Beyond the Bars: Ethical Enclosures for Captive Animals (Part Three)

"Westland kassen". Licensed under CC BY 1.0 via Commons -

Climate-controlled greenhouses. (Photo source. CC by SA 1.0)


I’ve already discussed (in the previous article) ways to think about the amount of three-dimensional space an animal’s enclosure should have. The next consideration should be climate, both macro and micro.

Climate, as most of us know, is the general range of weather in an area: this can include average temperatures, humidity, precipitation, and sunlight. In terms of animal housing, climate refers not only to these things, but to ambient things such as noise, artificial light, and vibration.

You might think that climate would only apply to outdoor animal exhibits rather than those within temperature-controlled buildings, but this is incorrect. A pleasant indoor climate for humans is obviously not the ideal one for all animal species, and this needs to be taken into account when designing an enclosure. In fact, climate is one of the most important things to consider for any enclosure, more important than space, or enrichment, or social interaction. Why? Well, previously I talked about the difference between acute and chronic stressors. Housing an animal in a climate it is poorly adapted to is one of the greatest causes of chronic stress for animals in captivity, if not outright death.

Climate for animals in captivity can be divided into two parts: macroclimate and microclimate. The macroclimate is the general climate of the area at large: generally, it would be the weather with respect to animals in outdoor enclosures, and the climate of the building surrounding indoor enclosures. Ideally, the macroclimate will match the animal’s needs and no adjustments will have to be made to the microclimate. Of course, this doesn’t always happen.

Sometimes, even having the appropriate macroclimate isn’t enough due to the way animal enclosures are built. Glass aquariums have poor ventilation and will retain more heat, cold, and humidity compared to, say, a wire cage. This is great if you have an animal with high heat and humidity requirements, like a reptile, but not so great for a small mammal like a mouse. Not only will they find the lack of airflow stifling, but the smell of their feces and urine will be greatly magnified due to the higher humidity. I’ll discuss the benefits and drawbacks of different barrier materials in a later article, but for now let’s break down the different components of climate.


Speaking of humidity, this is an aspect of climate that many pet owners forget or neglect. But it is important for almost every group of animals- certainly for reptiles and amphibians, but also for insects, mammals, birds, and even fish. Yup, fully submerged fish can be affected by humidity! At higher humidities, water from aquariums will evaporate more slowly. Since surface evaporation causes cooling, aquariums at high humidities tend to have warmer water.

For mammals, high humidity causes the aforementioned issue with feces and urine that linger and stink up the air, but it’s not just the stink- it’s the increased likelihood of bacterial infection. Wetter bedding will also need to be changed more frequently. Conversely, extremely dry bedding due to low humidity is dustier and can cause irritation and respiratory infections.

These issues with dirty bedding also plague birds, and to a lesser extent reptiles and amphibians (cold-blooded animals are less frequent with their, er, eliminations than warm-blooded ones are). Amphibians obviously prefer to have wet skin, given that it is a breathing surface for them, so high humidity is generally a must- you don’t see many frogs in deserts or tundras! Reptiles require particularly high humidity when they are shedding their skin, because it can crack or become painful when too dry. Insects, too, need high humidity when shedding, and also because many of them quench their thirst using water suspended in the air.

Some animals, particularly reptiles, may appreciate the opportunity to visit areas of their enclosure with varying rates of humidity. These micro-micro climates can be as simple as plastic hideaways lined with moist bedding such as peat moss- a mini-sauna, if you will.

A leopard gecko enjoying this humidity hide. (Photo source.)

A leopard gecko enjoying a humidity hide. (Photo source.)

High humidity can contribute to heat stress in animals that sweat, such as cows, horses, and humans, as well as animals that pant, like dogs. Much like how an aquarium will be hotter if water can’t evaporate from the surface, animals that sweat or pant rely on moisture evaporating from the surface of their skin to cool them down. If the air is already saturated with water, sweating or panting becomes ineffective and the animals will overheat in high humidities at temperatures they might be comfortable at in low humidities.


Every animal species has what’s called a “thermoneutral zone.” This refers to the range of temperature where the animal’s metabolism works the most efficiently and no chemical or physical changes are needed to make it more comfortable (i.e., in the human thermoneutral zone we wouldn’t sweat or shiver). Being kept for long periods of time out of the thermoneutral zone is obviously quite stressful- the microclimate should always be kept within this range, except in special circumstances such as a mother with newborns who cannot maintain body heat as effectively as adults.

Ectothermic animals (i.e., cold-blooded animals) obviously need to be housed in microclimates with carefully managed temperatures. But mammals and birds are also sensitive to temperature, particularly tropical species. If a tropical animal is housed outdoors in a temperate climate, there needs to be a sufficiently warm indoor space for them to retreat to during the colder months. Likewise, if an animal adapted for extreme cold is being kept in a warmer climate, adjustments must be made. Some zoos feed their polar bears special diets so that they do not build up the insulating layer of fat that keeps them warm in Arctic regions.

The pet industry is most accommodating to reptiles as far as temperature adjustment devices go, with fish following as a close second. Yet many pet owners are under-educated about how to appropriately manage temperature. For many species (including our own) the difference of a few degrees can lead to great discomfort. Very rapid changes in temperature are also highly stressful, even if it’s a change from an inappropriate temperature to a more appropriate one. Heating and cooling should take place gradually, particularly for aquatic animals, which can go into temperature shock if too-hot or too-cold water is added to their tanks. Water temperature also affects how much dissolved oxygen is present, the types of filter bacteria, the toxicity of ammonia, et cetera, et cetera.

As with humidity, making areas with multiple ambient temperatures available for an animal can be highly beneficial, so long as they’re all in the thermoneutral zone. Again, reptiles in particular need varied temperature spots in order to micromanage their own body temperature- basking spots are important for initially raising body temperature, but the whole enclosure should not be basking temperature or the animal risks overheating. Having multiple ambient temperatures available may be particularly important for snakes, which use different temperatures for basking, resting, eating, digesting, and other activities that require different amounts of energy.

Experienced hobbyists encourage reptile owners to have not one but multiple thermometers placed throughout their enclosures in order to manage different temperature ‘strips’. Hides should be available in each area of different temperature as well so the animal does not have to flee to a too-hot or too-cold area in order to feel safe.

While access to multiple temperature zones is crucial for ectothermic animals, all animals may benefit from having the option to move to areas with different temperatures. Having the option to retreat under cool shade or into a warm den is critical for animals housed outdoors, and even animals housed indoors at a stable, comfortable temperature might prefer warmer temperatures while sleeping and/or cooler temperatures while active. This can be achieved fairly simply by creating ‘den’ areas lined with soft bedding as well as areas with strong airflow to generate cooler temperatures.

Cattle utilizing some artificial shade. (Photo source.)

Cattle utilizing some artificial shade. (Photo source.)

For species adapted to temperate environments, seasonal temperature variation will greatly affect behavior, and many animal caretakers try to mimic this (particularly for animals that use seasonal cues to breed). However, if the caretaker wants to imitate normal a seasonal cycle, they should be ready to facilitate other temperature-based changes, such as changes in coat, sleep, and most importantly, diet composition. Changes in temperature and photoperiod can greatly affect an animal’s metabolism. Speaking of photoperiod…


Humidity and temperature are, perhaps surprisingly, two of the most crucial factors to any captive animal’s welfare, but there is a third factor that often goes unnoticed or underappreciated: lighting. Light intensity and duration can actually have quite profound effects on both an animal’s behavior and their internal chemistry. It may make sense intuitively that too much light would bother a nocturnal or burrowing animal. But light can affect everything from metabolism to sex drive as well.

Let me go into a little more detail, since this science isn’t well known to many people. The photoperiod, or length of day, is what many animals instinctively use to determine what season it is. Longer days = summer, shorter days = winter, for example. For years, scientists and farmers have been able to manipulate photoperiods in order to make certain species hibernate or go into estrous. By the way, humans are affected by photoperiods as well, though exactly how is poorly understood.

Hours of day length by latitude and day of the year. "Hours of daylight vs latitude vs day of year cmglee" by Cmglee - Own work. Licensed under CC BY-SA 3.0 via Commons -

Day length by latitude and time of year. (Photo source. CC BY-SA 3.0)

Because so many animals undergo seasonal changes in body chemistry and behavior, light control is crucial to welfare, and applies mainly to animals in indoor enclosures who aren’t exposed to natural daylight. These animals should be subjected to the appropriate number of hours of light each day, and seasonal changes in photoperiods should be done gradually so that they have time to adjust. Note: it is extremely cruel to expose any animal to 24-hour-a-day lighting. This destroys their normal circadian rhythm and can lead to the refusal of food, inability to sleep, and even death. This goes for fish as well- I implore you to turn off the lights on your fishtank at night! The only exception to this would be for animals that live in polar regions during periods of 24-hour daylight.

Photoperiods aren’t the only welfare-relevant aspect of light. Like us, many animals are affected by the amount of ultraviolet light in the environment. Too much may give our sensitive skin burns or melanomas, but too little can also limit our body’s ability to manufacture vitamin D. Animals too use UV light to manufacture vitamins and to promote healthy bones- this is especially important for the development of delicate bird bones. When preening, birds actually spread a special oil secreted from a gland above their tail over their feathers. This oil reacts with sunlight to produce vitamin D, which the birds then consume during the next preening session. Birds kept in low-UV light enclosures are at increased risk for dull feathers, overgrown beak, wobbly legs, bone fractures, seizures, and a whole host of other health issues.

Reptiles have difficulty absorbing vitamin D through dietary changes and need UV light in their environment in order to synthesize it. Otherwise, they risk getting metabolic bone disease. Reptiles need both UVA (short-wavelength) and UVB (long-wavelength) light in order to stay healthy. (The one exception to this is snakes, who have evolved a different means of synthesizing calcium.)

A tortoise showing irregualr shell growth (pyramiding) due to low UV light exposure. "Gopherus agassizii - Buffalo Zoo" by Dave Pape - Own work. Licensed under Public Domain via Commons -

A tortoise showing irregular shell growth (pyramiding) due to low UV light exposure.

While the health of birds and reptiles crucially depends on the availability of UV light, it is important to remember that mammals benefit from it too. In fact, research is showing more and more that UV light exposure greatly affects the vitamin D levels of primates and rodents. Unfortunately, glass absorbs UVB wavelengths, so simply placing the cage near a window is not sufficient.

It is important to remember that even though exposure to UV light is required for the health of many animals, too much of it can be as bad for them as it is for us. Overzealous reptile owners can easily give their pets carcinomas if they aren’t careful about the composition of their light.

Beyond all the health benefits (and dangers), an animal keeper should note that birds, reptiles, amphibians, and even some mammals can actually see UV light, and providing it enables them to see certain colors in their environment. Many birds and reptiles have areas on their body that fluoresce in UV light that are used for signalling, and rodents that can see UV light use it to detect urine splashes while scent-marking.

Cockatiels and their eggs look dramatically different to the avian eye. (Photo by

Cockatiels and their eggs look dramatically different to the avian eye than the human eye. Left- human vision. Center- bird fluorescing under UV light. Right- simulated avian vision. (Image by Dr. Klaus Schmitt.)

Light intensity and shading is another factor to consider when designing animal enclosures. The intensity of the sunlight in the desert, for example, is likely greater than the intensity of the sunlight in the British countryside. Logically, it follows that animals from these different environments should be exposed to different light intensities. Animals that live in shaded forests are also more apt to prefer dappled and muted light than those from the open plains. Shadows will be comfortable for an animal like a rat, which depends upon them to hide in, compared to a meerkat, which seeks unshaded ground in order to watch for aerial predators. As ever: look to the wild behavior of an animal for answers.

Little shade present in the natural habitat of the meerkat.

There’s little shade present in the natural habitat of the meerkat. (Photo source. CC BY-SA 3.0)

Noise and Vibration

Keeping in mind that animals have different physiology than humans, we can’t assume that just because a noise is at a comfortable hearing level for us, it is safe for animals. Research suggests that noise over 85 decibels causes acute stress and can even damage the hearing of rodents and nonhuman primates. That’s about the loudness of a lawn mower.

Those aware of human limitations will note that hearing noises over 85 db for sustained periods isn’t particularly good for humans, either, and they’d be right. But animals such as rodents can also hear noises in frequencies we cannot, which makes it harder to control the noise in their environments. Much modern machinery, for example, consistently produces sounds in these frequencies, particularly video monitors.

Noise is an especially serious problem for animals exhibited in zoos. At peak traffic, zoo visitors can generate continuous noise of over 70 db, which is much louder than even the noisiest rainforest habitat. Unfortunately, the long-term effects of this on zoo animals are understudied, but what has been observed is that larger crowds increase vigilance, territoriality, and stress levels in many species. Some zoos are attempting to tackle this issue by launching campaigns to change visitor behavior or by adding sound-dampening material to habitats.

You probably expect to hear this by now, but noise levels do not just affect mammals and birds, but reptiles, amphibians, and aquatic animals as well. Snakes, despite their lack of external ears, can still pick up sound through vibrations that pass through their skull, through a single inner ear bone, and into the inner ear itself. This inner ear can still be damaged by excessive noise, so snake owners who like playing very loud music should take note of this.

Frogs appear not to have ears, but actually use the circular timpanic membranes located behind their eyes to hear with. "North-American-bullfrog1" by Carl D. Howe - Carl D. Howe, Stow, MA USA. Licensed under CC BY-SA 2.5 via Commons -

Frogs may appear not to have ears, but they actually use the circular tympanic membranes located behind their eyes to transmit sound waves to their inner ears. (Photo by Carl D. Howe. CC BY-SA 2.5)

Fish and fully-aquatic amphibians can also still hear, and are particularly sensitive to vibrations as well. (All sound, as a matter of fact, is simply rapid air vibration.) Given how noisy pumps, bubblers, and other underwater devices commonly used in tanks are, the effect of vibration and sound on aquatic animals is very understudied. Some preliminary studies suggest, unsurprisingly, that high, constant noise levels increase stress and weaken fish immune systems.

Noise pollution does not just lead to stress and hearing loss, however. It can also have a negative effect on signalling behaviors dependant on sound. Obviously, if a signal is hard to hear, it’s not going to be effective; this is why many studies have found that birds living near noisy roads tend to be extra-loud and extra-high-pitched. In many cases, animals also find constant noise distracting or frightening, which can further inhibit normal behaviors.

Of course, not all noise is bad- an absolutely silent environment would be nearly if not more cruel to keep an animal in than a very loud one. Environmental noise can function as positive enrichment when played at the appropriate levels. In fact, some studies suggest that soft music played during active hours actually promotes positive behaviors in laboratory animals and reduces stress. Elephants reduce stereotypic behavior when listening to classical music and dogs staying in veterinary hospitals show reduced stress responses when listening to someone play a harp. Cows produce more milk when listening to slow-tempo music than when listening to fast-tempo music.

One study on how music affected the behavior of members of a chimpanzee colony found that listening to music reduced aggressive and agitated behavior between individuals and increased prosocial behavior. Interestingly, it also correlated with a decrease in overall activity levels. The authors of the paper suggest that music is an effective way to calm animals during more stressful, active times of the day, but is better off not played during times of the day when the animals are already low-activity. As a side note, apparently these chimps particularly enjoyed a live concert from a classical orchestra.

Music may be effective in calming animals because it masks other, more stressful sounds. Allowing the animals to choose what sounds they hear can also be considered a form of enrichment. The authors of the chimpanzee study are already investing in ways to allow the chimps to pick which types of music they want to hear.

Auditory enrichment is not limited to just music. Many animal species contact their neighbors of the same species via long-distance calls, such as the wolf’s howl or the songbird’s song. By playing back these calls, zookeepers can induce captive animals to respond with their own calls as they would in the wild.

Sound can also be used in combination with other forms of enrichment- one of my favorite examples of this is one study where an author used recorded bird calls to induce predatory behavior in a leopard named Sabrina. Speakers with motion detectors were placed throughout her enclosure, so that when Sabrina located the source of one bird call, the next speaker would start playing- causing her to run and jump from area to area until the sounds finally led her to a food chute. If Sabrina didn’t trigger all the motion sensors and then reach the food chute fast enough, she wouldn’t get her treat. Essentially, what this all does is mimic natural hunting and foraging behavior: chase sound, get food. By running each part of this complex enrichment system on a random, varied schedule, the researchers kept Sabrina very well entertained for over a year and a half.


We don’t often think of climate as something that greatly affects our own health, but we, like every other animal, depend on certain temperatures, light levels, and humidity levels for our own survival. That we take most of these for granted is only because we have been able to engineer ideal indoor climates for ourselves. If animals are taken from their natural environment and placed somewhere that is the equivalent of plopping a naked human into the Sahara desert, there are obviously going to be consequences that we need to control for.

In terms of health, it is crucial to have an understanding and a means of replicating a captive animal’s ideal temperature, humidity, and light conditions within its enclosure. But this series of articles is about more than just physical health: it is about thriving, not just living. By offering animals choices and variation in all three of these aspects, as well as sound, we can greatly improve their mental health.

That’s about it for this article in the collection. Next time, we’ll hopefully get to talk about enclosure substrate, topography, shelters, barriers, and more!

Previously in this series:


Stress and Space

To view a complete listing of all the scientific articles I’ve written, check out my Nonfiction section.

References and Further Reading

Almazán‐Rueda, P., Van Helmond, A. T., Verreth, J. A. J., & Schrama, J. W. (2005). Photoperiod affects growth, behaviour and stress variables in Clarias gariepinus. Journal of Fish Biology, 67(4), 1029-1039.

Anderson, P. A., Berzins, I. K., Fogarty, F., Hamlin, H. J., & Guillette, L. J. (2011). Sound, stress, and seahorses: the consequences of a noisy environment to animal health. Aquaculture, 311(1), 129-138.

Bruintjes, R., & Radford, A. N. (2013). Context-dependent impacts of anthropogenic noise on individual and social behaviour in a cooperatively breeding fish. Animal Behaviour, 85(6), 1343-1349.

Chávez, A. E., Bozinovic, F., Peichl, L., & Palacios, A. G. (2003). Retinal spectral sensitivity, fur coloration, and urine reflectance in the genus Octodon (Rodentia): implications for visual ecology. Investigative Ophthalmology & Visual Science, 44(5), 2290-2296.

Dickinson, H. C., & Fa, J. E. (1997). Ultraviolet light and heat source selection in captive spiny-tailed iguanas (Oplurus cuvieri). Zoo Biology, 16(5), 391-401.

Emerson JA, Whittington JK, Allender MC, Mitchell MA. Effects of ultraviolet radiation produced from artificial lights on serum 25-hydroxyvitamin D concentration in captive domestic rabbits (Oryctolagus cuniculi). Am J Vet Res. April 2014, Vol. 75, No. 4 , 380-384

Ferguson, G. W., Brinker, A. M., Gehrmann, W. H., Bucklin, S. E., Baines, F. M., & Mackin, S. J. (2010). Voluntary exposure of some western‐hemisphere snake and lizard species to ultraviolet‐B radiation in the field: how much ultraviolet‐B should a lizard or snake receive in captivity?. Zoo biology, 29(3), 317-334.

Institute of Laboratory Animal Resources (US). Committee on Care, Use of Laboratory Animals, & National Institutes of Health (US). Division of Research Resources. (1985). Guide for the care and use of laboratory animals. National Academies.

Junge, R. E., Gannon, F. H., Porton, I., McAlister, W. H., & Whyte, M. P. (2000). Management and prevention of vitamin D deficiency rickets in captive-born juvenile chimpanzees (Pan troglodytes). Journal of Zoo and Wildlife Medicine, 31(3), 361-369.

Kendall, P. E., Nielsen, P. P., Webster, J. R., Verkerk, G. A., Littlejohn, R. P., & Matthews, L. R. (2006). The effects of providing shade to lactating dairy cows in a temperate climate. Livestock Science, 103(1), 148-157.

Kenny, D. E. (1999). The role of sunlight, artificial UV radiation and diet on bone health in zoo animals. In Biologic Effects of Light 1998 (pp. 111-119). Springer US.

Klaphake, E. (2010). A fresh look at metabolic bone diseases in reptiles and amphibians. Veterinary Clinics of North America: Exotic Animal Practice, 13(3), 375-392.

Kuiken, T., Fox, G. A., & Danesik, K. L. (1999). Bill malformations in double‐crested cormorants with low exposure to organochlorines. Environmental Toxicology and Chemistry, 18(12), 2908-2913.

Manser, C. E. (1996). Effects of lighting on the welfare of domestic poultry: a review. Animal Welfare, 5(4), 341-360.

Markowitz, H., Aday, C., & Gavazzi, A. (1995). Effectiveness of acoustic prey?: Environmental enrichment for a captive African leopard (Panthera pardus). Zoo Biology, 14(4), 371-379.

Oppedal, F., Juell, J. E., & Johansson, D. (2007). Thermo-and photoregulatory swimming behaviour of caged Atlantic salmon: implications for photoperiod management and fish welfare. Aquaculture, 265(1), 70-81.

Ortavant, R., Bocquier, F., Pelletier, J., Ravault, J. P., Thimonier, J., & Volland-Nail, P. (1988). Seasonality of reproduction in sheep and its control by photoperiod. Australian journal of biological sciences, 41(1), 69-86.

Patterson-Kane, E. G., & Farnworth, M. J. (2006). Noise exposure, music, and animals in the laboratory: a commentary based on Laboratory Animal Refinement and Enrichment Forum (LAREF) discussions. Journal of applied animal welfare science, 9(4), 327-332.

Rajchard, J. (2009). Ultraviolet (UV) light perception by birds: a review.Veterinarni Medicina, 54(8), 351-359.

Silanikove, N. (2000). Effects of heat stress on the welfare of extensively managed domestic ruminants. Livestock production science, 67(1), 1-18.

Smith, M. E., Kane, A. S., & Popper, A. N. (2004). Noise-induced stress response and hearing loss in goldfish (Carassius auratus). Journal of Experimental Biology, 207(3), 427-435.

Wells, D. L., & Irwin, R. M. (2008). Auditory stimulation as enrichment for zoo-housed Asian elephants (Elephas maximus). Animal Welfare.

Surmacki, A., & Nowakowski, J. K. (2007). Soil and preen waxes influence the expression of carotenoid-based plumage coloration. Naturwissenschaften,94(10), 829-835.

Warwick, C., Frye, F. L., & Murphy, J. B. (Eds.). (2001). Health and welfare of captive reptiles. Springer Science & Business Media.

Wells, D. L., Graham, L., & Hepper, P. G. (2002). The influence of auditory stimulation on the behaviour of dogs housed in a rescue shelter. Animal Welfare, 11(4), 385-393.


About Koryos

Writer, ethology enthusiast, axolotl herder. Might possibly just be a Lasiurus cinereus that types with its thumbs.
Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *