagonicus) need to conserve energy, it is possible that they exhibit particularly low metabolic rates during periods of rest. We investigated the behavioral and physiological aspects of periods of minimum metabolic rate in king penguins under different circumstances. Heart rate ( f H ) measurements were recorded to estimate rate of oxygen consumption during periods of rest. Furthermore, apparent respiratory sinus arrhythmia (RSA) was calculated from the f H data to determine probable breathing frequency in resting penguins. The most pertinent results were that mini- mum f H achieved (over 5 min) was higher during respirometry experiments in air than during periods ashore in the field; that minimum f H during respirometry experiments on water was similar to that while at sea; and that RSA was apparent in many of the f H traces during periods of minimum f H and provides accurate estimates of breathing rates of king penguins resting in specific situations in the field. Inferences made from the results include that king penguins do not have the capacity to reduce their metabolism to a particularly low level on land; that they can, however, achieve surprisingly low metabolic rates at sea while resting in cold water; and that during respirometry experiments king penguins are stressed to some degree, exhib- iting an elevated metabolism even when resting. Introduction Resting metabolic rate (RMR) is an important component of total energy expenditure in birds, representing, on average, at least 25%–30% of daily energy turnover (estimate based on allometric predictions of metabolic rate for maintenance costs only; McKechnie and Wolf 2004; Nagy 2005). Understanding the nature of RMR is important for a full knowledge of the energy budgeting of birds and their energetic constraints. For example, king penguins (Aptenodytes patagonicus) undergo a number of prolonged fasts during their breeding cycle, where changes in body mass can be substantial (around 25% for a 29-d fast during courting and first incubation shift by the male (Gauthier-Clerc et al. 2001a). This challenging life cycle means that king penguins must limit energy use during periods of fasting to maximize the chances of survival and of successfully rearing a chick (Cherel et al. 1988, Le Maho et al. 1993; Challet et al. 1994). Thus, during these periods it is likely that king penguins will maintain their metabolic rate at a low resting level whenever possible, that is, when they are not forced to be active, for example, when defending their territory (Challet et al. 1994) or walking to and from the shoreline (Halsey et al. 2007c). Resting behavior is clearly an essential element of the time budgeting of king penguins (Challet et al. 1994). Fur- thermore, if king penguins are also adapted physiologically to long fasts, it might be expected that minimum metabolic rate (minimum RMR) is particularly low in this species (i.e., a low survival metabolic rate to increase fasting endurance; Lindstedt and Boyce 1985). Sleep represents periods when muscle tonus (Dewasmes et al. 1989) and alertness are at a minimum. Periods of sleep when the bird is within its thermoneutral zone (e.g. Speakman et al. 1993) presumably incorporate periods of minimum RMR. Dewasmes et al. (1989) found that there is an increase in the amount of short-wave sleep in emperor penguins (Aptenodytes forsteri) as a fasting period progresses, perhaps to decrease the rate at which energy stores are used as they gradually diminish. During other periods of rest, RMR can often be considerably higher than minimum RMR because of a heightened physio- logical and behavioral state (Speakman et al. 1993). For ex- ample, territorial king penguins, although sitting for many hours at a time, sleep very little when incubating and brooding because they must be alert to defend their nest and offspring,