Circannual rhythm of resting metabolic rate of a small Afrotropical bird Lindy J. Thompson n , Mark Brown, Colleen T. Downs n School of Life Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3201, South Africa article info Article history: Received 26 January 2015 Received in revised form 13 April 2015 Accepted 13 April 2015 Available online 14 April 2015 Keywords: Acclimatization Circannual rhythm Resting metabolic rate Evaporative water loss Afrotropical Cape white-eye Zosterops virens abstract Seasonal variation in avian metabolic rate is well established in Holarctic and temperate species, while trends in Afrotropical species are relatively poorly understood. Furthermore, given the paucity of data on circannual rhythm in avian metabolism, it is not known whether seasonal measurements made in summer and winter correspond with annual peaks and troughs in avian metabolic rate. Thus, we in- vestigated how mean body mass, resting metabolic rate (RMR) and evaporative water loss (EWL) of a small Afrotropical bird, the Cape white-eye (Zosterops virens), changed monthly over the course of a year at 20 °C and 25 °C. Mean body mass was 12.2 71.0 g throughout the study period. However, both EWL and RMR varied monthly, and peaks and troughs in RMR occurred in March and October respectively, which did not correspond to peaks and troughs in mean monthly outdoor ambient temperatures. These results suggest that measuring RMR at the height of summer and winter may underestimate the flex- ibility of which birds are capable in terms of their metabolic rate. We encourage further studies on this topic, to establish whether the lag between environmental temperature and RMR is consistent in other species. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Basal metabolic rate (BMR) is the ‘obligatory cost of living for endotherms' (Barceló et al., 2009), and it is measured in resting, post-absorptive, non-reproductive, adult endotherms at thermo- neutrality during their inactive period (McKechnie et al., 2006; McNab, 1997). BMR is one of the most commonly measured phy- siological variables of endotherms (Bech et al., 1999; Rønning et al., 2007), and is a useful metric for comparing metabolic power output among avian taxa (Liknes and Swanson, 1996; McKechnie, 2008; McKechnie et al., 2006). In birds, BMR is influenced by a multitude of factors (McKechnie and Swanson, 2010), however, it is not fixed, as was previously implied by studies that reported a single BMR value for a species (McKechnie, 2008; Speakman et al., 2004). On the con- trary, avian BMR is now known to be a highly flexible trait (Piersma, 2002; Swanson et al., 2014; Van de Ven et al., 2013a; Vézina et al., 2006; Zheng et al., 2013), and changes in avian BMR are temporary, reversible and repeatable (McKechnie et al., 2007; Piersma and Drent, 2003), which helps birds to adapt to spatially or temporally heterogeneous environments (DeWitt, 1998; DeWitt et al., 1998; Schlichting and Pigliucci, 1998; Tieleman et al., 2003; Via et al., 1995). For example, there are a plethora of studies showing seasonal flexibility in avian metabolic rate (Bech, 1980; Bush et al., 2008a; Cooper et al., 2002; Cooper and Swanson, 1994; Hart, 1962; McKechnie, 2008; Piersma et al., 1995; Pohl and West, 1973; Thabethe et al., 2013; Van de Ven et al., 2013b), and these changes in avian metabolism usually follow seasonal changes in energy expenditure (Dawson, 2003; Smit et al., 2008). The higher winter metabolic rates of some avian species sug- gest increased thermogenic capacity, and are correlated with im- proved cold tolerance (Cooper and Swanson, 1994). For example, increased thermogenic capacity was found to be the main feature of winter acclimatization in American goldfinches (Carduelis tristis) (Carey et al., 1978; Dawson and Carey, 1976), and winter-accli- matized house sparrows (Passer domesticus) are more tolerant of low temperatures than summer-acclimatized birds (Davis Jr., 1955; Kendeigh, 1949; Nzama et al., 2010). Avian BMR may show sig- nificant seasonal adjustments of up to 64% and 120% for whole animal and mass-specific BMR respectively (McKechnie, 2008; Nzama et al., 2010), and in environments with extremely cold winters, birds usually have higher BMR, cold tolerance, metabolic capacity, standard and peak metabolic rates, and resting metabolic rate (RMR), in winter than in summer (Dawson and Carey, 1976; Dawson and Marsh, 1989; Downs and Brown, 2002; Hart, 1962; Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jtherbio Journal of Thermal Biology http://dx.doi.org/10.1016/j.jtherbio.2015.04.003 0306-4565/& 2015 Elsevier Ltd. All rights reserved. Abbreviations: BMR, Basal metabolic rate; EWL, Evaporative water loss; RMR, Resting metabolic rate; RER, Respiratory exchange ratio; V ̇ CO 2 , Rate of CO 2 pro- duction; V ̇ O 2 , Rate of O 2 consumption n Corresponding authors. E-mail addresses: lindojano@yahoo.com (L.J. Thompson), downs@ukzn.ac.za (C.T. Downs). Journal of Thermal Biology 51 (2015) 119–125