Lizards in hot weather: what to do when the cool end disappears

Quick answer

A reptile does not have a body temperature of its own. It has the temperature of wherever it is standing, and it stays alive by moving between a hot place and a cool place.

That is why hot weather is dangerous in a way that is easy to miss. The lamp is not the problem. The problem is that when the room gets hot, the cool end of the enclosure disappears, and the animal has nowhere left to go.

• Turning the basking lamp off does not create a cool end. It removes the hot end and leaves a uniformly hot box.
• Measure the cool end, not the basking spot. A rising cool end is the number that matters in a heatwave.
• A thermostat cannot cool. It can only switch a heat source off, and it has already done that before you notice a problem.
• Gaping at the basking spot is normal thermoregulation. Gaping at the cool end, away from the lamp, is an emergency sign.
• Never use ice, ice water or a freezer to cool an overheated reptile. Rapid cooling causes shock.

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A glass or wooden vivarium in direct sunlight becomes a greenhouse and can kill within an hour.

Glass and clear plastic let short-wave solar radiation in and trap the re-radiated heat, so an enclosure that sits in a sunbeam climbs far above room temperature with every heat source switched off. A conservatory, a loft room, a car, or any position where a patch of sun crosses the enclosure during the day is not a survivable location, however good the equipment is.
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Decide in 60 seconds

Why the cool end is the thing that keeps a reptile alive

An ectotherm produces almost no usable body heat of its own. It sets its temperature by choosing where to be: on the hot rock, in the shade, in the burrow, back on the rock.

A captive enclosure works because it reproduces that choice. A basking zone at one end, a cool retreat at the other, and a range in between. The animal shuttles between them all day, and in doing so it holds itself within a preferred range far more precisely than any thermostat could.

Remove one end of that gradient and the whole system fails.

In a heatwave, the end that disappears is the cool one.

Ambient room temperature is the floor of the enclosure's range. If the room reaches 30°C, nothing inside the enclosure will be cooler than roughly 30°C, no matter what is switched off. The animal can no longer choose to be cool, and its body temperature rises to whatever the room decides.

This is why turning off the lamp is not a plan.

Switching the basking lamp off removes the top of the gradient. It does nothing at all to the bottom. The result is an enclosure at a uniform, elevated temperature with no basking site and no refuge, which is worse for the animal than a correct gradient would have been.

And it is why the thermostat is not reassurance.

A thermostat is a switch on a heat source. When the room warms, the thermostat cuts the lamp, which is exactly what it should do and exactly what makes the situation invisible. The owner sees a thermostat behaving normally, the basking bulb off, and assumes the enclosure is under control, while the cool end has quietly climbed to room temperature.

The desert argument, and why it is wrong.

Desert lizards do experience extreme surface temperatures. They also spend the hottest hours of the day in burrows, in rock crevices and under vegetation, where the temperature is dramatically lower than at the surface. A wild bearded dragon is not out on a rock at midday in high summer. The heat tolerance people attribute to desert species is really burrow-seeking behaviour, and a captive animal with no burrow does not have it.

Species changes the answer completely

The phrase "reptiles like heat" hides an enormous range.

Desert and arid-zone lizards.

Bearded dragons, uromastyx, leopard geckos and similar species tolerate high basking surface temperatures. They still have an upper limit for whole-body temperature, and they still need a genuinely cool retreat. Their advantage in a heatwave is a wider margin, not immunity.

Tropical forest and montane species.

Crested geckos, gargoyle geckos, day geckos and most chameleons come from environments that are warm but not hot, and often cooled at night by altitude or canopy. Crested geckos in particular are widely regarded by keepers as being at risk once ambient temperatures reach the high twenties Celsius, and prolonged exposure above that is a common cause of sudden loss in this species.

The practical consequence is uncomfortable: in a hot climate or a heatwave, a room that is entirely fine for a uromastyx is lethal for a crested gecko in the same house.

Humidity-dependent species.

Species needing high humidity face an additional trap. The little cooling a reptile gets from gaping depends on water evaporating from the mouth and airway, and that route closes in humid air. Warm and humid is the most dangerous combination there is, and it is the normal summer condition across much of the world.

Size and life stage.

Small animals and hatchlings heat up and dehydrate faster than large adults. Gravid females, animals in shed, and animals that are already unwell all have less margin.

Reading the animal

Watch the mouth and where the animal is when it opens it. Gaping under the lamp is thermoregulation. Gaping at the far end of the enclosure is not.

Early, behavioural.

The lizard spends all its time at the cool end. It presses its body flat against the coolest surface it can find, often the glass. It digs into the substrate. It stops using the basking area at all. Appetite falls.

Established.

Gaping away from the basking site. Restless glass surfacing or frantic attempts to climb out. Colour change: many bearded dragons pale in an attempt to reflect radiation, and a dragon that is unusually washed out and sitting at the cool end is not relaxed. Increased breathing rate.

Late.

Lethargy that does not lift when the animal is disturbed. Loss of the righting reflex, meaning the animal does not correct itself promptly if gently tilted. Muscle tremor, twitching, or seizures. Unresponsiveness.

At this stage the risk continues after cooling, because heat damages organs and the nervous system, and some of that damage declares itself over the following days.

Dehydration alongside it.

Sunken eyes, wrinkled or loose-looking skin, sticky or stringy saliva, and urates that become small, hard and yellow rather than soft and white. Skin tenting is unreliable in a scaled animal, so use the eyes, the mouth and the urates instead.

Emergency cooling, in order

1. Turn off every heat source and remove the animal from the enclosure if the enclosure itself is the hot place.

2. Move to the coolest room in the building. A tiled bathroom, a north-shaded room, a cellar. Lower and shaded is better.

3. Put the animal on a cool solid surface. A ceramic or slate tile that has been kept somewhere cool, or a tiled floor. Conduction removes heat from a reptile far more effectively than moving air does.

4. Offer a shallow soak in water that feels cool to your hand, not cold, for species that tolerate water. Shallow enough that the head is well clear. Ten to fifteen minutes, watching continuously.

5. Never use ice, iced water, cold packs against the skin or a freezer. Rapid surface cooling drives blood away from the skin, which slows heat loss from the core and causes circulatory shock. It can kill an animal that would have survived gradual cooling.

6. Offer water to drink but never pour water into the mouth of a collapsed animal.

7. Phone an exotics vet. Take the animal in if it is lethargic, unresponsive, twitching, or has lost the righting reflex, and take it in even after apparent recovery if it was severely affected.

Building the plan before the weather arrives

A hand mister and a towel have a place, but they are the smallest part of the answer. Where the enclosure sits and how the room is shaded matter far more.

Position the enclosure for the worst day, not the average one.

Choose the room that never receives direct afternoon sun, on the shaded side of the building, away from conservatories, loft spaces, kitchens, tumble dryers and rooms above boilers. Track where the sun actually falls across the day before deciding, because a spot that is shaded at nine is not necessarily shaded at four.

Shade the room from outside where you can.

External shutters, awnings, blinds outside the glass and reflective film all stop solar energy before it enters. Internal curtains and blinds stop light but the heat is already inside. If external shading is not possible, close internal blinds early in the day rather than reacting once the room is hot.

Give the cool end thermal mass.

A large slate or ceramic tile, a substantial water bowl, a stone hide, or a section of substrate deep enough to burrow into. Dense materials change temperature slowly, so they lag behind the room and give the animal somewhere genuinely cooler to lie during the peak.

Use a wrapped frozen bottle correctly.

A frozen water bottle wrapped in a towel, placed against the outside of the cool end of the enclosure, cools by conduction through the wall without the animal ever contacting it. Placed loose inside, it risks cold injury where the animal lies against it, and it drips.

Move air across the room, not at the animal.

A fan lowers room temperature by exchanging air, which helps. Directing a fan straight at a reptile does very little cooling, because reptiles have negligible evaporative loss through the skin, and it accelerates dehydration.

Air conditioning, where it exists, is the real answer.

In genuinely hot climates, reliable cooling of the room is not a luxury for tropical and montane species. If a species cannot be housed safely in your climate without continuous air conditioning, that is information worth having before acquiring it.

Measure the right things, in the right places.

A digital probe thermometer with the sensor at the cool end, at animal height, is the single most important instrument. Add a maximum and minimum function so you can see what happened while you were out. Use an infrared gun for basking surfaces only. Check both daily in hot weather, and do not rely on the dial thermometers supplied with many enclosures.

Decide your thresholds in advance.

Write down the cool end temperature at which you move the animal to another room, and the one at which you phone the vet. Making that decision while watching a struggling animal is much harder.

Think about UVB before you switch everything off.

Turning off a combined heat and UVB lamp for a few days during a heatwave is a reasonable trade. Leaving it off for weeks is not, because UVB drives vitamin D and calcium metabolism and metabolic bone disease follows its absence.

Feeding in heat.

An overheated reptile usually stops eating, and that is appropriate. Do not push food, and do not offer a large meal to an animal you cannot maintain at a proper gradient, because digestion depends on reaching basking temperature.

Travel and transport.

Never leave a reptile in a parked car. Move animals at the coolest part of the day, in an insulated container, with a wrapped cool pack outside the animal's reach, and never in a sealed box without ventilation.

What the vet will do

Expect hydration to be the first priority. Fluids are given under the skin or into the coelomic cavity, warmed to an appropriate temperature, because cold fluids into a compromised reptile cause their own problems.

Expect a neurological assessment: righting reflex, response to handling, pupil responses, and whether tremor or seizure activity is present.

Expect questions about numbers rather than impressions. Species, cool end temperature, basking surface temperature, how each was measured and where the sensor sits, how long the exposure lasted, and what the room reached.

Expect a discussion about delayed damage. Severe hyperthermia injures the gut lining, the kidneys and the nervous system, and an animal that looks better on the evening can deteriorate over the following days. Follow-up matters.

Bring photographs of the whole setup including where the thermostat probe sits, the equipment list, and the maximum and minimum readings if you have them.

What never to do

Do not rely on turning the lamp off as your heatwave plan.

Do not put ice, ice packs or frozen items where the animal can lie against them, and never immerse an overheated reptile in cold water.

Do not point a fan directly at a reptile.

Do not leave an enclosure where any sunlight reaches it, even briefly.

Do not trust the stick-on dial thermometer that came with the enclosure.

Do not assume a species is heat-tolerant because it is a reptile. Check that species.

Do not leave a reptile in a car, a conservatory or an unventilated room in warm weather, for any length of time.

Do not force-feed or force water into a collapsed animal.

Do not assume recovery is complete because the animal is moving again. Severe overheating has a delayed phase.

A cool, solid, shaded surface removes heat by conduction far more effectively than moving air. This is what emergency cooling looks like: unremarkable, gradual, and supervised.

When to get help

Contact an exotics or reptile vet immediately for:

• An unresponsive or collapsed lizard
• Loss of the righting reflex
• Tremor, twitching or seizures
• Gaping and laboured breathing that continues after cooling has begun
• Any animal that reached a temperature you know was well above its safe range, even if it now seems normal

Book within a day or two for:

• An animal that stopped eating during hot weather and has not restarted
• Sunken eyes, hard yellow urates or other signs of dehydration
• Lethargy that persists after the temperature has been corrected
• Any collapse that recovered on its own

Bring the numbers. Species, cool end and hot end temperatures, how they were measured, the maximum reached if you know it, and how long the exposure lasted. Reptile medicine runs on husbandry detail, and a vet given accurate temperatures will get to the answer far faster than one given an impression of the weather.