Effects of ozone on evaporative water loss and thermoregulatory behavior of marine toads (Bufo marinus)

Increased tropospheric ozone levels enhance pathogen infection levels of amphibians

As a result of anthropogenic activities, changes to the chemistry of Earth's atmosphere pose a threat to ecosystem health and biodiversity. One such change is the increase in tropospheric ozone (O₃), which is particularly severe in the Mediterranean basin area, where the levels of this pollutant are chronically high during spring and summer time. Within this region, Mediterranean mountain ecosystems are hot spots for biodiversity which may be especially vulnerable to changes in O₃ levels. Declines in montane amphibian populations have been recorded worldwide, including the Mediterranean basin. A significant driver of these declines is the emerging infection disease, chytridiomycosis, caused by the aquatic fungus Batrachochytrium dendrobatidis (Bd). Chytridiomycosis has negatively affected populations of several amphibian species in the Spanish Central Range, including in the Sierra Guadarrama, and interactions with other biotic and abiotic factors are an important part of these declines. However, there is little evidence or knowledge of whether tropospheric O₃ levels may be another factor in the outbreaks of this disease. To test the hypothesis that O₃ levels are another interactive driver of Bd infection dynamics, two different approaches were followed: 1) an experimental study in open top chambers was used to quantify the aspects of how Bd infection progressed throughout the metamorphic process under four different O₃ levels; and 2) a field epidemiological study was used to analyse the relationship between the Bd infection load in the Sierra de Guadarrama and tropospheric O₃ levels during a 9 year period. Our results suggest that high O₃ levels significantly delayed the rate of development of tadpoles and increased Bd infection, providing empirical evidence of two new separate ways that may explain population declines of montane amphibians.

Skin resistance to water gain and loss has changed in cane toads (Rhinella marina) during their Australian invasion

The water-permeable skin of amphibians renders them highly sensitive to climatic conditions, and interspecific correlations between environmental moisture levels and rates of water exchange across the skin suggest that natural selection adapts hydroregulatory mechanisms to local challenges. How quickly can such mechanisms shift when a species encounters novel moisture regimes? Cutaneous resistance to water loss and gain in wild-caught cane toads (Rhinella marina) from Brazil, USA (Hawai'i) and Australia exhibited strong geographic variation. Cutaneous resistance was low in native-range (Brazilian) toads and in Hawai'ian populations (where toads were introduced in 1932) but significantly higher in toads from eastern Australia (where toads were introduced in 1935). Toads from recently invaded areas in western Australia exhibited cutaneous resistance to water loss similar to the native-range populations, possibly because toads are restricted to moist sites within this highly arid landscape. Rates of rehydration exhibited significant but less extreme geographic variation, being higher in the native range than in invaded regions. Thus, in less than a century, cane toads invading areas that impose different climatic challenges have diverged in the capacity for hydroregulation.

The role of ambient temperature and body mass on body temperature, standard metabolic rate and evaporative water loss in southern African anurans of different habitat specialisation

Temperature and water availability are two of the most important variables affecting all aspects of an anuran’s key physiological processes such as body temperature (T_b), evaporative water loss (EWL) and standard metabolic rate (SMR). Since anurans display pronounced sexual dimorphism, evidence suggests that these processes are further influenced by other factors such as vapour pressure deficit (VPD), sex and body mass (M_b). However, a limited number of studies have tested the generality of these results across a wide range of ecologically relevant ambient temperatures (T_a), while taking habitat use into account. Thus, the aim of this study was to investigate the role of T_a on T_b, whole-animal EWL and whole-animal SMR in three wild caught African anuran species with different ecological specialisations: the principally aquatic African clawed frog (Xenopus laevis), stream-breeding common river frog (Amietia delalandii), and the largely terrestrial raucous toad (Sclerophrys capensis). Experiments were conducted at a range of test temperatures (5–35 °C, at 5 °C increments). We found that VPD better predicted rates of EWL than T_a in two of the three species considered. Moreover, we found that T_b, whole-animal EWL and whole-animal SMR increased with increasing T_a, while T_b increased with increasing M_b in A. delalandii and S. capensis but not in X. laevis. Whole-animal SMR increased with increasing M_b in S. capensis only. We did not find any significant effect of VPD, M_b or sex on whole-animal EWL within species. Lastly, M_b did not influence T_b, whole-animal SMR and EWL in the principally aquatic X. laevis. These results suggest that M_b may not have the same effect on key physiological variables, and that the influence of M_b may also depend on the species ecological specialisation. Thus, the generality of M_b as an important factor should be taken in the context of both physiology and species habitat specialisation.

Temperature and dehydration effects on metabolism, water uptake, and the partitioning between respiratory and cutaneous evaporative water loss in a terrestrial toad

Terrestrial anurans often experience fluctuations in body temperature and hydration state, which are known to influence evaporative water loss through the skin (EWLSkin) and lungs (EWLResp). These effects arises from associated changes in skin permeability, metabolism and lung ventilation. Herein, we determined the rates of EWLSkin and EWLResp in the terrestrial toad, Rhinella schneideri, at different temperatures and hydration states. We measured oxygen uptake rates to verify whether alterations in the partitioning between EWLSkin and EWLResp were associated to metabolic induced changes in pulmonary gas exchange. We also measured the influence of hydration and temperature on water uptake (WU) through the skin. Finally, since estimates of skin resistance to evaporation (Rs) are usually inferred from total evaporative water loss (EWLTotal), under the assumption of negligible EWLResp, we calculate the potential error in accepting this assumption, under different temperature and hydration states. EWLSkin and EWLResp increased with temperature, but this response was greater for EWLResp, which was attributed to the temperature-induced elevation in metabolism and lung ventilation. Dehydration caused a decrease in the relative contribution of EWLSkin to EWLTotal, mirrored by the concurrent increase in the contribution of EWLResp, at all temperatures. Thus, Rs increased with dehydration. WU rates were dictated by dehydration with little influence of temperature. The partitioning between EWLSkin and EWLResp was affected by both temperature and hydration state and, under some set of conditions, considering EWLResp as negligible led to significant errors in the assessment of skin resistance to evaporation.

The interplay of cutaneous water loss, gas exchange and blood flow in the toad,Bufo woodhousei:adaptations in a terrestrially adapted amphibian

Toads experiencing dehydrating conditions exhibit complex physiological and behavioral responses, some of which can potentially impact cutaneous gas exchange, an important component of total gas exchange. We measured the effect of dehydration on cutaneous gas exchange in the xeric-adapted toad Bufo woodhousei. First, two pharmacological agents were used to stimulate cutaneous blood flow - phentolamine (an α-blocker) and isoproterenol, aβ-stimulant and powerful cardio-accelerator - to determine a relationship between cutaneous blood flow and water loss. Both drugs increased heart rate and blood pressure, and caused visually evident extensive vasodilation of the skin. Untreated toads in a dry air stream took an average of 10.1±0.7 h to dehydrate to 80% body mass, while animals treated with isoproterenol and phentolamine requires only 7.2±0.8 h and 7.4±0.9 h,respectively. Rehydration, which was more rapid than dehydration, was similarly accelerated in pharmacologically treated toads.

Cutaneous gas exchange(ṀO, ṀCO) and C18O diffusing capacity (DSkinC18O) were then examined in unanesthetized toads under different states of body hydration. Blood gases and hematocrit were measured separately but under identical conditions. In fully hydrated toads at 23-25°C, cutaneous gas exchange values were: ṀO =1.43±0.47 μmol g-1 h-1, ṀCO = 1.75±0.85μmol g-1 h-1, and the respiratory exchange ratio R =1.36±0.56 (N=6, mean + 1s.d.). DSkinC18O was 0.48±0.03 μmol g body mass-1 h-1 kPa. Following an enforced 20-25% loss of body water, DSkinC18O fell by nearly 50% to 0.28±0.09 μmol g-1 h-1 kPa. However, cutaneous ṀO, ṀCO and R were unchanged at 1.48±0.15 μmol g-1 h-1, 1.72±0.29μmol g-1 h-1 and 1.13±0.08 μmol g-1 h-1, respectively. Partial pressure of arterial(sciatic) oxygen, PaO2, normally about 12-13 kPa, remained unchanged by dehydration, but PaCO2 increased about 250% from 0.93±0.27 up to 2.27±0.93 kPa. The fall in DSkinC18O during dehydration presumably results at least in part from decreased cutaneous blood flow, possibly in an attempt to reduce the transcutaneous water loss that would otherwise result during dehydrating conditions. Concurrently, cutaneous ṀCO is maintained under dehyrdating conditions by a greatly increased PaCO2 diffusion gradient across the skin. Thus, Bufo woodhousei appears able to restrict cutaneous blood flow without compromising vital cutaneous CO2 loss.

Respiratory and behavioral effects of ozone on a lizard and a frog

Ozone at concentrations found in urban air pollution is known to have significant physiological effects on humans and other mammals. Exposure of the lizard, Sceloporus occidentalis, to 0.6 ppm ozone for 4 h at 25 degrees C induced 1.6 degrees C of behavioral hypothermia immediately following exposure, but selected body temperature recovered to control 35.3 degrees C the next day. Lizards exposed at 35 degrees C to 0.6 ppm ozone for 4 h selected body temperatures 1.9 degrees C below controls after exposure, and the behavioral hypothermic response persisted and increased to 3.3 degrees C the following day. Four-hour exposures of the frog, Pseudacris cadaverina, to 0.2 to 0.8 ppm ozone resulted in concentration-dependent alterations of respiration including depression of lung ventilation and oxygen consumption and the adoption of a low profile posture that reduced the exposed body surface. Ozone levels in wilderness habitats downwind of urban sources can potentially have stressful physiological effects on wildlife. Defensive physiological and behavioral reactions to ozone exposure may interfere with routine activities, and oxidant air pollution may be in part responsible for observed wildlife population declines.