Hypoxia progressively lowers thermal gaping thresholds in bearded dragons,Pogona vitticeps

Abstract

Most animals, including reptiles, lower body temperature (T b ) under hypoxic conditions. Numerous physiological and behavioural traits significant to the regulation of T b are altered by hypoxia in ways that suggest an orchestrated adjustment of T b at a new and lower regulated level. We examined this matter in bearded dragons, Pogona vitticeps, a species of reptile that naturally exhibits open mouth gaping at high temperatures, presumably in order to promote evaporation and thus prevent or avoid further increases in T b . The threshold for the onset of gaping (assessed as the temperature at which lizards spent 50% of their time gaping) was reduced from 36.9°C in normoxia to 35.5°C at 10% and 34.3°C at 6% O 2 . The overall magnitude or degree of gaping, measured qualitatively, was more pronounced at lower temperatures in hypoxia. Females consistently had lower gaping threshold temperatures than did males, and this difference was retained throughout exposure to hypoxia. In addition to gaping, evaporative water loss from the cloaca may also play a significant role in temperature regulation, since the ambient temperature at which cloacal discharge occurred was also reduced significantly in hypoxia. The results reported herein strongly support the view that hypoxia reduces temperature set-point in lizards and that such changes are coordinated by specific behavioural thermoeffectors that modulate evaporative water loss and thus facilitate a high potential for controlling or modifying T b .

Behavioural responses of naked mole rats to acute hypoxia and anoxia

“…In support of this interpretation, our T b measurements from NMRs that had lost consciousness in anoxia agree well with the T b measurements in [3]. Interestingly, in our experiments, all animals were observed urinating on themselves with the onset of anoxia, which is a strategy that supports very rapid evaporative cooling in ectotherms [13], and likely contributed to the rapid heat loss in anoxic NMRs, along with higher rates of air flow in the chamber.…”

Behavioural responses of naked mole rats to acute hypoxia and anoxia

“…In support of this interpretation, our T b measurements from NMRs that had lost consciousness in anoxia agree well with the T b measurements in [3]. Interestingly, in our experiments, all animals were observed urinating on themselves with the onset of anoxia, which is a strategy that supports very rapid evaporative cooling in ectotherms [13], and likely contributed to the rapid heat loss in anoxic NMRs, along with higher rates of air flow in the chamber.…”

Control of Breathing in Ectothermic Vertebrates

“…The elevation in body temperature from 35 to 40°C seems nevertheless to be associated with a rise in ventilation relative to metabolism as end‐tidal PO 2 increases and end‐tidal PCO 2 decreases [Figure 39 ; (4) ]. Consistent with Crawford's (234) early studies, Tattersall and Gerlach (1115) demonstrated that gaping leads to decreased surface temperature on the head in bearded dragons at high ambient temperatures. The regulation of this response seems to involve a yet to be determined cue based on plasma osmolarity and/or dehydration stress (295, 1030).…”

Size-Related Differences in the Thermoregulatory Habits of Free-Ranging Komodo Dragons

“…In large Komodo dragons, these events coincided with that of their preferred T b plateau suggesting that it may be a method for fine tuning their body temperature regulation [33,35,47]. Indeed, big Komodo dragons may use mouth gaping as an inexpensive way to thermoregulate without moving about to sun shuttle [48] as employed by the smaller dragons. While we do not suggest that smaller dragons are incapable of expressing this behavior, our data does imply that larger dragons display it more readily as a thermoregulatory mechanism.…”

Behavioural responses of naked mole rats to acute hypoxia and anoxia

“…In support of this interpretation, our T b measurements from NMRs that had lost consciousness in anoxia agree well with the T b measurements in [3]. Interestingly, in our experiments, all animals were observed urinating on themselves with the onset of anoxia, which is a strategy that supports very rapid evaporative cooling in ectotherms [13], and likely contributed to the rapid heat loss in anoxic NMRs, along with higher rates of air flow in the chamber.…”

Control of Breathing in Ectothermic Vertebrates

“…The elevation in body temperature from 35 to 40°C seems nevertheless to be associated with a rise in ventilation relative to metabolism as end‐tidal PO 2 increases and end‐tidal PCO 2 decreases [Figure 39 ; (4) ]. Consistent with Crawford's (234) early studies, Tattersall and Gerlach (1115) demonstrated that gaping leads to decreased surface temperature on the head in bearded dragons at high ambient temperatures. The regulation of this response seems to involve a yet to be determined cue based on plasma osmolarity and/or dehydration stress (295, 1030).…”

Size-Related Differences in the Thermoregulatory Habits of Free-Ranging Komodo Dragons

“…In large Komodo dragons, these events coincided with that of their preferred T b plateau suggesting that it may be a method for fine tuning their body temperature regulation [33,35,47]. Indeed, big Komodo dragons may use mouth gaping as an inexpensive way to thermoregulate without moving about to sun shuttle [48] as employed by the smaller dragons. While we do not suggest that smaller dragons are incapable of expressing this behavior, our data does imply that larger dragons display it more readily as a thermoregulatory mechanism.…”

Behavioural responses of naked mole rats to acute hypoxia and anoxia

“…In support of this interpretation, our T b measurements from NMRs that had lost consciousness in anoxia agree well with the T b measurements in [3]. Interestingly, in our experiments, all animals were observed urinating on themselves with the onset of anoxia, which is a strategy that supports very rapid evaporative cooling in ectotherms [13], and likely contributed to the rapid heat loss in anoxic NMRs, along with higher rates of air flow in the chamber.…”

Control of Breathing in Ectothermic Vertebrates

“…The elevation in body temperature from 35 to 40°C seems nevertheless to be associated with a rise in ventilation relative to metabolism as end‐tidal PO 2 increases and end‐tidal PCO 2 decreases [Figure 39 ; (4) ]. Consistent with Crawford's (234) early studies, Tattersall and Gerlach (1115) demonstrated that gaping leads to decreased surface temperature on the head in bearded dragons at high ambient temperatures. The regulation of this response seems to involve a yet to be determined cue based on plasma osmolarity and/or dehydration stress (295, 1030).…”

Size-Related Differences in the Thermoregulatory Habits of Free-Ranging Komodo Dragons

“…In large Komodo dragons, these events coincided with that of their preferred T b plateau suggesting that it may be a method for fine tuning their body temperature regulation [33,35,47]. Indeed, big Komodo dragons may use mouth gaping as an inexpensive way to thermoregulate without moving about to sun shuttle [48] as employed by the smaller dragons. While we do not suggest that smaller dragons are incapable of expressing this behavior, our data does imply that larger dragons display it more readily as a thermoregulatory mechanism.…”