Physiological and behavioural adaptations of two Australian finch species living in hot environments

<p dir="ltr">The current climate crisis poses great challenges for endotherms inhabiting arid and semi-arid ecosystems. In these environments, daylight temperatures and water scarcity are expected to intensify, leading to more frequent and intense heatwaves and droughts. Maximum temperatures are becoming increasingly extreme and diurnal passerines are at risk of exceeding their lethal temperature limit. The Australian zebra finch (<i>Taeniopygia castanotis</i>) stands out for its resilience to extreme heat, which combined with the relative ease with which it can be kept in the laboratory, offers the opportunity to experimentally test their adaptive responses to high temperatures in controlled environments. In this thesis I investigate a range of physiological and behavioural components underlying the response to heat of the zebra finch, and another close relative, the long-tailed finch <i>Poephila acuticauda</i>.</p><p dir="ltr">Before the experimental component of my thesis, I start by commenting on a recently published article, highlighting discrepancies with the authors’ modelling which omits key aspects of zebra finches’ ability to endure extreme heat. Then I present individualised zebra finches’ drinking behaviour data from a wild population to reveal a threefold drinking increase for a given population when comparing hot against warm days obtained in the same summer season. Next, I expose zebra finches to two artificial heat trials: one in the presence of water and one in absence of water, measuring birds’ body temperature and thermoregulatory behaviours and drinking patterns over time. This work characterised the birds’ behavioural and physiological plasticity in relation to water availability and demonstrate the variability in drinking, and the use of panting – an evaporative cooling behaviour – over time.</p><p dir="ltr">In addition, my work investigates the impact of mitochondrial respiration on thermoregulation. Through exposure to experimental heatwaves in the laboratory I characterised I expose adult zebra finches raised under two different thermal regimes to an artificial heatwave-like treatment measuring mitochondrial metabolism before and after heat exposure, identifying changes in mitochondrial performance related to rearing conditions and the heat treatment in adulthood. These findings corroborate the physiological plasticity of zebra finches in both the short-term and long-term with respect to the environment experienced during early development.</p><p dir="ltr">Finally, I experimentally test for possible differences in mitochondrial performance between two subspecies adapted to different climatological conditions in their wild distribution. To do so, I compare the mitochondrial metabolism of two subspecies of long-tailed finch – <i>P. a. hecki </i>and <i>P. a. acuticauda</i>, living in adjacent but climatologically different areas in Australia’s northern savannah – before and after an artificial heat treatment. Both subspecies show increased metabolism in response to the heat treatment in most of the metabolic stages measured, with <i>P. a. hecki </i>showing a significantly more plastic response revealing a role for mitochondrial performance in both climate adaptation and speciation processes.</p><p dir="ltr">Together, this research provides important new insights into the mechanisms underlying thermoregulation in endothermic species and opens up new avenues for research.</p>