Standing Variation and the Capacity for Change: Are Endocrine Phenotypes More Variable That Other Traits?

Circulating steroid hormone levels exhibit high variation both within and between individuals, leading some to hypothesize that these phenotypes are more variable than other morphological, physiological, and behavioral traits. This should have profound implications for the evolution of steroid signaling systems, but few studies have examined how endocrine variation compares to that of other traits or differs among populations. Here we provide such an analysis by first exploring how variation in three measures of corticosterone (CORT)—baseline, stress-induced, and post-dexamethasone injection—compares to variation in key traits characterizing morphology (wing length, mass), physiology (reactive oxygen metabolite concentration [d-ROMs] and antioxidant capacity), and behavior (provisioning rate) in two populations of tree swallow (Tachycineta bicolor). After controlling for measurement precision and within-individual variation, we found that only post-dex CORT was more variable than all other traits. Both baseline and stress-induced CORT exhibit higher variation than antioxidant capacity and provisioning rate, but not oxidative metabolite levels or wing length. Variation in post-dex CORT and d-ROMs was also elevated in the higher-latitude population in that inhabits a less predictable environment. We next studied how these patterns might play out on a macroevolutionary scale, assessing patterns of variation in baseline testosterone (T) and multiple non-endocrine traits (body length, mass, social display rate, and locomotion rate) across 17 species of Anolis lizards. At the macroevolutionary level, we found that circulating T levels and the rate of social display output are higher than other behavioral and morphological traits. Altogether, our results support the idea that within-population variability in steroid levels is substantial, but not exceptionally higher than many other traits that define animal phenotypes. As such, circulating steroid levels in free-living animals should be considered traits that exhibit similar levels of variability from individual to individual in a population

Interspecific Variation in Temperature Effects on Embryonic Metabolism and Development in Turtles

We measured temperature-induced differences in metabolic rates and growth by embryos of three turtle species, Macrochelys temminckii, Trachemys scripta, and Apalone spinifera, at different, constant, temperatures. Oxygen consumption rate (VO2) was measured during development and used to characterize changes in metabolism and calculate total O2 consumption. Results from eggs incubated at different temperatures were used to calculate Q10s at different stages of development and to look for evidence of metabolic compensation. Total O2 consumption over the course of incubation was lowest at high incubation temperatures, and late-term metabolic rate Q10s were \u3c2 in all three species. Both results were consistent with positive metabolic compensation. However, incubation temperature effects on egg mass-corrected hatchling size varied among species. Apalone spinifera hatchling mass was unaffected by temperature, whereas T. scripta mass was greatest at high temperatures and M. temminckii mass was lowest at high temperatures. Hatchling mass : length relationships tended to correlate negatively with temperature in all three species. Although we cannot reject positive metabolic compensation as a contributor to the observed VO2 patterns, there is precedence for drawing the more parsimonious conclusion that differences in yolk-free size alone produced the observed incubation temperature differences without energetic canalization by temperature acclimation during incubation

Impact of oxytetracycline exposure on the digestive system microbiota of Daphnia magna.

Antibiotics are used to treat serious illness, but may also be used extraneously or as a preventative measure in many farm animals. This usage increases the potential for unintentional exposure to a variety of organisms. When antibiotics enter aquatic environments, Daphnia magna are especially vulnerable as they filter-feed in freshwater environments. Oxytetracycline (OTC) is a commonly-used broad-spectrum antibiotic used to treat a variety of mammalian diseases. In this study, the impact of OTC on D. magna mortality and gut biota were studied using both cultivation and sequencing-based approaches. Mortality rates were extremely low with the LD50 >2,000ppm. However, OTC impacted abundance and species diversity of intestinal microorganisms in the gut of the D. magna in abundance as well as species diversity. In control organisms, Pseudomonas putida and Aeromonas hydrophila were both present while only P. putida was found in OTC-exposed organisms. Disruption of the intestinal biota in D. magna could have implications on long-term survival, energy expenditure, and reproduction

Acute and Persistent Effects of Pre- and Posthatching Thermal Environments on Growth and Metabolism in the Red-Eared Slider Turtle, Trachemys scripta elegans

Many ectotherms possess the capacity to survive a wide range of thermal conditions. Long-term exposure to temperature can induce acclimational and/or organizational effects, and the developmental stage at which temperature exposure occurs may affect the type, degree, and persistence of these effects. We incubated red-eared slider turtle embryos at three different constant temperatures (Tinc; 26.5, 28.5, 30.5°C), then divided the resulting hatchlings between two water temperatures (Twater; 25, 30°C). We calculated growth rates to assess the short- and long-term effects of thermal experience on this metabolically costly process. We also measured resting metabolic rate (RMR) at three body temperatures (Tbody; 26.5, 28.5, 30.5°C) shortly after hatching and 6 months post hatching to characterize the degree and persistence of acclimationto Tinc and Twater. Hatchling RMRs were affected by Tbody and Tinc, and fit a pattern consistent with positive but incomplete metabolic compensation to Tinc. Average growth rates over the first 11 weeks posthatching were strongly affected by Twater but only marginally influenced by Tinc, and only at Twater= 30°C. Six-month RMRs exhibited strong acclimation to Twater consistent with positive metabolic compensation. However, within each Twater treatment, RMR fits patterns indicative of inverse metabolic compensation to Tinc, opposite of the pattern observed in hatch-lings. Average growth rates calculated over 6 months continued to show a strong effect of Twater, and the previously weak effect of Tinc observed within the 30°C Twater treatment became more pronounced. Our results suggest that metabolic compensation was reversible regardless of the life stage during which exposure occurred, and therefore is more appropriately consider edacclimational than organizational

Data from: Experimentally decoupling reproductive investment from energy storage to test the functional basis of a life-history tradeoff

The ubiquitous life-history trade-off between reproduction and survival has long been hypothesized to reflect underlying energy-allocation trade-offs between reproductive investment and processes related to self-maintenance. Although recent work has questioned whether energy-allocation models provide sufficient explanations for the survival cost of reproduction, direct tests of this hypothesis are rare, especially in wild populations. This hypothesis was tested in a wild population of brown anole lizards (Anolis sagrei) using a two-step experiment. First, stepwise variation in reproductive investment was created using unilateral and bilateral ovariectomy (OVX) along with intact (SHAM) control. Next, this manipulation was decoupled from its downstream effects on energy storage by surgically ablating the abdominal fat stores from half of the females in each reproductive treatment. As predicted, unilateral OVX (intermediate reproductive investment) induced levels of growth, body condition, fat storage and breeding-season survival that were intermediate between the high levels of bilateral OVX (no reproductive investment) and the low levels of SHAM (full reproductive investment). Ablation of abdominal fat bodies had a strong and persistent effect on energy stores, but it did not influence post-breeding survival in any of the three reproductive treatments. This suggests that the energetic savings of reduced reproductive investment do not directly enhance post-breeding survival, with the caveat that only one aspect of energy storage was manipulated and OVX itself had no overall effect on post-breeding survival. This study supports the emerging view that simple energy-allocation models may often be insufficient as explanations for the life-history trade-off between reproduction and survival

Data from: Experimentally decoupling reproductive investment from energy storage to test the functional basis of a life-history tradeoff

The ubiquitous life-history trade-off between reproduction and survival has long been hypothesized to reflect underlying energy-allocation trade-offs between reproductive investment and processes related to self-maintenance. Although recent work has questioned whether energy-allocation models provide sufficient explanations for the survival cost of reproduction, direct tests of this hypothesis are rare, especially in wild populations. This hypothesis was tested in a wild population of brown anole lizards (Anolis sagrei) using a two-step experiment. First, stepwise variation in reproductive investment was created using unilateral and bilateral ovariectomy (OVX) along with intact (SHAM) control. Next, this manipulation was decoupled from its downstream effects on energy storage by surgically ablating the abdominal fat stores from half of the females in each reproductive treatment. As predicted, unilateral OVX (intermediate reproductive investment) induced levels of growth, body condition, fat storage and breeding-season survival that were intermediate between the high levels of bilateral OVX (no reproductive investment) and the low levels of SHAM (full reproductive investment). Ablation of abdominal fat bodies had a strong and persistent effect on energy stores, but it did not influence post-breeding survival in any of the three reproductive treatments. This suggests that the energetic savings of reduced reproductive investment do not directly enhance post-breeding survival, with the caveat that only one aspect of energy storage was manipulated and OVX itself had no overall effect on post-breeding survival. This study supports the emerging view that simple energy-allocation models may often be insufficient as explanations for the life-history trade-off between reproduction and survival