Thermoregulatory consequences of salt loading in the lizard, Pogona vitticeps

Abstract

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 such as shuttling and gaping are similarly influenced. Shuttling, as an active behavioural response, is considered one of the most effective thermoregulatory behaviours, whereas 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 (UET a and LET a ), the percentage of time spent gaping, the metabolic rate (V˙O 2 ), the evaporative water loss (EWL) during gaping and non-gaping intervals and the evaporative effectiveness (EWL/V˙O 2 ) of gaping. All experiments were performed under isotonic (154 mmol l −1 ) and hypertonic saline injections (625, 1250 or 2500 mmol l −1 ). Only the highest concentration of hypertonic saline altered the UET a and LET a , 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; V˙O 2 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/V˙O 2 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.

Thermal biology of the toadRhinella schneideriin a seminatural environment in southeastern Brazil

“… 8,9 Most anurans, however, offer little or no resistance to evaporative water loss. 9-12 It is known that thermoregulatory adjustments need to accommodate the potentially conflicting compromises related to water balance 13 and the prominent tendency of amphibians to lose water is thought to prevent a more efficient regulation of T b . As a consequence, amphibians, and particularly terrestrial anurans, may have their activities limited, daily and seasonally, by the availability of adequate thermal microhabitats that allow for thermoregulation without serious consequences to water balance.…”

Bearded dragons (Pogona vitticeps) with reduced scalation lose water faster but do not have substantially different thermal preferences

“…In lepidosaurs, one mechanism that triggers this decrease in thermal preference is the neuropeptide arginine vasotocin (AVT), secreted by the pars nervosa of the brain in response to high blood sodium ion concentration (Bradshaw et al, 2007). There has even been past work on bearded dragons specifically to suggest that they thermoregulate differently when they are dehydrated compared with when they are hydrated (da Silveira Scarpellini et al, 2015). However, da Silveira Scarpellini et al (2015) demonstrated that dehydrated bearded dragons do not strictly exhibit an anapyrexia (i.e.…”

“…There has even been past work on bearded dragons specifically to suggest that they thermoregulate differently when they are dehydrated compared with when they are hydrated (da Silveira Scarpellini et al, 2015). However, da Silveira Scarpellini et al (2015) demonstrated that dehydrated bearded dragons do not strictly exhibit an anapyrexia (i.e. lowered thermal preference) in response to dehydration.…”

“…Additionally, bearded dragons have been used for years as model organisms for studies of how terrestrial ectotherms can cope with a complex physical environment and changing physiological states. These studies include those on physiological gaping behaviour (da Silveira Scarpellini et al, 2015;Tattersall and Gerlach, 2005), behavioural thermoregulation (Black and Tattersall, 2017;Cadena and Tattersall, 2009a,b;da Silveira Scarpellini et al, 2015;Khan et al, 2010) and colour change (Cadena et al, 2017(Cadena et al, , 2018de Velasco and Tattersall, 2008;Fan et al, 2014;Smith et al, 2016a,b). Furthermore, a mutation has arisen in the bearded dragon captive breeding population that affects the animal's scalation (de Vosjoli et al, 2017;Di-Poï and Milinkovitch, 2016).…”

Thermal tachypnea in avian embryos

“…Eventually, as the heat load progresses, thermal tachypnea wanes into thermal hyperventilation, a deep and slow breathing pattern oriented to boost heat dissipation at the expense of blood gas homeostasis (Mortola and Maskrey, 2011). Interestingly, several lizard species also use thermal tachypnea or gaping (increased heat loss with opened mouth) to protect the brain from a rise in temperature (Tattersall et al, 2006;Scarpellini et al, 2015).…”