Open-flow Respirometry Research Articles

Open-flow respirometry under field conditions: How does the airflow through the nest influence our results?

Open-flow respirometry is a common method to measure oxygen-uptake as a proxy of energy expenditure of organisms in real-time. Although most often used in the laboratory it has seen increasing application under field conditions. Air is drawn or pushed through a metabolic chamber or the nest with the animal, and the O2 depletion and/or CO2 accumulation in the air is analysed to calculate metabolic rate and energy expenditure. Under field conditions, animals are often measured within the microclimate of their nest and in contrast to laboratory work, the temperature of the air entering the nest cannot be controlled. Thus, the aim of our study was to determine the explanatory power of respirometry in a set-up mimicking field conditions. We measured O2 consumption of 14 laboratory mice (Mus musculus) using three different flow rates [50L*h-1 (834mL*min-1), 60L*h-1 (1000mL*min-1) and 70L*h-1 (1167mL*min-1)] and two different temperatures of the inflowing air; either the same as the temperature inside the metabolic chamber (no temperature differential; 20°C), or cooler (temperature differential of 10°C). Our results show that the energy expenditure of the mice did not change significantly in relation to a cooler airflow, nor was it affected by different flow rates, despite a slight, but significant decrease of about 1.5°C in chamber temperature with the cooler airflow. Our study emphasises the validity of the results obtained by open-flow respirometry when investigating energy budgets and physiological responses of animals to ambient conditions. Nevertheless, subtle changes in chamber temperature in response to changes in the temperature and flow rate of the air pulled or pushed through the system were detectable. Thus, constant airflow during open-flow respirometry and consequent changes in nest/chamber temperature should be measured.

Measuring Energy Expenditure Independent Of The Respiratory Quotient During Rest And Exercise

Widely used measures of energy expenditure (EE) are based on Weir’s (1949) equation that relies on oxygen utilization and carbon dioxide production to derive a respiratory quotient (RQ). However, Weir’s RQ-free equation, presented in the same article, was only recently validated to measure home-cage EE in rodents. It has not yet been used to measure EE in humans. PURPOSE: To evaluate an RQ-free method for measuring EE at rest and during submaximal and maximal intensity treadmill running. METHODS: A convenience sample of 27 physically-active college students (17 women) were recruited to perform a maximal treadmill test until volitional exhaustion (Bruce protocol). EE (kcal/min) was measured continuously at 10 Hz using an open-flow respirometry system (Sable Systems Intl.; Las Vegas, NV). EE was down-sampled and averaged at rest (EERest), during the first stage (EE1), during the last completed stage (EESubmax), and during the final stage (EEMax). The intensity (VO2; ml/kg*min) of the last completed stage and the final stage were calculated based on speed and incline using ACSM equations. Heart rate (HR) was measured at rest and every minute during the test. Participants rated their perceived exertion (RPE) at the end of each stage. VO2max was estimated based on the total time spent walking/running on the treadmill. Four multiple regression models were used to predict EE from participant traits (PT; gender, age, weight), stage intensity, HR, RPE, and estimated VO2max (EEMax only). A repeated-measures ANOVA was used to test the degree to which changes in EE were explained by changes in HR (treated as a time-varying covariate). RESULTS: EERest was predicted by PT and resting HR [p<.001, R2 = .658]. EE1 was predicted by PT and HR and RPE in Stage 1 [p<.001, R2 = .871]. EESubmax was predicted by PT, the intensity of, and HR and RPE response to, the last completed stage [p<.001, R2 = .844]. EEMax was predicted by PT, VO2max, and the intensity of, and HR and RPE response to, the final stage [p<.001, R2 = .746]. 83% of the increase in EE, from rest to the final stage, was explained by increasing HR. CONCLUSIONS: The reliability (R2) of our models for predicting EE at rest and during exercise are comparable to published RQ-dependent models, supporting the utility of an RQ-free method for measuring EE during submaximal exercise.

Aerobic Metabolism Alterations as an Evidence of Underlying Deltamethrin Resistance Mechanisms in Triatoma infestans (Hemiptera: Reduviidae)

Triatoma infestans (Klug, 1834), the main vector of Chagas disease in Latin America, is regularly controlled by spraying the pyrethroid deltamethrin, to which some populations have developed resistance. The three main mechanisms of resistance are 1) metabolic resistance by overexpression or increased activity of detoxifying enzymes, 2) target site mutations, and 3) cuticle thickening/modification. We use open-flow respirometry to measure real-time H2O loss rate (V˙H2O) and CO2 production rate (V˙CO2), on nymphs from susceptible and resistant populations before and after exposure to the insecticide to understand the underlying mechanisms of resistance in live insects. Lack of differences in V˙H2O between populations suggested that cuticular thickness/composition is not acting as a relevant resistance mechanism. Similarly, there was no difference in resting V˙CO2, suggesting a trade-off between resistance mechanisms and other physiological processes. The increment in V˙CO2 after application of deltamethrin was similar in both populations, which suggested that while enhanced enzymatic detoxification may play a role in resistance expression in this population, the main mechanism involved should be a passive one such as target site mutations. Open-flow respirometry provided useful evidence for evaluating the mechanisms involved in deltamethrin resistance. Using this technique could improve efficiency of scientific research in the area of insecticide resistance management, leading to a faster decision making and hence improved control results.

Evaporative water loss in Kuhl's pipistrelles declines along an environmental gradient, from mesic to hyperarid

Intraspecific variation in animal energy and water balances may play an important role in local adaptation of populations to specific habitats such as deserts. We examined Kuhl's pipistrelle (Pipistrellus kuhlii), a common bat in Israel that ranges in distribution from mesic Mediterranean to hyperarid desert habitats, for intraspecific differences in metabolic rate (MR) and evaporative water loss (EWL) among populations along a climatic gradient. We tested the prediction that EWL, especially at high ambient temperatures is lower in Kuhl's pipistrelles from desert habitats than from mesic habitats. We measured MR and total evaporative water loss (TEWL) at four ambient temperatures (10 °C, 20 °C, 30 °C and 35 °C) in three groups of bats using open-flow respirometry. We fitted the bats with a mask to separate cutaneous water loss (CWL) from respiratory water loss (RWL) at 35 °C. At 35 °C, mean TEWL in the southernmost group, from the hyperarid location, was significantly lower than in the other two groups, with no apparent difference in mean MR. The source of difference TEWL was that the southern group had significantly lower CWL than the other two groups; RWL did not differ among them. This suggests that there are mechanisms that reduce EWL from the skin of the bats; a likely candidate is modification of the lipids in the outer layer of the dermis that make the skin possibly less permeable to water as has been described in birds and a few other species of bat.

Migration Takes Extra Guts for Juvenile Songbirds: Energetics and Digestive Physiology During the First Journey

Many birds undertake long migrations when they are only a few months of age. Although they are typically of adult body size, their performance and survival are often poor compared to adults. This differential performance could be due to lack of experience, selection against poor-performing cohort members, or inherent constraints of continuing physiological and morphological maturation of juveniles. Limited evidence suggests that digestive and muscle physiology of juveniles during their first migration may differ from that of adults. We compared body composition, metabolic rate, and digestive physiology between juvenile and adult passerines during fall migration. First, we measured fat and lean masses by quantitative magnetic resonance, and organ and muscle masses of salvaged carcasses of fall migrants from four passerine species. In general, juveniles had more lean mass and heavier digestive organs (especially liver) than adults in hermit thrushes (Catharus guttatus), Swainson’s thrushes (Catharus ustulatus), ovenbirds (Seiurus aurocapilla), and white-throated sparrows (Zonotrichia albicollis). Principal components analysis of all organs and muscles revealed that juveniles for three of four species had overall larger digestive components and smaller flight muscles than adults. We then used open-flow respirometry to measure basal metabolic rates (BMRs) of juvenile and adult Swainson’s thrushes and white-throated sparrows captured in fall at a migratory stopover site. Controlling for a significant effect of body mass, juveniles had 6% higher BMRs than adults in both species. We then conducted total collection mass balance feeding trials with fall migratory Swainson’s thrushes and white-throated sparrows. Juvenile thrushes had greater metabolizable energy intake than adults, which was achieved through higher food intake rather than greater utilization efficiency. Age classes of white-throated sparrows did not differ in these measures of digestive performance, although juveniles had greater food intake capacity at low lean body masses. We propose that age-related differences in foraging ecology, diet composition and energy requirements may be responsible for larger digestive organs of juvenile migrants. Larger guts may allow juveniles to consume more food or a more dilute diet, but may contribute to higher BMRs.

Low ambient temperature reduces the time for fuel switching in the ruby-throated hummingbird (Archilochus colubris)

Physiological adaptations that enhance flux through the sugar oxidation cascade permit hummingbirds to rapidly switch between burning lipids when fasted to burning ingested sugars when fed. Hummingbirds may be able to exert control over the timing and extent of use of ingested sugars by varying digestive rates when under pressure to accumulate energy stores or acquire energy in response to heightened energy demands. We hypothesized that hummingbirds would modulate the timing of a switch to reliance on ingested sugars differently when facing distinct energetic demands (cool versus warm ambient temperatures). The timing of the oxidation of a single nectar meal to fuel metabolism was assessed by open-flow respirometry, while the time to first excretion following the meal was used as a proxy for digestive throughput time. As predicted, birds showed a more rapid switch in respiratory exchange ratio (RER = rate of O2 consumption/CO2 production) and excreted earlier when held at cool temperatures compared to warm. In both cases, RER peaked barely above 1.0 indicating ingested sugar fueled ≈100% of resting metabolism. Our findings suggest that energetic demands modulate the rate of fuel switching through shifts of the sugar oxidation cascade. The speed of this shift may involve decreases in gut passage times which have previously been thought to be inflexible, or may be caused by changes in circulation as a result of low ambient temperature.

Evaporative cooling and vasodilation mediate thermoregulation in naked mole-rats during normoxia but not hypoxia.

Naked mole-rats are among the most hypoxia-tolerant mammals but have a poor thermoregulatory capacity due to their lack of insulating fur and fat, and small body size. In acute hypoxia, naked mole-rat body temperature (Tb) decreases to ambient temperature (Ta) but the mechanisms that underlie this thermoregulatory response are unknown. We hypothesized 1) that naked mole-rat blood vessels vasodilate during hypoxia to shunt heat toward the body surface and/or 2) that they augment heat loss through evaporative cooling. Using open-flow respirometry (indirect calorimetry) we explored metabolic and thermoregulatory strategies of naked mole-rats exposed to hypoxia (7% O2 for 1 h) at two relative humidities (RH; 50 or 100% water saturation), and in two Ta's (25 and 30 °C), alone, and following treatment with the vasoconstrictor angiotensin II (ANGII). We found that Tb and metabolic rate decreased in hypoxia across all treatment groups but that neither RH nor ANGII effected either variable in hypoxia. Conversely, both Tb and metabolic rate were reduced in 100% RH or by ANGII treatment in normoxia at 25 °C, and therefore the absolute change in both variables with the onset of hypoxia was reduced when vasodilation or evaporative cooling were prevented. We conclude that naked mole-rats employ evaporative cooling and vasodilation to thermoregulate in normoxia and in 25 °C but that neither mechanism is involved in thermoregulatory changes during acute hypoxia. These findings suggest that NMRs may employ passive strategies such as reducing thermogenesis to reduce Tb in hypoxia, which would support metabolic rate suppression.

The influence of meal size on the digestive energetics of Gould’s wattled bat, Chalinolobus gouldii

Although variation in meal size is known to have an impact on digestive energetics, there is limited information on how it influences metabolic rate and energy assimilation in insectivorous bats. We investigated the influence of meal size, representing 10% or 20% of an individual’s weight, on the digestive energetics of Gould’s wattled bat, Chalinolobus gouldii (n = 61 bats). Using open-flow respirometry, we recorded a median resting metabolic rate of 2.0 mL g–1 h–1 (n = 51, range = 0.4–4.8) at an air temperature of 32°C. Median postprandial metabolic rate peaked at 6.5 (range = 3.4–11.6, n = 4) and 8.2 (range = 3.8–10.6, n = 7), representing 3.3- and 4.1-fold increases from resting metabolic rate for the two meal sizes. Using bomb calorimetry, we calculated the calorific value of the two meal sizes, and the calories lost during digestion. Following gut passage times of 120 min (range = 103–172, n = 15) and 124 min (range = 106–147, n = 12), C. gouldii assimilated 88.0% (range = 84.6–93.8, n = 5) and 93.3% (range = 84.0–99.4, n = 10) of the kilojoules available from the 10% and 20% meal sizes, respectively. When fed ad libitum, C. gouldii consumed a mean of 23.2% of their body weight during a single sitting (n = 18, range = 6.3–34.1%). Overall, digestive energetics were not significantly different between 10% or 20% meal sizes. The ability to ingest small and large meals, without compromising the rate or efficiency of calorie intake, indicates that free-ranging C. gouldii are likely limited by food available in the environment, rather than the ability to assimilate energy.

Changes in metabolic rate of spiny lobster under predation risk

Animals exposed to high levels of predation risk may exhibit a variety of changes in life history, behaviour, physiology and morphology that can affect survival. Under predation threat, prey individuals may increase their aerobic metabolism to allocate energy toward escaping behaviours (e.g. ‘fight or flight’), although the associated energetic cost of such behaviour remains largely unknown. Lobsters display different anti-predatory responses, such as sheltering and/or escaping, but the underlying energetic cost of such responses has not been examined. Here, we tested the aerobic metabolic response of southern rock lobsters Jasus edwardsii in the presence of predator (Maori octopus Octopus maorum) olfactory cues (kairomones) using open-flow respirometry. We examined the routine metabolic rate of lobsters in response to predator kairomones during the active phase of their diurnal cycle (at night) to investigate the physiological anti-predator response when lobsters are most vulnerable. Our findings revealed that lobsters strongly reduced their routine metabolism for 3 h by 31.4% when exposed to kairomones in comparison to controls. Our findings suggest that under laboratory conditions, lobsters exposed to predation risk during the night reduce their activity to avoid predators, i.e. the anti-predator mechanism is to be immobile or inactive rather than showing a fight-or-flight response. Lobster immobility may be an energetically advantageous anti-predator response in the short term; however, prolonged or regular predator exposure could have significant consequences on foraging time and foraging area, with an overall impact on lobster performance, particularly in environments with high predator presence such as fishing grounds.

Sand type influences the energetics of nest escape in Brisbane river turtle hatchlings

Freshwater turtles can construct their nest in a wide range of soil types, and because different soil types have different physical characteristics such as particle size distribution and compactness, soil type presumably affects digging performance and the energetics of nest escape of turtle hatchlings. Previous studies have reported how cohort size affects the energetic cost of nest escape in turtle hatchlings, but no studies have reported the influence of substrate type on the energetic cost of nest escape. The time taken and the energy required by the same number of hatchlings to dig through two different sand types were quantified by open-flow respirometry. Brisbane river turtle hatchlings digging through fine sand escaped faster and spent less energy than hatchlings digging through coarse sand, and a larger cohort size provided a clear energetic advantage while digging in both sand types. Across all group sizes, hatchlings digging through fine sand spent 33.8% less energy compared with hatchlings digging through coarse sand. We conclude that hatchlings emerging from nests constructed in fine sand have an energetic advantage over hatchlings emerging from nests constructed in course sand because they would have greater energy reserves upon reaching the nest’s surface.

Eurasian Red Squirrels Show Little Seasonal Variation in Metabolism in Food-Enriched Habitat.

Energy expenditure and ambient temperature (Ta) are intrinsically linked through changes in an animal's metabolic rate. While the nature of this relationship is stable, the breadth of change in thermoregulatory cost varies with body size and physiological acclimatization to season. To explore seasonal metabolic changes of small mammals, we studied a population of Eurasian red squirrels (Sciurus vulgaris) in a seminatural environment with a year-round supply of natural and supplemented food. In each season we measured the metabolic rate of wild-caught red squirrels, using open-flow respirometry, and hypothesized that individuals would make adjustments to contend with seasonal weather conditions. In comparison to summer animals, we predicted that winter squirrels would show (1) an increase in metabolic rate within the thermoneutral zone, (2) a decrease in the lower critical temperature of the thermoneutral zone, (3) a shallower slope of resting metabolic rate with decreasing Ta, and (4) lower thermal conductance. Surprisingly, we observed only minor changes in resting metabolic rate, and energetic modeling suggested that the scope of change was unlikely to be of ecological consequence. Hair area density was higher in winter than in summer, corresponding to a slightly elevated thermal conductance in summer, while body mass was reasonably constant year-round. We conclude that the scope of physiological seasonal adaptation is minimal when food is abundant and that squirrels instead rely on adjustments in activity to reduce exposure to low Ta. We suggest that this may explain the squirrel's success in a wide range of habitats, including urban areas, which require a rapid and flexible response to environmental changes and may indicate the capacity of other small mammal species to cope with environmental disturbance.

Seasonal energetics and torpor use in North American flying squirrels

Seasonal cold temperatures require mammals to use morphological, behavioural, or physiological traits to survive periods of extreme cold and food shortage. Torpor is a physiological state that minimizes energy requirements by decreasing resting metabolic rate (MR) and body temperature (Tb). Many rodent species are capable of torpor, however, evidence in northern and southern flying squirrels (Glaucomys sabrinus and Glaucomys volans, respectively) has remained anecdotal. We experimentally attempted to induce torpor in wild-caught flying squirrels by lowering ambient temperature (Ta) and measuring MR using open-flow respirometry. We also studied seasonal differences in MR and Tb at various Ta. Both MR and Tb provided evidence for torpor in flying squirrels, but only infrequent, shallow torpor. MR decreased infrequently and any decreases were rarely sustained for longer than one hour. We found a significant positive relationship between Ta and Tb only in G. volans, which suggests that G. volans is more susceptible to low Ta compared with G. sabrinus, possibly due to their small body size. We observed no substantive seasonal or interspecific differences in the relation between MR and Ta, with the exception that northern flying squirrels expended more energy at cold Ta during warm season trials than other species-season combinations. The infrequency of torpor use in our experiments suggests that other energy-saving strategies, such as social thermoregulation, may limit the reliance on torpor in this lineage.

Seasonal adaptations in energy budgeting in the primate Lepilemur leucopus.

The spiny forest of South Madagascar is one of the driest and most unpredictable habitats in Africa. The small-bodied, nocturnal primate Lepilemur leucopus lives in this harsh habitat with high diurnal and seasonal changes in ambient temperature. In this study, we investigated seasonal adaptions in energy budgeting of L. leucopus, which allow it to live under these conditions by measuring resting metabolic rate using open-flow respirometry. No signs of heterothermy were detected, and resting metabolic rate was significantly lower in the warmer wet season than in the colder dry season. In fact, L. leucopus possesses one of the lowest mass-specific metabolic rates measured so far for an endotherm, probably the result of adaptations to its habitat and folivorous and potentially toxic diet. Surprisingly, we identified a shift of the thermoneutral zone from between 25 and 30 °C in the wet season to between 29 and 32 °C in the cool dry season. L. leucopus seems to be more affected by the hot daytime temperatures during the dry season and thermoregulation seems to be more costly during this time, which makes this shift of the thermoneutral zone advantageous. Our findings suggest that L. leucopus has a very small scope to unfavorable conditions, making it highly vulnerable, e.g., to changing conditions due to climate change.

Swimming and diving energetics in dolphins: a stroke-by-stroke analysis for predicting the cost of flight responses in wild odontocetes

Exponential increases in hydrodynamic drag and physical exertion occur when swimmers move quickly through water, and underlie the preference for relatively slow routine speeds by marine mammals regardless of body size. Because of this and the need to balance limited oxygen stores when submerged, flight (escape) responses may be especially challenging for this group. To examine this, we used open-flow respirometry to measure the energetic cost of producing a swimming stroke during different levels of exercise in bottlenose dolphins (Tursiops truncatus). These data were then used to model the energetic cost of high-speed escape responses by other odontocetes ranging in mass from 42 to 2738 kg. The total cost per stroke during routine swimming by dolphins, 3.31±0.20 J kg-1stroke-1, was doubled during maximal aerobic performance. A comparative analysis of locomotor costs (LC; in Jkg-1stroke-1), representing the cost of moving the flukes, revealed that LC during routine swimming increased with body mass (M) for odontocetes according to LC=1.46±0.0005M; a separate relationship described LC during high-speed stroking. Using these relationships, we found that continuous stroking coupled with reduced glide time in response to oceanic noise resulted in a 30.5% increase in metabolic rate in the beaked whale, a deep-diving odontocete considered especially sensitive to disturbance. By integrating energetics with swimming behavior and dive characteristics, this study demonstrates the physiological consequences of oceanic noise on diving mammals, and provides a powerful tool for predicting the biological significance of escape responses by cetaceans facing anthropogenic disturbances.

A disparity between locomotor economy and territory-holding ability in male house mice.

Both economical locomotion and physical fighting are important performance traits to many species because of their direct influence on components of Darwinian fitness. Locomotion represents a substantial portion of the total daily energy budget of many animals. Fighting performance often determines individual reproductive fitness through the means of resource control, social dominance and access to mates. However, phenotypic traits that improve either locomotor economy or fighting ability may diminish performance in the other. Here, we tested for a predicted disparity between locomotor economy and competitive ability in wild-derived house mice (Mus musculus). We used 8week social competition trials in semi-natural enclosures to directly measure male competitive ability through territorial control and female occupancy within territories. We also measured oxygen consumption during locomotion for each mouse using running trials in an enclosed treadmill and open-flow respirometry. Our results show that territory-holding males have higher absolute and mass-specific oxygen consumption when running (i.e. reduced locomotor economy) compared with males that do not control territories. This relationship was present both before and after 8week competition trials in semi-natural enclosures. This disparity between physical competitive ability and economical locomotion may impose viability costs on males in species for which competition over mates is common and may constrain the evolution of behavioral and phenotypic diversity, particularly in natural settings with environmental and resource variability.

Examining the metabolic cost of otariid foraging under varying conditions

In order to understand the effects of changing prey availability on dive behavior and energetic costs, behavior and metabolism must be measured simultaneously. Video dive analysis and open-flow respirometry were used to assess changes in dive behavior and energetics of two captive trained California sea lions (Zalophus californianus) in response to high and low prey densities and increased cost of swimming. Differences in dive duration, foraging duration, dive efficiency and foraging efficiency were examined between high and low prey encounter rates, and between standard and cost-increased dives. Changes in foraging metabolic rate (FVO2) and foraging carbon dioxide elimination (FVCO2) were also assessed across all four conditions. Both animals had longer dive durations, greater dive efficiency and lower FVO2 and FVCO2 levels at high prey encounter rates. Foraging metabolic rate and FVCO2 showed a slight increasing trend in cost-increased bouts for both animals, and surface interval durations were shorter. The results suggest that at high prey encounter rates animals minimize their metabolic rate to extend dive duration. These results also suggest that although FVCO2 decreases under high prey encounter rates, increasing the cost of transport keeps VCO2 levels elevated and increases the animal's oxygen debt. The authors hypothesize that as the level of work increases, the ability of the body to sequester CO2 into bicarbonate diminishes due to an inability of the proton buffering process within the blood and muscles to keep pace with increasing CO2 production. Therefore, CO2 may be a more sensitive physiological marker than O2 when examining the metabolic cost of foraging.

443 - The Measurement of Cellular Bioenergetics at Defined Steady-State Oxygen Concentrations: A Novel Microplate Based Method

Although often overlooked, most cell-based in vitro assays are conducted under hyperoxic conditions, while the impact of oxygen concentration ([O2]) on experimental outcome is typically ignored. It is known however that for cells cultured at lower than ambient [O2], there exists a potential for higher [O2] to significantly impact cellular bioenergetics and, by extension, data output. While there is a growing appreciation of this deficiency, technical limitations have prevented broad uptake of [O2]-informed in vitro assay design. These limitations have been ameliorated by recent the advances in both instrumentation and assay technologies: integrated atmospheric control units (ACUs) now facilitate measurement at defined [O2] while advanced phosphorescent intracellular O2 and fluorescent extracellular pH probes, allowing simultaneous multiparametric measurements of cellular oxygenation, electron transport chain activity and glycolytic flux. Described herein is a high-throughput method for measuring all three key metabolic parameters under defined [O2] thereby further addressing such technical limitations. The method utilizes open-flow respirometry, a technique that allows for the measurement of cellular respiration at steady state (e.g. O2 supply = O2 demand). Preliminary data on respiration rate and media acidification at predefined [O2] is presented, facilitating a more detailed interrogatioen for the effect of [O2] on the balance between oxidative phosphorylation and glycolytic activity. This is further delineated using known metabolic effectors including the mitochondrial modulators, oligomycin, antimycin A and FCCP and the CPT-1 inhibitor Etomoxir. The method therefore complements existing technologies, and as a cost-effective, efficient means to measure cellular bioenergetics at an [O2] most relevant to a given cell type or experimental model.

Seasonal metabolic acclimatization in the herbivorous desert lizard Uromastyx philbyi (Reptilia: Agamidea) from western Saudi Arabia

Many ectotherms adjust their metabolic rate seasonally in association with variations in environmental temperatures. The range and direction of these seasonal changes in reptilian metabolic rates are thought to be linked to the seasonality of activity and energy requirements. The present study was conducted to measure the standard metabolic rate (SMR) of seasonally-acclimatized Uromastyx philbyi with different body masses at 20, 25, 30, 35 and 40°C using open-flow respirometry during the four seasons. SMR was mass-dependent. The mean exponent of mass, “b”, in the metabolism-body mass relation was 0.76 (variance=0.0007). Likewise, SMR increased as temperature increased with low Q10 values at high temperatures and high Q10 values at low temperatures. The lowest and highest Q10 values were achieved for temperature ranges of 30–35°C for summer-acclimatized dhabbs (Q10=1.6) and 20–25°C for winter-acclimatized dhabbs (Q10=3.9). Seasonal acclimatization effects were obvious at all temperatures (20–40°C). Winter-acclimatized dhabbs had the lowest metabolic rates at all temperatures. The seasonal acclimatization patterns displayed by U. philbyi may represent a valuable adaptation for herbivorous desert lizards that inhabit subtropical deserts to facilitate activity during their active seasons and to conserve energy during inactivity at low temperatures.

Synchronous activity lowers the energetic cost of nest escape for sea turtle hatchlings.

A potential advantage of group movement in animals is increased locomotion efficiency. This implies a reduced energetic cost for individuals that occur in larger groups such as herds, flocks and schools. When chelonian hatchlings hatch in the underground nest with finite energy for their post-hatching dispersal phase, they face the challenge of minimizing energetic expenditure while escaping the nest. The term 'social facilitation' has been used to describe the combined digging effort of sea turtle hatchlings during nest escape. Given that in a normal clutch, a substantial part of the energy reserve within the residual yolk is used by hatchlings in the digging out process, a decreased cohort size may reduce the energy reserve available to cross the beach and sustain the initial swimming frenzy. This hypothesis was experimentally tested by varying cohort size in hatchling green turtles (Chelonia mydas) and measuring energy expenditure during the nest escape process using open-flow respirometry. The energetic cost of escaping through 40 cm of sand was calculated to vary between 4.4 and 28.3 kJ per individual, the cost decreasing as the number of individuals in the cohort increased. This represents 11-68% of the energy contained in a hatchling's residual yolk at hatching. The reduced energetic cost associated with large cohorts resulted from both a lower metabolic rate per individual and a shortened nest escape time. We conclude that synchronous digging activity of many hatchlings during nest escape evolved not only to facilitate rapid nest emergence but also to reduce the energetic cost to individuals.

Bigger clutch sizes save offspring energy during nest escapes

Hatchling turtles typically emerge from underground nests in groups, so the nest escape process may represent another example of animals sharing a task (in this case, digging out of a nest) to save on individual energy expenditure. Previous studies have reported the energetic cost of embryonic development across chelonian taxa, but none has quantified the extra amount of energy needed to escape the nest. Brisbane river turtle (Emydura macquarii signata) hatchlings were found to fuel this activity by using approximately 50 % of their residual yolk energy content. An open-flow respirometry system was used to quantify the effect of clutch size on an individual’s energetic cost while digging out of the nest. The energetic cost of nest escaping 15 cm upward in the fine moist sand was calculated to be between 0.34 and 2.32 kJ per individual depending upon the number of hatchlings digging together. The energetic cost decreased as the number of individuals digging together increased and thus supports the ‘social facilitation’ hypothesis which suggests hatchlings cooperate to share the workload of digging out of the nest amongst clutch mates to reduce individual energy expenditure. The reduced energetic cost associated with large cohorts was chiefly caused by the shorter time taken to dig out of the nest by larger numbers of individuals. We conclude that synchronous digging activity of many individuals during nest escape evolved not only to facilitate quicker nest emergence but also reduce the energetic cost to individuals.