Causes and consequences of individual variation in energy turnover rates

Abstract: Individual energy turnover rates can vary remarkably, even within a species. In this thesis I study the causes and consequences of this pronounced variation and show that individual energy turnover rates can be affected by a large number of factors. I here show that both the early conditions during the incubation period and prenatal levels of the hormone testosterone can have consequences on individual energy turnover rates later in life. Furthermore do I show that individual metabolic rate is highly heritable in the wild. This variation in individual metabolism can be of great importance since I also show that basal metabolic rate (BMR) can affect both survival and breeding start in blue tits (Cyanistes caeruleus). Interestingly did the effect on survival fluctuate due to the prevailing winter conditions, with a high BMR being beneficial in cold winters but not in mild. The fact that selection on BMR fluctuated between years can explain why we see such large variation in metabolic rates in the wild, especially since different selection pressures also seems to operate on BMR during different times of the year. Even though I here show that variation in BMR can affect individual fitness, very little is known about the physiological pathways that are responsible for this connection. Oxidative stress has been suggested to be one important link. However, in this thesis I can not present any evidence that this would be the case, since I found that protection against oxidative stress not was connected to variation in BMR. Furthermore did supplementation of carotenoids, a group of antioxidants widely used by birds, have a very limited effect metabolic intensity. An individual’s metabolic rate is however not static and many animals have evolved ways of conserving energy. In the last paper in this thesis, it is shown that blue tits use hypothermia to decrease their energy demands during demanding winter nights. This process was found to be very dynamic, governed by both short term, stochastic cues and more long term, predictable cues. Since the amplitude of hypothermia in this study was quite high (~ 5 ºC) despite the relatively mild winter, nocturnal hypothermia should definitely be regarded as an important energy conserving mechanisms, also at lower latitudes. Together, these studies show that metabolic rate is affected by a large number of factors and that variation in metabolic intensity can be of great importance for individual fitness.