![]() Several studies have reported flexibility in organ size and metabolic physiology over short timescales depending on environmental conditions and physiological status (see Piersma and Lindström, 1997). A long period will normally involve larger environmental variation compared to a shorter period, and this may result in a greater variation of the trait, which in turn will decrease the repeatability. Because of this phenotypic flexibility the length of the measurement period could affect the repeatability of a physiological trait in such a way that repeatability decreases with the length of the measurement period ( Chappell et al.,1996 Hõrak et al.,2002 Vèzina and Williams, 2005). Variations in BMR between individuals are affected by genetic differences in addition to the external environment. This intra-individual phenotypic variance is mainly affected by the external environment that the animal experiences( Falconer and Mackay, 1996). In addition to inter-individual variations ( Rezende et al.,2004a), there also exist intra-individual variations in BMR during an animal's life cycle ( Langseth et al.,2000 Lindström and Rosen, 2002 Lindström and Klaassen, 2003). Significant repeatabilities in those studies indicate permanent inter-individual differences in BMR. Despite the large abundance of data,we are aware of only very few studies reporting repeatability of BMR( Bech et al., 1999 Hõrak et al., 2002 Labocha et al., 2004 Vézina and Williams,2005). Studies on repeatability of metabolism in vertebrates have mainly been focused on maximum metabolic rate ( Hayes and Chappell,1990 Chappell et al., 1995, 1996 Hayes and O'Connor, 1999),although daily energy expenditure( Speakman et al., 1994 Potti et al., 1999), field metabolic rate ( Berteaux et al.,1996) and resting metabolic rate (RMR Vézina and Thomas,2000) have also been studied. Because of this relationship with heritability, repeatability can give an indication of how effective natural selection will be in changing the trait. Repeatability provides important information by setting the upper limit for heritability ( Falconer and Mackay,1996), although this may not always be the case (see Dohm, 2002). Differences in BMR between species seem to be closely linked to different habitats and way of life( McNab, 1994 Bryant and Furness, 1995),indicating that BMR has been, and probably still is, a subject for natural selection ( Rezende et al.,2002 Furness,2003). A similar variation is also found in the standard metabolic rate(SMR) of invertebrates (e.g. Burness et al.,1998 Bech et al.,1999 Kvist and Lindström, 2001 Nilsson,2002 Lindström and Klaassen, 2003), and shows great variation both between and within species. ![]() BMR is one of the most widely measured physiological traits in birds (e.g. Basal metabolic rate (BMR) is defined as the rate of energy transformation in an endothermic organism at rest in a postabsorptive state,measured within its thermoneutral zone( IUPS Thermal Commission,2001). Metabolism reflects the cost of living of an organism, and energy is therefore thought to play an important role in shaping behaviour, ecology and physiology in animals ( Berteaux et al.,1996). Our results show that there are consistent between-individual variations in BMR on which natural selection can work, provided that this trait is heritable. In contrast to earlier studies on metabolism, our calculated repeatability ( R) did not change significantly from the short to the long period in either males( R from 0.501 to 0.465), females ( R from 0.413 to 0.522) or the pooled data ( R from 0.571 to 0.567). BMR residuals showed significant repeatabilities over a short (1.5 months) and a long (2.5 years) period for each sex as well as for both sexes pooled. ![]() BMR was measured six times over a 2.5 year period in captive zebra finches. We tested whether BMR was repeatable over a considerable time of the lifespan of a small passerine bird: the zebra finch Taeniopygia guttata. ![]() Repeatability indicates that the trait might be heritable and therefore a possible target for natural selection. ![]() Few studies have tested whether BMR shows stable between-individual variations. Basal metabolic rate (BMR) is a physiological trait believed to show adaptational changes. ![]()
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