Elevation of the panting threshold of the desert iguana, Dipsosaurus dorsalis, during dehydration: potential roles of changes in plasma osmolality and body fluid volume

Dehydration alters behavioral thermoregulation and the geography of climatic vulnerability in two Amazonian lizards

High temperatures and low water availability often strike organisms concomitantly. Observing how organisms behaviorally thermohydroregulate may help us to better understand their climatic vulnerability. This is especially important for tropical forest lizards, species that are purportedly under greater climatic risk. Here, we observed the influence of hydration level on the Voluntary Thermal Maximum (VTmax) in two small Amazonian lizard species: Loxopholis ferreirai (semiaquatic and scansorial) and Loxopholis percarinatum (leaf litter parthenogenetic dweller), accounting for several potential confounding factors (handling, body mass, starting temperature and heating rate). Next, we used two modeling approaches (simple mapping of thermal margins and NicheMapR) to compare the effects of dehydration, decrease in precipitation, ability to burrow, and tree cover availability, on geographic models of climatic vulnerability. We found that VTmax decreased with dehydration, starting temperature, and heating rates in both species. The two modeling approaches showed that dehydration may alter the expected intensity, extent, and duration of perceived thermal risk across the Amazon basin for these forest lizards. Based on our results and previous studies, we identify new evidence needed to better understand thermohydroregulation and to model the geography of climatic risk using the VTmax.

Thermoregulatory consequences of salt loading in the lizard, Pogona vitticeps

Previous research has demonstrated that dehydration increases the threshold temperature for panting and decreases the thermal preference of lizards. Conversely, it is unknown whether thermoregulatory responses like shuttling and gaping are similarly influenced. Shuttling, as an active behavioural response, is considered one of the most effective thermoregulatory behaviours, while gaping has been proposed to be involved in preventing brain over-heating in lizards. In this study we examined the effect of salt loading, a proxy for increased plasma osmolality, on shuttling and gaping in Pogona vitticeps. Then, we determined the upper and lower escape ambient temperatures (UETa and LETa), the percentage of time spent gaping, the metabolic rate (V̇O2), the evaporative water loss (EWL) during gaping and non-gaping intervals and the evaporative effectiveness (EWL/V̇O2) of gaping. All experiments were performed under isotonic (154 mM) and hypertonic saline injections (625, 1250 or 2500 mM). Only the highest concentration of hypertonic saline altered the UETa and LETa, but this effect appeared to be the result of diminishing the animal's propensity to move, instead of any direct reduction in thermoregulatory set-points. Nevertheless, the percentage of time spent gaping was proportionally reduced according to the saline concentration; V̇O2 was also decreased after salt-loading. Thermographic images revealed lower head than body surface temperatures during gaping; however this difference was inhibited after salt loading. Our data suggest that EWL/V̇O2 is raised during gaping, possibly contributing to an increase in heat transfer away from the lizard, and playing a role in head or brain cooling.

Effect of hypernatraemia and the neurohypophysial peptide, arginine vasotocin (AVT) on behavioural thermoregulation in the agamid lizard, Ctenophorus ornatus

Hypernatraemia induced by chronic injections of sodium chloride provokes thermal depression in the agamid lizard, Ctenophorus (formerly Amphibolurus) ornatus, with a fall of two degrees Celsius in the mean body temperature selected behaviourally in a photo-thermal gradient. The placement of an electrolytic lesion in the base of the hypothalamus, designed to eliminate secretion of the neuropeptide arginine vasotocin (AVT), did not affect the lizards' thermoregulatory behaviour and their Preferred Body Temperature (PBT) was not significantly different from that of unoperated controls. Saline loading, however, did not induce thermal depression in these tract-operated individuals and their PBT was significantly higher than that of salt-loaded intact individuals. When AVT was injected into operated, salt-loaded, animals, however, thermal depression was observed, supporting the hypothesis that thermal depression brought about by hypernatraemia is mediated through the action of AVT. AVT similarly significantly depressed the PBT of injected intact individuals by 3.2 degrees C when compared with hydrated controls. Immunostaining for AVT confirmed that the lesions placed in the region of the median eminence virtually eliminated AVT located in the neurohypophysial tract, and the pars nervosa. This is the first report of an effect of this peptide on behavioural thermoregulation in a lizard.

Physiological and hormonal control of thermal depression in the tiger snake, Notechis scutatus

Plasma sodium concentrations in field-caught Western tiger snakes, Notechis scutatus, from semi-arid Carnac Island (CI) varied seasonally, with snakes exhibiting significant hypernatraemia during summer and normal concentrations following autumn rain. In contrast, field-caught tiger snakes from a perennial fresh-water swamp (Herdsman Lake, HL) exhibited no significant increase in plasma sodium concentrations during summer. Laboratory-induced hypernatraemia caused thermal depression in both populations; there was a weak negative relationship between plasma sodium concentration and temperature selection that was significant for CI snakes. Hypernatraemia significantly elevated circulating concentrations of the neuropeptide arginine vasotocin (AVT) in both CI and HL snakes. CI snakes injected with a physiological dosage of AVT also evidenced thermal depression. Despite the positive correlation between AVT and both plasma sodium concentration and osmolality for laboratory snakes, field samples from CI snakes indicate that circulating levels of AVT may be influenced more by plasma osmolality than sodium levels. The data suggest that, in CI snakes, chronic dehydration in the field leads to hypernatraemia which may lead to elevated levels of AVT if plasma osmolality also increases. This will in turn invoke a depression in thermal behaviour that may improve the water economy and survival of snakes on semi-arid CI. Although HL snakes do not experience seasonal dehydration, physiological changes away from the stable homeostatic state appear to prompt the same behavioural shifts, illustrating the intrinsic nature of the thermal behaviour in different populations of the same species of snake.