Thermal Imaging to Study Stress Non-invasively in Unrestrained Birds

Stress, a central concept in biology, describes a suite of emergency responses to challenges. Among other responses, stress leads to a change in blood ...

University of Glasgow

Using featural cues such as colour to identify ephemeral food can increase foraging efficiency. Featural cues may change over time however; therefore, animals should use spatial cues to relocate food that occurs in a temporally stable position. We tested this hypothesis by measuring the cue preferences of captive greenfinches Carduelis chloris when relocating food hidden in a foraging tray. In these standardised associative learning trials, greenfinches favoured colour cues when returning to a foraging context that they had encountered before only once ("one-trial test") but switched to spatial cues when they had encountered that scenario on ten previous occasions ("repeated-trial test"). We suggest that repeated encounters generated a context in which individuals had a prior expectation of temporal stability, and hence context-dependent cue selection. Next, we trained birds to find food in the absence of colour cues but tested them in the presence of visual distracters. Birds were able to learn spatial cues after one encounter, but only when visual distracters were identical in colouration. When a colourful distracter was present in the test phase, cue selection was random. Unlike the first one-trial test, birds were not biased towards this colourful visual distracter. Together, these results suggest that greenfinches are able to learn both cue types, colour cue biases represent learning, not simply distraction, and spatial cues are favoured over colour cues only in temporally stable contexts.

Colour cues or spatial cues? Context-dependent preferences in the European greenfinch (Carduelis chloris).

Where behavioural responses differ consistently between individuals, this is termed 'personality'. There is the suggestion, but with little supporting data, that personality traits reflect underlying variation in physiology. Here, we tested whether greenfinches Carduelis chloris differing in personality traits differed in various plasma indices of oxidative profile: antioxidant capacity (OXY), pro-oxidant status (reactive oxygen metabolites, ROMs), oxidative stress (OS) and an end-product of oxidative damage: malondialdehyde (MDA). We measured two personality traits: neophobia (latency to approach food near novel objects) and object exploration (latency to approach novel objects). These traits were uncorrelated. ROMs, OXY, OS and MDA were also uncorrelated with each other. Highly neophobic birds had lower OXY, higher ROMs and higher OS than less neophobic birds. Fast exploring birds had higher OXY than slow explorers, but did not differ in ROMs or OS. Variation in MDA was described by a quadratic relationship with neophobia: birds with extremely high or low neophobia had lower MDA than birds with intermediate neophobia, despite highly neophobic birds exhibiting lower OS than intermediately neophobic birds. Additively in that model, fast explorers had lower MDA than slower explorers. To conclude: first, personality types can differ in oxidative profile. Second, although physiological differences (e.g. hormonal stress responsiveness) between personality types generally range along a linear continuum, physiological costs may not. Finally, relationships with oxidative profile differed between neophobia and object exploration. Understanding how oxidative profile and thus physiological costs vary within and between personality traits may explain how differences in personality traits can predict fitness.

Oxidative profile varies with personality in European greenfinches.

Exposure to stressors early in life is associated with faster ageing and reduced longevity. One important mechanism that could underlie these late life effects is increased telomere loss. Telomere length in early post-natal life is an important predictor of subsequent lifespan, but the factors underpinning its variability are poorly understood. Recent human studies have linked stress exposure to increased telomere loss. These studies have of necessity been non-experimental and are consequently subjected to several confounding factors; also, being based on leucocyte populations, where cell composition is variable and some telomere restoration can occur, the extent to which these effects extend beyond the immune system has been questioned. In this study, we experimentally manipulated stress exposure early in post-natal life in nestling European shags (Phalacrocorax aristotelis) in the wild and examined the effect on telomere length in erythrocytes. Our results show that greater stress exposure during early post-natal life increases telomere loss at this life-history stage, and that such an effect is not confined to immune cells. The delayed effects of increased telomere attrition in early life could therefore give rise to a 'time bomb' that reduces longevity in the absence of any obvious phenotypic consequences early in life.

Stress exposure in early post-natal life reduces telomere length: an experimental demonstration in a long-lived seabird.

The transfer of antibodies from mother to offspring is key to protecting young animals from disease and can have a major impact on responses to infection and offspring fitness. Such maternal effects also allow young that may be exposed to disease in early life to focus resources on growth and development at this critical period of development. Maternally transferred antibodies are therefore an important source of phenotypic variation in host phenotype as well as influencing host susceptibility and tolerance to infection across generations. It has previously been assumed the transfer of antibodies is passive and invariant and reflects the level of circulating antibody in the mother at the time of transfer. However, whether females may vary in the relative amount of protection transferred to offspring has seldom been explored.

Factors affecting the levels of protection transferred from mother to offspring following immune challenge.

Many wildlife species forage on sewage-contaminated food, for example, at wastewater treatment plants and on fields fertilized with sewage sludge. The resultant exposure to human pharmaceuticals remains poorly studied for terrestrial species. On the basis of predicted exposure levels in the wild, we administered the common antidepressant fluoxetine (FLUOX) or control treatment via prey to wild-caught starlings (Sturnus vulgaris) for 22 weeks over winter. To investigate responses to fluoxetine, birds were moved from their group aviaries into individual cages for 2 days. Boldness, exploration and activity levels showed no treatment effects but controls and FLUOX birds habituated differently to isolation in terms of the concentration of corticosterone (CORT) metabolites in faeces. The controls that excreted higher concentrations of CORT metabolites on day 1 lost more body mass by day 2 of isolation than those which excreted lower levels of CORT metabolites. CORT metabolites and mass loss were unrelated in FLUOX birds. When we investigated the movements of birds in their group aviaries, we found the controls made a higher frequency of visits to food trays than FLUOX birds around the important foraging periods of sunrise and sunset, as is optimal for wintering birds. Although individual variability makes interpreting the sub-lethal endpoints measured challenging, our data suggest that fluoxetine at environmentally relevant concentrations can significantly alter behaviour and physiology.

Behavioural and physiological responses of birds to environmentally relevant concentrations of an antidepressant.

Parasites play key ecological and evolutionary roles through the costs they impose on their host. In wild populations, the effect of parasitism is likely to vary considerably with environmental conditions, which may affect the availability of resources to hosts for defense. However, the interaction between parasitism and prevailing conditions is rarely quantified. In addition to environmental variation acting on hosts, individuals are likely to vary in their response to parasitism, and the combined effect of both may increase heterogeneity in host responses. Offspring hierarchies, established by parents in response to uncertain rearing conditions, may be an important source of variation between individuals. Here, we use experimental antiparasite treatment across 5 years of variable conditions to test how annual population productivity (a proxy for environmental conditions) and parasitism interact to affect growth and survival of different brood members in juvenile European shags (Phalacrocorax aristotelis). In control broods, last-hatched chicks had more plastic growth rates, growing faster in more productive years. Older siblings grew at a similar rate in all years. Treatment removed the effect of environment on last-hatched chicks, such that all siblings in treated broods grew at a similar rate across environmental conditions. There were no differences in nematode burden between years or siblings, suggesting that variation in responses arose from intrinsic differences between chicks. Whole-brood growth rate was not affected by treatment, indicating that within-brood differences were driven by a change in resource allocation between siblings rather than a change in overall parental provisioning. We show that gastrointestinal parasites can be a key component of offspring's developmental environment. Our results also demonstrate the value of considering prevailing conditions for our understanding of parasite effects on host life-history traits. Establishing how environmental conditions shape responses to parasitism is important as environmental variability is predicted to increase.

Parasitism in early life: environmental conditions shape within-brood variation in responses to infection.

Parasitic infection has a direct physiological cost to hosts but may also alter how hosts interact with other individuals in their environment. Such indirect effects may alter both host fitness and the fitness of other individuals in the host's social network, yet the relative impact of direct and indirect effects of infection are rarely quantified. During reproduction, a host's social environment includes family members who may be in conflict over resource allocation. In such situations, infection may alter how resources are allocated, thereby redistributing the costs of parasitism between individuals. Here, we experimentally reduce parasite burdens of parent and/or nestling European shags (Phalacrocorax aristotelis) infected with Contracaecum nematodes in a factorial design, then simultaneously measure the impact of an individual's infection on all family members. We found no direct effect of infection on parent or offspring traits but indirect effects were detected in all group members, with both immediate effects (mass change and survival) and longer-term effects (timing of parents' subsequent breeding). Our results show that parasite infection can have a major impact on individuals other than the host, suggesting that the effect of parasites on population processes may be greater than previously thought.

Indirect effects of parasitism: costs of infection to other individuals can be greater than direct costs borne by the host.

Acute stress triggers peripheral vasoconstriction, causing a rapid, short-term drop in skin temperature in homeotherms. We tested, for the first time, whether this response has the potential to quantify stress, by exhibiting proportionality with stressor intensity. We used established behavioural and hormonal markers: activity level and corticosterone level, to validate a mild and more severe form of an acute restraint stressor in hens (Gallus gallus domesticus). We then used infrared thermography (IRT) to non-invasively collect continuous temperature measurements following exposure to these two intensities of acute handling stress. In the comb and wattle, two skin regions with a known thermoregulatory role, stressor intensity predicted the extent of initial skin cooling, and also the occurrence of a more delayed skin warming, providing two opportunities to quantify stress. With the present, cost-effective availability of IRT technology, this non-invasive and continuous method of stress assessment in unrestrained animals has the potential to become common practice in pure and applied research.

Katherine Herborn

Institute of Biodiversity,
Animal Health and Comparative Medicine,
College of Medical,
Veterinary & Life Sciences,
Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary & Life Sciences

University of Glasgow

Thermal Imaging to Study Stress Non-invasively in Unrestrained Birds