Increased activity correlates with reduced ability to mount immune defenses to endotoxin in zebra finches

Associating with kin selects for disease resistance and against tolerance

Behavioural and physiological resistance are key to slowing epidemic spread. We explore the evolutionary and epidemic consequences of their different costs for the evolution of tolerance that trades off with resistance. Behavioural resistance affects social cohesion, with associated group-level costs, while the cost of physiological resistance accrues only to the individual. Further, resistance, and the associated reduction in transmission, benefit susceptible hosts directly, whereas infected hosts only benefit indirectly, by reducing transmission to kin. We therefore model the coevolution of transmission-reducing resistance expressed in susceptible hosts with resistance expressed in infected hosts, as a function of kin association, and analyse the effect on population-level outcomes. Using parameter values for guppies, Poecilia reticulata, and their gyrodactylid parasites, we find that: (1) either susceptible or infected hosts should invest heavily in resistance, but not both; (2) kin association drives investment in physiological resistance more strongly than in behavioural resistance; and (3) even weak levels of kin association can favour altruistic infected hosts that invest heavily in resistance (versus selfish tolerance), eliminating parasites. Overall, our finding that weak kin association affects the coevolution of infected and susceptible investment in both behavioural and physiological resistance suggests that kin selection may affect disease dynamics across systems.

Shifts in immune responses of an invasive alien species: A test of the evolution of increased competitive ability hypothesis using American Eastern gray squirrels in Italy

Based on the Evolution of Increased Competitive Ability (EICA) hypothesis, a reduced investment in immunity, consequent to parasite loss, could partly explain the success of invasive alien species. We investigated variation in parasite load and immune responses of alien Eastern gray squirrels (Sciurus carolinensis) along the invasion wave of an expanding population. We first verified by fecal analyses that 1) parasite abundance decreased moving from the core towards the invasion front. Next, we used multiple measures of immunity to investigate whether, in response to the lower parasite pressure, individuals at the invasion front 2) dampened their costly inflammatory response, and 3) increased their investment in less expensive acquired immunity. We first explored variation in hematological variables related either to the inflammatory or the acquired response. On a subset of individuals, we carried out ex vivo cell cultures to analyse the basal expression of MHC class II genes and the expression of TNF-α genes in response to an immune challenge. Platelet counts and TNF-α expression suggested higher inflammation in individuals living at the invasion core, whereas parameters associated with an acquired response (lymphocyte counts and MHC II expression by spleen cells), conversely, were higher in squirrels at the front. Overall, our results suggest a shift between different immune strategies along the invasion wave, supporting a reduced investment in costly inflammatory responses and an increased investment in acquired immunity in individuals at the expanding edge of the range, which are subjected to high selective pressures for dispersal and reproduction.

The social environment alters neural responses to a lipopolysaccharide challenge

Sick animals display drastic changes in their behavioral patterns, including decreased activity, decreased food and water intake, and decreased interest in social interactions. These behaviors, collectively called "sickness behaviors", can be socially modulated. For example, when provided with mating opportunities, males of several species show reduced sickness behaviors. While the behavior is known to change, how the social environment affects neural molecular responses to sickness is not known. Here, we used a species, the zebra finch, Taeniopygia guttata, where males have been shown to decrease sickness behaviors when presented with novel females. Using this paradigm, we obtained samples from three brain regions (the hypothalamus, the bed nucleus of the stria terminalis, and the nucleus taeniae) from lipopolysaccharide (LPS) or control treated males housed under four different social environments. Manipulation of the social environment rapidly changed the strength and co-expression patterns of the neural molecular responses to the immune challenge in all brain regions tested, therefore suggesting that the social environment plays a significant role in determining the neural responses to an infection. In particular, brains of males paired with a novel female showed muted immune responses to LPS, as well as altered synaptic signaling. Neural metabolic activity in response to the LPS challenge was also affected by the social environment. Our results provide new insights into the effects of the social environment on brain responses to an infection, thereby improving our understanding of how the social environment can affect health.

Changes in Body Surface Temperature Play an Underappreciated Role in the Avian Immune Response

AbstractFever and hypothermia are well-characterized components of systemic inflammation. However, our knowledge of the mechanisms underlying such changes in body temperature is largely limited to rodent models and other mammalian species. In mammals, high dosages of an inflammatory agent (e.g., lipopolysaccharide [LPS]) typically leads to hypothermia (decrease in body temperature below normothermic levels), which is largely driven by a reduction in thermogenesis and not changes in peripheral vasomotion (i.e., changes in blood vessel tone). In birds, however, hypothermia occurs frequently, even at lower dosages, but the thermoeffector mechanisms associated with the response remain unknown. We immune challenged zebra finches (Taeniopygia guttata) with LPS, monitored changes in subcutaneous temperature and energy balance (i.e., body mass, food intake), and assessed surface temperatures of and heat loss across the eye region, bill, and legs. We hypothesized that if birds employ thermoregulatory mechanisms similar to those of similarly sized mammals, LPS-injected individuals would reduce subcutaneous body temperature and maintain constant surface temperatures compared with saline-injected individuals. Instead, LPS-injected individuals showed a slight elevation in body temperature, and this response coincided with a reduction in peripheral heat loss, particularly across the legs, as opposed to changes in energy balance. However, we note that our interpretations should be taken with caution owing to small sample sizes within each treatment. We suggest that peripheral vasomotion, allowing for heat retention, is an underappreciated component of the sickness-induced thermoregulatory response of small birds.

Greater chronic morbidity is associated with greater fatigue in six countries

Background and objectives

Human susceptibility to chronic non-communicable disease may be explained, in part, by mismatches between our evolved biology and contemporary environmental conditions. Disease-induced fatigue may function to reduce physical activity during acute infection, thereby making more energy available to mount an effective immune response. However, fatigue in the context of chronic disease may be maladaptive because long-term reductions in physical activity increase risks of disease progression and the acquisition of additional morbidities. Here, we test whether cumulative chronic morbidity is associated with subjective fatigue.

Methodology

We constructed a cumulative chronic morbidity score using self-reported diagnoses and algorithm-based assessments, and a subjective fatigue score based on four questionnaire items using cross-sectional survey data from the Study on global AGEing and adult health, which features large samples of adults from six countries (China, Ghana, India, Mexico, Russia and South Africa).

Results

In a mixed-effects linear model with participants nested in countries (N = 32 455), greater cumulative chronic morbidity is associated with greater subjective fatigue (β = 0.34, SE = 0.005, P < 2e−16). This association replicates within each country and is robust to adjustment for key sociodemographic and physical covariates (sex, age, household wealth, physical function score, habitual physical activity, BMI and BMI²).

Conclusions and implications

Fatigue is a common but perhaps maladaptive neuropsychological response to chronic morbidity. Disease-induced fatigue may mediate a self-perpetuating cycle, in which chronic morbidity reduces physical activity, and less physical activity increases cumulative chronic morbidity. Longitudinal research is needed to test whether chronic morbidity, fatigue and physical activity form a cyclical feedback loop.

Lay Summary: Fatigue during acute illness may promote recovery, but persistent fatigue in the context of chronic disease may make matters worse. We present evidence from six countries that more chronic disease is associated with more fatigue. This fatigue may reduce physical activity, which increases risks of acquiring additional chronic health problems.

Why we don't move: The importance of somatic maintenance and resting

A compelling ecological theory of movement and vigor must explain why humans and other animals spend so much time not moving. When we rest, our somatic maintenance systems continue to work. When our somatic maintenance requirements increase, we place greater subjective value on resting. To explain variation in movement and vigor, we must account for the subjective value of resting.

Energy expenditure across immune challenge severities in a lizard: consequences for innate immunity, locomotor performance and oxidative status

Reptiles, like other vertebrates, rely on immunity to defend themselves from infection. The energetic cost of an immune response is liable to scale with infection severity, prompting constraints on other self-maintenance traits if immune prioritization exceeds energy budget. In this study, adult male side-blotched lizards (Uta stansburiana) were injected with saline (control) or high (20 µg g-1 body mass) or low (10 µg g-1 body mass) concentrations of lipopolysaccharide (LPS) to simulate bacterial infections of discrete severities. The costs and consequences of the immune response were assessed through comparisons of change in resting metabolic rate (RMR), energy metabolites (glucose, glycerol, triglycerides), innate immunity (bactericidal ability), sprint speed and oxidative status (antioxidant capacity, reactive oxygen metabolites). High-LPS lizards had the lowest glucose levels and greatest sprint reductions, while their RMR and bactericidal ability were similar to those of control lizards. Low-LPS lizards had elevated RMR and bactericidal ability, but glucose levels and sprint speed changes between those of high-LPS and control lizards. Levels of glycerol, triglycerides, reactive oxygen metabolites and antioxidant capacity did not differ by treatment. Taken together, energy expenditure for the immune response varies in a non-linear fashion with challenge severity, posing consequences for performance and self-maintenance processes in a reptile.

Isotopic niche variation in Tasmanian devils Sarcophilus harrisii with progression of devil facial tumor disease

Devil facial tumor disease (DFTD) is a transmissible cancer affecting Tasmanian devils Sarcophilus harrisii. The disease has caused severe population declines and is associated with demographic and behavioral changes, including earlier breeding, younger age structures, and reduced dispersal and social interactions. Devils are generally solitary, but social encounters are commonplace when feeding upon large carcasses. DFTD tumors can disfigure the jaw and mouth and so diseased individuals might alter their diets to enable ingestion of alternative foods, to avoid conspecific interactions, or to reduce competition. Using stable isotope analysis (δ13C and δ15N) of whiskers, we tested whether DFTD progression, measured as tumor volume, affected the isotope ratios and isotopic niches of 94 infected Tasmanian devils from six sites in Tasmania, comprising four eucalypt plantations, an area of smallholdings and a national park. Then, using tissue from 10 devils sampled before and after detection of tumors and 8 devils where no tumors were detected, we examined whether mean and standard deviation of δ13C and δ15N of the same individuals changed between healthy and diseased states. δ13C and δ15N values were generally not related to tumor volume in infected devils, though at one site, Freycinet National Park, δ15N values increased significantly as tumor volume increased. Infection with DFTD was not associated with significant changes in the mean or standard deviation of δ13C and δ15N values in individual devils sampled before and after detection of tumors. Our analysis suggests that devils tend to maintain their isotopic niche in the face of DFTD infection and progression, except where ecological conditions facilitate a shift in diets and feeding behaviors, demonstrating that ecological context, alongside disease severity, can modulate the behavioral responses of Tasmanian devils to DFTD.

Sickness behaviors across vertebrate taxa: proximate and ultimate mechanisms

There is nothing like a pandemic to get the world thinking about how infectious diseases affect individual behavior. In this respect, sick animals can behave in ways that are dramatically different from healthy animals: altered social interactions and changes to patterns of eating and drinking are all hallmarks of sickness. As a result, behavioral changes associated with inflammatory responses (i.e. sickness behaviors) have important implications for disease spread by affecting contacts with others and with common resources, including water and/or sleeping sites. In this Review, we summarize the behavioral modifications, including changes to thermoregulatory behaviors, known to occur in vertebrates during infection, with an emphasis on non-mammalian taxa, which have historically received less attention. We then outline and discuss our current understanding of the changes in physiology associated with the production of these behaviors and highlight areas where more research is needed, including an exploration of individual and sex differences in the acute phase response and a greater understanding of the ecophysiological implications of sickness behaviors for disease at the population level.

Urinary Cortisol Increases During a Respiratory Outbreak in Wild Chimpanzees

Abstract: In mammals, the excretion of cortisol can provide energy toward restoring homeostasis and is a major component of the stress response. However, chronically elevated cortisol levels also have suppressive effects on immune function. As mounting an immune response is energetically costly, sick individuals may conserve energy by exhibiting certain sickness behaviors, such as declining activity levels. Due to the complex interplay between immune function and sickness behaviors, endocrinological correlates have received growing attention in the medical community, but so far, this subject was investigated rarely. Furthermore, given the complexities of studying illnesses and immunity in natural settings, correlates of sickness behaviors have yet to be studied in non-human primates in the wild.

Methods: We measured urinary cortisol levels using liquid chromatography–mass spectrometry in a group of wild habituated chimpanzees in Taï National Park, Côte d'Ivoire, before, during, and after a respiratory disease outbreak (main causative pathogen: human respiratory syncytial virus A, with coinfections of Streptococcus pneumoniae). Changes in cortisol levels were then related to urinary neopterin levels, a biomarker of immune system activation.

Results: Urinary cortisol levels were found to be more than 10-fold higher during the outbreak in comparison with levels before and after the outbreak period. Increasing cortisol levels were also associated with increasing neopterin levels. Interestingly, rather atypical patterns in a diurnal decline of cortisol levels were found during infection periods, such that levels remained raised throughout the day.

Conclusion: In conclusion, cortisol increase was related to cellular immune response. Our results suggest that cortisol is a mediator of infectious disease pathogenicity through its impact on the immune system and that wild chimpanzees may be facing energetic stress when sick. By monitoring immune challenges in wild-living animals, our study demonstrates that immune defenses have costs and that these costs are context-specific.

Flexibility in an emergency life-history stage: acute food deprivation prevents sickness behaviour but not the immune response

The emergency life-history stage (ELHS) can be divided into two subcategories that describe distinct, coordinated responses to disease- or non-disease-related physiological challenges. Whether an individual can simultaneously express aspects of both subcategories when faced with multiple challenges is poorly understood. Emergency life-history theory suggests that disease- and non-disease-related responses are coordinated at the level of the whole organism and therefore cannot be expressed simultaneously. However, the reactive scope and physiological regulatory network models suggest that traits can be independently regulated, allowing for components of both disease- and non-disease-related responses to be simultaneously expressed within a single organism. To test these ideas experimentally, we subjected female zebra finches to food deprivation, an immune challenge, both, or neither, and measured a suite of behavioural and physiological traits involved in the ELHS. We examined whether the trait values expressed by birds experiencing simultaneous challenges resembled trait values of birds experiencing a single challenge or if birds could express a mixture of trait values concurrently. We find that birds can respond to simultaneous challenges by regulating components of the behavioural and immune responses independently of one another. Modularity within these physio-behavioural networks adds additional dimensions to how we evaluate the intensity or quality of an ELHS. Whether modularity provides fitness advantages or costs in nature remains to be determined.

Presence of mother prompts dissociation of sickness behavior, fever, and hypothalamic gene expression in lipopolysaccharide-injected guinea pig pups

During infection, sickness behaviors, such as a hunched stance with piloerection, can facilitate host resistance by supporting the generation and maintenance of fever. Fever, in turn, is mediated by hypothalamic neuroimmune signaling. Sickness behaviors, however, can also be influenced by social stimuli. In this study, guinea pig pups were injected with lipopolysaccharide to simulate a bacterial infection and then exposed to a novel, threatening environment while either with their mother or alone. We found that the presence of the mother suppressed sickness behavior, but enhanced fever, and had no measureable effect on gene expression of hypothalamic mediators of fever. This 3-way dissociation induced by the mother's presence is interpreted in terms of the differential adaptive consequences of behavioral and febrile responses for pups in this situation. The results contribute to a growing literature linking immunological and social processes.

A Return to Wisdom: Using Sickness Behaviors to Integrate Ecological and Translational Research

Sickness is typically characterized by fever, anorexia, cachexia, and reductions in social, pleasurable, and sexual behaviors. These responses can be displayed at varying intensities both within and among individuals, and the adaptive nature of sickness responses can be demonstrated by the context-dependent nature of their expression. The study of sickness has become an important area of investigation for researchers in a wide range of areas, including psychoneuroimmunology (PNI) and ecoimmunology (EI). The general goal of PNI is to identify key interactions among the nervous, endocrine and immune systems and behavior, and how disruptions in these processes might contribute to disease states. EI, in turn, has been established more recently within the perspectives of ecology and evolutionary biology, and is aimed more at understanding natural variation in immune function and sickness responses within a broadly integrative, organismal, and evolutionary context. The goal of this review is to examine the literature on sickness from both basic and biomedical perspectives within PNI and EI and to demonstrate how the integrative study of sickness behavior can serve as an integrating agent to connect ecological and translational approaches to the study of disease. By focusing on a set of specific exemplars, including the energetics of sickness, social context, and environmental influences on sickness, we hope to accomplish the larger goal of developing a common synthetic framework to understand sickness from multiple levels of analysis and varying perspectives across the fields of PNI and EI. By applying this integrative approach to sickness, we will be able to develop a more comprehensive view of sickness as a suite of adaptive responses rather than the simply deleterious consequences of illness.

Indices of stress and immune function in Arctic barnacle goslings (Branta leucopsis) were impacted by social isolation but not a contaminated grazing environment

In many areas around the Arctic remains and spoil heaps of old mines can be found, which have been abandoned after their heydays. Runoff from tailings of these abandoned mines can directly contaminate the local environment with elevated concentrations of trace metals. Few studies have investigated the possible negative effects of contaminants on Arctic terrestrial animals that use these areas. Trace metals can accumulate in animals and this accumulation has been linked to negative effects on fitness. Both, the hypothalamus-pituitary-adrenal (HPA) axis and/or the immune system have been named as possible underlying causes for these observations. Free-living animals are often exposed to multiple stressors simultaneously, however, and this is often not considered in studies on the effects of contaminants on animal physiology. Here, we performed a study on Spitsbergen (Svalbard) taking both potential effects of trace metal contamination and social stress into account. We investigated experimentally effects of exposure to contaminants from a historic coal mine area on plasma corticosterone levels and on four innate immune parameters (haemolysis, haemagglutination, haptoglobin-like activity and nitric oxide) before and after social isolation in human-raised barnacle goslings (Branta leucopsis). Baseline corticosterone and immune parameters were not affected by mine-exposure. After social isolation, mine goslings tended to show decreased haemagglutination in comparison with control goslings, but we detected no difference in the other measures. Social isolation increased corticosterone and decreased haptoglobin-like activity in all goslings. Immunology and corticosterone levels of barnacle goslings thus seem unaffected, at least on the short term, by Arctic coal mining contamination.

Why are behavioral and immune traits linked?

Through behavior, animals interact with a world where parasites abound. It is easy to understand how behavioral traits can thus have a differential effect on pathogen exposure. Harder to understand is why we observe behavioral traits to be linked to immune defense traits. Is variation in immune traits a consequence of behavior-induced variation in immunological experiences? Or is variation in behavioral traits a function of immune capabilities? Is our immune system a much bigger driver of personality than anticipated? In this review, I provide examples of how behavioral and immune traits co-vary. I then explore the different routes linking behavioral and immune traits, emphasizing on the physiological/hormonal mechanisms that could lead to immune control of behavior. Finally, I discuss why we should aim at understanding more about the mechanisms connecting these phenotypic traits.

Elevational variation in body-temperature response to immune challenge in a lizard

Immunocompetence benefits animal fitness by combating pathogens, but also entails some costs. One of its main components is fever, which in ectotherms involves two main types of costs: energy expenditure and predation risk. Whenever those costs of fever outweigh its benefits, ectotherms are expected not to develop fever, or even to show hypothermia, reducing costs of thermoregulation and diverting the energy saved to other components of the immune system. Environmental thermal quality, and therefore the thermoregulation cost/benefit balance, varies geographically. Hence, we hypothesize that, in alpine habitats, immune-challenged ectotherms should show no thermal response, given that (1) hypothermia would be very costly, as the temporal window for reproduction is extremely small, and (2) fever would have a prohibitive cost, as heat acquisition is limited in such habitat. However, in temperate habitats, immune-challenged ectotherms might show a febrile response, due to lower cost/benefit balance as a consequence of a more suitable thermal environment. We tested this hypothesis in Psammodromus algirus lizards from Sierra Nevada (SE Spain), by testing body temperature preferred by alpine and non-alpine lizards, before and after activating their immune system with a typical innocuous pyrogen. Surprisingly, non-alpine lizards responded to immune challenge by decreasing preferential body-temperature, presumably allowing them to save energy and reduce exposure to predators. On the contrary, as predicted, immune-challenged alpine lizards maintained their body-temperature preferences. These results match with increased costs of no thermoregulation with elevation, due to the reduced window of time for reproduction in alpine environment.

Using accelerometers to remotely and automatically characterize behavior in small animals

Activity budgets in wild animals are challenging to measure via direct observation because data collection is time consuming and observer effects are potentially confounding. Although tri-axial accelerometers are increasingly employed for this purpose, their application in small-bodied animals has been limited by weight restrictions. Additionally, accelerometers engender novel complications, as a system is needed to reliably map acceleration to behaviors. In this study, we describe newly developed, tiny acceleration-logging devices (1.5-2.5 g) and use them to characterize behavior in two chipmunk species. We collected paired accelerometer readings and behavioral observations from captive individuals. We then employed techniques from machine learning to develop an automatic system for coding accelerometer readings into behavioral categories. Finally, we deployed and recovered accelerometers from free-living, wild chipmunks. This is the first time to our knowledge that accelerometers have been used to generate behavioral data for small-bodied (<100 g), free-living mammals.

How animals glide: from trajectory to morphology

Animals that glide produce aerodynamic forces that enable transit through the air in both arboreal and aquatic environments. The relative ease of gliding compared with flapping flight has led to a large diversity of taxa that have evolved some degree of flight capability. Glide paths are curved, reflecting the changing forces on the animal as it progresses through its aerial trajectory. These changing forces can be under control of the glider, which uses specific aspects of anatomy to modulate lift, drag, and rotational moments on the body. However, gliders share no single anatomical or behavioral feature, and some species are unspecialized for gliding, producing aerodynamic forces using posture and orientation alone. Animals use gliding in a broad range of ecological roles, suggesting that multiple performance metrics are relevant for consideration, but we are only beginning to understand how gliders produce and control their flight from takeoff to landing. In this review, we focus on the physical aspects of how glide trajectories are produced, and additionally discuss the range of morphologies and postures that are used to control aerial movements across the broad diversity of animal gliders.

When is it socially acceptable to feel sick?

Disease is a ubiquitous and powerful evolutionary force. Hosts have evolved behavioural and physiological responses to disease that are associated with increased survival. Behavioural modifications, known as 'sickness behaviours', frequently involve symptoms such as lethargy, somnolence and anorexia. Current research has demonstrated that the social environment is a potent modulator of these behaviours: when conflicting social opportunities arise, animals can decrease or entirely forgo experiencing sickness symptoms. Here, I review how different social contexts, such as the presence of mates, caring for offspring, competing for territories or maintaining social status, affect the expression of sickness behaviours. Exploiting the circumstances that promote this behavioural plasticity will provide new insights into the evolutionary ecology of social behaviours. A deeper understanding of when and how this modulation takes place may lead to better tools to treat symptoms of infection and be relevant for the development of more efficient disease control programmes.

Interactions between social behaviour and the acute phase immune response in house finches

In social organisms, immune-mediated behavioural changes (sickness behaviours) can both influence and respond to social dynamics. We tested whether social status in house finches (Haemorhous mexicanus) modulates the acute phase response or aggressive interactions with flockmates. We treated subordinate or dominant finches within captive flocks with lipopolysaccharide (LPS) to stimulate an acute phase response (APR), and quantified mass loss, activity, foraging behaviours, and agonistic interactions. Subordinate finches lost more mass than dominants in response to LPS, but social status did not influence the expression of sickness behaviours (activity and foraging) upon LPS injection. LPS-injected subordinate birds experienced reduced aggression from mid-ranking but not dominant flockmates, indicating status-mediated effects of sickness behaviour on agonistic interactions. Our results suggest that social status in house finches influences one component of the APR (mass loss) and can interact with the APR to modulate intraspecific agonistic interactions in ways likely relevant for disease transmission.