Hot and scared: how do heatwaves and predation risk impact resource acquisition and allocation?

Heatwaves are increasingly prevalent and can constrain investment into important life-history traits. In addition to heatwaves, animals regularly encounter threats from other organisms in their environments, such as predators. The combination of these two environmental factors introduces a decision-making conflict-heat exposure requires more food intake to fuel investment into fitness-related traits, but foraging in the presence of predators increases the threat of mortality. Thus, we used female variable field crickets (Gryllus lineaticeps) to investigate the effects of heatwaves in conjunction with predation risk (exposed food and water sources, and exposure to scent from black widow spiders, Latrodectus hesperus) on resource acquisition (food intake) and allocation (investment into ovarian and somatic tissues). A simulated heatwave increased food intake and the allocation of resources to reproductive investment. Crickets exposed to high predation risk reduced food intake, but they were able to maintain reproductive investment at an expense to investment into somatic tissue. Thus, heatwaves and predation risk deprioritized investment into self-maintenance, which may impair key physiological processes. This study is an important step towards understanding the ecology of fear in a warming world.

Stress tolerance is influenced by artificial light at night during development and life-history strategy.

Artificial light at night (ALAN) is increasingly prevalent worldwide, but life-history strategy may mitigate the costs of ALAN for animals. Yet, interactions among ALAN, life-history strategy and tolerance to climate-related stressors are unknown. We determined if developmental ALAN exposure (1) affects development, (2) affects adult phenotype, including heat and desiccation tolerance, and (3) affects and/or interacts with life-history strategy. We used the variable field cricket (Gryllus lineaticeps) because its geographic range is increasingly exposed to ALAN, heat, and drought conditions, and it exhibits different life-history strategies (flight-capability versus flight-incapability). ALAN affected adult phenotype, with positive effects on body mass (and size) and female reproductive investment, and a negative effect on heat tolerance. Life-history strategy also affected stress tolerance; flight-incapable females had greater heat tolerance and their desiccation tolerance was improved by ALAN exposure. Key features of environmental change (i.e. exposure to ALAN, heat and drought) may favor some life-history strategies over others.

Flight capacity drives circadian patterns of metabolic rate and alters resource dynamics.

Animals must acquire, use, and allocate resources, and this balancing act may be influenced by the circadian clock and life-history strategy. Field (Gryllus) crickets exhibit two distinct life-history strategies during early adulthood-flight-capable females invest in flight muscle at a cost to ovary mass, whereas flight-incapable females instead invest solely into ovaries. In female Gryllus lineaticeps, I investigated the role of life-history strategy in resource (food) acquisition and allocation, and in circadian patterns of energy use. Flight capacity increased the standard metabolic rate (SMR) due to greater late-day SMR and flight-capable crickets exhibited greater circadian rhythmicity in SMR. Flight-capable crickets also ate less food and were less efficient at converting ingested food into body or ovary mass. Thus, investment into flight capacity reduced fecundity and the amount of resources available for allocation to other life-history traits. Given the increasing uncertainty of food availability in many global regions, work in Gryllus may clarify the important roles of food and circadian patterns in life-history evolution in a changing world.

Evaluating the effects of water and food limitation on the life history of an insect using a multiple-stressor framework.

Many environmental stressors naturally covary, and the frequency and duration of stressors such as heat waves and droughts are increasing globally with climate change. Multiple stressors may have additive or non-additive effects on fitness-related traits, such as locomotion, reproduction, and somatic growth. Despite its importance to terrestrial animals, water availability is rarely incorporated into multiple-stressor frameworks. Water limitation often occurs concurrently with food limitation (e.g., droughts can trigger famines), and the acquisition of water and food can be linked because water is necessary for digestion and metabolism. Thus, we investigated the independent and interactive effects of water and food limitation on life-history traits using female crickets (Gryllus firmus), which exhibit a wing dimorphism mediating a life-history trade-off between flight and fecundity. Our results indicate that traits vary in their sensitivities to environmental factors and factor-factor interactions. For example, neither environmental factor affected flight musculature, only water limitation affected survival, and food and water availability non-additively (i.e., interactively) influenced body and ovary mass. Water availability had a larger effect on traits than food availability, affected more traits than food availability, and mediated the effects of food availability. Further, life-history strategy influenced the costs of multiple stressors because females investing in flight capacity exhibited greater reductions in body and ovary mass during stress relative to females lacking flight capacity. Therefore, water is important in the multiple-stressor framework, and understanding the dynamics of covarying environmental factors and life history may be critical in the context of climate change characterized by concurrent environmental stressors.

When it's hot and dry: life-history strategy influences the effects of heat waves and water limitation.

The frequency, duration and co-occurrence of several environmental stressors, such as heat waves and droughts, are increasing globally. Such multiple stressors may have compounding or interactive effects on animals, resulting in either additive or non-additive costs, but animals may mitigate these costs through various strategies of resource conservation or shifts in resource allocation. Through a factorial experiment, we investigated the independent and interactive effects of a simulated heat wave and water limitation on life-history, physiological and behavioral traits. We used the variable field cricket, Gryllus lineaticeps, which exhibits a wing dimorphism that mediates two distinct life-history strategies during early adulthood. Long-winged individuals invest in flight musculature and are typically flight capable, whereas short-winged individuals lack flight musculature and capacity. A comprehensive and integrative approach with G. lineaticeps allowed us to examine whether life-history strategy influenced the costs of multiple stressors as well as the resulting cost-limiting strategies. Concurrent heat wave and water limitation resulted in largely non-additive and single-stressor costs to important traits (e.g. survival and water balance), extensive shifts in resource allocation priorities (e.g. reduced prioritization of body mass) and a limited capacity to conserve resources (e.g. heat wave reduced energy use only when water was available). Life-history strategy influenced the emergency life-history stage because wing morphology and stressor(s) interacted to influence body mass, boldness behavior and immunocompetence. Our results demonstrate that water availability and life-history strategy should be incorporated into future studies integrating important conceptual frameworks of stress across a suite of traits - from survival and life history to behavior and physiology.

City limits: Heat tolerance is influenced by body size and hydration state in an urban ant community.

Cities are rapidly expanding, and global warming is intensified in urban environments due to the urban heat island effect. Therefore, urban animals may be particularly susceptible to warming associated with ongoing climate change. We used a comparative and manipulative approach to test three related hypotheses about the determinants of heat tolerance or critical thermal maximum (CT max) in urban ants-specifically, that (a) body size, (b) hydration status, and (c) chosen microenvironments influence CT max. We further tested a fourth hypothesis that native species are particularly physiologically vulnerable in urban environments. We manipulated water access and determined CT max for 11 species common to cities in California's Central Valley that exhibit nearly 300-fold variation in body size. There was a moderate phylogenetic signal influencing CT max, and inter (but not intra) specific variation in body size influenced CT max where larger species had higher CT max. The sensitivity of ants' CT max to water availability exhibited species-specific thresholds where short-term water limitation (8 hr) reduced CT max and body water content in some species while longer-term water limitation (32 hr) was required to reduce these traits in other species. However, CT max was not related to the temperatures chosen by ants during activity. Further, we found support for our fourth hypothesis because CT max and estimates of thermal safety margin in native species were more sensitive to water availability relative to non-native species. In sum, we provide evidence of links between heat tolerance and water availability, which will become critically important in an increasingly warm, dry, and urbanized world that others have shown may be selecting for smaller (not larger) body size.

Life History and Immune Challenge Influence Metabolic Plasticity to Food Availability and Acclimation Temperature.

Animals vary in their rates of energy expenditure for self-maintenance (standard metabolic rate [SMR]). Yet we still lack a thorough understanding of the determinants of SMR, potentially because of complex interactions among environmental, life-history, and physiological factors. Thus, we used a factorial design in female sand field crickets (Gryllus firmus) to investigate the independent and interactive effects of food availability (unlimited or limited access), acclimation temperature (control or simulated heat wave), life-history strategy (flight-capable or flight-incapable wing morphology), and immune status (control or chronic immune activation) on SMR (CO2 production rate) measured at 28°C. Both environmental factors independently affected SMR where heat wave and food limitation reduced SMR. Furthermore, wing morphology and immune status mediated the plasticity of SMR to food and temperature. For example, the hypermetabolic effect of food availability was greater in flight-capable crickets and reduced in immune-challenged crickets. Therefore, although SMR was directly affected by food availability and acclimation temperature, interactive effects on SMR were more common, meaning several factors (e.g., life history and immune status) influenced metabolic plasticity to food and temperature. We encourage continued use of factorial experiments to reveal interaction dynamics, which are critical to understanding emergent physiological processes.

Dehydration enhances cellular and humoral immunity in a mesic snake community.

The immunocompetence of a community of free-living animals can be affected by seasonality, sex, and parasite burden. However, each of these factors is often examined independently. Recent studies have also found that dehydration can enhance aspects of immunocompetence in drought-adapted species. To explore how all of these factors interact, and their effect on the immune system in mesic-adapted species, we collected blood samples from a community of free-ranging snakes in coastal South Carolina, United States, across 2 years. We specifically examined (a) how sex and seasonality influence humoral and cellular immunocompetence and parasite burden, (b) the dynamics among hydration state, parasite burden, and immunocompetence, and (c) whether mesic-adapted species also show enhanced innate immunity with dehydration. Consistent with previous work on drought-adapted species, we found that dehydration enhances multiple aspects of humoral immunity in mesic species, and we are the first to report that dehydration also enhances aspects of cellular immunocompetence. Contrary to previous results in other squamates, sex and season did not impact immunocompetence or parasite prevalence. Our results also reveal complex interactions among parasite prevalence, immunocompetence, and hydration state demonstrating that hydration state and parasitism are two ubiquitous factors that should continue to be considered in future studies examining ecoimmunological variation.

Eating increases oxidative damage in a reptile.

While eating has substantial benefits in terms of both nutrient and energy acquisition, there are physiological costs associated with digesting and metabolizing a meal. Frequently, these costs have been documented in the context of energy expenditure while other physiological costs have been relatively unexplored. Here, we tested whether the seemingly innocuous act of eating affects either systemic pro-oxidant (reactive oxygen metabolite, ROM) levels or antioxidant capacity of corn snakes (Pantherophis guttatus) by collecting plasma during absorptive (peak increase in metabolic rate due to digestion of a meal) and non-absorptive (baseline) states. When individuals were digesting a meal, there was a minimal increase in antioxidant capacity relative to baseline (4%), but a substantial increase in ROMs (nearly 155%), even when controlling for circulating nutrient levels. We report an oxidative cost of eating that is much greater than that due to long distance flight or mounting an immune response in other taxa. This result demonstrates the importance of investigating non-energetic costs associated with meal processing, and it begs future work to identify the mechanism(s) driving this increase in ROM levels. Because energetic costs associated with eating are taxonomically widespread, identifying the taxonomic breadth of eating-induced ROM increases may provide insights into the interplay between oxidative damage and life history theory.

Plasticity of immunity in response to eating.

Following a meal, an animal can exhibit dramatic shifts in physiology and morphology, as well as a substantial increase in metabolic rate associated with the energetic costs of processing a meal (i.e. specific dynamic action, SDA). However, little is known about the effects of digestion on another important physiological and energetically costly trait: immune function. Thus, we tested two competing hypotheses. (1) Digesting animals up-regulate their immune systems (putatively in response to the increased microbial exposure associated with ingested food). (2) Digesting animals down-regulate their immune systems (presumably to allocate energy to the breakdown of food). We assayed innate immunity (lytic capacity and agglutination) in cornsnakes (Pantherophis guttatus) during and after meal digestion. Lytic capacity was higher in females, and (in support of our first hypothesis) agglutination was higher during absorption. Given its potential energetic cost, immune up-regulation may contribute to SDA.

Consequences of complex environments: Temperature and energy intake interact to influence growth and metabolic rate.

The field of comparative physiology has a rich history of elegantly examining the effects of individual environmental factors on performance traits linked to fitness (e.g., thermal performance curves for locomotion). However, animals live in complex environments wherein multiple environmental factors co-vary. Thus, we investigated the independent and interactive effects of temperature and energy intake on the growth and metabolic rate of juvenile corn snakes (Pantherophis guttatus) in the context of shifts in complex environments. Unlike previous studies that imposed constant or fluctuating temperature regimes, we manipulated the availability of preferred thermal microclimates (control vs. relatively warm regimes) for eight weeks and allowed snakes to behaviorally thermoregulate among microclimates. By also controlling for energy intake, we demonstrate an interactive effect of temperature and energy on growth-relevant temperature shifts had no effect on snakes' growth when energy intake was low and a positive effect on growth when energy intake was high. Thus, acclimation to relatively warm thermal options can result in increased rates of growth when food is abundant in a taxon in which body size confers fitness advantages. Temperature and energy also interactively influenced metabolic rate-snakes in the warmer temperature regime exhibited reduced metabolic rate (O2 consumption rate at 25 °C and 30 °C) if they had relatively high energy intake. Although we advocate for continued investigation into the effects of complex environments on other traits, our results indicate that warming may actually benefit important life history traits in some taxa and that metabolic shifts may underlie thermal acclimation.

Physiological state influences evaporative water loss and microclimate preference in the snake Vipera aspis.

Animals typically respond to environmental variation by adjusting their physiology, behavior, or both. Ectothermic animals are particularly sensitive to microclimatic conditions and behaviorally thermoregulate to optimize physiological performance. Yet, thermoregulation can be costly and may obligate a physiological tradeoff with water loss. Presumably, this tradeoff intensifies when animals undergo necessary life-history events (e.g., pregnancy or digestion) that impose significant behavioral and physiological changes, including shifts in behavioral thermoregulation and increased metabolic rate. Thus, behavioral responses, such as modified microclimatic preferences, may help mitigate the physiological tradeoff between thermoregulation and water loss. Herein, we examined the influence of major physiological states (specifically, pregnancy, ecdysis, and digestion) on evaporative water loss and on behavioral adjustments in a viviparous snake, Vipera aspis. First, we used open-flow respirometry to measure the effects of physiological states and microclimatic conditions (temperature and humidity) on the rate of total evaporative water loss (TEWL) and metabolic rate (rate of O2 consumption, V˙O2). Then, we experimentally tested the influence of physiological state on microclimate selection. We found that energy-demanding physiological states were associated with i) an increased rate of TEWL and V˙O2 compared to control states and ii) a slight preference (statistically marginal) for both warm and humid conditions compared to controls, suggesting a state-specificity in behavioral response. Overall our results underline the impact of physiological state on water loss and demonstrate the potential for behavior to mitigate the physiological tradeoff between thermoregulation and water balance.

Morphological and physiological changes during reproduction and their relationships to reproductive performance in a capital breeder.

Current reproductive effort typically comes at a cost to future reproductive value by altering somatic function (e.g., growth or self-maintenance). Furthermore, effects of reproduction often depend on both fecundity and stage of reproduction, wherein allocation of resources into additional offspring and/or stages of reproduction results in increased costs. Despite these widely accepted generalities, interindividual variation in the effects of reproduction is common-yet the proximate basis that allows some individuals to mitigate these detrimental effects is unclear. We serially measured several variables of morphology (e.g., musculature) and physiology (e.g., antioxidant defenses) in female Children's pythons (Antaresia childreni) throughout reproduction to examine how these traits change over the course of reproduction and whether certain physiological traits are associated with reduced effects of reproduction in some individuals. Reproduction in this capital breeder was associated with changes in both morphology and physiology, but only morphological changes varied with fecundity and among specific reproductive stages. During reproduction, we detected negative relationships between morphology and self-maintenance (e.g., increased muscle allocation to reproduction was related to reduced immune function). Additionally, females that allocated resources more heavily into current reproduction also did so during future reproduction, and these females assimilated resources more efficiently, experienced reduced detriments to self-maintenance (e.g., lower levels of oxidative damage and glucocorticoids) during reproduction, and produced clutches with greater hatching success. Our results suggest that interindividual variation in specific aspects of physiology (assimilation efficiency and oxidative status) may drive variation in reproductive performance.

Thermal sensitivity of immune function: evidence against a generalist-specialist trade-off among endothermic and ectothermic vertebrates.

Animal body temperature (Tbody) varies over daily and annual cycles, affecting multiple aspects of biological performance in both endothermic and ectothermic animals. Yet a comprehensive comparison of thermal performance among animals varying in Tbody (mean and variance) and heat production is lacking. Thus, we examined the thermal sensitivity of immune function (a crucial fitness determinant) in Vertebrata, a group encompassing species of varying thermal biology. Specifically, we investigated temperature-related variation in two innate immune performance metrics, hemagglutination and hemolysis, for 13 species across all seven major vertebrate clades. Agglutination and lysis were temperature dependent and were more strongly related to the thermal biology of species (e.g., mean Tbody) than to the phylogenetic relatedness of species, although these relationships were complex and frequently surprising (e.g., heterotherms did not exhibit broader thermal performance curves than homeotherms). Agglutination and lysis performance were positively correlated within species, except in taxa that produce squalamine, a steroidal antibiotic that does not lyse red blood cells. Interestingly, we found the antithesis of a generalist-specialist trade-off: species with broader temperature ranges of immune performance also had higher peak performance levels. In sum, we have uncovered thermal sensitivity of immune performance in both endotherms and ectotherms, highlighting the role that temperature and life history play in immune function across Vertebrata.

Taxonomic chauvinism revisited: insight from parental care research.

Parental care (any non-genetic contribution by a parent that appears likely to increase the fitness of its offspring) is a widespread trait exhibited by a broad range of animal taxa. In addition to influencing the fitness of parent(s) and offspring, parental care may be inextricably involved in other evolutionary processes, such as sexual selection and the evolution of endothermy. Yet, recent work has demonstrated that bias related to taxonomy is prevalent across many biological disciplines, and research in parental care may be similarly burdened. Thus, I used parental care articles published in six leading journals of fundamental behavioral sciences (Animal Behaviour, Behavioral Ecology, Behavioral Ecology and Sociobiology, Ethology, Hormones and Behavior, and Physiology & Behavior) from 2001-2010 (n = 712) to examine the year-to-year dynamics of two types of bias related to taxonomy across animals: (1) taxonomic bias, which exists when research output is not proportional to the frequency of organisms in nature, and (2) taxonomic citation bias, which is a proxy for the breadth of a given article-specifically, the proportion of articles cited that refer solely to the studied taxon. I demonstrate that research on birds likely represents a disproportionate amount of parental care research and, thus, exhibits taxonomic bias. Parental care research on birds and mammals also refers to a relatively narrow range of taxonomic groups when discussing its context and, thus, exhibits taxonomic citation bias. Further, the levels of taxonomic bias and taxonomic citation bias have not declined over the past decade despite cautionary messages about similar bias in related disciplines--in fact, taxonomic bias may have increased. As in Bonnet et al. (2002), my results should not be interpreted as evidence of an 'ornithological Mafia' conspiring to suppress other taxonomic groups. Rather, I generate several rational hypotheses to determine why bias persists and to guide future work.

Sexual variation in assimilation efficiency: its link to phenotype and potential role in sexual dimorphism.

Sex-specific variation in morphology (sexual dimorphism) is a prevalent phenomenon among animals, and both dietary intake and resource allocation strategies influence sexually dimorphic traits (e.g., body size or composition). However, we investigated whether assimilation efficiency (AE), an intermediate step between dietary intake and allocation, can also vary between the sexes. Specifically, we tested whether sex-based differences in AE can explain variation in phenotypic traits. We measured morphometric characteristics (i.e., body length, mass, condition, and musculature) and AE of total energy, crude protein, and crude fat in post-reproductive adult Children's pythons (which exhibit a limited female-biased sexual size dimorphism) fed both low and high dietary intakes. Meal size was negatively related to AE of energy. Notably, male snakes absorbed crude protein more efficiently and increased epaxial (dorsal) musculature faster than females, which demonstrates a link between AE and phenotype. However, females grew in body length faster but did not absorb any nutrient more efficiently than males. Although our results do not provide a direct link between AE and sexual size dimorphism, they demonstrate that sexual variation in nutrient absorption exists and can contribute to other types of sex-based differences in phenotype (i.e., sexual dimorphism in growth of musculature). Hence, testing the broader applicability of AE's role in sexually dimorphic traits among other species is warranted.

The role of python eggshell permeability dynamics in a respiration-hydration trade-off.

Parental care is taxonomically widespread because it improves developmental conditions and thus fitness of offspring. Although relatively simplistic compared with parental behaviors of other taxa, python egg-brooding behavior exemplifies parental care because it mediates a trade-off between embryonic respiration and hydration. However, because egg brooding increases gas-exchange resistance between embryonic and nest environments and because female pythons do not adjust their brooding behavior in response to the increasing metabolic requirements of developing offspring, python egg brooding imposes hypoxic costs on embryos during the late stages of incubation. We conducted a series of experiments to determine whether eggshells coadapted with brooding behavior to minimize the negative effects of developmental hypoxia. We tested the hypotheses that python eggshells (1) increase permeability over time to accommodate increasing embryonic respiration and (2) exhibit permeability plasticity in response to chronic hypoxia. Over incubation, we serially measured the atomic and structural components of Children's python (Antaresia childreni) eggshells as well as in vivo and in vitro gas exchange across eggshells. In support of our first hypothesis, A. childreni eggshells exhibited a reduced fibrous layer, became more permeable, and facilitated greater gas exchange as incubation progressed. Our second hypothesis was not supported, as incubation O(2) concentration did not affect the shells' permeabilities to O(2) and H(2)O vapor. Our results suggest that python eggshell permeability changes during incubation but that the alterations over time are fixed and independent of environmental conditions. These findings are of broad evolutionary interest because they demonstrate that, even in relatively simple parental-care models, successful parent-offspring relationships depend on adjustments made by both the parent (i.e., egg-brooding behavioral shifts) and the offspring (i.e., changes in eggshell permeability).

Effect of nest temperature on egg-brooding dynamics in Children's pythons.

Parental care meets several critical needs of developing offspring. In particular, egg brooding in pythons is an exceptional model for examining environmental influences on specific parental behaviors because brooding behaviors are dynamic yet simple and modulate embryonic temperature, respiration, and water balance. We used captive Children's pythons (Antaresia childreni) to assess their facultative endothermic capability, the influence on egg-brooding behaviors of the gradient between the nest temperature (T(nest)) and clutch temperature (T(clutch)), and the effect of these behaviors on the developmental micro-environment. We monitored maternal egg-brooding behavior, rates of brooding unit (i.e., female and associated clutch) respiratory gas exchange, T(nest), T(clutch), and intra-clutch oxygen tension (PO(2)clutch) during acute changes among four incubation temperature conditions: constant preferred temperature (31.5 degrees C); 'cooling' (T(nest)T(clutch)). We demonstrated that A. childreni are not facultatively endothermic because brooding unit temperature coefficient (Q(10)) for V(O2) and V(CO2) was similar to other ectothermic boid snakes (1.9-5.7) and T(clutch) conformed to T(nest) at the constant, cool temperature treatment. Females coiled tightly around eggs more often during cooling than during warming. Further, the amount of time that females spent tightly coiled during warming significantly affected the T(nest)-T(clutch) gradient. Together these results indicate that, although female A. childreni are not facultatively endothermic, they are capable of assessing the T(nest)-T(clutch) gradient and making behavioral adjustments to enhance the thermal micro-environment of their developing offspring.

Alternating egg-brooding behaviors create and modulate a hypoxic developmental micro-environment in Children's pythons (Antaresia childreni).

Parental care is a widespread and ecologically relevant adaptation known to enhance the developmental environment of offspring. Parental behaviors, however, may entail both costs and benefits for developing offspring. In Children's pythons (Antaresia childreni), we monitored both maternal egg-brooding behavior and intra-clutch oxygen partial pressure (PO2) in real-time to assess the effects of various brooding behaviors on PO2 in the clutch micro-environment at three stages of development. Furthermore, at the same developmental stages, we measured O2 consumption rates (VO2) of eggs at varying PO2 to determine their critical oxygen tension (i.e. the minimal PO2 that supports normal respiratory gas exchange) and to predict the impact that naturally brooded intra-clutch PO2 has on embryonic metabolism. At all three stages of development, a tightly coiled brooding posture created an intra-clutch PO2 that was significantly lower than the surrounding nest environment. Maternal postural adjustments alleviated this hypoxia, and the magnitude of such corrections increased with developmental stage. Mean intra-clutch PO2 decreased with stage of development, probably because of increasing egg VO2. Additionally, embryo critical oxygen tension increased with developmental stage. Together, these results suggest that python embryos are unable to maintain normal metabolism under brooded conditions during the final 10% of incubation. These results demonstrate that specific parental behaviors can impose obligatory costs to developing offspring and that balancing these behaviors can mediate deleterious consequences.