The EK Guide to Humidity

One of the most over-simplified but intrinsically vital elements of captive husbandry is humidity. Some species benefit from vast fluctuations, whilst other species will perish without consistently high readings; getting your head around humidity control is a very important aspect of exotics keeping.

An example of humidity within a plant terrarium

What is humidity?

Relative humidity, simply put, is the amount of water in the air. However, the percentage is in relation to the amount of water that the air can hold. So, 100% relative humidity does not necessarily mean the hygrometer (used to measure humidity) is underwater, but that the air in the environment cannot hold any more water.

Water comes in all three forms, liquid (obviously), solid (ice) and gas (vapour). Warmer air can hold more vapour because there is more energy available to hold the water within the atmosphere. Dry locations are hot, but are not humid, because there is not enough water to evaporate, and cold places are dry because there is not much energy to evaporate water, and much of this is stored as ice.

Correct humidity is vital for the wellbeing of all species, but tropical reptiles, amphibians and invertebrates can be the most reliant. Given the way we house these animals, often in glass or wooden enclosures, it is of paramount importance that we get this right. Researching the species’ natural history is key to understanding what works and using resources online to determine the daily or seasonal fluctuations in humidity is a great starting point. This method is not fool-proof however, as some species will occupy ecological niches that have higher (or lower) humidity than the readings of our weather stations.

 There are also several ways of recording humidity, but in the exotic pet world, we are most concerned with relative humidity.

High humidity

Most amphibians require very high humidity. Amphibian comes from the Greek ‘amphi’ meaning both and ‘bios’ meaning life and suggests that most amphibians require water to develop. It also points towards the fact that amphibians receive much of their oxygen through cutaneous respiration and thus needs water in their environment (either in the form of vapour or liquid) to breathe. The increasingly popular subculture of dart frog keeping has paved the way for an entire spectrum of products designed to keep humidity as high as possible, with most dart frogs requiring at least 80% relative humidity. Some species of these frogs come from high altitudes and thus require reasonably cool temperatures as well as good ventilation to prevent diseases. This can create challenges for the keeper.

Increasing humidity to +80% is best achieved with several components. Firstly, a drainage layer beneath the substrate will create a reservoir of water and thus increase humidity gradually. In some cases, this can be warmed with a heating cable to increase the dispersion of water. Air, which is comprised of nitrogen and oxygen (weighing around 28g per mole and 32g per mole) is heavier than water vapour (18g per mole), meaning humidity will rise. This is where having an excellent quality bioactive substrate is key, as having a loose composition allows more water to rise. For non-bioactive setups, a deeper substrate can hold more water and is therefore better at raising the humidity.

Secondly, live plants undergo respiration and photosynthesis which eventually releases over 90% of their required water back into the atmosphere. A heavily planted terrarium will maintain humidity better than an artificial setup and live plants are almost essential to reaching high humidity. Leaf litter, whilst not photosynthesising, will also support humidity levels as it traps moisture and provides vital microclimates (areas of differing humidity for the animal to move between).

Pools of water placed in the warmest part of the terrarium will also release water into the atmosphere as they are heated via IR-C. Bromeliads are perfect for tropical setups, but even a strategically placed water dish can help increase humidity in semi-arid enclosures. Moving water also evaporates quicker than still water as molecules rub together and heat up over time. Artificial waterfalls are rarely necessary for most terrestrial frog species but do serve a function in increasing humidity whilst being visually appealing.

Ventilation will allow humidity to escape and therefore striking a balance that allows fresh air into the enclosure without releasing humidity is key. Perspex, glass and plastic are frequently used to keep humidity in, but keepers should be very cautious that these will also block out any UVB radiation from overhead lighting. Naturally, some homes will be drier than others, which is why it is sometimes easier to increase the humidity or temperature of the entire room. This is often why breeding facilities or ‘tropical houses’ in zoos prefer to achieve an ambient humid environment. Keepers should still consider the ambient humidity in their home and aim to maintain a suitable enclosure for several weeks before adding any animals. This gives the keeper time to make any necessary adjustments to the setup without disturbing the inhabitants.   

Finally, the easiest way of increasing humidity is ensuring there is always ample water in the enclosure to be evaporated into the atmosphere. This is usually done by spraying or misting. In most cases, high humidity enclosures will need to be sprayed or misted at least every other day (although some may need more, others may need less, depending on species and ventilation). Automated misting systems are majorly beneficial to anyone with several high-humidity enclosures. They can be programmed to the specific requirements of the inhabitants. For example, a 20-second morning mist might be suitable at times, or an ongoing ‘rain chamber’ might be required on other days to stimulate breeding behaviours. A reliable misting system allows the keeper to leave their animals longer than 48 hours, which is crucial for unexpected emergencies or a busy social life.

Boophis viridis in the rain

Low humidity

On the opposite side of the spectrum are low humidity, arid enclosures. These pose a different set of challenges, but in many ways are more forgiving than high humidity enclosures. Most desert species will still benefit from the occasional humidity spike, whereas tropical forests will never fully dry out.

Relative humidity can be lowered by essentially reversing all the ways in which it can be increased. Lots of ventilation, dry substrate and infrequent spraying will ensure humidity remains low. Using a smaller water bowl and moving it towards the cool end can also reduce humidity.

If only one animal (or animals with the same humidity requirement) is kept in the room, a dehumidifier will help bring the overall humidity down. It is then much easier to increase the humidity within the enclosure to meet the animals needs than to battle with a high-humidity room. Here in the UK, average outdoor humidity jumps between 40% in the day and 90% at night, with coastal areas being most humid. This means most homes should not struggle to provide very high or very low humidity environment if the correct elements are used within the enclosure.  

On occasions where an arid species is having to be kept in a humid room, there are some tricks including adding absorbent materials or even dry rice to a hidden area within the enclosure. This is only a short-term solution but may alleviate some issues if the keeper is stuck.

Ornate Uromastyx (Uromastyx ornata) occupy some of the driest regions on Earth.

Maintaining humidity

At night, the humidity will increase. Just turning the warm lights of an enclosure off will see humidity increase drastically in a reasonably short space of time. Providing it is not to the detriment of the animal, a shorter photoperiod can help support healthy humidity fluctuations. For species that are particularly sensitive to humidity, it is a good idea to invest in a hygro/thermometer that records the highest and lowest levels over a 24-hour period. These are inexpensive and gives the keeper insight into their enclosure throughout the night. 

Providing choice is key to maintaining good animal welfare. Whilst it’s widely understood that reptiles should be provided with temperature gradients and most people will understand the benefits of providing UV gradients, humidity is slightly different. Humid hides are majorly beneficial to many species, particularly during times of shedding. However, different substrates used in different areas of the enclosure can create important microclimates that allow the animal to move between humidity levels. This is crucial for Mediterranean tortoise species which require periods of higher humidity to ensure healthy carapace growth. For keepers opting to use a tortoise table to house their animals, using multiple substrates (with a primary soil-based substrate) that receives regular misting will help maintain some humidity, even in an open-topped enclosure.

Spot bulbs or compact UVB lighting can dry a ‘basking spot’ out whilst humidity across the rest of the enclosure remains high. The most well-ventilated and most exposed area of the enclosure will therefore have the lowest humidity. Keepers should consider whether their animal is particularly sensitive to lower or higher humidity and consider placing the hygrometer accordingly. For example, it makes sense to record the lowest humidity location in a frog enclosure, as this poses the greatest risk to their health. Those that have adjusted the tops of their enclosure to only allow some ventilation from a specific area beneath the lighting will see noticeable differences in humidity readings across the entire tank. Having two hygrometers at either end may be useful in these cases.

A frog enclosure designed to create microclimates and humidity gradients

Microclimates and ‘clean air’

Bioactive set ups have a huge wealth of benefits when it comes to maintaining humidity. A clever choice of plants will provide microclimates that can be utilised by the inhabitants. For example, a well-planted enclosure that sits at 70% ambient humidity will likely contain pockets of higher humidity amongst the ground cover. Bromeliads such as cryptanthus will also hold pockets of water, which will naturally raise the humidity in those areas. Broad-leaved plants such as Monstera will also hold pockets of higher humidity beneath their leaves as water is released upwards into the environment. Choosing a good quality bioactive substrate can help support this cycle, as ingredients such as activated charcoal, decaying wood, or even a healthy colony of isopods will aerate the soil. Humidity will be released from the water in the drainage layer gradually, at different rates across the enclosure. This is extremely important for arboreal snakes such as green tree pythons (Morelia azurea), which require clean air and naturally fluctuating humidity levels.

Cryptanthus gather pools of water at ground level