Susan Barnard, Curator
From her book, Reptile Keeper's Handbook, Chapter 3, The Captive Environment (p. 26; Krieger Publishing Co., Melbourne FL, 1996).

"Snake usually tend to maintain lower body temperatures than do lizards from the same geographical area, and tropical reptiles generally have less tolerance for low temperatures than those from temperate regions. Some reptiles, especially, tropical forest dwellers, may not display heat-avoidance behavior. Do not, therefore, expose these reptiles to basking spots where temperatures would rise above their selected body temperature. Keep all heating devices on the cage exterior to prevent accidental burns. I do not favor heating devices that require animals to make contact for warmth. Such devices do not increase the surrounding temperature, but provide dangerously localized heat on an animal. Reptiles can suffer burns before they become aware that they have been injured when thermostats are set to high or when they fail altogether. Conversely, thermostats that fail may produce no heat at all, and tropical forest dwellers may develop respiratory problems when temperatures fall below the range that is best for them."

Stephen L. Barten, DVM
From his Burns chapter, in Reptile Medicine and Surgery (Douglas Mader, DVM, ed. W.B. Saunders. 1996. pp. 9-19).

"Thermal burns may result from any source of heat. Heat sources that allow direct contact between the captive reptile and t he source, such as hot rocks, heating pads, or spot lights within the cage, are most often implicated in burn cases. Burns may be prevented by keeping heat sources outside of the cage or shielding them to prevent contact with the captive reptile. Backup thermostats should be wired into the system in case the primary thermostat malfunctions."

Fredric L. Frye, DVM
Quotes from this noted reptile veterinarian's Reptiles: An Atlas of Diseases and Treatments, 1992.

Page 157, Figure 5-6a-e. "Some devices that have been used to create convective and radiant warmth for cages. Some are useful; others are not approved."

Figure a (photo of a human heating pad with a 3-way heat switch): "A common household heating pad used for warming sore muscles and sprains. This device in inexpensive, adjustable, and has three heat settings."

Figures d-e (photos of two types of hot rocks): "Two examples of electrical resistance heating devices that are commonly used to provide basking sites for caged reptiles. Often these devices either provide insufficient or excessive heat. Their use should be discouraged in favor of more efficiently and safer sources of warmth."

Lee Kelly, Zoo Med Customer Service
The following appeared in a series of posts in rec.pets.herp that got into discussing hot rocks and the fact that they are not suitable for all reptiles, especially iguanas. He was unable to explain, even though he acknowledged that hot rocks were inappropriate for use with green iguanas, why Zoo Med's advertising and packaging all shows iguanas sitting on hot rocks.

"Finally, I understand your views on rock heaters and yes, it would be nice to put lists of appropriate animals on every product out there. It would also be nice if everyone out there with a reptile were properly educated before they obtained their animal and were interested in going to all lengths to obtain this knowledge instead of listening to the pet shop selling iguanas for $9.99 or giving them away for free with the purchase of a 10 gal. tank (please, no offense meant).

"95% of all rock heater burn complaints I have ever heard or received in my 20+ years experience with reptiles was the result of using them inappropriately as primary heat sources. They are not meant to have an animals body covering up their surface for extended periods of time. [Worse] yet, people cover them up with socks or pillow cases; this causes the heat to become encapsulated and is a good way to burn your house down! All items that produce heat have the ability to produce a burn. Were exactly is the line between manufacturer liability and consumer responsibility?"

The subject of hot rocks and their appropriateness for use with green iguanas was hotly discussed on the AOL Pets Green Iguana forum during the summer of 2000. Henry Lizardlover, who has advocated the use of hot rocks for green iguanas, finally contacted Zoo Med himself. He wrote to me on 09/02/00, summarizing what he heard back from Zoo Med:

I just received a letter back to me from the Director of Consumer Relations and he stated that Zoo Med does not officially say that Rock heaters are not appropriate for Green Iguanas, but that Rock heaters should never be used as a primary or secondary heat source for Green Iguanas. Zoo Med also recommends the Deluxe Rock heater with built in Rheostat for any large lizards or snakes to ensure their safety when they cannot fit their entire bodies over the heat rocks.

So, again, the leading manufacturer of hot rocks says that they are not appropriate for green iguanas, whether used alone as the sole source of heat, or as a secondary source of heat, as in conjunction with room heaters or basking lights. Zoo Med apparently can't leave well enough alone, however, missing the point that, if a major part of the reptile's body is cold, laying with part of the body on a heat source, whether or not it has a working thermostat, is not an appropriate way to heat a reptile. Please see the Wawick-Arena discussion below on why this is a problem.

Bill Love
The following was published in the Herpetological Queries section in the March 1997 issue of Reptiles magazine.

What is your position on the use of heat rocks for captive snakes? My local herpetoculturist/store owner says belly heat is unimportant, and it's ambient heat that counts. LJ., Connecticut

As I start pondering the response to your letter, I'm thinking to myself, "What does the idea of substrate heating resemble in nature that snakes would normally utilize?" Unless we're using the odd example of a species that inhabits an area with subterranean steam vents or other such rare phenomena, where would most snakes ever have 24-hour access to a toasty warm spot on which to pass their time digesting a meal?

The closest analogy to heat rocks is a serpent basking on a warm road surface (or a sun-warmed rock) as its environment cools after dark. This might be a cozy solution for a few hours at best (and if no cars are coming down the road!). Eventually, though, the animal may have to tolerate some chilling before morning.

The sun is the chief source of heat, mostly in the direct form while radiating heat down from above. Snakes, and many other herps, have learned to adapt to this by developing routines of basking behavior to maximize the sun effectively. Many move into it for awhile in the morning, until they reach their desired temperature level for activity. During the rest of the day, diurnal species move in and out of shade to maintain the proper temperature levels. Even nocturnal snakes with meals in their stomachs may spend part of the day in this manner to assist digestive efficiency, knowing that the heat gets "switched off' at night. They simply don't have the option of keeping a perfect temperature all the time, and they've obviously evolved to be capable of handling it.

Artificial substrate heating came about as a convenient heat choice because the lack of such options in captivity is the norm. This method is effective for people who aren't able to maintain an ambient temperature high enough for heatloving reptiles that may be kept, for instance, in northern basements during the winter. Heating of this kind creates a warm spot in an otherwise cold cage, so digestion can occur, or simply so the herp doesn't have to endure uncomfortable or dangerous temperatures.

Heat rocks, pads and tapes are particularly handy when the goal is to keep a large number of specimens in rows of tanks or plastic boxes. This type of heating may be more economical than raising the ambient temperature of a large holding area, and it gives a range across the cage. ICs especially suited to offering heat to growing juveniles that may be sharing the same room with adults that you wish to cool for breeding. It can also speed digestion in sick animals.

Substrate heat is more effective for terrestrial species than arboreal ones, although I did see a new heating device at a show which incorporated heat cable into an artificial tree limb to accommodate arboreal herps.

For snakes, I prefer overhead lamps that more closely mimic the sun as heat sources in cages where its feasible to use them. They can be aimed at chosen sites to tempt snakes into comfortable, yet easily viewed, places in their cages. Simple spotlights, especially with directional hoods, can be set up over the ends of cages to give the inhabitants the choice of either basking under them or avoiding them by staying at the darker side of their enclosures. Controlled by timers, lights such as this can be set to coincide with the local conditions, offering a seasonal photoperiod in which natural basking (thermoregulation) is also possible. Healthy herps should continue to thrive when exposed to slightly suboptimal temperatures overnight, as long as they can obtain as much heat as desired during the daytime.

Although substrate heat can be valuable, I believe that snakes can better judge the warmth they need from heat coming down from above, rather than from heat rising up through their bellies at night, when it normally wouldn't be available at all. Captivity subjects animals to stresses that we're only beginning to fully comprehend, and keeping snakes at abnormally warm temperatures may be just as adverse as letting them get too cold. This may be why certain diseases and/or parasites that are normally held in check by the course of daily activity in the wild may gain the upper hand in captive animals.

Sean McKeown, Curator of Reptiles
From his General Husbandry and Management chapter, in Reptile Medicine and Surgery (Douglas Mader, DVM, ed. W.B. Saunders. 1996. Pp. 9-19).

"Historically, the hot rock has been the most misused heating source. It should never be used as the primary heat source. Hot rocks are made of clay, cement or hard plastic molded or formed around an electric resistor (heating coil). As the electric coil heats up, so does the surrounding mold. The reptile is expected to crawl on or coil around this rock and maintain its body temperature. These rocks do not heat the captive environment.; therefore, a reptile housed in a terrarium heated only with a hot rock will not have the proper warmth to meet its metabolic requirements and may receive sever e burns. Hot rocks are only effective when buried under the substrate an used as a secondary heat source. Most do not have any built-in means to control their temperature output, resulting in an "all or none" heating system. Although these have somewhat standardized construction, they typically vary in size and heat production. What may be an appropriate size and temperature for a large python may be dangerously hot for a small kingsnake. Likewise, a small, buried hot rock suitable as a secondary heat source for a small snake would be less effective for a large-bodied snake. Hot rocks are also notorious for having surface "hot spots" that can reach such high temperatures that severe thermal burns can result. Hot rocks are suitable only for ground dwelling species of snakes and must be insulated from the live animal."

Clifford Warwick, Director, Institute of Herpetology and Phillip Arena, School of Veterinary Studies
From their Miscellaneous factors affecting health and welfare chapter, in Health and Welfare in Captive Reptiles (Clifford Warwick, Fredric L. Frye, and James B. Murphy, eds. Chapman & Hall, London. 1996. pp. 263-283).

"An example of apparent but almost certainly misunderstood physical insensitivity and poor thermal environments concerns thermal burns, which occur when captive reptiles come into direct contact with heat sources while attempting to raise their body temperature to preferred levels. The general problem may be exacerbated somewhat by ambient temperatures that are too low and which result in animals being forced to raise their body temperature through extreme proximity to heat sources (J.B. Murphy, pers. comm.). Damage from thermal burns ranges from minor lesions and scarring of the skin to extensive injury such as fusion of the eyelids or burns that extend deep into the body tissues (Frye, 1991a). These injuries may lead to permanent defacement, disability or death.

"The point of particular interest here is that the individual may appear to be oblivious to gross trauma during the period of damaging injury, and in fact insensitive to pain. One current view is that these burns arise after an animal has rested against an inactive heat source, which is then activated and beats up rapidly, causing tissue and presumably local neural damage (Frye, 1991a). Anecdotal accounts suggest that reptiles also settle on already active heat sources and then suffer burns. We propose that a major reason for this behaviour is twofold. First, a large reptile may not be able to attain an optimal body temperature from a small intense heat source such as a lamp. Second, thermal provisions in captivity fail to simulate adequately the thermal diversity of the natural environment.

"In nature the thermal requirements of, for example, a heliotherm are satisfied by a radiant solar source which bathes the entire animal with heat. However, the efficiency with which a body absorbs warmth depends on not only its own properties but also other factors including the intensity of the heat source, the position of the body with respect to the heat source and the proximity and properties of other reflective surfaces (Geiger, 1959). Thus a thermally receptive body is subject to thermal inputs of a multidimensional and heterogeneous nature. Different regions of a reptile's body have different absorbency spectra and thus different heating rates (Heatwole and Taylor, 1987). In captivity, often the only source of heat available is one or two small and usually intense heat lamps. In order to raise their body temperature, reptiles move toward a heat source and bask. Especially where large reptile species and individuals are involved, with associated slower blood circulation (Coulson and Coulson, 1986) and thus heat dispersal, these animals must attempt to raise the temperature of the entire body using primarily diminutive heat sources. Thermal absorption is attempted while continually losing warmth from body surfaces that are not exposed to the heat source and which may, indeed, be in contact with 'cold' surfaces that conduct heat away from animals.

"Compensatory behaviour may include moving closer to the heat source where upon the peripheral nerve endings are damaged and desensitized. Once this occurs, the reptile moves closer still and eventually contacts the heat source in an attempt to raise its body temperature to an optimal level, a point it may never achieve. Thus an unnatural thermal environment and related 'biological confusion' may result in thermal burns. Clearly, more data are needed to clarify the reasons behind this aberrant behaviour, especially in consideration of body size and associated heating requirements of reptiles. If this hypothesis were supported, heliotherms of a small body size in particular, would be less likely to suffer thermal burns because a heat lamp is, to them, a relatively expansive source that may more appropriately 'saturate' their bodies entirely.

"Related considerations include the fact that large lizards are more reflective of solar radiation than smaller individuals (Norris, 1967) and that they heat and cool at a slower rate as a result of a low ratio of surface area to volume. Snakes, by the very nature of their morphology, may be compromised by inadequate thermal provisions."

Comments

Caveat Emptor
On the one hand, we have product manufacturers who expect the customer to buy their products based on the text and illustrations that appear in print media and on their packages. These product developers and manufacturers tout the research that supposedly goes into the development and improvements of their products. This research and development is used as a selling point to convince pet stores to buy the product and, through advertising, the consumer to search out those products.

The pet stores in turn use this marketing rhetoric to convince consumers to buy the product. The consumer, thinking that there is indeed valid research behind the product and that pet stores wouldn't sell products about which they know nothing, assume that the product manufacturers are producing safe products to be used as described and pictured on the packaging, in advertisements, and on their websites, which are in turn being sold to them by the supposed experts in the stores.

Yet, at the same time, the product manufacturers claim that it is the consumers fault if the consumers believe what they read and see and are told by the pet stores, or read or deduce from the manufacturers' or sellers' advertising and packaging. Thus, the manufacturers,and, by extension, the pet stores, abrogate all responsibility for injuries and deaths that results when the consumers of their products use those products as illustrated or described in advertisements and on packaging, and too often replicated in pet store enclosures.

The customer is just supposed to know that hot rocks are not suitable as a primary, and often secondary, heat source, despite the fact that they are pictured as such in their advertising and packaging.

Consumers are just supposed to know that wrapping a hot rock in a sock or towel or burying it under newspaper or woodchips is supposed to be a fire hazard when the packaging contains no such warnings.

Consumers are just supposed to know that even though an iguana is pictured happily roasting on a hot rock that hot rocks are not appropriate for iguanas.

Unfortunately, by failing to aggressively pursue both the pet stores and product manufacturers when their animals are injured or killed by the use of such products, pet stores and product manufacturers are able to shrug off undocumented complaints, since as far as they know, no injuries or deaths have occurred...and when they do, it is through the consumer's own ignorance, not any fault of the product or misleading advertising and packaging.

It is our animals who suffer from such circular logic, not the product manufacturers' bottom line.

Know Your Species
More precisely, know the natural habits of the species you keep. Reptiles utilize heat in different ways. While there are some similarities, how a species utilizes heat sources in its environment should be mimicked in captivity through the use of appropriate types of heat sources.

Reptiles absorb and lose heat in three different ways: radiation, convection, and conduction. Radiation is the heat that hits the body directly from an overhead heat source. Convection is the heat reflected or bounced from other surfaces. Conduction is soaking up heat from a surface contact. In reptilian terms, this means they get radiation from exposure to the sun or their basking light, convected heat that is bounced off surfaces like the outside wall and patio when the reptile is housed outside, or from some enclosure and furnishings surfaces inside their outdoor or indoor enclosure. Conducted heat is the heat they get from laying on a heated surface.

Heat sources in captivity can be a light bulb used to provide white light and heat or one that produces dim light and heat; an infrared bulb producing both dim light and heat; a ceramic heating element; an infrared panel; a human heating pad; a farrowing pad; central heating heating the entire building the reptile is housed in; a space heater heating only the room the reptile is housed in; and/or agricultural or plumbers flexible heat tape. Depending on the species and where you live (which affects the ambient room air temperature of the rooms the reptiles are kept in), you may need to use one or more of these heat sources to attain and maintain the temperatures your species require in the way the species requires it.

Products made for human use inside their homes and on their bodies tend to be safer than products made for reptiles. So, while a human heating pad may essentially do the same thing as a hot rock, when it comes to choosing one to use as a secondary heat source for a thigmothermic or heliothermic species.