The Paradoxical Importance of Humidity

[l4nsky]

Disclaimer: If you're a beginning tarantula hobbyist or an intermediate hobbyist starting with moisture dependent species, looking for specific care information, the theories in this thread aren’t for you. The maxim of "ignore humidity percentages, maintain proper soil moisture and ventilation instead" still stands as a golden rule for tarantula care, regardless of the species. You will still get value and a level of understanding out of the content of this thread, but the complexity around the debate on humidity won't change the golden rule on general tarantula care mentioned above. If you’re wanting general tips on how these concepts apply to husbandry in your collection, skip to the section titled “How All this Relates to Tarantula Husbandry”. Also, I'm not a formally trained expert in humidity, meteorology, zoology, or evolutionary biology and I make absolutely no claim to be. If anyone more knowledgeable in these subjects spots a flaw in any of my logic or facts, please point it out and I’m glad to discuss its implications. As such, this thread is detailing my understanding on the subject from my own research and how it relates to tarantula care in my collection. I'm going to simplify this subject a bit for clarity as well, to make this treatise more digestible for all without losing any of the key points.

On the Paradoxical Importance of Humidity:

Why it’s both Vital to Proper Husbandry and Why Specific Humidity Levels Should be the Last Thing a Keeper Worries About​

Alright, so I've been stating that I need to make a thread on humidity and how it is vital for proper tarantula husbandry for quite some time. My hope is that this thread will both help dispel the notion that humidity plays absolutely no role in tarantula husbandry and serve as the depository of my thoughts and research on the subject for posterity.

So, there are a lot of statements made on the boards along the lines of "humidity isn't important, just overflow the water bowl from time to time" or "humidity doesn't play any role in keeping tarantulas" that, in my belief, are just flat out wrong. Now, I understand and agree with the INTENT behind these statements, I completely disagree with the CONTENT of said statements. The intent of these statements is to simplify the care of tarantulas for novice keepers, to get them to completely disregard any recommended humidity levels found in care sheets, and to ensure the tarantula doesn't die from a new keeper trying to figure out the complexities around humidity. On the other hand, the content of these statements is factually wrong and spreading lies on tarantula husbandry (even white lies meant to do more good than harm) is antithetical to the purpose of these boards. We say "maintain proper soil moisture and ventilation" because both of these factors are major, easily manipulated components in determining the amount of water vapor in the air, and the current amount of water vapor in the air is one of two numbers used to calculate and manipulate the Relative Humidity (ex 60% humidity) of a given air volume. If humidity didn't matter to the care of tarantulas or wasn't important, then a G. porteri (a desert adapted species) would be able to survive and thrive in a tropical, high humidity, planted tank intended for an Asian moisture dependent species and an Asian moisture dependent species, like C. lividus, could likewise survive and thrive in a dry, low humidity, desert scaped enclosure intended for a G. porteri (DON'T try this, both tarantulas will die). It's just not possible.

First, we're going to get a general understanding of RH, the complexities involved, and the numerous environmental parameters that influence and modify the RH values and then we'll go over how this all relates to the care of tarantulas and why a proper RH range (NOT a specific RH value) is crucial to the care of any tarantula, whether it's a desert adapted species or a moisture dependent species.

On the Subject of Humidity​

Diving right in, humidity can be measured three different ways. It can be displayed as absolute humidity (displayed as grams of water per cubic meter of air), specific humidity (displayed as grams of water per kilogram of air), and relative humidity (displayed as a percentage of air saturation). When humidity is used in conversation or is used to describe the care of an animal, relative humidity is the measurement that is being referred to and is the value we will be dissecting (Note: Going forward, the terms humidity, relative humidity, RH, and RH value(s) will be used interchangeably). So, here’s where the complexity around humidity (RH values) starts. What most people don’t fully realize (or comprehend the implications of) is that relative humidity is a calculated value using two numbers (if either number changes, the change in humidity can be drastic). Those two numbers are the current water vapor content of the air AND the maximum amount of water vapor the air can contain. As such, relative humidity can be expressed and calculated as:

Relative Humidity (%) = Current Water Vapor Content of the Air / Maximum Amount of Water Vapor the Air Can Contain​

Both numbers have different influencing environmental parameters, some of which can be manipulated by a hobbyist, and both numbers will change the RH value if they change, regardless if the other number changes or not.

The current water vapor content of the air is influenced by the following parameters:

• Soil Moisture
• Precipitation
• Ventilation/Air Turnover

The maximum amount of water vapor the air can contain is influenced by the following parameters:

• Air Temperature
• Air pressure

Now, any change in these parameters will change the numbers used to calculate the RH values, which in turn changes the RH value itself. Diving deeper into the complexities, let’s look at what happens when each of these values are CHANGED IN ISOLATION (meaning that all other values don’t change).

For the current water vapor content of the air:

• If the soil moisture level goes up, the RH value goes up. This is because more moisture in the substrate means more moisture is evaporating from the substrate as water vapor, thus more water vapor is in the air.
• If the soil moisture level goes down, the RH value goes down. This is because less moisture is evaporating from the soil, or in some cases, the drier soil is pulling moisture from the air as nature always seeks to reach equilibrium.
• If precipitation occurs, the RH value goes up. This is because we are adding more water vapor to the air via rain or fog.
• If the ventilation/air turnover increases AND the incoming air is drier than the enclosed air, then the RH value goes down. This is because the more humid, enclosed air will lose some of it’s moisture to the drier, incoming air as nature balances itself out.
• If the ventilation/air turnover increases AND the incoming air is more humid than the enclosed air, then the RH value goes up. This is because the drier, enclosed air will pull moisture from the incoming, humid air as nature balances itself out.
• If the ventilation/air turnover decreases, the RH value will go up. This is because there is less air escaping, meaning any water vapor added to the air (like from soil moisture evaporation or precipitation) will build up as it can’t escape the enclosure as fast as if there was more ventilation/air turnover.

For the maximum amount of water vapor the air can contain:

• If the temperature goes up, then the RH value goes down. This is because warmer air can hold more water vapor at saturation (100% RH) than colder air can. This relationship seems illogical until you realize that the maximum amount of water vapor the air can hold is the denominator (bottom number) in the division equation that calculates relative humidity. Think about it, if you have 6 “units” of moisture currently in the air and the air mass can hold 10 ‘units’ max, then the RH is 6/10 or 60%. Consequently, if the “units” of moisture in the air stays at 6, but the temperature goes up, the max “units” of moisture the air can now hold is 12. This would put the RH at 6/12 or 50%.
• If the temperature goes down, then the RH value goes up. This is because cooler air can hold less water vapor at saturation (100% RH) then warmer air can. Using the same general example above, if the RH prior to lowering the temperature is 6/10 or 60%, then when the temperature lowers, the air can now hold only 8 “units” of water max, the RH becomes 6/8, or 75%.
• If the air pressure decreases, then the RH value goes down. This is because the air becomes less dense as the pressure drops, allowing it to be able to hold more water vapor (there’s a really neat and paradoxical effect as well when dealing with certain terrain called the mountain shadow effect. It’s beyond the scope of this discussion, but it produces the opposite effect, where the pressure decreases causing the air to let go of it’s water vapor)
• If the air pressure increases, then the RH value goes up. This is because the air molecules become compressed, decreasing the amount of water vapor the air can hold.

As you can see, this is getting more and more complex. Finally, to add to this complexity, it’s pretty rare that only one of these values changes. Usually, it’s a combination of multiple values changing that leads to a change in humidity. Take a thunderstorm for example. Thunderstorms are accompanied by cold air fronts, low air pressure, and precipitation. So simultaneously, a thunderstorm will drop the air temperature (RH goes up), drop the air pressure (RH goes up), and produce precipitation (RH goes up).

On the flip side, let’s look at a captive scenario. Say you take an enclosure off the bottom shelf, overfill the water bowl slightly, and then decide to put it on the top shelf. Opening the enclosure will increase air turnover (temporarily dropping the RH), overfilling the water bowl will increase the soil moisture (RH goes up), but putting the enclosure on the top shelf, where there is likely a higher temperature (as warm air rises), will decrease the RH. In this scenario, we’re not likely to get a humidity bump because of the cross cancellation of the effects of a slightly increased soil moisture and a higher temperature.

Alright, that’s enough background on RH. Hopefully you’re still with me. It’s not my intent to confuse you, only to fully express the complexity that is in a RH value and the difficulty one would have if they were chasing and trying to hold a specific RH value in their enclosures. Next, we’re going to dive into how RH relates to tarantula husbandry and why it is vital.

 

[l4nsky]

On Why Relative Humidity is Important to Tarantulas​

We’re going to start this off with a simple question that will lead to a complex answer.

• Why are some tarantula species moisture dependent and need a moist environment?

This question implies that there are two other questions needing answered as well. Those questions are:

• Why can some tarantula species go a very long time without moisture and require a dry environment?
• Why can some tarantulas survive in both relatively moist environments and relatively dry environments

So, boil it all down and the question really becomes:

• Why will some tarantula's quickly die in a dry environment whereas others will survive, and still yet other species will not only survive, but indeed thrive?

I guess the easiest answer is "well, that's how they evolved". This is a very broad and general answer. While technically correct, it doesn't really dive into the "why" of the question. Evolutionary adaptations usually occur either so an organism can cope with a particular environmental parameter or so an organism can better take advantage of an environmental resource. So how can we apply this thinking to better answer the question? I’m going to base my argument on three key facts going forward.

• To answer this question, we need to remember that life on earth is water based. Whether you believe that life started evolving on deep sea vents or in land-based chemical pools created from geothermal vapor vents, or somewhere else, it has evolved with a dependency on water. In turn, each organism needs a certain amount of water to be able to survive and function. Different organisms have developed different ways to collect, store, retain, or make do with less water for their continued existence.
• We also need to realize that different habitats are exposed to different environmental variables throughout the year. Some are quite stabile month over month (for example, rainforests are consistently moist, while deserts are consistently dry). Others can see much more variable conditions throughout the year (months of high moisture and/or months of low moisture like the edges of grasslands or more temperate forests).
• The final piece of knowledge we need to be aware of is that nature tends towards equilibrium. The fundamental science to all natural sciences is widely regarded to be physics as all other natural sciences adhere to the principles it puts forth. One of those principles is that of pressure. When pressure is achieving equilibrium, the greater the difference between the two starting values, the faster and more violent the rate of change to equilibrium. A good example can be shown using two balloons. Let’s take two smaller balloons and fill them up, pinching them closed when done. One will have enough air pressure to be roughly the size of a fist and one will be roughly the size of a basketball. Let them both go at the same time. While the fist sized one was faster to empty than the larger one, the large one will shoot away while the smaller one will fall to the floor. In other words, the increased flow rate from the air pressure change was higher for the larger balloon, causing the larger balloon to lose more air in the same time frame as the smaller balloon. This effect can be seen in a lot of gasses when going from areas of low concentration to high, including water vapor.

Knowing how important water is then, we can see that tarantulas from dry, desert environments must have evolved coping mechanisms for the high heat, low moisture environments they chose to exploit. Those tarantulas that chose to exploit environments that have high levels of moisture never had to be as concerned about storing and retaining water as it was readily available, thus they wouldn't have evolved the same specialized adaptations as desert species or retained those same costly adaptions when they moved into more hospitable environments. Finally, those species that seem more tolerant to a wider range of moisture (I'll refer to these as generalists going forward) would have some adaptations to conserve moisture, but they wouldn't be as efficient as those of a desert adapted species, but more efficient than those of a purely rainforest species.

So, what are these adaptations that allow a desert tarantula to retain water? To answer that, we need to identify how a tarantula loses water in the first place.

For tarantulas in general, water is lost by:

• evaporation of water from the body's surface area
• excretion
• egg production
• respiration
• molting

In general, tarantulas conserve water with the following adaptations and behaviors:

• Creation and use of an epicuticle. An epicuticle (aka as a waxy cuticle) is a thin, waxy layer of the exoskeleton that helps control evaporation from the body's surface area.
• Living in insulating burrows in the ground or insulating cavities in the trees. These enclosed spaces produce microclimates of temperature and humidity and maintain more stable parameters then the wide open and exposed environment.
• Lower levels of activity requiring less energy expenditure and water use/loss.

Desert adapted species of spiders and tarantulas have taken these moisture conservation measures to the extreme.

• Certain spiders (L. laeta and S. globula) have been theorized to have the ability to modify the permeability of their epicuticle, meaning that they can make it thicker during certain portions of the year to further reduce their water loss. This has also been theorized in the tarantula E. parvulus, as they paradoxically have a higher moisture loss during cooler periods of the year and lower moisture losses during egg sac rearing.
• G. rosea burrows have been found to be about 18” deep, which, while seeming to be quite shallow in comparison to other fossorials like C. lividum, provides a larger and more substantial difference in environmental parameters of the microhabitat vs the exposed air than that of a burrow in a rainforest.
• G. rosea, aka the pet rock, takes inactivity to the extreme. In addition to being extremely sedentary, even by tarantula standards, this species has the ability to go without food or water for months up to years at a time and has a molt cycle that can last years.

Even with all of these adaptive traits and behaviors, desert species still lose water. Of all the water loss measured from a E. parvulus at 40 degrees centigrade, 60% of the water loss occurred at the book lungs. This is a standard by product of respiration. The internal water vapor concentration and pressure (think back to our physics fact) of an almost completely, hermetically sealed and pressurized organism, like a tarantula is much higher than its outside environment. The best place for that water vapor to escape via evaporation is across the thin membranes in the book lungs that allow gas exchange. Now, tarantulas can control their respiration using their spiracles, which are the openings to the book lungs. By controlling how much time the dry, outside air is exposed to the gas membrane which water reserves evaporate from, tarantulas can control their water loss to a certain degree. To further reduce this loss, the extremely sedentary life of a G. rosea equates to a very low oxygen need. In addition to having to breathe less because of a reduced oxygen requirement, tarantula book lungs have a really high surface to volume ratio, allowing for a good gas exchange across the permeable surface. While this does equate to more moisture loss, it also means that one breath goes farther for a G. rosea in a hunger strike then for any of the comparatively more active tropical species.

Alright, let’s compare moisture dependent species and desert adapted species in their habits and habitats to see if it would make sense for any of these desert adaptions to be present or still be present in a rainforest species.

• Desert species possibly have the ability to increase or decrease the permeability of their epicuticle. They do this to decrease water loss from evaporation during the extended dry periods in their habitat. Those dry periods result in a much lower humidity, and since the water concentration in a tarantula is much higher than the water in the air, the environment naturally is more eager to steal that moisture. Consequently, the environment in a rainforest is saturated with moisture. Since the difference between the moisture content of a rainforest tarantula and the moisture content of the environment (the air is saturated with water or in other words has a high humidity) is much smaller, nature isn’t so quick to steal water. From an evolutionary perspective, it doesn’t make sense for a rainforest species to retain or create the costly adaption of body modifications to retain moisture. They simply have no need for it.
• All tarantulas live in hides in the wild, whether through creating their own burrows, stealing another species’ burrow or being opportunistic and using dead trees, tree crevices, or tree root structures. The purpose of these burrows is two-fold, both to have a safe place to retreat from predators and to create a stabilized microclimate for comfort. The latter is much more important in desert adapted species. A sealed deep burrow will retain the moisture lost from respiration, increasing the humidity inside the sealed chamber. Since the temperature inside is also lower, this also means the RH will be higher. A higher RH or water saturation of the air limits evaporative loss from respiration, which in turn helps to conserve the tarantula’s water reserves. For rainforest adapted species, the RH is consistently high and these levels are more or less stable for the entirety of the year. IMO, their hides are more for protection from predators, as rainforests are highly competitive environments and predators abound whereas moisture is plentiful and there is no great need to place such an emphasis on a hide for moisture retention.
• In comparison to desert adapted species that can go into a months long period of torpor, rainforest species are highly active. They don’t need to conserve as much moisture or energy as they live in both a prey and moisture saturated environment. Their next meal/water source is much closer in time then a desert adapted species. In addition, speed and mobility are prized evolutionary advancements in a rainforest, which enables a species to better capture prey and escape predators. This increased energy demand means increased respiration, but since the air is so saturated with moisture, even though a rainforest adapted species is breathing more, it has a lower rate of water loss per breath than a desert adapted species. As such, since their prey availability/water availability is more consistent and their water losses from respiration are minimal, it doesn’t make sense evolutionarily for them to create or retain any adaptations to conserve moisture like decreased activity levels and the coping mechanisms for them.

So, boiling all of this down to one coherent statement, we can answer the original question.

Some species are moisture dependent and require moist substrate because they never faced any evolutionary pressure to create or keep any moisture retainment adaptations. As such, when conditions are dry (low RH), moisture dependent species will lose water at a faster rate than a desert adapted species and will hit the point of no return faster.

Some species can tolerate a wide range of moisture parameters because they faced evolutionary pressure from their environment to adapt to variable conditions (the generalists). These adaptations like reduced energy expenditure and creation of microclimates allow them to weather small periods of unfavorable conditions and allow them to colonize a greater variety of habitats.

Some species can tolerate extremely low levels of moisture because they have acquired specific adaptations and behaviors to greatly minimize water loss during periods of uncertainty.

Or, to put it more succinctly, the average levels of RH in the natural environment directly correlates to whether or not a tarantula has the evolutionary adaptation to physically limit its moisture loss. As such, tarantulas that evolved in a high RH environment are at a much higher risk of desiccation in a low humidity environment than a generalist or desert adapted tarantula that evolved in environments with periods of low RH. In captivity, to combat this, we need to monitor both the soil moisture level and ventilation level in order to maintain a suitable humidity range for all tarantulas to reduce their primary water loss from respiration in line with what they have evolved to live in.

There’s one more argument that I want to address and I’ve seen it thrown around from time to time.

If humidity was important and not just soil moisture, then why can’t you keep a moisture dependent species in a high humidity environment with dry soil?

The first thing I want to point out is that soil moisture is a major component to humidity, therefore if soil moisture is important, then so is humidity and vice versa. To even achieve this environment in a confined enclosure, one would have to maintain an extremely high ambient humidity in the room housing the enclosure or live in a naturally humid environment. Keepers that I have talked to in the PNW (which has a consistently high ambient humidity) have stated they can keep their moisture dependent species on the drier side (Not dry, just not as moist as expected). Second, you have to remember that tarantulas will instinctively burrow and hide to either find a suitable microclimate or get away from predators. This includes moisture dependent species. When they make a burrow in dry soil, the humidity in the burrow will actually be lower than the higher enclosure air humidity. The reason is that the moisture content of the air in the burrow is less than the moisture content of the soil, meaning that dry soil will actually pull water vapor from the air. Even if they are well fed and have a water bowl (you can lead a horse to water, but you can’t make it drink), their moisture loss rate from this decreased humidity in their hide will still be greater than their intake in the long run as they have no adaptations to conserve water and they will perish.

Alright. That’s it. That’s my reasoning behind my stance on the debate on humidity and why it is vital to proper tarantula husbandry. Due to the controversial nature of this topic, I expect to get quite a few discussions and or disagree emotes from the subject of this thread (and that’s fine). My only ask is that you be prepared to backup your statements with facts so we can generate a constructive conversation for the benefit of all.

 

[l4nsky]

How All of this Relates to Tarantula Husbandry


On the Subject of Recommended Humidity Levels on Care Sheets​

• Don’t take them as gospel, as they are far, far from it. Those numbers come from two possible sources. One would be the climate data from their collection points. The other is probable measurements taken by humidity measuring devices. Both sources are inaccurate. Addressing the climate data first, unless you are trying to cycle females for breeding purposes, don’t worry about the climate numbers. First of all, if that humidity number is the max daytime high, you have to realize that the tarantula will never be exposed to that level. The hides they choose are insulated microclimates that maintain much stabler parameters. Further, if that number was the night time high when tarantulas are active outside the burrow, the specific number still wouldn’t be required in a captive scenario. Tarantulas get the majority of their moisture from their prey. While moisture dependent species certainly have a more available supply of prey, the frequency of catching said prey is still variable. In captivity, the frequency they get prey is much more consistent, meaning they are consistently gaining moisture while still feeding, which also means they can survive slightly greater moisture loss due to lower humidity levels then their natural environment (but not too low, it’s a balancing act where the net positive has to be water gained). This fact allows them to survive and thrive in a much larger RH range then they would in the wild. Addressing probable measurements of a captive environment, this is equally ineffective. Due to the complexities in RH that were previously discussed, these instruments are delicate and can fall out of calibration quite easily. If the device can’t be calibrated, it’s GARBAGE and the readings can’t be trusted at all. If it can be calibrated, the readings must still be taken with a grain of salt, and that sodium serving should increase the longer the sensor goes without calibration. This is why the best scientific tools for measuring RH cost $100’s of dollars. So that begs the questions, was the author’s gauge able to be calibrated and if so, when was it last calibrated? Is this humidity level consistent throughout both the day and night? Do I need a gauge with calibration to chase this number? How will I even achieve this number consistently when there are some influencing variables of RH that are out of my control (like air pressure or the ambient humidity of the author’s environment)? The safe answer to all of these questions is No. Keeping tarantulas is not hard, even moisture dependent species if you do the right research. The only time I’ve even worried about specific humidity levels is during breeding and I manipulated them the same way I do in my enclosures that don’t have gauges (through ventilation and soil moisture).

On the Subject of Soil Moisture​

• Get to know your substrate mix (and learn to pair it with the right amount of ventilation). Mix up your substrate and then take some out to a separate bowl. Add a bit of water and thoroughly mix it in. Take a fist full and SQUEEZE really hard. If more then a drop or two of water comes out, add a handful of dry soil, remix, and test again. If no water comes out, add some more water to the bowl, remix, and test again. You’re trying to achieve a test where one or two drops of water comes out of your clenched fist of soil. This level of soil moisture is the most a moisture dependent species or generalist species should ever be exposed to. When you have this result, really get to know it. Look at the color difference between this moist substrate and the dry substrate. Fill an enclosure with it and another with dry substrate and compare their weights. The ability to judge soil moisture levels is an important skill needed to keep moisture dependent species and is only learned through experience. Taking that same bowl of substrate, add 50% by volume of dry substrate and remix. This soil moisture level is what I am for to keep generalist species. It can be drier for a time and it can be wetter for a time, but this is what I consistently try to achieve. One important note here is that soil moisture levels in an enclosure will naturally stratify, or form layers. The substrate with the most moisture will be found furthest away from the evaporative surface area (meaning the bottom of the enclosure). If your tarantula has a burrow at these levels, you only need to be concerned with keeping these soil depths at the proper moisture. The top levels can be allowed to dry out a bit more (and decrease the RH) as long as the soil moisture level of the bottom layer and its accompanying microclimate with an increased humidity can be maintained.
• If you’re housing a moisture dependent species, consider substrate additives like vermiculite or sphagnum moss to increase the moisture capacity of the soil. This, along with a deep substrate, will maximize the amount of water in the soil, maintaining a stable humidity for a longer period of time.
• Soil compaction plays a role in regulating RH. Soil that is well compacted lacks air pockets. This greatly reduces the surface area for evaporation of water from the soil. Soil that is loose has a much larger surface area and will lose moisture faster through evaporation. Take advantage of this. In moisture dependent species, make sure the deepest layers are well compacted. Your tarantula will still be able to make a burrow in it and that burrow will maintain a much more stable humidity. For species like Avicularia that appreciate a little humidity bump, but not a consistently humid environment, utilize a loose substrate that will give up its moisture faster, returning to a lower level quicker than a compacted soil.
• DON’T FOLLOW SOMEONE ELSE’S STEP BY STEP CARE GUIDE ON WHEN TO WATER! There is no one size fits all schedule to maintaining a proper soil moisture. You have to gain this experience yourself, in your environment. The rate of evaporation of moisture from an enclosure is dependent on the ambient humidity of the air and the soil moisture capacity is dependent on the substrate mix. If the keeper writing the guide lived in FL with a high humidity and the reader of said guide was trying to follow it while living in the low humidity environment of the Arizona desert, assuming the ventilation is the same, then the keeper who wrote the guide will actually be watering less than what the keeper reading it in the desert would need to in order to keep moisture dependent species.

On the Subject of Ventilation​

• Controlling ventilation and the air turnover rate directly controls humidity in tandem with soil moisture. Humid air rises and escapes from the top of the enclosure. Take advantage of that. For species that like it on the drier side, use more top ventilation then cross ventilation to allow the humidity to escape quickly. For those that need to be kept on the moist side, utilize more cross ventilation then top ventilation. Restricting the humid air from escaping through the top increases the ambient humidity in the enclosure, while the ample cross ventilation prevents stagnate conditions from forming.
• If condensation forms at the top of the enclosure, then there is too much soil moisture or not enough ventilation. The moist air can’t escape quick enough and as it cools, it will lose the ability to hold moisture, but since the amount of moisture in the air doesn’t change, that moisture will be deposited on a surface through condensation (meaning you’re at or very close to 100% RH). If you know your soil moisture is on par with that described above, then you need to increase your ventilation a bit.
• If the soil moisture level is decreasing too rapidly and it dries out too much between the times of your husbandry schedule, you either need to add more water or decrease the ventilation. If you know your soil moisture is on par with that described about, then you need to reduce your ventilation a bit to retain more moisture and keep a good air turnover rate.

On the Subject of Misting and Maintaining Soil Moisture​

• Don’t mist to maintain soil moisture as it is ineffective. The water won’t penetrate deep into the soil and won’t be retained for long. Likewise, the water sprayed on the enclosure sides will quickly evaporate, producing a humidity spike instead of a consistent humidity level. Humidity spikes are more gradual and easier to control if a loose substrate is used instead.
• There are only three acceptable circumstances to mist that come to mind:
  • Providing an arboreal tarantula like an Avicularia with a chance to drink by misting their webbing
  • Providing a sling kept without a water bowl the ability to drink from a misted enclosure side
  • Potentially when being used to cycle reproductive females though a breeding cycle
• To add moisture to a substrate, use a spray bottle set to a stream or a water bottle with holes poked in the cap to create a small watering can. Moisten the top a little bit, but send the majority of the water against one of the enclosure sides so you can see it percolate to the bottom to judge when you’ve added enough.
• It’s easier to add moisture then take it back. If you’re in doubt, err on the side of caution, add a little less than you think is needed, and check on the soil moisture level of the enclosure in a day or so. If it needs more, then add it. If you’ve added too much, you’ll have to let it dry down and adjust accordingly the next time you add moisture

On the Subject of Sling Husbandry​

• Slings are much more prone to desiccation and death from water loss then their adult counterparts. The reason is twofold. For one, slings lack a fully formed epicuticle, and as such suffer higher water losses from evaporation off the body’s surface area. Secondly, and compounding on the first, slings have a much higher surface to mass ratio then their adult counterparts, meaning they have less water to lose before reaching the point of no return and they have a larger area comparatively over which to lose it. Its for this reason that slings need to be kept with a higher humidity then adults of the same species. Please note I’m not saying they need to be kept at high humidity in general. You shouldn’t be keeping a sling of a desert adapted species in the same conditions you will be keeping a sling of a rainforest adapted species. You just need to keep the sling at a consistently higher humidity then an adult of the same species. For desert adapted species, this might mean soaking a third of the cage every week. For a rainforest adapted species, this might mean adding a bit of moisture to the soil every 3-4 days to keep it at the max levels described above. For generalists, they can tolerate a middle ground between the two previously mentioned, but my recommendation is to err more towards the damper side of the spectrum (as they can tolerate it) to help prevent desiccation.
• Feed slings an appropriately sized meal often (my schedule is every 3-4 days). Tarantulas get a lot of moisture from their prey and a lot of slings aren’t kept with water dish. By getting consistent prey and moisture, combined with a decent humidity for the species, you’ll maintain a slings moisture reserves, giving it better odds to successfully molt and grow out of this delicate stage of life.
• If you can’t feed appropriately sized prey and have to give them larger, prekilled prey, be very proactive in maintaining a decent humidity level and offering a chance to drink from a misted enclosure wall from time to time. They will likely have a longer premolt (or time when they aren’t actively eating and taking in moisture) and they will lose more of their water reserves in premolt then a sling with a more consistent feeding schedule of smaller prey leading up to premolt.

On the Subject of Premolt and Molting​

• Humidity has an effect on the odds of your tarantula having a successful molt. When a tarantula molts, it has a fluid between the old exoskeleton and the new one which acts as a lubricant, allowing the tarantula to squeeze out of its exuvia. When this lubrication is exposed to the air, it starts to evaporate. If the air is bone dry, this lubrication is gong to evaporate a whole lot faster at lower RH levels. If it evaporates faster then the tarantula can extricate itself from the old molt, there’s a high probability the tarantula will become stuck.
• Humidity also has an effect during premolt that can lessen your tarantula’s odds of a successful molt. If a premolt tarantula is kept dry, then they will lose the moisture reserves they need for a molt faster than if they were kept more humid. The reason is that tarantulas get a lot of their moisture from their prey, and since they aren’t eating during this time, they are only losing water. If they lose too much, they have higher odds of getting stuck
• “But what about a water bowl?” The age-old adage “you can lead a horse to water but you can’t make it drink” applies. Just because a tarantula has access to a water bowl doesn’t mean it will use it. By all means, use one if you can, but don’t solely rely on it to keep your tarantula’s moisture reserves up.
• “I keep my premolt tarantula’s dry and I’ve never had a problem, so everyone doesn’t have to.” Notice that I’m saying there are higher odds for a bad molt, NOT your tarantula will have a bad molt. There are mitigating factors in this, and while it might just be true that your tarantula was able to keep an adequate moisture level for that molt to be successful, why play Russian roulette again or encourage someone else to play it? It’s so much easier to just bump the humidity a little bit for some peace of mind.

On the Husbandry of Aviculariinae​

• In my personal opinion (but admittingly limited experience as of this writing), I believe we should refer to Aviculariinae as “ventilation dependent species” much in the same vein as we call rainforest tarantula species “moisture dependent species”. My opinion is based on what we know of their natural habitat. These are the true arboreal spiders, pretty much always making an elevated hide utilizing leaves, bark, a small cavity, and plenty of webbing. They're not really adaptable like some other generalist species (ex Psalmopeous or Poecilotheria) who will burrow in captivity. Being strictly arboreal in their habits, they are exposed to much different environmental parameters then a terrestrial species or a more adaptable species that might make its home much lower. They still get exposed to a decent humidity level, but there is much more air movement in the less restricted canopy and sub canopy they call home. This fact also means that while they can be exposed to some really high humidity levels in the rainforests, the air turnover will mellow them out much quicker, resulting in temporary humidity spikes instead of prolonged increased humidity levels like terrestrial species would encounter on the forest floor. In short, stagnation, not high humidity, kills Avicularia. Also, slings and younger Avicularia can be found lower down in the trees, where the humidity is a bit higher which in turn helps prevent water loss from evaporation. For the six Avicularia slings I’m currently raising at this time, I'm keeping them in 5.5 oz deli's, with loads of cross ventilation and some top ventilation (about a 9 to 1 ratio), the bottom half of the sub is compacted and was originally moist, the top half is loose and dry, covering a few cross ventilation holes (getting more airflow into the loose soil increases evaporation, resulting in humidity levels that quickly return to normal after watering, producing spikes instead of consistently humid). They get care every 3-4 days. If the sub is completely dry, I wet half, otherwise I leave it be. If they're not in premolt, I'll feed them an appropriately sized lateralis nymph. If they are in premolt, then their webbing gets a slight mist and I’m more vigilant about keeping half the sub wet.

On the Husbandry of Pelinobius muticus​

• The commonly accepted husbandry strategy for KBB's has always been extremely deep, bone dry substrate as they dig deep burrows in the semi-arid grasslands they inhabit in the wild. The problem with this husbandry technique is it doesn't take into account why they dig such deep burrows. Like all tarantulas, they dig burrows for security AND to find the perfect microclimate to live in (ex temperature and humidity). In the drier areas they call home, they have to dig to great depths to find soil moisture. When we keep them in deep, dry substrate, they'll burrow to the bottom to try and find moisture, and they'll close off their burrow to retain the humidity that they find, just like in nature. Unlike nature though, in captivity, the KBB's don’t have access to the subterranean prey like worms and burrowing insects. As a result, while they wait for conditions to become more favorable (better moisture levels), they will wither away and perish (Tom has a good video on his YT channel Tom's Big Spiders where he makes a similar observation). With all of this in mind, I keep my KBB using a different husbandry method. I keep my female on relatively shallow substrate (1-2x DLS measurement) and I keep the substrate moist (not as moist as Asian species require, but definitely more than bone dry). This way, she never has to dig deep to establish the perfect microclimate. I have no problem getting her to eat and she'll only close her burrow off when she's in premolt. I've kept her this way since she was a ~3/4" sling with no issues.

​

 

[l4nsky]

Respectfully, I disagree. I fully understand why we tell new keepers to disregard RH values as specific values are impossible to maintain and they'll do more harm then good even attempting to maintain it. I don't believe I've caused confusion as we're all aligned on the proper husbandry methods around monitoring soil moisture vs monitoring RH values. I guess I'm just tired of seeing beginners fed the same white lie of "humidity doesn't matter, just overfill the water bowl" when one action directly begets the other. I understand the good intentions of the statement, but it's still a false statement that is antithetical to the purpose of these boards. All this being said, I do agree that this is not the thread to wade into these waters and I apologize for letting my frustrations get the better of me. My commitment is that I'll start working on a thread to explore this topic in depth and I'll tag you both for the discussion.

https://arachnoboards.com/threads/how-to-calm-down-a-human-specimen.345734/post-3183072

@cold blood, @Smotzer, as promised.

 

[viper69]

Would you provide a better idea on the formation of water in the air, I feel you’re missing a few key points

 

[l4nsky]

Would you provide a better idea on the formation of water in the air, I feel you’re missing a few key points

By all means sir, care to add those missing key points? Explicit is better than implicit

 

[mack1855]

I don't know what this means?..Im a retired truck driver,in the western US, who keeps big hairy spiders.
I worry about if my t will eat that B.lat I just gave it.Is this sling going to molt?.Should I rehouse now,or next month?.
And,will the Denver Broncos ever win another superbowl/?. .

 

[Arachnid Addicted]

Awesome thread.

I'll just comment in here to check how the "humidity is irrelevant" guys will react to this.

Thanks for all the patience you had to provide all these infos.

@Dorifto
Take a look at this.

 

[l4nsky]

I don't know what this means?..Im a retired truck driver,in the western US, who keeps big hairy spiders.
I worry about if my t will eat that B.lat I just gave it.Is this sling going to molt?.Should I rehouse now,or next month?.
And,will the Denver Broncos ever win another superbowl/?. .

Sounds like someone skipped past the disclaimer on the top lol. I mean, why even read those things

 

[mack1855]

I mean, why even read those things

What things?.

 

[vicareux]

I was looking for info such as this for a long time. I couldn't help but notice the polarization between other sources (Youtube,care guides and whatnot) and this website. Of course,this website has much more reliable info than Youtube/care guides online so i leaned on this forum's info,thankfully. But it just didn't seem right that humidity plays next to no role in a Tarantula's life,especially molting. Not much in my husbandry will change,but at least i have a better understanding now,which makes me a more confident keeper. Maybe i didn't understand every detail of the complexity behind humidity itself,but i can always come back and re-read the details.
Good text. Cheers!

 

[l4nsky]

Maybe i didn't understand every detail of the complexity behind humidity itself,but i can always come back and re-read the details.
Good text. Cheers!

All good, a complete understanding isn't necessary as it doesn't change the golden rule of using soil moisture and ventilation to control humidity. If anything, as long as you understand it's complex and that complexity makes maintaining specific values extremely challenging and not worth the time for the casual keeper, then you get the gist of it .

 

[cold blood]

I'll just comment in here to check how the "humidity is irrelevant" guys will react to this.

Its still irrelevant to the actual keeping of tarantulas.

 

[viper69]

I don't know what this means?..Im a retired truck driver,in the western US, who keeps big hairy spiders.
I worry about if my t will eat that B.lat I just gave it.Is this sling going to molt?.Should I rehouse now,or next month?.
And,will the Denver Broncos ever win another superbowl/?. .

You have this all wrong.

Dude stop worrying about rehouse, just let them free roam, it’s great with arboreals- don’t have to worry about stepping on them!!!

 

[KaroKoenig]

it’s great with arboreals- don’t have to worry about stepping on them!!!

But they'll jump onto your neck and bite you. That's what they do, I heard.

On topic: great Information, and from someone who has to deal with rh all the time in my job - pretty much spot on.

 

[moricollins]

In my other hobby, dart frogs, we're constantly telling new keepers not to chase an RH value in their tanks, and that moisture gradients and ventilation are more important than keeping the tank at a specific RH.

 

[l4nsky]

Its still irrelevant to the actual keeping of tarantulas.

Care to expand your opinion a bit sir? You know me, I like verbose posts with full explanations .

 

[DaveM]

Ahggg! @l4nsky, why did you have to do this now? I have a huge grant due Tuesday and then am traveling for a few days. I want to respond, but don't have time.
I appreciate your intentions and see a lot of value in what you've written, but I hope beginners don't read it and take it as gospel.
I'm not completely in the hardcore anti-humidity group, and agree with some of what you said, but there are also some assumptions up there... ...thanks for using language like "opinions" and "theories" --which is accurate as laypeople understand those terms.
One large point to consider: Think how differently we modern humans live today, from how we 'evolved' to live in prehistoric times. There are downsides in some cases (e.g. not enough exercise, obesity, etc.), but we generally live longer, healthier, safer lives.
I evolved to binge eat as much fat and sugar as my cave man hands could grab, to feed my gut load of parasites, to watch my wife die in childbirth, to lose all my teeth and die at 50 if I'm lucky.
I'll take my modern life in the 'captivity' we have created. "That's how they evolved" is not sound scientific reasoning, not technically correct.

 

[Arachnid Addicted]

Its still irrelevant to the actual keeping of tarantulas.

We will always disagree with that.

Although I agree care sheets and pursuing RH percentage are irrelevant, I also have in my mind that moist, damp, and other words that are used here, are nothing more than humid synonyms, therefore, I can't make such a bold statement like "humidity is irrelevant". But I got your point a long time ago, I was just joking.

(And of course, there are other reasons why I won't ever say humidity don't matter, but we've already debated about this a lot, and there's no point to talk about it again. Lol. Respect your opinions, though.)

 

[cold blood]

Care to expand your opinion a bit sir? You know me, I like verbose posts with full explanations .

Ive expanded on it so many times here on the boards...shouldnt be hard to find.