Effects of Body Mass, Feeding, and Circadian Cycles on Metabolism in the Lizard Sceloporus occidentalis

Divergent effects of climate change on the egg-laying opportunity of species in cold and warm regions

Climate warming can substantially impact embryonic development and juvenile growth in oviparous species. Estimating the overall impacts of climate warming on oviparous reproduction is difficult because egg-laying events happen throughout the reproductive season. Successful egg laying requires the completion of embryonic development as well as hatching timing conducive to offspring survival and energy accumulation. We propose a new metric-egg-laying opportunity (EO)-to estimate the annual hours during which a clutch of freshly laid eggs yields surviving offspring that store sufficient energy for overwintering. We estimated the EO within the distribution of a model species, Sceloporus undulatus, under recent climate condition and a climate-warming scenario by combining microclimate data, developmental functions, and biophysical models. We predicted that EO will decline as the climate warms at 74.8% of 11,407 sites. Decreasing hatching success and offspring energy accounted for more lost EO hours (72.6% and 72.9%) than the occurrence of offspring heat stress (59.9%). Nesting deeper (at a depth of 12 cm) may be a more effective behavioral adjustment for retaining EO than using shadier (50% shade) nests because the former fully mitigated the decline of EO under the considered warming scenario at more sites (66.1%) than the latter (28.3%). We advocate for the use of EO in predicting the impacts of climate warming on oviparous animals because it encapsulates the integrative impacts of climate warming on all stages of reproductive life history.

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.

In a field cricket, investment into flight capacity (1) increased the circadian rhythmicity of resource use (standard metabolic rate), (2) reduced resource acquisition (food intake), and (3) reduced the efficiency by which ingested food was converted to reproductive tissue.

Analyzing Time-Energy Constraints to Understand the Links between Environmental Change and Local Extinctions in Terrestrial Ectotherms

AbstractAccelerated extinction rates have prompted an increased focus on the interplay between environmental change and species response. The effects of environmental change on thermal opportunity are typically considered through a climate change context. However, habitat alteration can also have strong effects on the thermal environment. Additionally, habitat alteration is considered a leading factor of species extinction, yet few studies address the influence of habitat alteration on thermal opportunity and time-energy budgets in at-risk species. Here, we show the strong effects that habitat degradation can have on thermal opportunity, time-energy budgets, and life history demographics of local populations. In the Ozark Mountains of northern Arkansas, woody vegetation encroachment has resulted in a shift in life history traits that appears to play an important role in recent extirpations of eastern collared lizards (Crotaphytus collaris). Populations in degraded habitats experienced a decline in thermal opportunity and less time at body temperatures (time at T_b) suitable for digestion compared with those in intact habitats. We used our data to model the effect of reduced time at T_b on the net assimilated energy available for growth and reproduction. Our model predicts an ∼46% decline in the annual fecundity of individuals, which is similar to empirical observations of reproduction of C. collaris populations in degraded habitats (~49%). We conclude that C. collaris in degraded habitats experienced reduced growth and reproduction primarily as a result of constrained thermal opportunity leading to a decline in digestive processing rates. Our study applies an underappreciated approach to identify the biophysical and time-energy effects of habitat alteration.

Compensating for a stressful pregnancy? Glucocorticoid treatment during gravidity reduces metabolic rate in female fence lizards post-parturition

Reproduction is a critical part of an animal's life history, but one which incurs significant costs to survival and future reproductive potential. These physiological consequences are likely to be influenced by context - for example, if an individual is subject to environmental stressors, physiological and behavioral changes associated with reproduction may be altered. Glucocorticoids, hormones produced as part of the physiological response to stressors, may alter how reproduction affects female physiology and behavior, and therefore the outcomes of reproductive trade-offs. Glucocorticoids prioritize immediate survival over reproduction, for example through changes in immune function, metabolic rate, and foraging, which may reduce energy expenditure or increase energy gain. However, we previously found that female eastern fence lizards (Sceloporus undulatus) experiencing elevated glucocorticoid levels during gestation were nevertheless able to maintain reproductive output and body condition. Here we investigate compensatory mechanisms by which eastern fence lizard females may maintain reproduction under experimental increases in a glucocorticoid, corticosterone (CORT). We found that, although CORT-treated females had similar immune function and behavior, they had reduced metabolic rates 3-5 days post-parturition compared to control females. Given that CORT-treated females spent a similar time basking and had equal food intake compared to control females, we suggest that the reduced metabolic rate is a mechanism by which CORT-treated females maintain their energy balance and reduce the energetic costs of gestation during periods of stress. This study suggests that physiological responses to reproduction may be context-dependent and could act to minimize costs of reproduction in situations where CORT is elevated (such as during periods of environmental stress).

Body size impacts critical thermal maximum measurements in lizards

Understanding the mechanisms behind critical thermal maxima (CTmax; the high body temperature at which neuromuscular coordination is lost) of organisms is central to understanding ectotherm thermal tolerance. Body size is an often overlooked variable that may affect interpretation of CTmax, and consequently, how CTmax is used to evaluate mechanistic hypotheses of thermal tolerance. We tested the hypothesis that body size affects CTmax and its interpretation in two experimental contexts. First, in four Sceloporus species, we examined how inter- and intraspecific variation in body size affected CTmax at normoxic and experimentally induced hypoxic conditions, and cloacal heating rate under normoxic conditions. Negative relationships between body size and CTmax were exaggerated in larger species, and hypoxia-related reductions in CTmax were unaffected by body size. Smaller individuals had faster cloacal heating rates and higher CTmax, and variation in cloacal heating rate affected CTmax in the largest species. Second, we examined how body size interacted with the location of body temperature measurements (i.e., cloaca vs. brain) in Sceloporus occidentalis, then compared this in living and deceased lizards. Brain temperatures were consistently lower than cloacal temperatures. Smaller lizards had larger brain-cloacal temperature differences than larger lizards, due to a slower cloacal heating rate in large lizards. Both live and dead lizards had lower brain than cloacal temperatures, suggesting living lizards do not actively maintain lower brain temperatures when they cannot pant. Thermal inertia influences CTmax data in complex ways, and body size should therefore be considered in studies involving CTmax data on species with variable sizes.

The effect of temperature on standard metabolic rate of Brown Anoles

Understanding the influences of sex and ambient temperature on metabolic rates of reptiles is of fundamental interest to biologists because of the role that temperature-dependent metabolic rates play in shaping behaviour, life history evolution and geographic range. We investigated the effects of sex, body mass and temperature on standard metabolic rate, respiratory quotient (RQ), and Q10 in male and female Brown Anoles, Norops sagrei. When mass-adjusted, oxygen consumption increased linearly with temperature, and there was no effect of sex. RQ did not differ by sex or temperature. Q10 was within the range published for other lizards.

Elevated plasma corticosterone increases metabolic rate in a terrestrial salamander

Plasma glucocorticoid hormones (GCs) increase intermediary metabolism, which may be reflected in whole-animal metabolic rate. Studies in fish, birds, and reptiles have shown that GCs may alter whole-animal energy expenditure, but results are conflicting and often involve GC levels that are not physiologically relevant. A previous study in red-legged salamanders found that male courtship pheromone increased plasma corticosterone (CORT; the primary GC in amphibians) concentrations in males, which could elevate metabolic processes to sustain courtship behaviors. To understand the possible metabolic effect of elevated plasma CORT, we measured the effects of male courtship pheromone and exogenous application of CORT on oxygen consumption in male red-legged salamanders (Plethodon shermani). Exogenous application of CORT elevated plasma CORT to physiologically relevant levels. Compared to treatment with male courtship pheromone and vehicle, treatment with CORT increased oxygen consumption rates for several hours after treatment, resulting in 12% more oxygen consumed (equivalent to 0.33 J) during our first 2h sampling period. Contrary to our previous work, treatment with pheromone did not increase plasma CORT, perhaps because subjects used in this study were not in breeding condition. Pheromone application did not affect respiration rates. Our study is one of the few to evaluate the influence of physiologically relevant elevations in CORT on whole-animal metabolism in vertebrates, and the first to show that elevated plasma CORT increases metabolism in an amphibian.

Specific dynamic action: a review of the postprandial metabolic response

For more than 200 years, the metabolic response that accompanies meal digestion has been characterized, theorized, and experimentally studied. Historically labeled "specific dynamic action" or "SDA", this physiological phenomenon represents the energy expended on all activities of the body incidental to the ingestion, digestion, absorption, and assimilation of a meal. Specific dynamic action or a component of postprandial metabolism has been quantified for more than 250 invertebrate and vertebrate species. Characteristic among all of these species is a rapid postprandial increase in metabolic rate that upon peaking returns more slowly to prefeeding levels. The average maximum increase in metabolic rate stemming from digestion ranges from a modest 25% for humans to 136% for fishes, and to an impressive 687% for snakes. The type, size, composition, and temperature of the meal, as well as body size, body composition, and several environmental factors (e.g., ambient temperature and gas concentration) can each significantly impact the magnitude and duration of the SDA response. Meals that are large, intact or possess a tough exoskeleton require more digestive effort and thus generate a larger SDA than small, fragmented, or soft-bodied meals. Differences in the individual effort of preabsorptive (e.g., swallowing, gastric breakdown, and intestinal transport) and postabsorptive (e.g., catabolism and synthesis) events underlie much of the variation in SDA. Specific dynamic action is an integral part of an organism's energy budget, exemplified by accounting for 19-43% of the daily energy expenditure of free-ranging snakes. There are innumerable opportunities for research in SDA including coverage of unexplored taxa, investigating the underlying sources, determinants, and the central control of postprandial metabolism, and examining the integration of SDA across other physiological systems.

Effect of exogenous corticosterone on respiration in a reptile

Release of glucocorticoids (GCs) enables organisms to meet energy requirements during stressful situations by regulating intermediary metabolism. In the absence of compensatory mechanisms, increased metabolic activity (e.g., protein catabolism, lipolysis, and gluconeogenesis) should translate to increases in whole animal metabolism, and therefore energy expenditures, by organisms. However, to our knowledge, no study has estimated the total energy cost of elevated plasma GCs in any organism. Here we evaluated the effect of exogenous corticosterone (CORT) on metabolism in captive western fence lizards (Sceloporus occidentalis) by conducting two experiments. In experiment I we determined the dynamics of plasma CORT concentrations resulting from CORT injections. In experiment II we frequently measured changes in respiration for 24h before and after CORT injection. Injection of 0.025 microg/g (low CORT) and 0.40 microg/g CORT (high CORT) produced up to 26-fold increases in plasma CORT in lizards 3h following injection compared to baseline levels. Plasma CORT concentrations returned to baseline levels 6h after injection. CORT increased metabolism in both low and high CORT lizards (up to 121% and 245%, respectively) during the first 7.5h after CORT injections compared to pre-treatment trials. Respiration returned to baseline rates 7.5h after CORT administration. A surprising finding was that although high CORT males achieved higher plasma CORT concentrations than high CORT females, the metabolic response of high CORT females was 30% greater than high CORT males. Our results suggest that GC-induced changes in respiration may be important for understanding the overall energetic implications of stress.

Metabolic and digestive response to food ingestion in a binge-feeding lizard, the Gila monster (Heloderma suspectum)

The gastrointestinal tract possesses the capacity to change in form and function in response to fasting and feeding. Such plasticity can be dramatic for species that naturally experience long episodes of fasting between large meals (e.g. sit-and-wait foraging snakes, estivating anurans). By contrast, for active foraging species that feed more frequently on smaller meals, gastrointestinal responses are more modest in magnitude. The Gila monster Heloderma suspectum is an active foraging lizard that feeds infrequently on meals weighing up to one-third of its body mass. Additionally, Gila monsters possess a species-specific salivary peptide, exendin-4, which may be involved in the regulation of metabolic and digestive performance. To investigate the adaptive postprandial response of Gila monsters and the potential regulatory role of exendin-4, we measured metabolic and intestinal responses to feeding in the presence or absence of circulating exendin-4. Following the consumption of rodent or egg meals equivalent to 10% of lizard body mass, metabolic rates peaked at 4.0- to 4.9-fold of standard metabolic rates and remained elevated for 5-6 days. Specific dynamic action of these meals (43-60 kJ) was 13-18% of total meal energy. Feeding triggered significant increases in mucosal mass, enterocyte width and volume, and the upregulation of D-glucose uptake rates and aminopeptidase-N activity. Total intestinal uptake capacity for L-leucine, L-proline and D-glucose were significantly elevated within 1-3 days after feeding. Whereas the absence of circulating exendin-4 had no impact on postprandial metabolism or the postprandial response of intestinal structure and nutrient uptake, it significantly increased intestinal aminopeptidase-N activity. Within the continuum of physiological responses to feeding and fasting, Gila monsters occupy an intermediate position in experiencing moderate, though significant, regulation of intestinal performance with feeding.

Energy acquisition and allocation in an ectothermic predator exposed to a common environmental stressor

Stressors are commonly encountered by organisms and often prove to be energetically costly. Certain stressors can simultaneously affect multiple components of an animal's energy budget and can either exacerbate energetic costs to the individual or offset one another. Here we used a commonly encountered stressor, the pesticide carbaryl, to examine the complex effects that acute environmental disturbances can have on energy expenditure, allocation, and acquisition, important processes that influence growth and reproduction. After exposing lizards (Sceloporus occidentalis) to carbaryl, we measured their metabolism over a 48 h period and assessed their food consumption over 96 h. We found no difference in total energy expenditure among treatment groups, but lizards exposed to the highest dose of carbaryl allocated energy differently than other groups. Compared to controls, these lizards exhibited a 16-30% increase in standard metabolic rate (SMR), which was offset by a 45-58% decrease in additional energy expenditures. Lizards in the highest dose group also exhibited a 30-34% decrease in energy acquisition compared to controls. The net result was a 1.83 kJ decrease in energy assimilation, equivalent to 5 times their daily SMR requirements. Our results indicate that energetic consequences of stressors may result in complex energetic trade-offs, and emphasize the need to simultaneously examine the effect of stressors on multiple portions of an animal's energy budget.