Ecological & evolutionary role of resting metabolic rate in wild house sparrow populations

Abstract

Ongoing environmental changes cause worldwide population declines. To better preserve biodiversity we must understand factors that contribute to population viability and persistence. For populations to persist under environmental change, they must optimize fitness related traits to the environmental factors, either by adaptive evolution, or phenotypic plasticity, or both. Climate change is among the most prominent environmental changes, and global temperatures are projected to increase, together with changes in amount and intensity of precipitation. Metabolic rates are a group of traits that connect environmental temperatures with individual performance. Body mass (MB) is an important determinant for metabolic rates, but the connection between body mass is complicated and unresolved. Fitness consequences of metabolic rates are expected to change following climate change. Yet, how key metabolic traits may respond to environmental variation and selection, and the role of MB in shaping selection on metabolic traits is unclear but may be important for future population persistence. Here, I investigated the key metabolic traits basal metabolic rate (BMR) and minimum thermal conductance (Cmin), which describe key components relating resting metabolic rates to the environmental temperatures. I estimated the potential and constraints to adaptive evolution and explored the capacity for phenotypic plasticity in BMR, MB and Cmin in response to key weather variables. I quantified the combined fitness consequences of these traits combined in a multivariate quantitative genetic framework. I utilized a study system in wild populations of house sparrows (Passer domesticus) in Norway, whose lifestyle facilitate robust application of quantitative genetic methods. In Chapter I, I showed that house sparrows have moderate evolutionary potential in BMR and MB, and positive genetic correlation between them, all of which appear to be similar between populations. Thus, these traits appeared to be able to respond to selection, and the size of the response can be expected to be similar between them. In Chapter II, I show that following strong artificial selection on BMR, BMR did indeed respond, and MB showed a correlated response to the selection on BMR. Against expectations, the natural selection after the artificial selection did however generally not act against the artificial selection, but positive correlational selection acted to contribute to the positive genetic correlation between BMR and MB. In Chapter III, I showed that the RMR curve along a temperature gradient, described by BMR and Cmin, and MB, had capacity to be optimized to track environmental variation. Specifically, presence of evolutionary potential but no phenotypic plasticity in BMR and MB, combined with phenotypic plasticity but no evolutionary potential in Cmin, suggest that house sparrow populations may optimize individual resting metabolic rates to environmental variation within an individual’s lifespan mediated by plasticity in Cmin, and may evolve in response to selection mediated by the evolutionary potential in BMR and MB. In unmanipulated populations in Chapter III, selection appeared to be qualitatively different from findings in Chapter II. Specifically, selection acted stabilizing in a different direction, towards a particular combination of BMR and MB, and may have acted weakly against the genetic correlation between BMR and MB. Moreover, there appeared to be little selection on Cmin, possibly due to the large capacity to adjust and potentially optimize Cmin to track short term variation in weather. In total it appears Norwegian house sparrows are well adapted to the current thermal environment and appear to have sufficient capacity for optimizing their resting metabolic rates by a combination of evolutionary potential and phenotypic plasticity. However, to increase the generality of these findings, more taxa in wider ranges of environmental variation should be investigated, and limits to plasticity in Cmin, and the underlying physiological mechanisms remain unknown and should receive more attention in future research. And how multiple drivers of change may interact to shape future selection and influence evolutionary trajectories and population viability should be further investigated. This thesis contributes to describe the baseline to better understand how future climate change and other drivers of environmental changes may affect the role of metabolic rates in shaping population viability in endotherms.