Abstract
Basal metabolic rate (BMR) constitutes the lowest metabolic rate in a resting animal and is, therefore, considered to reflect the energetic cost of maintenance in endotherms. BMR is a reversible plastic trait that changes with environmental and ecological circumstances, albeit being heritable and susceptible to selection. Inter-individual variation within populations of small birds is substantial, and while many of the drivers of such variation have been identified, many remain unexplained. We studied winter BMR variation of juveniles over a 15-year period in a wild population of great tits Parus major at the northern border of their distribution. BMR during winter consistently changed between years, even after controlling for environmental factors, suggestive of a non-reversible developmental plasticity shaping the adult metabolic phenotype. BMR in cohorts of wintering great tits varied among winters as a response to minimum ambient temperatures experienced early in life, during the prehatching period. This developmental plasticity might be adaptive if temperatures experienced by growing embryos would metabolically prime them to an environment that they will likely encounter in future life. However, in line with a more unpredictable future climate, the risk of phenotype-environment mismatch is likely to lead to certain cohorts being poorly adapted to prevailing winter conditions, resulting in wider annual fluctuations in population size.
Highlights
Small birds acclimatize to winter conditions by improving their cold-endurance and thermogenic capacity, which correlates with a seasonal increase of internal energy reserves and basal metabolic rate (BMR) (Swanson 2010)
Of the factors that could be predicted to have a direct effect on the winter Basal metabolic rate (BMR) of first-year great tits, 4 were included in all 12 competing models (ΔAIC < 2) (Table 1; for a list of all models see Supplementary Table S2)
Individual winter BMR increased with decreasing minimum temperatures, suggesting a reversible effect of ambient temperature that has been commonly reported in other studies of tits (Bouwhuis et al 2011; Petit and Vézina 2014a; Broggi et al 2019)
Summary
Small birds acclimatize to winter conditions by improving their cold-endurance and thermogenic capacity, which correlates with a seasonal increase of internal energy reserves and basal metabolic rate (BMR) (Swanson 2010). The size and activity of these systems often vary in concert in relation to season (Zheng et al 2014a), BMR has been documented to respond faster to changes in ambient temperature (Dubois et al 2016) and show a different temporal adjustment to seasonal changes (Petit et al 2013) than maximal energy expenditure for thermogenesis. BMR is partly a genetically determined trait that remains individually repeatable (Versteegh et al 2008; Broggi et al 2009) with moderate but significant heritability in studies of passerine birds (Rønning et al 2007; Nilsson et al 2009; Tieleman et al 2009; Bushuev et al 2012; Mathot et al 2013) It is a labile trait (i.e. phenotypically flexible) that can change reversibly with environmental and ecological circumstances (see McKechnie (2008) and Swanson (2010) for reviews). Ambient temperature proved to be the main driver explaining interspecific variation in BMR in rodents (Naya et al 2013)
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