Cloacal evaporative cooling: a previously undescribed means of increasing evaporative water loss at higher temperatures in a desert ectotherm, the Gila monster Heloderma suspectum

Abstract

The Gila monster Heloderma suspectum is an active forager in an environment that, at times, can be extremely hot and arid. Thus, Gila monsters face extreme thermostatic and hydrostatic demands. For a desert ectotherm routinely risking dehydration, evaporative water loss (EWL) is typically viewed as detrimental. Yet evaporation simultaneously dehydrates and cools an animal. We explored EWL in Gila monsters by measuring cutaneous, ventilatory and cloacal EWL at five ambient temperatures between 20.5°C and 40°C. Our results show that Gila monsters have high EWL rates relative to body mass. Cutaneous EWL underwent a consistent, temperature-dependent increase over the entire range of test temperatures (Q10=1.61, with EWL ranging from 0.378 to 0.954·mg·g -1 ·h -1 ). Ventilatory EWL did not show a significant temperature-dependent response, but ranged from 0.304 to 0.663·mg·g -1 ·h -1 . Cloacal EWL was extremely low and relatively constant between 20.5°C and 35°C, but rose dramatically above 35°C (Q10 >8.3×10 7 , from 0.0008 at 35°C to 7.30·mg·g -1 ·h -1 at 40°C). This steep rise in cloacal EWL coincided with an increasing suppression of body temperature relative to ambient temperature. Dehydration to 80% of initial body mass led to a delay in the onset and an attenuation of the dramatic increase in cloacal EWL. These results emphasize the potential value of EWL for thermoregulation in ectotherms and demonstrate for the first time the role of the cloaca in this process.

The urinary bladder as a physiological reservoir that moderates dehydration in a large desert lizard, the Gila monsterHeloderma suspectum

“…In addition to providing a direct osmoregulatory advantage, the physiological reservoir may benefit other critical processes by balancing hydric costs associated with physiological tradeoffs. For example, energetic demands may stimulate Gila monsters to leave refugia where EWL and heat load are reduced (Beck and Jennings, 2003) to forage for widely dispersed prey, which exposes these lizards to xeric surface conditions for long activity periods, exacerbating EWL (Mautz, 1982;DeNardo et al, 2004). This critical surface activity can expose Gila monsters to temperatures substantially above their 29°C preferred body temperature (Bogert and Martín del Campo, 1956;Beck, 2005).…”

“…The Gila monster provides an ideal model for studies examining the potential contribution of the urinary bladder to osmoregulation because it occupies xeric environments and uses a novel suite of behavioral, morphological and physiological strategies to survive (Beck and Jennings, 2003;DeNardo et al, 2004;Beck, 2005). Notably, despite having to forage long distances to locate widely dispersed prey (vertebrate nestlings and eggs) (Bogert and Martín del Campo, 1956;Beck, 2005), these lizards reduce exposure to the most desiccating conditions by behavioral avoidance.…”

“…Perhaps to cope with these high temperatures, Gila monsters have a relatively high EWL rate compared with other lizards of their size. In fact, the Gila monster uses cloacal evaporative cooling to reduce body temperature when it becomes critically elevated (>37.5°C) (DeNardo et al, 2004). Gila monsters also conserve water by foraging infrequently (3-17% of the time during dry months) and selecting thermally and hydrically favorable refugia when inactive (Beck, 1990;Beck and Jennings, 2003) (J.R.D.…”

The stoichiometric approach in determining total evaporative water loss and the relationship between evaporative and non-evaporative heat loss in two resting bird species: passerine and non-passerine

“…Evaporation is loss of heat via loss of body mass. Experimental determination of total evaporation has been performed for many species of birds using various methods and terms to describe the evaporative routes (e.g., [11][12][13][14][15][16][17].…”

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

“…In the wild, it prefers temperatures of approximately 33°C (Melville and Schulte, 2001) and will adopt behavioural and physiological strategies to achieve this preferred body temperature. Lizards achieve these kinds of preferred temperatures in the wild through shuttling between warm and cold environments (Brattstrom, 1971), modifying heart rates during heat and cooling (Seebacher and Franklin, 2001), altering peripheral blood flow (Grigg and Seebacher, 1999), changing cloacal evaporative water loss (DeNardo et al, 2004) and via ventilatory mechanisms, manifesting as gaping or panting responses . Numerous lizard species open the mouth widely when heated.…”