Physiological variation in Amethyst Sunbirds (Chalcomitra amethystina) over an altitudinal gradient in summer Claire Lindsay, Colleen Downs à , Mark Brown School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Pietermaritzburg 3209, South Africa article info Article history: Received 21 November 2008 Accepted 21 January 2009 Keywords: Altitudinal variation Amethyst Sunbird Metabolic rates Phenotypic plasticity Phenotypic flexibility abstract Amethyst Sunbirds (Chalcomitra amethystina) occur over an altitudinal gradient within KwaZulu-Natal, South Africa, from the Drakensberg mountain range to the coast. Sunbirds were caught in summer (November–December 2006) at three altitudinal locations within KwaZulu-Natal; Underberg (1553 m above sea level (asl)), Howick (1075 m asl) and Oribi Gorge (541 m asl). Summer oxygen consumption (VO 2 ) was measured pre-acclimation at 5 and 25 1C. After post-acclimation to 25 1C and 660 m for 6 weeks, VO 2 measurements were taken at 8 different temperatures (15, 5, 10, 20, 30, 28, 25 and 33 1C). Summer resting metabolic rates (RMR) differed significantly between altitudinal subpopulations of Amethyst Sunbirds. Summer basal metabolic rate was observed to decrease as altitude decreased. Differences between pre-acclimation RMR (representing natural acclimatization to summer conditions in the field) and post-acclimation RMR (to 25 1C at 660 m asl for 6 weeks) were large. In particular, Sunbirds from the Underberg subpopulation increased RMR by 80.9% at 5 1C and 85.2% at 25 1C post- acclimation. This summer research complemented an earlier study on altitudinal variation in Amethyst Sunbirds in winter. It again emphasized the need to acknowledge altitudinal differences between subpopulations and not just use species means, as species means do not fully incorporate the effect of phenotypic plasticity/flexibility. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction Birds are considered to be homeothermic endotherms, which implies the ability to maintain a constant body temperature over a broad range of ambient temperatures by adjusting their metabo- lism (Chaui-Berlinck et al., 2002). Many birds display phenotypic flexibility in maintenance energy requirements, and are able to up- or down-regulate basal metabolic rate (BMR) over a period of time in response to thermal acclimation (McKechnie et al., 2007; Bush et al., 2008a), season (Smit et al., 2008; Bush et al., 2008a, b), altitude (Soobramoney et al., 2003; Lindsay et al., 2009a), origin (captive bred vs. wild-caught, McKechnie, 2008), and latitude (Hudson and Kimzey, 1966). Amethyst Sunbirds (Chalcomitra amethystina), relatively large African sunbirds (mean body mass 15 g) (Cheke et al., 2001; Tree, 2005), occupy a relatively broad geographical region within South Africa which includes an altitudinal gradient within KwaZulu-Natal (KZN), from the Drakensberg mountain range to the coastal regions (Cheke et al., 2001). The distribution of Amethyst Sunbirds over an altitudinal gradient makes them an ideal species in which to study plasticity in thermal physiology within a species, as well as flexibility within a population. This paper is a continuation of an investigation into phenotypic plasticity/flexibility over an altitu- dinal gradient as described earlier (Lindsay et al., 2009a), where sunbird subpopulations were found to differ in winter BMR and resting metabolic rates (RMR) over a range of ambient tempera- tures. However, in this study, the response of BMR and RMR of subpopulations to summer acclimatization was investigated. As in the winter study (Lindsay et al., 2009a), it was predicted that the summer metabolic rates of subpopulations of Amethyst Sunbirds would vary over the altitudinal gradient due to acclima- tization and adaptations to different temperatures, as well as pre- and post-acclimation due to innate physiological differences in approach to acclimation to 25 1C and the Pietermaritzburg altitude (660 m) (Lindsay et al., 2009a). BMR is generally thought to be species specific, however, it was predicted that during summer plasticity/flexibility would occur between subpopulations, within a species, due to temperature differences over altitude. 2. Materials and methods 2.1. Study site, capture and maintenance Amethyst Sunbirds were captured in the summer of 2006 (November–December) at three different locations in KZN, South ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jtherbio Journal of Thermal Biology 0306-4565/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jtherbio.2009.01.007 à Corresponding author at: School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa. Tel.: +27 33 2605127; fax: +27 33 2605105. E-mail address: downs@ukzn.ac.za (C. Downs). Journal of Thermal Biology 34 (2009) 190–199