Periodic Reporting for period 2 - THERMALIMAGING STATE (Thermal imaging to assess individual physiological state in wild animals)
Période du rapport: 2024-02-01 au 2025-01-31
Understanding variation in ability to survive and pass on genes (fitness) is vital for evolutionary biology and conservation ecology. Physiological state is especially important in this respect, because physiological processes are dynamically adjusted to maximise fitness in response to environmental changes - both predictable rhythmic environmental variation like the cycle of day and night and unpredictable challenges, such as an attack by a predator. As such, assessing physiological state is crucial to uncovering the fundamental reasons why some individuals perish, while others prosper, and therefore, to predicting which populations might face risk of extinction. However, assessing physiological state in wild animals still usually means subjects need to be trapped and handled, so blood can be sampled or measurement devices can be implanted or attached. While useful, these kinds of invasive techniques interrupt natural behaviour, can cause bias toward trappable individuals, and may alter subsequent performance. Additionally, welfare limits on repeated invasive sampling also restrict tracking of responses over time. This project sought to provide an alternative method for measuring physiological state by instead targeting body surface temperatures (BST). BST are expected to relate to underlying physiological processes in a predictable way, and can be measured non-invasively using infrared thermal imaging. BST in endothermic species (which generate heat internally, e.g. birds, mammals) have previously been shown to correlate with acute stress and energy metabolism. But, the mechanisms linking these traits are not fully understood, and have not been experimentally validated. Our overall objectives were to combine use of heart rate monitoring backpacks and thermal imaging to experimentally characterise the relationships between stress/energetic physiological states and body surface temperatures in the lab. This data was then used to inform experimental field tests aiming to demonstrate how body surface temperature responses to stress and energetic challenges are connected with fitness in wild animals.
The first two years of the project were predominantly spent comparing body surface temperature and cardiac responses to acute stress and energetic challenges in captive house sparrows. First, we subjected birds to the acute stress of capture and handling. We found that during acute stress, heart rate increased and intervals between successive heart beats (heart rate variability) became more regular, indicating that the physiological system which drives the ‘fight or flight’ response (the sympathetic nervous system) was activated. At the same time, eye region and bill surface temperatures dropped, due to a reduction of blood flow to the body surface (vasoconstriction). After the stressor had ceased, all values returned to approximately pre-stress levels, demonstrating that body surface temperatures track sympathetic nervous system activation. These results were published in the Journal of Experimental Biology, and presented at the Society of Integrative and Comparative Biology Annual Meeting (2023). Additionally, promotion of the publication on Twitter reached up to 164,000 users. For the energetics experiments, we compared eye region and bill surface temperatures from undisturbed birds with metabolic rate (estimated from heart rate) between moulting and non-moulting individuals, as feather regrowth is a considerable energetic challenge. In addition, we made measurements with and without food restriction within each group to assess whether body surface temperature responses to energetic challenge might be generalisable. In the final year, we compared body surface temperature responses to acute stress and differing energetic states with breeding success in wild great tits. We subjected parents using artificial nest boxes to the acute stress of great tit alarm call playback. And, we compared baseline body surface temperatures of parents between two areas with differing prey availability. Body surface temperatures were measured within the nest box using a custom camera setup, while parents were provisioning their offspring. Analysis and publication of data from the second-year lab energetic experiments and the final year field tests has been delayed due to researcher illness (although work will continue once the researcher is recovered).
As well as these practical studies, we also established the generalisability of links between body surface temperatures and physiology across settings and populations via a systematic review. Our review suggested thermoregulatory, metabolic and acute stress body surface temperature (BST) responses are likely to be broadly generalisable. However, BST dynamics during immune activation likely depend on discrete ranges of environmental conditions. These results will be published shortly in the journal Biological Reviews, and subsequently promoted on Bluesky.
As well as these practical studies, we also established the generalisability of links between body surface temperatures and physiology across settings and populations via a systematic review. Our review suggested thermoregulatory, metabolic and acute stress body surface temperature (BST) responses are likely to be broadly generalisable. However, BST dynamics during immune activation likely depend on discrete ranges of environmental conditions. These results will be published shortly in the journal Biological Reviews, and subsequently promoted on Bluesky.
Our work, unequivocally showing that body surface temperature dynamics during acute stress track sympathetic nervous system activation is the first to demonstrate the mechanistic link between these traits. Also, the scale of body surfaced temperature changes (particularly in the bill) was sufficient to facilitate detection with less sensitive, but more economical thermal imaging cameras. Consequently, the potential impacts of this research in providing a transformative and increasingly cost-effective new non-invasive tool for assessment of physiological stress in wild birds and mammals are considerable. The acute stress response is the major evolutionary adaptation to maximise fitness in the face of unpredictable environmental challenges. But, we still know surprisingly little about how variation in the acute stress response influences fitness in wild animals, mainly because it’s so difficult to assess in natural habitats. Our techniques will not only be valuable to fundamental researchers aiming to plug this knowledge gap, but also in numerous applied circumstances, such as identifying or monitoring at-risk populations. We expect the results of our energetic validations and field tests to be similarly clear, and prove correspondingly useful in expanding the toolkit available to investigate how wild animals use physiological mechanisms to cope with, and adapt to environmental change. Equally, we expect publication of the final year field tests will act as an inspirational demonstration of our novel methods, further promoting their take-up by researchers and conservation ecology practitioners.