Effects of manipulated food availability and seasonality on innate immune function in a passerine

The innate immune system is essential for survival, yet many immune traits are highly variable between and within individuals. In recent years, attention has shifted to the role of environmental factors in modulating this variation. A key environmental factor is food availability, which plays a major role in shaping life histories, and may affect resource allocation to immune function through its effect on nutritional state. We developed a technique to permanently increase foraging costs in seed-eating birds, and leveraged this technique to study the effects of food availability on the innate immune system over a 3-year period in 230 zebra finches housed in outdoor aviaries. The immune components we studied were haptoglobin, ovotransferrin, nitric oxide, natural antibodies through agglutination, complement-mediated lysis, and killing capacity of Escherichia coli and Candida albicans, covering a broad spectrum of the innate immune system. We explored the effects of food availability in conjunction with other potentially important variables: season, age, sex and manipulated natal brood size. Increased foraging costs affected multiple components of the immune system, albeit in a variable way. Nitric oxide and agglutination levels were lower under harsh foraging conditions, while Escherichia coli killing capacity was increased. Agglutination levels also varied seasonally, but only at low foraging costs. C. albicans killing capacity was lower in winter, and even more so for animals in harsh foraging conditions that were raised in large broods. Effects of food availability on ovotransferrin were also seasonal, and only apparent in males. Haptoglobin levels were independent of foraging costs and season. Males had higher levels of immune function than females for three of the measured immune traits. Innate immune function was independent of age and manipulated natal brood size. Our finding that food availability affects innate immune function suggests that fitness effects of food availability may at least partially be mediated by effects on the immune system. However, food availability effects on innate immunity varied in direction between traits, illustrating the complexity of the immune system and precluding conclusions on the level of disease resistance.

Food supply and individual quality influence seabird energy expenditure and reproductive success

Breeding animals trade off maximizing energy output to increase their number of offspring with conserving energy to ensure their own survival, leading to an energetic ceiling influenced by external, environmental factors or by internal, physiological factors. We examined whether internal or external factors limited energy expenditure by supplementally feeding breeding black-legged kittiwakes varying in individual quality, based on earlier work that defined late breeders as low-quality and early breeders as high-quality individuals. We tested whether energy expenditure increased when food availability decreased in both low- and high-quality birds; we predicted this would only occur in high-quality individuals capable of sustaining high levels of energy expenditure. Here, we find that food-supplemented birds expended less energy than control birds because they spent more time at the colony. However, foraging trips of food-supplemented birds were only slightly shorter than control birds, implying that food-supplemented birds were limited by food availability at sea similarly to control birds. Late breeders expended less energy, suggesting that low-quality individuals may not intake the energy necessary for sustaining high-energy output. Food-supplemented birds had more offspring than control birds, but offspring number did not influence energy expenditure, supporting the idea that the birds reached an energy ceiling. Males and lighter birds expended more energy, possibly compensating for relatively higher energy intake. Chick-rearing birds were working near their maximum, with highest levels of expenditure for early-laying (high-quality) individuals foraging at sea. Due to fluctuating marine environments, kittiwakes may be forced to change their foraging behaviors to maintain the balance between reproduction and survival.

Experimentally manipulated food availability affects offspring quality but not quantity in zebra finch meso-populations

Food availability modulates survival, reproduction and thereby population size. In addition to direct effects, food availability has indirect effects through density of conspecifics and predators. We tested the prediction that food availability in isolation affects reproductive success by experimentally manipulating food availability continuously for 3 years in zebra finches (Taeniopygia guttata) housed in outdoor aviaries. To this end, we applied a technique that mimics natural variation in food availability: increasing the effort required per food reward without affecting diet. Lower food availability resulted in a slight delay of start of laying and fewer clutches per season, but did not affect clutch size or number of offspring reared per annum. However, increasing foraging costs substantially reduced offspring growth. Thus, food availability in isolation did not impact the quantity of offspring reared, at the expense of offspring quality. Growth declined strongly with brood size, and we interpret the lack of response with respect to offspring number as an adaptation to environments with low predictability, at the time of egg laying, of food availability during the period of peak food demand, typically weeks later. Manipulated natal brood size of the parents did not affect reproductive success. Individuals that were more successful reproducers were more likely to survive to the next breeding season, as frequently found in natural populations. We conclude that the causal mechanisms underlying associations between food availability and reproductive success in natural conditions may be more complex than usually assumed. Experiments in semi-natural meso-populations can contribute to further unravelling these mechanisms.

Manipulation of Heat Dissipation Capacity Affects Avian Reproductive Performance and Output

Animal life requires hard work but the ability to endure such workload appears to be limited. Heat dissipation limit (HDL) hypothesis proposes that the capacity to dissipate the excess of body heat during hard work may limit sustained energy use. Experimental facilitations of heat loss rate via feather-clipping in free-living birds seem to support HDL hypothesis but testing of HDL through laboratory experiments under controlled conditions is not reported. We employed a two-factorial experimental design to test HDL hypothesis by manipulating the capacity to dissipate heat through exposure of captive zebra finches (Taeniopygia guttata) to a cold and warm ambient temperature (14°C and 25°C), and through manipulation of the insulating layer of feathers around the brood patch in females (clipped and unclipped). To simulate foraging costs encountered in the wild we induced foraging effort by employing a feeding system that necessitated hovering to access food, which increased energetic costs of reproduction despite ad libitum conditions in captivity. We quantified the outcome of reproductive performance at the level of both parents, females, and offspring. Thermal limitations due to warm temperature appeared at the beginning of reproduction for both parents with lower egg-laying success, smaller clutch size and lower egg mass, compared to the cold. After hatching, females with an enhanced ability to dissipate heat through feather-clipping revealed higher body mass compared to unclipped females, and these clipped females also raised heavier and bigger nestlings. Higher levels for oxidative stress in plasma of females were detected prior to reproduction in warm conditions than in the cold. However, oxidative stress biomarkers of mothers were neither affected by temperature nor by feather-clipping during the reproductive activities. We document upregulation of antioxidant capacity during reproduction that seems to prevent increased levels of oxidative stress possibly due to the cost of female body condition and offspring growth. Our study on reproduction under laboratory-controlled conditions corroborates evidence in line with the HDL hypothesis. The link between temperature-constrained sustained performance and reproductive output in terms of quality and quantity is of particular interest in light of the current climate change, and illustrates the emerging risks to avian populations.

Impact of developmental temperatures on thermal plasticity and repeatability of metabolic rate

Phenotypic plasticity is an important mechanism that allows populations to adjust to changing environments. Early life experiences can have lasting impacts on how individuals respond to environmental variation later in life (i.e., individual reaction norms), altering the capacity for populations to respond to selection. Here, we incubated lizard embryos (Lampropholis delicata) at two fluctuating developmental temperatures (cold = 23 ºC + / − 3 ºC, hot = 29 ºC + / − 3 ºC, ncold = 26, nhot = 25) to understand how it affected metabolic plasticity to temperature later in life. We repeatedly measured individual reaction norms across six temperatures 10 times over ~ 3.5 months (nobs = 3,818) to estimate the repeatability of average metabolic rate (intercept) and thermal plasticity (slope). The intercept and the slope of the population-level reaction norm was not affected by developmental temperature. Repeatability of average metabolic rate was, on average, 10% lower in hot incubated lizards but stable across all temperatures. The slope of the thermal reaction norm was overall moderately repeatable (R = 0.44, 95% CI = 0.035 – 0.93) suggesting that individual metabolic rate changed consistently with short-term changes in temperature, although credible intervals were quite broad. Importantly, reaction norm repeatability did not depend on early developmental temperature. Identifying factors affecting among-individual variation in thermal plasticity will be increasingly more important for terrestrial ectotherms living in changing climate. Our work implies that thermal metabolic plasticity is robust to early developmental temperatures and has the capacity to evolve, despite there being less consistent variation in metabolic rate under hot environments.

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.

Total energy expenditure is repeatable in adults but not associated with short-term changes in body composition

Low total energy expenditure (TEE, MJ/d) has been a hypothesized risk factor for weight gain, but repeatability of TEE, a critical variable in longitudinal studies of energy balance, is understudied. We examine repeated doubly labeled water (DLW) measurements of TEE in 348 adults and 47 children from the IAEA DLW Database (mean ± SD time interval: 1.9 ± 2.9 y) to assess repeatability of TEE, and to examine if TEE adjusted for age, sex, fat-free mass, and fat mass is associated with changes in weight or body composition. Here, we report that repeatability of TEE is high for adults, but not children. Bivariate Bayesian mixed models show no among or within-individual correlation between body composition (fat mass or percentage) and unadjusted TEE in adults. For adults aged 20-60 y (N = 267; time interval: 7.4 ± 12.2 weeks), increases in adjusted TEE are associated with weight gain but not with changes in body composition; results are similar for subjects with intervals >4 weeks (N = 53; 29.1 ± 12.8 weeks). This suggests low TEE is not a risk factor for, and high TEE is not protective against, weight or body fat gain over the time intervals tested.

Body temperature is a repeatable trait in a free-ranging passerine bird

Body temperature (Tb) affects animal function through its influence on rates of biochemical and biophysical reactions, the molecular structures of proteins and tissues, and, ultimately, organismal performance. Despite its importance in driving physiological processes, there are few data on how much variation in Tb exists within populations of organisms, and whether this variation consistently differs among individuals over time (i.e. repeatability of a trait). Here, using thermal radio-frequency identification implants, we quantified the repeatability of Tb, both in the context of a fixed average environment (∼21°C) and across ambient temperatures (6-31°C), in a free-living population of tree swallows (Tachycineta bicolor, n=16). By experimentally trimming the ventral plumage of a subset of female swallows (n=8), we also asked whether the repeatability of Tb is influenced by the capacity to dissipate body heat. We found that both female and male tree swallow Tb was repeatable at 21°C (R=0.89-92), but female Tb was less repeatable than male Tb across ambient temperature (Rfemale=0.10, Rmale=0.58), which may be due to differences in parental investment. Trimmed birds had on average lower Tb than control birds (by ∼0.5°C), but the repeatability of female Tb did not differ as a function of heat dissipation capacity. This suggests that trimmed individuals adjusted their Tb to account for the effects of heat loss on Tb. Our study provides a first critical step toward understanding whether Tb is responsive to natural selection, and for predicting how animal populations will respond to climatic warming.

Exposure to food insecurity increases energy storage and reduces somatic maintenance in European starlings (Sturnus vulgaris)

Birds exposed to food insecurity-defined as temporally variable access to food-respond adaptively by storing more energy. To do this, they may reduce energy allocation to other functions such as somatic maintenance and repair. To investigate this trade-off, we exposed juvenile European starlings (Sturnus vulgaris, n = 69) to 19 weeks of either uninterrupted food availability or a regime where food was unpredictably unavailable for a 5-h period on 5 days each week. Our measures of energy storage were mass and fat scores. Our measures of somatic maintenance were the growth rate of a plucked feather, and erythrocyte telomere length (TL), measured by analysis of the terminal restriction fragment. The insecure birds were heavier than the controls, by an amount that varied over time. They also had higher fat scores. We found no evidence that they consumed more food overall, though our food consumption data were incomplete. Plucked feathers regrew more slowly in the insecure birds. TL was reduced in the insecure birds, specifically, in the longer percentiles of the within-individual TL distribution. We conclude that increased energy storage in response to food insecurity is achieved at the expense of investment in somatic maintenance and repair.

Sex-specific energy management strategies in response to training for increased foraging effort prior to reproduction in captive zebra finches

Free-living animals often engage in behaviour that involves high rates of workload and results in high daily energy expenditure (DEE), such as reproduction. However, the evidence for elevated DEE accompanying reproduction remains equivocal. In fact, many studies have found no difference in DEE between reproducing and non-reproducing females. One of the hypotheses explaining the lack of difference is the concept of an 'energetic ceiling'. However, it is unclear whether the lack of increase in energy expenditure is due to the existence of an energetic ceiling and/or compensation by males during parental care. To investigate whether an energetic ceiling exists, we experimentally manipulated foraging effort in captive zebra finches, Taeniopygia guttata, creating two groups with high and low foraging efforts followed by both groups breeding in the low foraging effort common garden condition. DEE was measured in both sexes throughout the experiment. We show sex-specific energy management strategies in response to training for increased foraging effort prior to reproduction. Specifically, males and females responded differently to the high foraging effort treatment and subsequently to chick rearing in terms of energy expenditure. Our results also suggest that there is an energetic ceiling in females and that energetic costs incurred prior to reproduction can be carried over into subsequent stages of reproduction in a sex-specific manner.

Endocrinology of thermoregulation in birds in a changing climate

The ability to maintain a (relatively) stable body temperature in a wide range of thermal environments by use of endogenous heat production is a unique feature of endotherms such as birds. Endothermy is acquired and regulated via various endocrine and molecular pathways, and ultimately allows wide aerial, aquatic, and terrestrial distribution in variable environments. However, due to our changing climate, birds are faced with potential new challenges for thermoregulation, such as more frequent extreme weather events, lower predictability of climate, and increasing mean temperature. We provide an overview on thermoregulation in birds and its endocrine and molecular mechanisms, pinpointing gaps in current knowledge and recent developments, focusing especially on non-model species to understand the generality of, and variation in, mechanisms. We highlight plasticity of thermoregulation and underlying endocrine regulation, because thorough understanding of plasticity is key to predicting responses to changing environmental conditions. To this end, we discuss how changing climate is likely to affect avian thermoregulation and associated endocrine traits, and how the interplay between these physiological processes may play a role in facilitating or constraining adaptation to a changing climate. We conclude that while the general patterns of endocrine regulation of thermogenesis are quite well understood, at least in poultry, the molecular and endocrine mechanisms that regulate, e.g. mitochondrial function and plasticity of thermoregulation over different time scales (from transgenerational to daily variation), need to be unveiled. Plasticity may ameliorate climate change effects on thermoregulation to some extent, but the increased frequency of extreme weather events, and associated changes in resource availability, may be beyond the scope and/or speed for plastic responses. This could lead to selection for more tolerant phenotypes, if the underlying physiological traits harbour genetic and individual variation for selection to act on - a key question for future research.

Ecological and evolutionary consequences of metabolic rate plasticity in response to environmental change

Basal or standard metabolic rate reflects the minimum amount of energy required to maintain body processes, while the maximum metabolic rate sets the ceiling for aerobic work. There is typically up to three-fold intraspecific variation in both minimal and maximal rates of metabolism, even after controlling for size, sex and age; these differences are consistent over time within a given context, but both minimal and maximal metabolic rates are plastic and can vary in response to changing environments. Here we explore the causes of intraspecific and phenotypic variation at the organ, tissue and mitochondrial levels. We highlight the growing evidence that individuals differ predictably in the flexibility of their metabolic rates and in the extent to which they can suppress minimal metabolism when food is limiting but increase the capacity for aerobic metabolism when a high work rate is beneficial. It is unclear why this intraspecific variation in metabolic flexibility persists-possibly because of trade-offs with the flexibility of other traits-but it has consequences for the ability of populations to respond to a changing world. It is clear that metabolic rates are targets of selection, but more research is needed on the fitness consequences of rates of metabolism and their plasticity at different life stages, especially in natural conditions. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

Coupling lifespan and aging? The age at onset of body mass decline associates positively with sex-specific lifespan but negatively with environment-specific lifespan

Whether lifespan scales to age-associated changes in health and disease is an urgent question in societies with increasing lifespan. Body mass is associated with organismal functioning in many species, and often changes with age. We here tested in zebra finches whether two factors that decreased lifespan, sex and poor environmental quality, accelerated the onset of body mass declines. We subjected 597 birds for nine years to experimentally manipulated foraging costs (harsh = H, benign = B) during development (small vs large brood size) and in adulthood (easy vs hard foraging conditions) in a 2 × 2 design. This yielded four treatment combinations (HH, HB, BH, BB) for each sex. Harsh environments during development and in adulthood decreased average body mass additively. The body mass aging trajectory showed a short steep increase in early adulthood, followed by a plateau and then a decline after 5 years. This decline occurred in all groups except for HB females, which gained mass until death. Surprisingly, the onset of body mass decline was earlier in experimental groups with a longer lifespan. In contrast, the onset of body mass decline was one year earlier in females, which lived two months (4%) shorter than males. Thus, the onset of body mass aging associated positively with the sex-specific differences in lifespan, but negatively with the environmental modulation of lifespan. Thus, body mass aging trajectories did not generally scale to lifespan, and we discuss the possible causes and implications of this finding.

Corticosterone levels reflect variation in metabolic rate, independent of 'stress'

Variation in glucocorticoid hormones (GCs) is often interpreted as reflecting 'stress', but this interpretation is subject of intense debate. GCs induce gluconeogenesis, and we hypothesized therefore that GC variation can be explained by changes in current and anticipated metabolic rate (MR). Alternatively, GC levels may respond to psychological 'stress' over and above its effect on metabolic rate. We tested these hypotheses in captive zebra finches, by inducing an increase in MR using a psychological stressor (noise), and compared its effect on corticosterone (CORT, the primary avian GC) with the effect induced by a decrease in ambient temperature increasing MR to a similar extent. We found the increase in CORT induced by the psychological stressor to be indistinguishable from the level expected based on the noise effect on MR. We further found that a handling and restraint stressor that increased CORT levels also resulted in increased blood glucose levels, corroborating a key assumption underlying our hypothesis. Thus, GC variation primarily reflected variation in energy expenditure, independently of psychological stress. GC levels have many downstream effects besides glucose mobilization, and we propose that these effects can be interpreted as adjustments of physiological functions to the metabolic level at which an organism operates.

Baseline glucose level is an individual trait that is negatively associated with lifespan and increases due to adverse environmental conditions during development and adulthood

High baseline glucose levels are associated with pathologies and shorter lifespan in humans, but little is known about causes and consequences of individual variation in glucose levels in other species. We tested to what extent baseline blood glucose level is a repeatable trait in adult zebra finches, and whether glucose levels were associated with age, manipulated environmental conditions during development (rearing brood size) and adulthood (foraging cost), and lifespan. We found that: (1) repeatability of glucose levels was 30%, both within and between years. (2) Having been reared in a large brood and living with higher foraging costs as adult were independently associated with higher glucose levels. Furthermore, the finding that baseline glucose was low when ambient temperature was high, and foraging costs were low, indicates that glucose is regulated at a lower level when energy turnover is low. (3) Survival probability decreased with increasing baseline glucose. We conclude that baseline glucose is an individual trait negatively associated with survival, and increases due to adverse environmental conditions during development (rearing brood size) and adulthood (foraging cost). Blood glucose may be, therefore, part of the physiological processes linking environmental conditions to lifespan.

Glucocorticoid-temperature association is shaped by foraging costs in individual zebra finches

Glucocorticoid (GC) levels vary with environmental conditions, but the functional interpretation of GC variation remains contentious. A primary function is thought to be metabolic, mobilizing body reserves to match energetic demands. This view is supported by temperature-dependent GC levels, although reports of this effect show unexplained heterogeneity. We hypothesised that the temperature effect on GC concentrations will depend on food availability through its effect on the energy spent to gather the food needed for thermoregulation. We tested this hypothesis in zebra finches living in outdoor aviaries with manipulated foraging conditions (i.e. easy vs. hard), by relating within-individual differences in baseline GCs between consecutive years to differences in ambient temperature. In agreement with our hypothesis, we found the GC-temperature association to be significantly steeper in the hard foraging environment. This supports the metabolic explanation of GC variation, underlining the importance of accounting for variation in energy expenditure when interpreting GC variation.

Strong association between corticosterone and temperature dependent metabolic rate in individual zebra finches

Glucocorticoid hormones (GCs) are often assumed to be indicators of stress. At the same time, one of their fundamental roles is to facilitate metabolic processes to accommodate changes in energetic demands. While the metabolic function of GCs is thought to be ubiquitous across vertebrates, we are not aware of experiments which tested this directly, i.e., in which metabolic rate was manipulated and measured together with GCs. We therefore tested for a relationship between plasma corticosterone (CORT, ln transformed) and metabolic rate (MR, measured using indirect calorimetry) in a between- and within-individual design in captive zebra finches (Taeniopygia guttata) of both sexes. In each individual, CORT and MR were measured at two different temperature levels: ‘warm’ (22°C) and ‘cold’ (12 °C). CORT and MR were both increased in colder compared to warmer conditions, within individuals, but also across individuals. At the between-individual level, we found a positive relationship between CORT and MR, with an accelerating slope towards higher MR and CORT values. In contrast, the within individual changes in CORT and MR in response to colder conditions were linearly correlated between individuals. The CORT-MR relationship did not differ between the sexes. Our results illustrate the importance of including variation at different levels to better understand physiological modulation. Furthermore, our findings support the interpretation of CORT variation as indicator of metabolic needs.

Physiological effects of increased foraging effort in a small passerine

Foraging to obtain food, either for self-maintenance or at presumably elevated rates to provision offspring, is thought to be an energetically demanding activity but one that is essential for fitness (higher reproductive success and survival). Nevertheless, the physiological mechanisms that allow some individuals to support higher foraging performance, and the mechanisms underlying costs of high workload, remain poorly understood. We experimentally manipulated foraging behaviour in zebra finches (Taeniopygia guttata) using the technique described by Koetsier and Verhulst (2011). Birds in the “high foraging effort” (HF) group had to obtain food either while flying/hovering or by making repeated hops or jumps from the ground up to the feeder, behaviour typical of the extremely energetically-expensive foraging mode observed in many free-living small passerines. HF birds made significantly more trips to the feeder per 10min whereas control birds spent more time (perched) at the feeder. Despite this marked change in foraging behaviour we documented few short- or long-term effects of “training” (3 days and 90 days of “training” respectively) and some of these effects were sex-specific. There were no effects of treatment on BMR, hematocrit, hemoglobin, or plasma glycerol, triglyceride, glucose levels, and masses of kidney, crop, large intestine, small intestine, gizzard and liver. HF females had higher masses of flight muscle, leg muscle, heart and lung compared to controls. In contrast, HF males had lower heart mass than controls and there were no differences for other organs. When both sexes were pooled, there were no effects of treatment on body composition. Finally, birds in the HF treatment had higher levels of reactive oxygen metabolites (dROMs) and, consequently, although treatment did not affect total antioxidant capacity (OXY), birds in the HF treatment had higher oxidative stress.