Thermoregulation in Reptiles and Amphibians

Thermoregulation in Reptiles

Key References

• Vitt & Caldwell (2014): Herpetology, Chapter 7 (Thermoregulation)
• Angilletta (2009): Thermal Adaptation: A theoretical and Empirical Synthesis

Recap

• Origin of reptiles and amphibians
• Key adaptations of both groups
• Modern day herp diversity
• How reptiles and amphibians move
• A key trait that ties these groups together: Ectothermy!

Ectothermy: More Primitive?

• In chronological terms, ectothermy likely arose first.
• Avoid thinking of it as "primitive".

Thermoregulation: Endothermy versus Ectothermy

• Illustrative graph comparing body temperature vs. ambient temperature for bobcats (endotherm) and snakes (ectotherm).
• Ectotherms' body temperature fluctuates more with the environmental temperature than endotherms'.

Ectothermy

• "Cold-blooded"
• Graph depicting the relationship between body temperature and environmental temperature for ectotherms and endotherms.

The Effects of Extreme Temperatures

• Hottest human body temperature recorded: Willie Jones, Atlanta, T_b = 45°C (heatstroke).
• Raises the question of the hottest reptile body temperature.

Gaining and Losing Heat: Thermoregulation

• Radiation: Emission of electromagnetic waves by objects warmer than absolute zero.
  • Transfers heat between objects not in direct contact (e.g., a lizard absorbing heat from the sun).
• Conduction: The direct transfer of thermal motion (heat) between molecules of objects in direct contact with each other, as when a lizard sits on a hot rock.

Heliotherm

• Heliotherms gain heat primarily through RADIATION (directly from the sun).
• A little through conduction from hot substrate.
• Almost always diurnal species, especially in arid areas.
• Key consideration: basking in the open is very risky.

Heliotherm Strategies

• Modulate sun exposure carefully.
• Lizards can flatten their bodies laterally to maximize surface area and warm up faster.
• Often have black skin (between their scales) to protect organs from UV damage.
• Scales are often thick and impervious to UV.
• Colour change: darker in the morning, turn pale/into display colours as they reach optimal body temperatures.

Mosaic Basking

• Another strategy to minimise the risk is mosaic basking.
• Reptile leaves a part of its body in open sun and hides the rest.
• Classic example is a coil of a snake sticking out of a burrow/bush.

Heliotherm: Vitamin D3 Regulation

• Lizards need vitamin D3 to regulate calcium metabolism and various cellular processes.
• They obtain this through their skin from sunlight (same as humans!).
• Chameleons preferentially choose areas of high UV intensity to regulate D3.
• Reference to Ferguson et al. (2003) study on panther chameleons and basking for vitamin D3 production.

Gaining and Losing Heat: Conduction

• Radiation: Emission of electromagnetic waves by all objects warmer than absolute zero. Radiation can transfer heat between objects that are not in direct contact, as when a lizard absorbs heat radiating from the sun.
• Conduction: Direct transfer of thermal motion (heat) between molecules of objects in direct contact with each other, as when a lizard sits on a hot rock.

Thigmotherm

• Thigmotherms will gain heat primarily through CONDUCTION (indirectly from the sun).
• Often seen in nocturnal and fossorial (burrowing) species.
• Often used by heliotherms as supplemental heat in the evening (e.g., snakes basking on warm roads).
• Allows for thermoregulation with minimal risk of predation.

Adaptations of Thigmotherms

• Sand swimming species often have eyes/nostrils at the dorsal terminal extremes of their skulls.
• Allows them to conceal the majority of their bodies under the sand, protection from aerial predators while simultaneously thermoregulating AND hunting/ambushing prey

Supplemental Heat

• Often used by heliotherms as supplemental heat in the evening (e.g., snakes basking on warm roads).
• Roads hold heat after the sun goes down.
• Rattlesnakes seem to insulate this heat with their ventral surfaces well into the night?
• More research into this behaviour is needed.

Losing Heat: Evaporation

• Radiation: Emission of electromagnetic waves by all objects warmer than absolute zero. Radiation can transfer heat between objects that are not in direct contact, as when a lizard absorbs heat radiating from the sun.
• Evaporation: Removal of heat from the surface of a liquid that is losing some of its molecules as gas. Evaporation of water from a lizard’s moist surfaces that are exposed to the environment has a strong cooling effect.
• Conduction: Direct transfer of thermal motion (heat) between molecules of objects in direct contact with each other, as when a lizard sits on a hot rock.

Losing Heat

• Primarily through evaporative cooling.
• Tied to water loss.
• Reptiles must balance internal body heat with their water balance during thermoregulation (+hygroregulation).
• Most obvious is gaping in lizards (e.g., bearded dragons).
• If they need to warm up and conserve water? Squint!

Postural Regulation

• Postural regulation comes in too- standing on tiptoes or climbing off the ground.
• On thermal images the reptile’s eyes/nostrils are the coolest parts of the body.

Understanding Thermoregulation Parameters

• Diagram illustrating lethal minimums and maximums, critical thermal minimums and maximums, voluntary minimums and maximums, preferred temperature, mean body temperature, and activity range.

Controlling Thermoregulation

• Many reptiles have a pineal/parietal eye on the top of their heads.
• It was thought this controls thermoregulation accurately.
• But in reality it only detects light intensity/sequence of colours.
• In heliothermic species MAYBE?
• Probably used in tracking seasonal changes.

Melatonin and Thermoregulation

• For how important this is in reptiles it is surprisingly poorly studied!
• In diurnal species Melatonin has been shown to have an effect.
• Suggests some involvement by the pineal gland/eye.
• But no effect in several nocturnal toad and snake species?

Controlling Thermoregulation: Exercise and Hypoxia

• Exercise and hypoxic conditions decreased body temperature selection in Anolis sagrei (brown anole).
• Important to note this is just in a lab, not in the wild.

Controlling Thermoregulation: Genes

• Temperatures are sensed by transient receptor potential ion channels (TRP genes).
• Expressed as proteins in the brain, heart and liver.
• More research needed.
• Some species have more (cold areas) or less sensitive reception.

Heart Rate Hysteresis

• Reptiles use heart rate hysteresis.
• By speeding up their heart rate during warming and slowing down when cooling.
• They can modulate heat exchange throughout their bodies (to limbs etc.).

Exceptions to Ectothermy

• Tegus in the breeding season: Shown to be able to prewarm themselves in their burrows.
• Some other snakes generate heat via muscle action (seemingly passively).
• Rattlesnakes!
• Some snakes vibrate their muscles to generate heat.
• Incubating pythons.

Mesothermy

• Introduction of a third category between ectothermy and endothermy.

What is Mesothermy?

• A species that can modulate body temperature with metabolic heat, but does not defend a fixed body temperature.
• Usually large bodied species.
• Many Dinos thought to be mesothermic.
• Rare extant herp example is leatherback turtle (Dermochelys coriacea).

Benefits of Mesothermy

• Leatherbacks can occupy habitats too cool for other marine turtles.
• But must periodically dive in warmer areas to avoid overheating at the surface (Okuyama et al., 2021).

Thermoregulation in Amphibians

• Thermoregulation ßà Desiccation; amphibians face unique challenges with permeable skin.
• Some can ‘sweat’ (Shoemaker et al., 1987); allowing T_b to reach 40°C before glands activate that increase evaporative water loss.

Temperature Preferences in Amphibians

• Preferred temperatures: Arboreal > Terrestrial > Aquatic (Tracy & Christian , 2005).
• Inverse correlation with permeability of the skin.

Character Select: Ectotherm vs. Endotherm

• Visual slide comparing ectotherms and endotherms.

Advantages and Disadvantages

• Ectothermy
  • − Performance affected by T_e
  • − Restricted in cold habitats
  • − Behaviour constrained by thermoregulation
  • − Poor endurance
  • + Low energy requirements
  • + Higher population density/food availability – e.g., deserts
  • + Greater flexibility in terms of body shape
• Endothermy
  • + Performance largely unaffected by T_e
  • + Better in colder habitats
  • + Few behavioural constraints due to thermoregulation
  • + Sustained power
  • − High energy requirements
  • − Lower population density/food availability
  • − Surface:volume ratio restricted

Cellular Differences

• ATPase activity in cow mitochondrial membranes is vastly higher than in a croc.
• Major organs are proportionally smaller in reptiles (Why?).

Energetic Benefits of Ectothermy

• Energetic benefits of ectothermy most pronounced at small sizes.
• Resting metabolic rate as a function of body size in terrestrial vertebrates.

An Animal’s Use of Energy

• Is partitioned to Basal Metabolic Rate (or SMR), activity, homeostasis, growth, and reproduction. Energy consumption comparison between endotherms and ectotherm.

Thermoregulation and Food Chains

• Endotherms are often the top predator in food chains.
• Food chains with lots of ectotherms are often longer in length.

Rethinking the Endotherm vs. Ectotherm Dichotomy

• Depiction of graph shown previously is inaccurate.

A More Accurate View

• Not as simple as Endotherm VS Ectotherm!

Ectothermy: A Continuum of Adaptations

• Ectothermy is part of a continuum of physiological adaptations.
• It is a far more effective process than endothermy in/when:
  • Warm environments
  • Food/water are limited
  • Resources are seasonal
  • Aquatic species
  • Small-bodied species
• Think of them as any other adaptations- different evolutionary solutions for different environmental problems.
• Ectothermy is extremely efficient.