Explaining Ectothermy | Thermophysiology of Herps

Authored by: Christina Miller CAHT/RVT, RLAT, BSc

Herps Aren’t Cold-Blooded

What is the most defining feature of our pet reptiles and amphibians? Many would argue that what unites reptiles and amphibians in the artificial grouping of “herps” is their cold-blooded nature, but this is not exactly accurate. “Cold-blooded” is an archaic term and its use implies that you do not understand their physiology. In fact, there are many species of reptile and amphibian whose preferred body temperature is higher than most mammals’ (Lillywhite and Gatten 1995).

Characterizing reptile and amphibian thermophysiology
This is one of the most significant features of reptile and amphibian biology that differs from our mammalian companions, and is important to understand in order to provide them with an appropriate captive environment. Thermal homeostasis is the process (or series of processes) that maintain an appropriate body temperature that is required for many physiological processes (e.g. enzyme activity, dissolved gas balance, etc.) (O’Malley 2005).

Most mammals and birds are homeothermic endotherms: Their body temperature is primarily the result of their metabolism (endothermy), and their body temperature varies within a relatively small range (homeothermy) (Colville and Bassert 2002).

ThermoregulationIn contrast, the most common mode in reptiles and amphibians is heterothermic ectothermy. Ectotherms use heat from the environment to achieve a preferred body temperature (Vitt and Caldwell 2009). To do this, they employ behavioural, morphological, and physiological means of maintaining the preferred body temperature (Allaby 2009, Vitt and Caldwell 2009). Heterothermy refers to switching between thermal strategies to maximize their use of the environment, varying between a homeothermic strategy using external heat sources when active (Hertz 1992, Hertz et al. 1993, Seebacher and Shine 2004) and becoming poikilothermic (conforming to the immediate environmental temperature, also called a thermoconformer) when inactive (Purves et al. 2004, Vitt and Caldwell 2009). Cellular processes in reptiles and amphibians do produce heat like in endotherms, but at a much smaller scale that is essentially negligible (Vitt and Caldwell 2009). Some completely aquatic reptiles and amphibians, as well as species inhabiting very specific microhabitats may be strictly poikilotherms as there is no thermal gradient to thermoregulate within.

Heterothermic ectotherms have a body temperature that may fall within a relatively larger range compared to most endotherms (Lillywhite and Gatten 1995, O’Malley 2005). Consider that the normal body temperature ranges (taken rectally) in cats is 38.0-39.5 ⁰C, and in dogs 37.5-39.0 ⁰C (Smith 2013), a range of only 1.5 ⁰C in a healthy animal. Most reptiles and amphibians can maintain some body functions within a much wider range, even greater than a 20 ⁰C difference in some species (Vitt and Caldwell 2009). These temperature differences would be fatal to many endotherm species.

It seems that vocabulary pertaining to thermoregulation and energetics is not always consistent in the literature.

Poikilothermy (sometimes also called exothermy) is used interchangeably with ectothermy in some texts; however this is not entirely accurate. As explained above, ectotherms are not always poikilothermic, and may certainly maintain a body temperature greater than the immediate environment.

Thermoregulation and metabolism

The preferred body temperature of any given herp is species-specific and may vary between individuals depending on their physiological state and metabolic needs. A preferred optimum temperature range (POTR)* exists for every species, which is a range of body temperatures in which the animal active and fluctuates within to achieve a preferred body temperature (PBT). The POTR has been experimentally determined some species by observing the temperature extremes that an animal will tolerate with other external influences removed (Vitt and Caldwell 2009).

An individual’s preferred body temperature varies depending on what’s needed. Physiological processes like gravidity, seasonal dormancy, or digestion of a large meal all have different metabolic needs (Vitt and Caldwell 2009). Considering this, ectotherms have much lower energy needs compared to endotherms, since they do not rely on metabolic energy to generate body heat resulting in a comparatively lower basal metabolic rate. It has been estimated that they need only a tenth of the energy of a similar-sized mammal (Bennett and Nagy 1977); however this rough estimate probably cannot be applied to any and all species in any metabolic state. This lesser need for calories to feed a fast-burning metabolism allows them to live on a much smaller quantity of food, and to tolerate cooling at night compared to many endotherms (Lillywhite and Gatten 1995, O’Malley 2005).

Achieving the preferred body temperature
Ectotherms will thermoregulate by behavioural, morphological, and physiological means. We need to consider this to understand many of their behaviours, and when designing an appropriate habitat.

With behavioural thermoregulation, the animal employs behavioural adaptations in order to achieve the preferred body temperature. This is generally the main method of thermoregulation in ectotherms. Usually, the animal shuttles between areas of different temperature in its environment, a thermal gradient or thermal mosaic, to “aim” for the preferred body temperature (meaning the ectotherm moves to a warm spot when it is cold, and moves to a cooler spot when it is too warm) (Lillywhite and Gatten 1995). This includes variations in the macrohabitat and any microhabitats that the individual may exploit (Cowles and Bogert 1944, Gvozdik 2002, Seebacher 1999, Vitt and Caldwell 2009). They may also alter their body posture in order to increase or reduce exposure to sunlight (DeWitt 1967, Vitt and Caldwell 2009). Aggregation with conspecifics to reduce heat loss has been observed in some species (Goldberg et al. 1987, Khan et al. 2010)

Physiological processes and morphological features are the main method of thermoregulation in endotherms: Producing body heat via the metabolism, and conserving that body heat with thermal insulation like fur or feathers (Kauffman et al. 2001, Kvadsheim and Aarseth 2002). Ectotherms also use morphology and physiology for thermoregulation, although in very different ways. Many reptiles will gape their mouths when too warm, effectively “panting” to increase evaporative water loss and cool down (Vitt and Caldwell 2009), and this has further been explored as a means of maintaining a cooler temperature of blood in the animal’s head (Tattersall et al. 2006), possibly to avoid heat stress of delicate brain tissue. Lightening or darkening of the body to alter reflectance of heat occurs in many species (O’Malley 2005, Vitt and Caldwell 2009). Some species of boid snakes (family Boidae) will encircle their clutch of eggs, and shiver to incubate their brood (Bartholomew 1982). Amphibians, especially terrestrial caudates, largely regulate body temperature by controlling evaporative heat losses through skin secretions (Vitt and Caldwell 2009).

What is perhaps most impressive is their capacity for cardiovascular control, contributing to thermoregulation. It has been demonstrated that herps alter their heart rate and cardiac output during heating and cooling to maximize efficiency (Bartholomew and Tucker 1963, Grigg et al. 1979, O’Connor 1999, Seebacher 2000, Seebacher and Franklin 2004, Vitt and Caldwell 2009). Vascular shunting to change blood flow to the extremities occurs to also aid in warming up or cooling down (Dzialowski and O’Connor 2004, Vitt and Caldwell 2009).

Next time, we will look a bit further into how ectotherms interact with the environment to thermoregulate, and how this demonstrates their needs in a captive environment.

*Authors Note: NB: Several similar acronyms exist and are used in the literature for terms relating to preferred temperature ranges. Preferred optimum temperature zone and activity temperature range are two such synonymous terms.

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