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Heatmats - A guide to them and their use in the vivarium

Posted 8/7/2012

Heatmats are a common and inexpensive method of providing heat to ectotherms such as insects and reptiles in captivity. They and their use are commonly misunderstood, so the aim of this note is to provide some explanations based on real science and observation, rather than hearsay, supposition and common mythology

An ectotherm is an organism that regulates its body temperature by exchanging heat with its surroundings. When it needs to warm up it moves to a warmer environment, when it needs to cool down it must move to a cooler one.

 

In the pet trade the most commonly seen ectotherms are the reptiles, so, for this note I'll describe heating in relation to these animals.

 

Most of the reptiles we see in the hobby require a body temperature higher than our natural climate to have sufficient energy to hunt and to digest their prey. We have a number of methods by which we can provide this, and heatmats are probably the most popular because they are cheap, and the characteristics of the mat mean it can fulfil this role without the need for extra equipment. To use any equipment effectively and safely, we should understand how it works, it's characteristics, advantages and shortcoming

 

A heatmat works in one of two ways - by passing an electric current through a series of resistive printed "wire" elements to generate heat, or by passing an electric current through one or more bulk resistance elements, usually composed of a carbon impregnated cloth. To electrically insulate the elements the mat is laminated between two heavy duty plastic sheets. The mat is designed to provide a low power diffuse heat across its entire surface area with very little heat generation at any one point on the mat. Power densities of around 200W per square meter are common. This wide gentle heating allows the creation of a warm patch in close proximity to the mat, but generally does not allow for the raising of the overall temperature inside the vivarium by much more than a few degrees Celsius in the average British home. Clearly this gives heat mats a few great advantages, assuming you pick the right one for the size of vivarium. They provide a gentle diffuse heat and therefore the area where it is fitted becomes a warm zone within the vivarium allowing the animal to seek and gain heat when it wants it, and to easily escape the heat when it doesn't simply by moving away from the mat.There's three main downsides to mats though;

 

  1. The gentle diffuse heat they generate along their surface can be intensified by insulating the mat with substrate or with the body of the reptile itself. This phenomenon, known as "thermal blocking" can be very easily simulated. Take a heat mat, put it on a piece of polystyrene and cover half the mat with another piece of polystyrene weighted down. After an hour or so, you should be able to feel the temperature difference easily. Therefore, it should be obvious that mats are not suitable for base mounting inside vivaria where the substrate is a good insulator either because of its depth, its composition, or its dryness. Water bowls, hides and decorations shouldn't be placed directly over the heat mat either for the same reason.

  2. They don't heat the air of the vivarium to the same temperature as some heavy bodied reptiles would require to gain sufficient body temperature from the surrounding air without seeking out the warmth of the heat mat. The mat itself may reach very high temperatures at the point where the reptiles body contacts it, but may not put in sufficient heat energy to warm the reptile's body core enough. The high temperature at the point of contact between the reptile and mat is just a repeat of the thermal blocking experiment we conducted in (1) above. Less well understood is why the reptile doesn't move. Put simply, it doesn't move because it's cold, it needs to warm up (or die), and that need to warm up over-rides any pain signals it might be getting. We've all heard of people severing their own limbs to escape life threatening peril, so enduring a burn to avoid death by hypothermia is quite logical. The fact that the reptile wouldn't get so cold as to die is a level of reasoning beyond its mental capabilities - it's just programmed as a matter of survival to warm up when it's cold.

    If you care to experiment, I have one which simulates quite well the situation of a heavy bodied reptile on a heat mat; get a lump of steak (simulated reptile) and fry it on a high heat. Observe that the steak is rare in the middle, cold even, and yet the surface in contact with the pan is very much cooked (If you don't like rare steak, just put it on a  plate and bring it to me and I'll dispose of it for you). Now, if you cook that same steak outside on a camping stove at its lowest setting, you will see the bottom of it burn and the heat never build up enough to cook it through because the colder ambient air is cooling the steak all the time, faster than the low flamed camping stove can heat it. This is a reasonable analogy for our heavy bodied reptile on a low power mat in a cold room (relative the the ideal ambient temperature for that species). The low powered heat of a heat mat just doesn't penetrate into the reptile's body enough for it to feel warm and move away, so its skin burns.

  3. It's impossible to regulate the heat output of a mat across a wide area. Thermostats can only regulate the temperature at the site of the sensor. If the heat mat is not raising the ambient temperature of the vivarium sufficiently than the thermostat will never switch it off. Also, from (1) and (2) above, it's easy to see that all manner of localised thermal blocking can occur and the thermostat will not sense it. For this reason I hesitate to recommend thermostats for use with heat mats, except in such cases as they may raise the ambient air temperature sufficiently to harm the animal during hot weather, because they tend to foster a false sense of security in the owner. It's better to select a heat mat where the electrical rating of the mat is insufficient to cause dangerous overheating, and to protect the whole viv with basic thermostat providing a full electrical shut-off of all electrical equipment in the event of disaster¹ ²

So, should we even use heat mats? Well yes, they're an excellent solution to the problem of providing heat and a temperature gradient necessary for ectotherms to be kept in captivity. What we need to do is understand how they work and how to mitigate their shortcomings. We can minimise thermal blocking by mounting heatmats in such a way as to maintain an air gap to prevent overheating, something as simple as a piece of corrugated cardboard is often sufficient. An air gap is only effective if air can circulate through it though, so care must be taken in designing and placing it. The probe for any thermostat used should be placed on the mat underneath whatever you're using to maintain the air gap. Heatmats are also an excellent background heat source, to assist your main heater during cold weather. In the presence of heavy bodied reptiles though they should not used as the sole heat source in normal domestic situations.³

 

Heat mats should be checked regularly. All vivaria should have at least one thermometer and from this it should be easy to detect a "no-heat" failure of your heat mat. "Over-heat" failures can only be checked for by actually feeling the mat for hot-spots or scanning its surface with a non-contact infra-red thermometer.

 

I intend to write a few of these notes. Next I'm going to focus on heavy-bodied reptiles, specifically fat snakes like pythons, and how we provide heat for them in a safe and effective manner. I welcome all feedback and discussion on this and related subjects. Debate is how we learn.

 

¹ When I originally published this article on facebook, a well respected reptile shop proprietor commented thus "I have been saying this to customers for years. that in MOST CASES a mat stat is only good for making us shop keepers profit.".

² It's no good switching off the heater if the light is still heating the viv. Even in the UK I have seen viv temperatures reach 15°C above ambient room temperature from nothing more than the fluorescent lamp plus sunlight exposure. I use bimetallic thermal cut-outs of the type used to prevent overheating of microwave ovens and the like. They're available at trip temperatures of 35-40°C from electronic component supply houses such as Farnell and Maplin.

³The article is aimed at normal pet reptile keeping houses with one or two vivaria in a bedroom or living room. The advice herein is not applicable to RUB stacks, dense viv stacks etc as one might find in a more "serious" herpetologist's premises