Factors Affecting the Immune System of the Ornate Box Turtle (\u3ci\u3eTerrapene ornata\u3c/i\u3e)

While many studies have detailed the complex intricacies of the endothermic immune system, relatively little is known about the immune system of ectotherms--specifically, reptiles. In an attempt to gain more knowledge about the factors affecting reptilian immune function, ornate box turtles (Terrapene ornata) were subjected to chronic stress in the form of high ambient temperatures in a semi-natural environment. It was hypothesized that chronic stress would lead to elevated levels of corticosterone in the blood, which would, in turn, suppress immune activity. It was found that body temperature and body mass in particular were significantly affected by chronic heat stress. Among turtles subjected to chronic heat stress, white blood cell to red blood cell ratios decreased and numbers of natural antibodies decreased--all of which suggests lowered immune activity. This decrease in immune activity correlated with increases in body temperature as well as body mass. Changes in corticosterone levels within and between both treatment groups were insignificant. Taken together, these results suggest that turtles subjected to chronic heat stress experienced improved health due to increased body mass, increased body temperature, and decreased immune function. This study reinforces the complexity of the stress response, and it highlights its far-reaching effects not only on immune function, but also on the body as a whole

Contributions of phenotypic plasticity to differences in thermogenic performance between highland and lowland deer mice

Small mammals face especially severe thermoregulatory challenges at high altitude because the reduced O2 availability constrains the capacity for aerobic thermogenesis. Adaptive enhancement of thermogenic performance under hypoxic conditions may be achieved via physiological adjustments that occur within the lifetime of individuals (phenotypic plasticity) and/or genetically based changes that occur across generations, but their relative contributions to performance differences between highland and lowland natives are unclear. Here, we examined potentially evolved differences in thermogenic performance between populations of deer mice (Peromyscus maniculatus) that are native to different altitudes. The purpose of the study was to assess the contribution of phenotypic plasticity to population differences in thermogenic performance under hypoxia. We used a common-garden deacclimation experiment to demonstrate that highland deer mice have enhanced thermogenic capacities under hypoxia, and that performance differences between highland and lowland mice persist when individuals are born and reared under common-garden conditions, suggesting that differences in thermogenic capacity have a genetic basis. Conversely, population differences in thermogenic endurance appear to be entirely attributable to physiological plasticity during adulthood. These combined results reveal distinct sources of phenotypic plasticity for different aspects of thermogenic performance, and suggest that thermogenic capacity and endurance may have different mechanistic underpinnings. Includes Supplementary material

The combined effects of reactant kinetics and enzyme stability explain the temperature dependence of metabolic rates

A mechanistic understanding of the response of metabolic rate to temperature is essential for understanding thermal ecology and metabolic adaptation. Although the Arrhenius equation has been used to describe the effects of temperature on reaction rates and metabolic traits, it does not adequately describe two aspects of the thermal performance curve (TPC) for metabolic rate—that metabolic rate is a unimodal function of temperature often with maximal values in the biologically relevant temperature range and that activation energies are temperature dependent. We show that the temperature dependence of metabolic rate in ectotherms is well described by an enzyme-assisted Arrhenius (EAAR) model that accounts for the temperature-dependent contribution of enzymes to decreasing the activation energy required for reactions to occur. The model is mechanistically derived using the thermodynamic rules that govern protein stability. We contrast our model with other unimodal functions that also can be used to describe the temperature dependence of metabolic rate to show how the EAAR model provides an important advance over previous work. We fit the EAAR model to metabolic rate data for a variety of taxa to demonstrate the model’s utility in describing metabolic rate TPCs while revealing significant differences in thermodynamic properties across species and acclimation temperatures. Our model advances our ability to understand the metabolic and ecological consequences of increases in the mean and variance of temperature associated with global climate change. In addition, the model suggests avenues by which organisms can acclimate and adapt to changing thermal environments. Furthermore, the parameters in the EAAR model generate links between organismal level performance and underlying molecular processes that can be tested for in future work

Food Supplements Modulate Changes in Leucocyte Numbers in Breeding Male Ground Squirrels

Immunosuppression may be an important cost of reproduction in breeding males. It can result from elevated levels of testosterone or stress hormones and may serve to lower the energetic cost of maintaining immune function at a time of high demand. This suggests that greater access to energy resources could reduce immunosuppression as a cost of reproduction, minimizing the trade-off between energetic investment in current reproductive effort and survival. I examined the impact of food availability on immune function by provisioning male Belding’s ground squirrels in the field from the time they emerged from hibernation to the start of breeding. Temporal changes in immune status, measured by leucocyte counts, differed between provisioned males and un-provisioned controls. Provisioning advanced the increase in lymphocytes and neutrophils from after breeding to before. At the start of breeding, the leucocyte count was three times greater in provisioned males than in controls and was still nearly twice as great at the end of breeding. Control males increased all leucocyte numbers after breeding. This experiment demonstrates that variation in food intake can lead to individual variation in the extent of immunosuppression during breeding and therefore that reduced immune function may not be an obligatory cost of reproduction

Food Restriction Effects on the Body Composition of Free-living Ground Squirrels, \u3ci\u3eSpermophilus belding\u3c/i\u3e

I conducted afield experiment to examine the use of body fat as an energy re- serve by juvenile female Belding\u27s ground squirrels, Spermophilus beldingi, prior to hibernation. Squirrels were divided into two groups: the squirrels in one group had their foraging times restricted by being placed in a cage forpart of each day for four consecutive days (deprived group), and the squirrels in the other group foraged ad lib. (ad lib. group). The deprived group foraged in its normal habitat morning and evening but had access only to water and lettuce while captive. Total body electrical conductivity (TOBEC), an in vivo method of estimating fat-free mass, was used to follow changes in body composition. The deprived group lost mass, while the ad lib. group gained mass. Total body electrical conductivity measurements indicated that the deprived group lost fat-free mass and not fat, whereas mass gained by the ad lib. group was mostly fat. This is consistent with an emphasis on fat anabolism prior to hibernation. I also show that the TOBEC method is affected by variation in stomach contents, and I evaluate conditions under which an intraspecific calibration equation is needed

Relationships between Body Composition, Body Size and Alternative Reproductive Tactics in a Lekking Sandpiper, the Ruff (\u3ci\u3ePhilomachus pugnax\u3c/i\u3e)

1. The Ruff is a lekking sandpiper in which males have two genetically determined alternative mating strategies: ‘satellite’ or ‘independent’. Satellite males are non-territorial, following groups of females on and off leks. Independent males attempt to establish territories on leks and can be either ‘resident’ territory holders, or non-territorial ‘marginals.’ The time budgets of the three behavioral tactics (resident, marginal and satellite) differ notably in time allocated to foraging, aggression and display activity. These differences have led to the suggestion that the benefits of size and energy stores may vary with mating tactic. 2. In this paper in vivo estimates of body composition (fat, fat-free mass) for breeding male Ruffs using total body electrical conductivity (TOBEC) are presented. 3. Satellite males have significantly shorter tarsi and wings than independent males. 4. After correcting for size, independent males are significantly heavier and fatter than satellites, and marginals in particular are heavier and fatter than both residents and satellites. 5. Estimates of energy expenditure during flight suggest that satellites may maintain reduced energy reserves to minimize flight costs, while the larger fat stores of independent males are consistent with the benefits of endurance as a territory holder

Exercise Capacity of House Wren Nestlings: Chicks Are Not Working as Hard as They Can

We measured the aerobic capacity for exercise in house wren (Troglodytes aedon) nestlings from 3 to 10 days of age. Exercise data were compared with previous measurements of the energy cost of begging (Ebeg) to determine if begging chicks are working at maximal exercise capacity. We also compared exercise with the peak metabolic rate during digestion and, in older chicks, with thermogenic heat production. Rates of oxygen consumption (VO2) during exercise increased rapidly with age, but the factorial aerobic scope for exercise (exercise VO2/resting VO2) averaged only 1.4 (at three to four days) to 1.7 (at 8 to 10 days). The mean Ebeg was consistently lower than exercise VO2, but maximal Ebeg was similar to exercise VO2. The peak VO2 during digestion was higher than exercise VO2 for young chicks (three to six days) but not significantly different for older chicks. In older chicks (8 to 10 days), regulatory thermogenesis at 22°C was significantly higher than exercise VO2. These results suggest that some begging by house wren chicks can be at maximal muscular effort, but the average output during begging probably is not constrained by exercise capacity. The metabolic capacity for muscular activity of any kind is lower than the metabolic capacity for digestion (at least in young chicks), which presumably reflects a high priority for food processing and growth

The Costs of Female Choice in a Lekking Bird

We investigated the costs of active female choice in sage grouse, Centrocercus urophasianus, a lekking species in which females make repeated, lengthy visits to leks to assess males before mating. Several potential costs were measured by monitoring changes in hens\u27 ranging behavior, time budgets, and encounter rates with predators when they visited leks. Two costs were identified: hens moved further per day and encountered golden eagles, Aquila chrysaetos, more frequently when visiting leks. However, extra travel due to visiting leks increased predicted daily energetic expenditure by only about 1%, and the risk of predation by golden eagles over a typical series of lek visits (compared to a single short visit for mating) was estimated to reduce annual survival by \u3c 0.1%. Two other potential costs were not supported: visiting leks did not depress foraging time or conflict with nest defense. These results indicate that any costs of mate choice are slight and imply that even very small benefits could be sufficient to maintain female choice. We present calculations which suggest that increased offspring viability due to choosing fitter males could balance predation costs even if the heritability of fitness is low and if females identify fitter males with only moderate accuracy. Despite recent emphasis on the direct benefits of mate choice, we conclude that either indirect or direct benefits could provide a plausible solution to the lek paradox

Severe Drought Impacts Female Pheasant Physiology in Southwest Nebraska

In 2012, Nebraska experienced one of the worst droughts since the 1930s, accompanied by abnormally high temperatures. We studied the impacts of the 2012 summer drought on female ring-necked pheasant (Phasianus colchicus) body condition and baseline and stress-induced corticosterone concentrations (CORT). We hypothesized that drought conditions would reduce pheasant body condition, increase chronic stress resulting in elevated baseline CORT levels, and down-regulate pheasant stress response to acute stressors, resulting in reduced stress-induced CORT concentrations. In southwestern Nebraska, we captured female pheasants in 2012 (pre-drought) and 2013 (post-drought). Pheasants had poorer body condition after the drought. Although female CORT measures were similar among years (baseline: F1,8 = 0.591, P = 0.465; stress-induced: F1,26 = 1.118, P = 0.300), females in poorer condition had elevated baseline CORT (F1,26 = 6.446, P = 0.018) and stress-induced CORT (F1,26 = 8.770, P = 0.006) with potential negative consequences for reproduction. Our results suggest that it is critical for managers to consider how to buffer the negative impacts of drought on pheasant physiology and population growth, as droughts are likely to occur more frequently in southwest Nebraska in the next century

Repeatability of Maximal Aerobic Performance in Belding’s Ground Squirrels, \u3ci\u3eSpermophilus beldingi\u3c/i\u3e

1. The repeatability of a trait is a critical factor in determining how that trait is affected by natural selection. We examined the repeatability of a key physiological trait, maximum oxygen consumption (VO2max), in a wild population of Belding’s Ground Squirrels, Spermophilus beldingi. VO2max is an integrated measure of organismal metabolic performance. It can be intuitively related to fitness because it sets an upper limit to sustainable power output during ecologically important activites such as locomotion and thermoregulatory heat production. 2. We used respirometry to determine VO2max during exercise and thermogenesis. Exercise VO2max was elicited in an enclosed running wheel. Thermogenic VO2max was obtained with acute cold exposure in a helium-oxygen gas mixture. 3. Repeatability of both exercise and thermogenic VO2max was high over 2 h intervals but declined over longer test periods (6-18 days and 1-2 years). In general, repeatability was higher for exercise VO2max than for thermogenic VO2max. 4. We found no repeatability for animals tested initially as juveniles and then 1 or 2 years later as adults; evidently there is sufficient plasticity in VO2max to decouple aerobic performance between these life stages. A small number of adults tested in successive years showed significant repeatability of exercise VO2max but no repeatability of thermogenic VO2max