An increase in minimum metabolic rate and not activity explains field metabolic rate changes in a breeding seabird

Abstract

SUMMARYThe field metabolic rate (FMR) of a free-ranging animal can be considered as the sum of its maintenance costs (minimum metabolic rate, MMR) and additional costs associated with thermoregulation, digestion, production and activity. However, the relationships between FMR and BMR and how they relate to behaviour and extrinsic influences is not clear. In seabirds, FMR has been shown to increase during the breeding season. This is presumed to be the result of an increase in foraging activity, stimulated by increased food demands from growing chicks, but few studies have investigated in detail the factors that underlie these increases. We studied free-ranging Australasian gannets (Morus serrator) throughout their 5month breeding season, and evaluated FMR, MMR and activity-related metabolic costs on a daily basis using the heart rate method. In addition, we simultaneously recorded behaviour (flying and diving) in the same individuals. FMR increased steadily throughout the breeding season, increasing by 11% from the incubation period to the long chick-brooding period. However, this was not accompanied by either an increase in flying or diving behaviour, or an increase in the energetic costs of activity. Instead, the changes in FMR could be explained exclusively by a progressive increase in MMR. Seasonal changes in MMR could be due to a change in body composition or a decrease in body condition associated with changing the allocation of resources between provisioning adults and growing chicks. Our study highlights the importance of measuring physiological parameters continuously in free-ranging animals in order to understand fully the mechanisms underpinning seasonal changes in physiology and behaviour.

Associations between Resting, Activity, and Daily Metabolic Rate in Free-Living Endotherms: No Universal Rule in Birds and Mammals

“…However, the relationship between a c tiv ity /, and m in im u m /H for Australasian gannets is significantly negative. This is a product of the curvilinear relationship between V02 and fH (Green et al 2013) and the greater proportion of the day spent in flight by this species (25%-35%; Green et al 2013) compared to the other volant bird species included in this study (usually <5%;Gremillet et al 2005;Pelletier et al 2008;Portugal et al 2012). For any species, a curvilinear relationship results in the difference in heart rate between m inim um fH and mean / H, equating to a greater A 0 50 100 150 200…”

“…In total, data sets from seven bird and four mammal species were available for analysis (table 1). Information and details regarding the devices used, implantation and removal procedures, and data collection protocols are avail able from the following sources: great cormorants Phalacrocorax carbo (Gremillet et al 2005), barnacle geese Branta leucopsis (Portugal et al 2009), Australasian gannets (Green et al 2013), little penguins Eudyptula minor (J. A.…”

“…See Guillemette and for an alternative method. We then calculated means of each of the three variables for time periods equal to the largest of these three intervals for each species (see table 1) to give the following three variables: m ean/H, m inim um /H, and activity f H. For birds, mean / H and minimum f H were converted into estimates of mean V0, and minimum V02, respectively, using calibration equations from the following sources: great cormorants (White et al 2011), barnacle geese (Portugal et al 2009), Australasian gannets (Green et al 2013), little penguins (Green et al 2008), king penguins (Halsey et al 2007), macaroni penguins (Green et al 2005), com mon eiders (Guillemette et al 2007), and, where necessary, animal masses reported therein. Activity V0, was given as minimum Vo2 subtracted from mean V02.…”

“…For two of the bird species (great cormorants and Australasian gannets) these calibrations account for the curvilinear relation ships in / H and V02 observed in flying birds (White et al 2011;Green et al 2013). However, it has not yet been possible to account for this in the calibration equations available for barna cle geese or eider; it is known that calibrations based on walking may not accurately estimate V0, from /H during flight in the geese at least (Ward et al 2002).…”

“…An example of the application of such analysis is demon strated in a study of Australasian gannets Morus senator by Green et al (2013) using heart rate ( /H) as a calibrated proxy for metabolic rate. The positive correlation between RMR and DEE in this species reflects variation in the magnitude of RMR, but there is no variation in AEE with RMR, suggesting the in dependent model of energy management.…”