One of the most important keys to success when keeping tarantulas is getting their environmental conditions correct.
This means that they must have the correct temperature and – the topic of this article – the right humidity.
So important is this subject that in this section we’re going to discuss the topic in some detail, so that you know everything necessary to keep your new pet in the best possible health.
Why is Humidity Important?
Humidity can be thought of as the microscopic droplets of moisture in the air. When humidity gets high it may also condense out of the air onto other surfaces – like the way your bathroom mirror steams up after a shower.
And just like our bathrooms, humidity is generally raised when fresh water is combined with a warm environment; the water droplets evaporate, and the humidity level rises.
There are a number of ways why specifically humidity, and more generally moisture, are important to keeping tarantulas.
First of all, the right humidity makes moulting – or changing their skins – easier. When a tarantula is kept in an environment that is too try, it may struggle to escape from its old skin. This can lead to malformed tarantulas, or even death.
Secondly, the amount of moisture around can affect your tarantulas’ need to drink. In tiny hatchlings in particular, it is impractical to offer them an open water dish as they are so small. However, ensuring that there are tiny droplets of water in the cage that they can drink can be a suitable alternative.
Thirdly, it is important to understand how tarantulas breath. The theraphosid spiders breath through a device known as a “book lung”. Like the pages of a book, so tarantula lungs have lots of sections, through which oxygen passes.
The thing is, the surface of the lungs need to remain moist for the oxygen to be effectively absorbed. An environment that is too dry can therefore even effect the ability of a tarantula to breath.
You might imagine, based on what’s been said so far, that a higher humidity is therefore better. While in many cases this is correct, there are a few provisos. Tarantulas actually don’t tend to do very well in really soggy conditions. Not only do they struggle, but such an environment also increases the chances of fungi and other microscopic pathogens growing.
This is why humidity is such a big and important topic; we need enough moisture in the cage, but not too much. We’re looking for the “goldie locks” number…
How Do You Measure and Monitor Humidity?
There are a number of ways that the tarantula enthusiast can measure moisture levels in a tarantula cage.
The first of these is using a device known as a hygrometer. It’s basically a thermometer that measures moisture rather than temperature. They come in a range of forms, from low-cost dial versions, to slightly more expensive digital hygrometers.
My personal choice is for the latter option, because on average I find them considerably more accurate than dial-based hygrometers.
The second method of monitoring moisture in a tarantula cage is by observation. Moisture is visible as it condenses on the walls of a tarantula cage. A small amount of condensation is fine – good even.
However a cage which routinely has a waterfall rolling down the walls is probably too damp for the inhabitant.
The final way is to pay attention to the substrate itself. A slightly moist substrate it fine, but if you could pick up a handful and squeeze running water out then the substrate is likely far moister than is ideal.
The goal is to use these methods – basically a hygrometer and general observation – to keep the moisture within your tarantula cage to within acceptable limits.
What is a “Good” Humidity?
Tarantulas are a surprisingly diverse group of invertebrates, present on all but one continent. Here they inhabit everything from dry scrubland and semi-arid desert areas through to moist tropical jungles with exceptionally high annual rainfall.
The best bet when setting up a tarantula tank is therefore to research the natural habitat of the species in question, to accurately mimic these conditions as far as is possible. This is why many of our tarantula care sheets start with an analysis of the wild habitat of each species we cover.
But what if you can’t find suitable information, or the authorities you refer to all disagree? What are some “general rules” for what a good humidity should look like?
In general a healthy humidity for tarantulas is in the range of 75-85% relative humidity (RH). The substrate can be gently moist but should not allowed to become dripping wet.
It can be a good idea to lightly spray a tarantula cage once or twice a week, and then allow the cage to dry out in between. This prevents fungi being able to get a hold in a constantly damp and warm environment.
What is a “Bad” Humidity?
A bad humidity varies too much from this optimum. A bad humidity involves either keeping your spider bone-dry, or in an environment that is too damp, and so leads to sickness or death.
Possibly the most pressing element is how damp the substrate is. If this is gently moist, like potting compost straight out of a fresh bag, then this is unlikely to be anything to worry about. However if there are visible pools of water on the floor, or the substrate becomes soggy and “slimy” then it is too moist.
How to Increase Humidity
Increasing humidity largely involves adding more water to the cage in a controlled manner. There are three simple ways to do this. Firstly, a houseplant mister can be used to gently mist the cage.
Care should be taken under such circumstances to use a mister bought and kept specially for the purpose; you don’t want to accidentally spray any harmful chemicals into the tarantula tank. Ideally the water should be at room temperature too, rather than cold and icy from the tap.
Lukewarm water not only evaporates quicker (raising the humidity more swiftly) but is also less likely to startle your pet. Try to avoid spraying your tarantula directly with the mister, which can cause stress.
A second method is to gently tip a small amount of water onto the substrate. Here it should quickly be absorbed, and will then be released slowly over time as the water warms up.
Lastly, particularly with larger tarantulas, their water bowl can be moved to the warm end of the cage, where it will evaporate more rapidly. Under these circumstances, however, be certain to monitor the water level carefully, replenishing it as often as necessary.
How to Decrease Humidity
In tanks that are too damp, there are a number of possible solutions. The best answer really depends on how extreme the moisture levels are, and how regularly you suffer from such problems.
Firstly, of course, stop adding any more moisture. Frequently, increasing the ventilation in the cage will help excess water vapour to escape – so open up any vents, or cut extra holes for air exchange.
Some substrates tend to do a better job of absorbing excess moisture than others, so opt for one that is well-known for it’s moisture control abilities, such as coir fibre.
Lastly, while this is an extreme solution, it may pay to actually clean out your tarantula entirely, removing the sodden substrate.
Thoroughly clean the cage and all the decor, allow it to dry out, and then set the cage up again. This allows you to start from scratch again, controlling moisture more carefully in the future.
Understanding Humidity Cycles
The last topic worthy of discussion here is that humidity rarely stays fixed at one figure. We spray the cage and a few minutes later the humidity increases.
A few days later the cage is drying out and the moisture levels are dropping again.
This is perfectly normal, and nothing to worry about. Indeed, some might argue that this is a good thing, as it makes it harder for fungi to gain a hold in the cage.
The goal is really that the moisture on average should hit the recommended figures, with a particular emphasis around the time of moulting, when relative humidity is of the greatest importance.