@phdthesis{e7e13d62-c990-482d-be73-a7eef6a99a16,
  abstract     = {{The rate of energy turnover in an animal, metabolic rate, is fundamental to life and determines the energy available for growth, reproduction, and survival. Thus, metabolic rate has been proposed as a mechanism linking physiology to broader ecological patterns. Numerous adaptive frameworks have been proposed to explain how selection acts on metabolic rate, how it relates to life histories, and the ecological significance of variation in metabolic rate. However, questions remain about how metabolic rate is determined, what constrains or limits metabolic rate, and how metabolic rate is connected to fitness. I aimed to understand how phenotypic variation and flexibility of metabolic rates manifest in both wild and captive birds, and to identify potential mechanisms and implications of those patterns. First, I investigated how the early environment affects the developmental trajectory of metabolic rate. Second, I measured the lower and upper limits to metabolic rates within individuals to determine if these traits were phenotypically correlated and to describe variation among individuals. Third, I measured the upper limit to metabolic rate at different points in time and in different environmental and ecological contexts, to determine if individual variation was consistent or flexible over time, or a combination of both. Fourth, I asked what sets the upper limits to metabolic rate. Fifth and finally, I investigated how individual variation in metabolic rates was associated with performance and fitness in different contexts. In this thesis, I show that the nest environment affects the development of metabolic rate. I found that minimal and maximal metabolic rates co-varied, although some of these patterns were also explained by variation in body mass. Maximum metabolic rate was both individually consistent between seasons and flexible, responding to changing environmental conditions and workload. During exercise, aerobic capacity was limited by increased air temperature, suggesting that the upper limits to metabolic rate shift depending on environmental conditions. As for links between metabolic performance and fitness, my results suggest that associations between metabolic phenotypes and fitness were dependent on the ecological and environmental context. Taken together, this thesis shows that metabolic traits can be phenotypically correlated, flexible, and repeatable, but their upper limits and links to fitness depend on the physiological and environmental context. Going forward, knowledge of physiological constraints, ecological consequences, and life history theory of metabolic rates could be leveraged to gain a better understanding of their ecological significance and how fitness outcomes could change in the context of the environment.}},
  author       = {{Engert, Elana Rae}},
  isbn         = {{978-91-90202-83-8}},
  keywords     = {{birds; ecophysiology; fitness; life history; metabolic rate; natural selection; phenotypic flexibility; thermoregulation; exercise capacity; seasonality}},
  language     = {{eng}},
  month        = {{10}},
  publisher    = {{Department of Biology, Lund University}},
  school       = {{Lund University}},
  title        = {{Variation and flexibility of metabolic performance in birds : Ecological significance and physiological constraints}},
  url          = {{https://lup.lub.lu.se/search/files/261065685/Thesis_Elana_Rae_Engert_LUCRIS.pdf}},
  year         = {{2026}},
}