Contents lists available at ScienceDirect Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa Altitudinal variation in metabolic parameters of a small Afrotropical bird Lindy J. Thompson, Colleen T. Downs ⁎ School of Life Sciences, University of KwaZulu-Natal, P/Bag X01, Scottsville, Pietermaritzburg, 3209, South Africa ARTICLE INFO Keywords: Altitude Altitudinal variation Seasonal variation Cape White-eye Zosterops virens Resting metabolic rate Evaporative water loss Respiratory exchange ratio ABSTRACT Of the numerous factors affecting avian metabolic rate, altitude is one of the least studied. We used mass-flow respirometry to measure resting metabolic rate (RMR), evaporative water loss (EWL) and respiratory exchange ratio (RER) in two populations of a small (10–12 g) Afrotropical bird, the Cape White-eye (Zosterops virens), in summer and in winter. In total, 51 freshly wild-caught adult Cape White-eyes were measured overnight. Altitude was included as a source of variation in the best approximating models for body mass, whole-animal RMR, RER, whole-animal standard EWL and whole-animal basal EWL. RER was significantly lower in winter, suggesting a greater proportion of lipid oxidation at lower ambient temperatures (T a ). Cape White-eyes were 0.8 g heavier at the higher altitude site and 0.5 g heavier in winter, suggesting they may have increased their metabolic ma- chinery to cope with cooler temperatures. EWL was generally significantly lower in winter than in summer, suggesting that birds may increase EWL with increasing T a , as the need for evaporative cooling increases. Our results support the argument that the subtle and complex effects of altitude (and ambient temperature) should be taken into account in studies on avian metabolic rate. What is already known: Of the numerous studies known to affect avian metabolic rate, altitude is one of the least studied. Although trends are not always clear, generally, at higher altitudes, avian metabolic rate increases. What the study adds: There were statistically significant seasonal and altitudinal differences in various physio- logical parameters of Cape White-eyes. These results highlight the importance of accounting for altitude in studies of avian metabolic rate. 1. Introduction There are numerous environmental factors that may reversibly af- fect avian metabolic rate (see reviews by McKechnie, 2008; McKechnie and Swanson, 2010). One of these factors is altitudinal variation, which can significantly affect various avian physiological parameters, in- cluding rate of oxygen consumption (V ̇ O 2 ), rate of carbon dioxide production (V ̇ CO 2 ), respiratory exchange ratio (RER) and body tem- perature (Chappell and Bucher, 1987), however studies on the effects of altitudinal variation on metabolic rate within species are few (see Table 1). For example, resting metabolic rates (RMR) of Rosy Finches (Leucosticte arctoa), House Finches (Carpodacus mexicanus) and Ame- thyst Sunbirds (Chalcomitra amethystina) increased significantly with increased altitude (Clemens, 1988; Lindsay et al., 2009). Similarly, in 13 bird of paradise species (Family Paradisaeidae), those that were limited to lower altitudes had lower basal metabolic rates (BMRs) than those found at higher altitudes (McNab, 2003; McNab, 2005). In each case, this increase in metabolic rate with increased altitude may be due to variation in ambient temperature (T a ) along the altitudinal gradient. In contrast to the results of the aforementioned studies, Londoño et al. (2015) found that avian BMR does not always increase with an increase in altitude; contrary to expectations, the BMR of 253 Peruvian forest bird species did not differ significantly between altitudes of 400, 1500 and 3000 m above sea level. The authors concluded that tropical birds have a ‘consistently ‘slow’ energy metabolism’ across a wide range of altitudes (and thus temperatures). Furthermore, a study on four al- titudinal subpopulations of Common Fiscals (Lanius collaris) showed that birds from lower altitudes had higher BMR and EWL than con- specifics from higher altitudes (Soobramoney et al., 2003), a finding these authors related to unpredictable climate and food availability. Thus altitudinal trends in avian metabolism are not always clear, and warrant further investigation. Relatively few studies have examined the phenotypic plasticity of avian metabolic rate in terms of both season and altitude, despite in- creasing recognition of avian RMR and BMR as a highly flexible traits (Piersma, 2002; Vézina et al., 2006), reversibly affected by a multitude of factors (Broggi et al., 2009). Furthermore, future work predicting the effects of climate change on avian physiology will require baseline data http://dx.doi.org/10.1016/j.cbpa.2017.07.015 Received 2 May 2017; Received in revised form 24 July 2017; Accepted 26 July 2017 ⁎ Corresponding author. E-mail addresses: lindojano@yahoo.com (L.J. Thompson), downs@ukzn.ac.za (C.T. Downs). Abbreviations: BMR, Basal metabolic rate; EWL, Evaporative water loss; Mb, Body mass; RER, Respiratory exchange ratio; RMR, Resting metabolic rate; T a , Ambient temperature; V ̇ CO 2 , Rate of CO 2 production; V ̇ O 2 , Rate of O 2 consumption Comparative Biochemistry and Physiology, Part A 212 (2017) 88–96 Available online 31 July 2017 1095-6433/ © 2017 Elsevier Inc. All rights reserved. MARK