The Physiological and Behavioural Consequences of Reduced Scalation in Captive-bred Phenotypes of the Bearded Dragon (Pogona vitticeps Ahl 1926)

Abstract

Lepidosaurs as a group are known for their tough, scaled integument and low rates of evaporative water loss. Whether or not there is a causal relationship between the two has been a contentious issue. There also remains the question of whether the lepidosaur scale forms a barrier to ultraviolet (UV) light. Thirdly, there is evidence to suggest that rate of evaporative water loss influences behavioural thermoregulation in lepidosaurs. Lepidosaurs with higher rates of evaporative water loss should be expected to choose cooler temperatures than lepidosaurs with lower rates of evaporative water loss in order to reduce water loss. To investigate these ideas, I used three captive-occurring phenotypes of the bearded dragon (Pogona vitticeps Ahl 1926): Wild Type, animals exhibiting scales of reduced prominence (“Leatherback”), and scaleless animals (“Silkback”). I a priori expected that Silkbacks would have the highest rates of evaporative water loss, the lowest thermal preferences, and the lowest UV light intensity preferences. By the same token, I expected Wild Types to be at the opposite end of the spectrum from Silkbacks for each of these measurements, and I expected Leatherbacks to be intermediate between the two. I used respirometry to measure the animals’ rates of evaporative water loss, a thermal gradient to measure their thermal preferences, and a UV light intensity gradient to measure their UV light intensity preferences. Silkbacks on average lost water at about twice the rate that Wild Types did, with Leatherbacks being intermediate in their water loss rates. The three phenotypes did not visibly differ in their thermal preference. Silkbacks had lower UV light intensity preferences than either Leatherbacks or Wild Types. These results suggest that the lepidosaur scale is indeed a barrier to evaporative water loss and suggest that it is also a barrier to UV light. However, the lack of obvious difference in thermal preference suggests that thermal preference in bearded dragons is not plastic enough to respond to a phenotype that increases the animal’s rate of evaporative water loss. In addition to answering basic questions about lepidosaur biology, my data have relevance to the fields of animal welfare and conservation.