Optimal Air Temperature in an Amphibious Enclosure

Temperature, humidity, precipitation, and lighting are environmental parameters that profoundly affect the health of an amphibian, as well as its behavior and reproductive success. Successful husbandry depends on controlling these parameters to produce the functional equivalent of the amphibian’s natural microhabitat in captivity. The captive environment should include any daily fluctuations of these parameters encountered by the amphibian in the wild.

For many species of amphibians, it may be necessary to simulate the seasonal fluctuations in certain environmental variables to stimulate reproductive cycles. This includes conditions appropriate for hibernation or estivation in many temperate species. However, eliminating these cycles of dormancy will not harm the amphibians themselves. In fact, creating appropriate conditions for hibernation or estivation and making sure that the amphibians are physically prepared to handle it can be difficult, and these arrangements are best left in the hands of experienced herpetoculturists.

Information on climatic conditions around the world (temperature, humidity, wind, precipitation) is available on computer disc (WeatherDisc Associates, 1995). The library of the National Oceanic and Atmospheric Administration in Silver Springs, MD (301/713-2600 ext. 124) will fax a limited amount of this climatological data to any caller at no charge. Many public and university libraries also have access to this material. This data can be useful in setting up appropriate conditions in the captive environment.

Temperature. The optimal air temperature in an amphibian enclosure will depend on the species being housed and on the specimen’s place of origin. Approximate temporal-spatial temperature gradients suggested for keeping postlarval terrestrial amphibians are presented according to broad habitat type: tropical lowland species 24-30°C (75-85°F); tropical montane forms 18-24°C (65-75°F); subtropical forms 21-27°C (70-80°F); and temperate forms, summer 18-24°C (65-75°F), winter hibernation 10-16°C (50-60°F). These ranges are only rough guidelines, and temperatures may need to be adjusted higher or lower depending on a particular amphibian’s response to the temperatures provided.

The optimal water temperature in an aquatic amphibian enclosure will depend on the species being housed and on the specimen’s place of origin. Approximate temporal-spatial temperature gradients suggested for keeping aquatic larval and adult amphibians are presented according to broad habitat type: tropical lowland species 24-30°C (75-85°F), tropical montane species 18-24°C (65-75°F), subtropical species 21-27°C (70-80°F), temperate stream species in summer 16-21°C (60-70°F), and temperate pond species in summer 18-24°C (65-75°F). Most temperate species of aquatic amphibians experience a drop of 9°C (15°F) or more in the winter, so a chilling unit may be needed. Alternatively, the entire room can be cooled with an air conditioner. These ranges are only rough guidelines, and temperatures may need to be adjusted higher or lower depending on a particular amphibian’s response to the temperatures provided.

Some amphibians undergo a daily shift in thermal preference. Diurnal species may seek lower temperatures at night while nocturnal species may seek such temperatures in the daytime. When placed in a thermal gradient in the laboratory, the mudpuppy, Necturus maculosus, a completely aquatic, nocturnally active species, selects considerably higher temperatures at night than it does during the day (Hutchison & Spriestersbach, 1986).

A maximum-minimum thermometer should be maintained at the warmest and coolest points in the enclosure at all times to make sure that the daily temperature range (temporal and spatial) is appropriate. Digital maximum-minimum thermometers, with a remote sensor for a second location, are available at some electronics/appliance stores (e.g., Dual Display In/Outdoor Thermometer, Cat. No. 63-1020, Radio Shack, Ft. Worth, TX) as well as through laboratory and herpetological supply companies. Some models also include a hygrometer for measuring relative humidity.

Tropical lowland and subtropical species of amphibians can normally be kept and bred at a fairly constant temperature year-round. However, many temperate species of amphibians are stimulated to reproduce only after a period of hibernation at cooler temperatures. Seasonal drops of 6-8°C (10- 15°F) or more may be necessary to induce the cycle of gonad atrophy and recrudescence needed for breeding.

Amphibians control their body temperature by both behavioral and physiological mechanisms (for a review, see Duellman & Trueb, 1986; Stebbins & Cohen, 1995). Behavioral control involves movement from place to place within the temperature mosaic of the environment and postural adjustments that increase or decrease contact with environmental temperature sources. Physiological control includes changing the rate and amount of evaporative water loss from the skin, changing skin color to affect the absorption and reflectance of solar radiation, and possibly peripheral vasodilation and constriction.

For many species of amphibians, a thermal gradient within the enclosure is desirable throughout most of the year to enable the animals to thermoregulate. In most cases the warmest area should be no more than 5-8°C (10-15°F) above the coolest area in the enclosure. Substrate heating can be accomplished by using a low wattage undertank heating pad, or flexible heating tape or cable (see Fogel, 1993 for a review of heating equipment). Electrical “hot rocks” are too hot for amphibians and should never be used. For temperate amphibians, the thermal gradient in the enclosure can be reduced in magnitude or eliminated during the winter hibernation.

Basking appears to be important in body temperature regulation in some species of amphibians (for a review, see Stebbins & Cohen, 1995). Basking may take place in direct sunlight, or under rocks or other material warmed by the sun. Many newly metamorphosed anurans seem especially prone to sitting in the sun when there is adequate substrate moisture or the opportunity to move quickly into water to compensate for evaporative water loss. Juveniles of the green toad, Bufo debilis, accelerate feeding, digestion, and growth by elevating their body temperature through basking (Seymour, 1972).

A basking spot can be provided by a low wattage ceramic heating lamp or an incandescent spotlight. The former type of lamp produces no visible light and is preferable where nighttime heating is needed, or where the basking amphibian is photophobic. Incandescent lights are preferable for daytime use with species of amphibians that normally bask in sunlight. Heliophilic lizards can be confused by unnatural combinations of light and temperature (e.g., bright light with low temperatures) and fail to thermoregulate properly (Sievert & Hutchison, 1991). It is possible that basking amphibians could experience the same difficulty.

Care must be taken that any heat-generating light does not overly dry the air. Also, it is important that amphibians do not come into direct contact with the heat source, which could cause desiccation and thermal burns; however, fluorescent tubes emit only small amounts of heat (unlike the starter unit, which can become quite warm). Agile treefrogs will often rest on lit tubes, presumably for the warmth. This is not known to be deleterious to these amphibians. It is best to shield the starter unit with screening, or to use a remote ballast located outside the cage. Used inside the enclosure, however, the starter unit can serve as a source of ambient heat.

Heating requirements for aquatic species and the larvae of terrestrial species are easily met using standard aquarium heaters. A simple way to prevent the amphibian from directly contacting the heater is to sheath it in a section of PVC pipe. Holes should be drilled in the pipe to improve water flow through it. The pipe should be small enough in diameter to keep the amphibian out, but large enough so that the heating element does not touch the interior wall of the pipe. Clear plastic tubes are also available, albeit more expensive than PVC pipe, and have the advantage that the heater’s indicator light can be seen without obstruction. These clear tubes must be ordered from plastic specialty supply companies.

Aquarium heaters can also be used to add heat to terrestrial enclosures that have a pond or stream. Again, the heater should be sheathed in a piece of highly perforated PVC pipe or clear plastic, or otherwise located so that the cage inhabitants cannot directly contact the heating element.