Thermal Adaptation - Basking

Reptiles are ectothermic animals and rely on external heat sources to regulate their internal temperature (Bogert 1949). This suggests that their means of thermoregulation is primarily behavioural, other than obvious physical structures affecting heat transfer such as skin type/thickness, and skin colour. These physical structures likely contribute to thermoregulation of reptiles in conjunction with behavioural adaptations, but will not be discussed in this post. The primary focus of this blog post is to evaluate the environmental conditions that reptiles are subject to, and discuss the adaptations that enable the effective operation of biological systems within reptiles. An understanding of the adaptations allowing the survival of reptiles in their habitat enables inference into past, present and future evolutionary patterns.

(Fig.1 - Thermoregulation time for reptiles of varying weights)

Reptiles regardless of species or size tend to share common behavioural thermoregulation patterns. Most activity is recorded in the morning with the rising sun (finding an appropriate position and substrate to bask upon), with decrease of activity or normal foraging/resting behaviour from approximately mid-day onwards. This was found in the lace monitor (Varanus varius), which would emerge from shelter and bask in morning sun until reaching 35-36°C, then reduce activity to forage and return to shelter (Seebacher 2004). The 2004 study by Seebacher also found variances in basking time between reptiles of varying weights; the five kilogram lace monitor required over 90 minutes to reach body temperature equilibrium, where a small lizard (0.01kg) required less than 15 minutes (Fig. 1). This is significant in determining the costs of achieving thermal preference, in that the risk of predation increases if basking time increases (Blouin-Demers & Weatherhead 2001). The time in which reptiles reach their thermal preferences is varied throughout species and weights, but the techniques used work the same. It would make logical sense that larger reptiles which take longer to behaviourally thermoregulate have evolved with different adaptations to deal with costs of thermoregulation (predation, oxygen availability, energy costs) than those which can achieve optimum internal temperature relatively quickly and easily.

Reptiles rely on behavioural thermoregulation (basking) in the early hours of the day to maintain optimal internal temperature (Bogert 1949). Basking in the morning enables reptiles to spend the remainder of the day engaging in routine activities (foraging, mating etc.) which require energy and warm, active cells. Larger reptiles require more time basking, presenting risks such as predation, energy loss, and dehydration. However, it is likely that larger reptiles have evolved with very different defense mechanisms to smaller ones to compensate for the cost-benefit scenarios present in reptilian ecological communities.

References:

Blouin-Demers, G. Weatherhead, P. 2001, ‘Thermal ecology of Black Rat Snakes (Elaphe Obsoleta) in a thermally challenging environment’, Ecology, Vol. 82, No.11, pp. 30125-3043

Bogert, C. 1949, ‘Thermoregulation in Reptiles, A Factor in Evolution’, Evolution, Vol. 3, No. 3, pp. 195-211

Seebacher, F. Shine, R. 2004, ‘Evaluating Thermoregulation in Reptiles: The Fallacy of the Inappropriately Applied Method’, Physiological and Biochemical Zoology: Ecological and Evolutionary Approaches, Vol. 77, No. 4, pp. 688-695

Figures:

Fig.1 - Seebacher, F. Shine, R. 2004, ‘Evaluating Thermoregulation in Reptiles: The Fallacy of the Inappropriately Applied Method’, Physiological and Biochemical Zoology: Ecological and Evolutionary Approaches, Vol. 77, No. 4, page 690