Blood mercury concentrations in four sympatric gull species from South Western France: Insights from stable isotopes and biologging

Mercury (Hg) is a toxic trace element widely distributed in the environment, which particularly accumulates in top predators, including seabirds. Among seabirds, large gulls (Larus sp) are generalist feeders, foraging in both terrestrial and marine habitats, making them relevant bioindicators of local coastal Hg contamination. In the present study, we reported blood Hg concentrations in adults and chicks of four different gull species breeding on the French Atlantic coast: the European herring gull (Larus argentatus), the Lesser black-backed gull (L. fuscus), the Great black-backed gull (L. marinus) and the Yellow-legged gull (L. michahellis). We also investigated the potential role of foraging ecology in shaping Hg contamination across species, using the unique combination of three dietary tracers (carbon, nitrogen and sulfur stable isotopes) and biologging (GPS tracking). A high concentration of Hg was associated with high trophic position and a marine diet in gulls, which was corroborated by birds' space use strategy during foraging trips. Adults of all four species reached Hg concentrations above reported toxicity thresholds. Specifically, adults of Great black-backed gulls had a high trophic marine specialized diet and significantly higher Hg concentrations than the three other species. Blood Hg was 4-7 times higher in adults than in chicks, although chicks of all species received mainly marine and high trophic position prey, which is expected to be the cause of blood Hg concentrations of toxic concern. By using both stable isotopes and GPS tracking, the present study provides compelling insights on the main feeding habits driving Hg contamination in a seabird assemblage feeding in complex coastal environments.

Environmental conditions and male quality traits simultaneously explain variation of multiple colour signals in male lizards

Male lizards often display multiple pigment‐based and structural colour signals which may reflect various quality traits (e.g. performance, parasitism), with testosterone (T) often mediating these relationships. Furthermore, environmental conditions can explain colour signal variation by affecting processes such as signal efficacy, thermoregulation and camouflage. The relationships between colour signals, male quality traits and environmental factors have often been analysed in isolation, but simultaneous analyses are rare. Thus, the response of multiple colour signals to variation in all these factors in an integrative analysis remains to be investigated.

Here, we investigated how multiple colour signals relate to their information content, examined the role of T as a potential mediator of these relationships and how environmental factors explain colour signal variation.

We performed an integrative study to examine the covariation between three colour signals (melanin‐based black, carotenoid‐based yellow–orange and structural UV), physiological performance, parasitism, T levels and environmental factors (microclimate, forest cover) in male common lizards Zootoca vivipara from 13 populations.

We found that the three colour signals conveyed information on different aspects of male condition, supporting a multiple message hypothesis. T influenced only parasitism, suggesting that T does not directly mediate the relationships between colour signals and their information content. Moreover, colour signals became more saturated in forested habitats, suggesting an adaptation to degraded light conditions, and became generally brighter in mesic conditions, in contradiction with the thermal melanism hypothesis.

We show that distinct individual quality traits and environmental factors simultaneously explain variations of multiple colour signals with different production modes. Our study therefore highlights the complexity of colour signal evolution, involving various sets of selective pressures acting at the same time, but in different ways depending on colour production mechanism.

Seasonality impacts collective movements in a wild group-living bird

BACKGROUND

A challenge faced by animals living in groups with stable long-term membership is to effectively coordinate their actions and maintain cohesion. However, as seasonal conditions alter the distribution of resources across a landscape, they can change the priority of group members and require groups to adapt and respond collectively across changing contexts. Little is known about how stable group-living animals collectively modify their movement behaviour in response to environment changes, such as those induced by seasonality. Further, it remains unclear how environment-induced changes in group-level movement behaviours might scale up to affect population-level properties, such as a population's footprint.

METHODS

Here we studied the collective movement of each distinct social group in a population of vulturine guineafowl (Acryllium vulturinum), a largely terrestrial and non-territorial bird. We used high-resolution GPS tracking of group members over 22 months, combined with continuous time movement models, to capture how and where groups moved under varying conditions, driven by seasonality and drought.

RESULTS

Groups used larger areas, travelled longer distances, and moved to new places more often during drier seasons, causing a three-fold increase in the area used at the population level when conditions turned to drought. By contrast, groups used smaller areas with more regular movements during wetter seasons.

CONCLUSIONS

The consistent changes in collective outcomes we observed in response to different environments raise questions about the role of collective behaviour in facilitating, or impeding, the capacity for individuals to respond to novel environmental conditions. As droughts will be occurring more often under climate change, some group living animals may have to respond to them by expressing dramatic shifts in their regular movement patterns. These shifts can have consequences on their ranging behaviours that can scale up to alter the footprints of animal populations.

Intense nocturnal warming alters growth strategies, colouration and parasite load in a diurnal lizard

In the past decades, nocturnal temperatures have been playing a disproportionate role in the global warming of the planet. Yet, they remain a neglected factor in studies assessing the impact of global warming on natural populations. Here, we question whether an intense augmentation of nocturnal temperatures is beneficial or deleterious to ectotherms. Physiological performance is influenced by thermal conditions in ectotherms and an increase in temperature by only 2°C is sufficient to induce a disproportionate increase in metabolic expenditure. Warmer nights may expand ectotherms' species thermal niche and open new opportunities for prolonged activities and improve foraging efficiency. However, increased activity may also have deleterious effects on energy balance if exposure to warmer nights reduces resting periods and elevates resting metabolic rate. We assessed whether warmer nights affected an individual's growth, dorsal skin colouration, thermoregulation behaviour, oxidative stress status and parasite load by exposing yearling common lizards (Zootoca vivipara) from four populations to either ambient or high nocturnal temperatures for approximately 5 weeks. Warmer nocturnal temperatures increased the prevalence of ectoparasitic infestation and altered allocation of resources towards structural growth rather than storage. We found no change in markers for oxidative stress. The thermal treatment did not influence thermal preferences, but influenced dorsal skin brightness and luminance, in line with a predicted acclimation response in colder environments to enhance heat gain from solar radiation. Altogether, our results highlight the importance of considering nocturnal warming as an independent factor affecting ectotherms' life history in the context of global climate change. ​.

Short-term changes in air humidity and water availability weakly constrain thermoregulation in a dry-skinned ectotherm

Thermoregulation is critical for ectotherms as it allows them to maintain their body temperature close to an optimum for ecological performance. Thermoregulation includes a range of behaviors that aim at regulating body temperature within a range centered around the thermal preference. Thermal preference is typically measured in a thermal gradient in fully-hydrated and post-absorptive animals. Short-term effects of the hydric environment on thermal preferences in such set-ups have been rarely quantified in dry-skinned ectotherms, despite accumulating evidence that dehydration might trade-off with behavioral thermoregulation. Using experiments performed under controlled conditions in climatic chambers, we demonstrate that thermal preferences of a ground-dwelling, actively foraging lizard (Zootoca vivipara) are weakly decreased by a daily restriction in free-standing water availability (less than 0.5°C contrast). The influence of air humidity during the day on thermal preferences depends on time of the day and sex of the lizard, and is generally weaker than those of of free-standing water (less than 1°C contrast). This shows that short-term dehydration can influence, albeit weakly, thermal preferences under some circumstances in this species. Environmental humidity conditions are important methodological factors to consider in the analysis of thermal preferences.

Temperature as a constraint on the timing and duration of African elephant foraging trips

In arid and semiarid environments, water is a key resource that is limited in availability. During the dry season, perennial water sources such as water pans often are far apart and shape the daily movement routines of large herbivores. In hot environments, endotherms face a lethal risk of overheating that can be buffered by evaporative cooling. Behavioral adjustments are an alternative way to reduce thermal constraints on the organism. The trade-off between foraging and reaching water pans has been studied widely in arid environments; however, few studies have looked into how ambient temperature shapes individual trips between two visits to water. In this study, we tracked during the dry season the movement of eight GPS-collared African elephants (Loxodonta africana) cows from different herds in Hwange National Park, Zimbabwe. This species, the largest extant terrestrial animal, is particularly sensitive to heat due to its body size and the absence of sweat glands. We show that most foraging trips depart from water at nightfall, lowering the average temperature experienced during walking. This pattern is conserved across isolated elephant populations in African savannas. We also observed that higher temperatures at the beginning of the trip lead to shorter trips. We conclude that elephants adjust the timing of foraging trips to reduce the thermal constraints, arguing that further considerations of the thermal landscape of endotherms are important to understand their ecology.

Short-term change in water availability influences thermoregulation behaviours in a dry-skinned ectotherm

Mechanistic models of terrestrial ectotherms predict that climate warming will induce activity restriction due to heat stress and loss of shade, leading to the extinction of numerous populations. Such models rely on the assumption that activity patterns are dictated by simple temperature thresholds independent of changes in water availability. However, changes in water availability may further influence thermoregulation behaviour of ectotherms through dehydration risk perception, changes in water balance or changes in microclimatic conditions. Here, we experimentally assess the interactive effects of thermal conditions and water availability on activity patterns, shade selection and thermoregulation efficiency in a model ectothermic species. Thermoregulation behaviour of adult common lizards Zootoca vivipara was monitored in outdoor mesocosms as we manipulated water availability, providing water as mist in the morning and free-standing water during the daytime. We recorded operative temperatures and micro-meteorological conditions to infer thermal constraints and dehydration risk. Activity and shade selection were better predicted by continuous changes in thermal conditions and dehydration risk, respectively, than by threshold functions. In addition, water supplementation increased activity in males and reduced shade selection in both sexes, most probably as a behavioural response to the perception of a stronger dehydration risk. Water supplementation also influenced the thermal quality of the environment, which in turn altered daily activity patterns and thermoregulation statistics. This demonstrates that dual effects of heat and water stress on activity patterns may lead to stronger activity restriction as a result of climate change than currently predicted.

Acclimation to Water Restriction Implies Different Paces for Behavioral and Physiological Responses in a Lizard Species

Chronic changes in climate conditions may select for acclimation responses in terrestrial animals living in fluctuating environments, and beneficial acclimation responses may be key to the resilience of these species to global changes. Despite evidence that climate warming induces changes in water availability, acclimation responses to water restriction are understudied compared with thermal acclimation. In addition, acclimation responses may involve different modes, paces, and trade-offs between physiological and behavioral traits. Here, we tested the dynamical acclimation responses of a dry-skinned terrestrial ectotherm to chronic water restriction. Yearling common lizards (Zootoca vivipara) were exposed to sublethal water restriction during 2 mo of the summer season in laboratory conditions, then released in outdoor conditions for 10 additional months. Candidate behavioral (exploration, basking, and thermal preferences) and physiological (metabolism at rest and standard water loss rate) traits potentially involved in the acclimation response were measured repeatedly during and after water restriction. We observed a sequential acclimation response in water-restricted animals (yearlings spent less time basking during the first weeks of water deprivation) that was followed by delayed sex-specific physiological consequences of the water restriction during the following months (thermal depression in males and lower standard evaporative water loss rates in females). Despite short-term negative effects of water restriction on body growth, annual growth, survival, and reproduction were not significantly different between water-restricted and control yearlings. This demonstrates that beneficial acclimation responses to water restriction involve both short-term flexible behavioral responses and delayed changes in thermal and water biology traits.

Chronic water restriction triggers sex-specific oxidative stress and telomere shortening in lizards

Animals use a variety of strategies to avoid acute dehydration and death. Yet, how chronic exposure to sub-lethal dehydration may entail physiological and fitness costs remains elusive. In this study, we experimentally tested if water restriction causes increased oxidative stress (OS) and telomere length (TL) shortening, two well-described mediators of environment-fitness relationships. We exposed 100 yearling female and male common lizards (Zootoca vivipara) either to a 51-day period of water restriction or to water ad libitum, followed by 45 days in common garden outdoor conditions. We measured the kinetic changes in OS and TL and found that water-restricted males had enhanced antioxidant defences and decreased oxidative damage at day 36, whereas females did not immediately respond. A month and a half after water restriction, both sexes experienced a drop in antioxidant capacity but only males exhibited significant TL shortening. In the following 3 years, we found that lizards with longer initial TL and those who maintained stronger antioxidant defences experienced higher longevity, irrespective of sex and water restriction. Together, these results unravelled sex-specific responses to water restriction, with potential applications in better understanding the physiological costs of increasing summer droughts as a result of global climate change.