Thermoregulation in tarantulas

Here we go again! I have been working on writing about tarantula welfare, and saw this topic has come up on The Tarantula Collective/Exotic Pet Collective podcast. I just happen to be mid-literature review looking for papers and texts that can help keepers understand animal welfare for tarantulas. I need to read and understand some of these papers anyway, so I thought I would make plain-language summaries for what I’ve found. Before reading, I also want to mention that there are lots of folks working towards better husbandry and welfare, and this is a synthesis of information and ideas from many people.

First, why is this interesting? Aren’t tarantulas comfortable at the same temperature as people? I had a C. versicolor that passed away last year after a rehouse. After reviewing what could have gone wrong, I thought that perhaps my rehouse location was too cool. Maybe my spiderling wasn’t comfortable where I was comfortable after all. I have tried again with a new sling, and he is now a beautiful subadult on his way to maturity. The big difference I made is that I made sure he could thermoregulate as a sling which included two main components: 1) Offering an appropriate level of heat and 2) and enclosure large enough that he can choose for himself where to be. Aside from that experience, I have spent the last two summers recording data on A. hentzi burrows and, this summer, had the opportunity to observe many ectotherms in four different habitat types in Costa Rica over 6 weeks. I think our conventional wisdom – that tarantulas are happy at a single temperature – is probably incorrect. Since then, I have added low level heat for my adult tarantulas. They are in enclosures with two burrows (warm and cold) or, in the case of my C. versicolor, large enough that he can escape warm and/or cool areas. It’s important to note that my personal observations are very low on the food chain of scientific evidence. What we really need is a long term study with periodic welfare assessments of many tarantulas: half living in enclosures where they can thermoregulate and half where they cannot.

Secondly, animal welfare frameworks used for captive animals in zoos, meat production facilities, and in research often focus on providing captive animals with five freedoms (more recently five domains). They are the freedom 1) from hunger and thirst 2) from discomfort 3) from pain, injury, and disease 4) to express normal behavior 5) from fear and distress. Providing tarantulas with multiple burrows, water dishes, and the opportunity for thermoregulation falls under freedom two and four. Many welfare researchers believe that access to these freedoms benefits captive animals regardless of their cognitive ability. The following quote is from Browning and Veit (2021)’s paper “Freedom and Animal Welfare.” Their paper is a great read and not too technical. I recommend it for anyone keeping pets.

“The second way we might conceptualize the relationship between freedom and welfare is in a purely instrumental sense. That is, that being free allows animals to pursue those things that are in their own interests—those things which create positive subjective experiences—as well as allowing them to avoid those things that would harm them. This will then lead to a correlation between increased freedom and increased welfare that is a result of the welfare opportunities freedom provides, rather than the intrinsic welfare benefits of freedom itself.”

Browning, H., & Veit, W. (2021). Freedom and Animal Welfare. Animals, 11(4), 1148. https://doi.org/10.3390/ani11041148

While holistic welfare assessment for tarantulas is in its infancy, we can use frameworks developed for vertebrates (animals with a backbone) like the five freedoms, to asses our own husbandry. I contend, that the more choice we can provide tarantulas – the better their overall health and welfare will be. They have preferences for food, water, shelter, space, and temperature just like any other animal. Their enclosures should provide them with many opportunities to choose their own comforts and avoid discomfort. This approach works very well, and is accepted for many other animals – why ignore invertebrates?

Five Freedoms	Five Domains
1. From hunger and thirst	1. Nutrition
2. From discomfort	2. Environment
3. From pain, injury and disease	3. Health
4. To express normal behaviour	4. Behavioural interactions
5. From fear and distress	5. Mental state/experiences

The five freedoms and five domains keepers, farmers, and researchers use to evaluate animal welfare. This table is copied from RSPCA.

Here I am going to summarize four studies that show clear thermoregulatory behavior in tarantulas. I do not know these scientists personally, and they did not make any statements in their papers about husbandry practices. I also want to point out that scientists in South America do really excellent tarantula (and arachnid) research. Much of what we know about tarantulas is because of dedicated latino and latina researchers.

Veloso, C., Luhr, D., Marfull, R., Torres-Contreras, H., Pérez, D. F., Sabat, P., & Canals, M. (2012). Characterization of the thermal micro-environment of Paraphysa parvula Pocock 1903 (Araneae: Theraphosidae), a spider from the Chilean Andes. Journal of Arachnology, 40(1), 34–38. https://doi.org/10.1636/B10-46.1

In this study, Veloso et al. (2012) observed wild Euathlus parvulus and brought some into the lab. They used thermocouples to measure the body temperature of the tarantulas and the temperatures of the air, surrounding substrate, and the rocks that covered their burrows. In the lab, they placed spiders into a tube with a temperature controlled floor that varied from 10 to 70 degrees Celsius (C). For my North American friends, that’s 50 – 158 degrees Fahrenheit (F). So, the hottest point was more than enough to kill the tarantula during prolonged exposure. The researchers wanted to know if the tarantulas had a preferred temperature, if that varied with sex or size, and how they might be getting warmth from their environment. E. parvulus lives in the Chilean Andes above 2000m (6500 ft). Considering that many tarantulas live at lower elevations, E. parvulus is probably more cold-tolerant than your average tarantula.

So, what did they find? Both tarantulas in the wild and the lab maintained their temperatures at around 31 C (87 F). Females that were in some part of their reproductive phase, had slightly cooler body temperatures at 29 C (84 F). The size of the tarantula did not affect their preferred temperature, but there was a lot more variation in the temperature of subadult tarantulas. Their body temperatures were correlated with all the other temperature variables they measured, air, substrate/dirt, and rock temp. Generally, the spiders were a little cooler than these variables, but there was a positive relationship. Warmer environmental temperatures meant warmer spiders. Interestingly, the researchers did NOT observe any of their tarantulas cooking themselves in their lab experiment. So, the tarantulas did not wander over to the 158 F side and die. Furthermore, the researchers concluded that their tarantulas were regulating their body temperatures behaviorally, which means the spiders were choosing where to go to maintain a comfortable body temperature. Lastly, they noted that the rocks the wild tarantulas used for cover were less associated with tarantula body temperature than the air temperature under the rocks. That’s important to think about! It means that, given a deep burrow, the tarantulas could adjust their temperature by moving up and down their burrow. It also means that habitat disturbance, like rock flipping, can change the microenvironment where tarantulas live. So, it’s important to always put rocks back exactly how you find them if you are legally looking for ectotherms.

Schwerdt, L., de Villalobos, A. E., & Pérez-Miles, F. (2019). Thermal preferences and effects of temperature on fitness parameters of an endemic Argentinean tarantula (Grammostola vachoni). Canadian Journal of Zoology, 98(2), 134–141.

In this series of experiments, de Villalobos and Pérez Miles (2019) studied G. vachoni in the lab. They were interested in learning more about their temperature preferences in light of climate change and conservation. They made a tank with a temperature gradient that varied from 3 C (37 F) to 60 C (140 F). The tarantulas were placed in the center of the tank and the scientists measured their temperatures every half hour for four hours. Just like E. parvulus, adults and juvenile tarantulas had similar temperature preferences, spending just under half their time between 25-29 C (77-84 F). For the second part of their experiment, they measured locomotor performance (sprinting speed) at different temperatures. Performance for both adults and juveniles decreased above 40 C (104 F) and 45 C (113 F) respectively, and in a few cases the tarantulas movements become spasmodic (abnormal). This is important because extremely high and low temperatures could affect where and how tarantulas move from place to place. The researchers concluded here that their tarantulas were good at behaviorally regulating their temperature around their thermal optimum. Thermal optimums are calculated based on the metabolic rate of animals, and give scientists the temperature where the animal is metabolizing most efficiently. This is really important for ectotherms that don’t produce their own body heat. Additionally, the researchers were worried about how the tarantulas moved in cold and warm temperatures, and made some comments about how climate change could restrict their movements if the environment outside their burrows becomes too hot or cold. There was much more to this paper in terms of math modelling and and subtle details. However, the big takeaway was – again – the tarantulas did not cook themselves and they regulated their temperature behaviorally.

Shillington, C. (2002). Thermal ecology of male tarantulas (Aphonopelma anax) during the mating season. Canadian Journal of Zoology, 80(2), 251–259. https://doi.org/10.1139/z01-227

Shillington (2002) is a very nice study on wild male A. anax. She did a really excellent job estimating temperature parameters used in for ecophysiology studies. This type of work is especially important for understanding the preferred environmental temperatures of animals and how that relates to energy expenditure. Remember that ectotherms’ metabolic rates are directly associated with temperature and it can get too low or too high. To figure out some of these temperatures, Shillington (2002) spent a few field seasons following male tarantulas around in Texas. She used a thermocouple arrangement to measure their body temperature and measured air temperature and temperature in shade. She also observed a few males along a thermal gradient in the lab that ranged from 12C (53 F) to 46C (114 F). One of her main figures is very easy to understand (figure 1). The bars represent the % of recordings that were at each temperature. (a) is wild tarantulas, (b) are the lab tarantulas, and (c) are random temperatures at her field site. You can see in both conditions, the tarantulas were choosy and their temperatures weren’t random. In this case, the males needed to stay cool rather than warm. Shillington (2002) hypothesized that males should conserve energy by hiding during the hottest parts of the day, thereby keeping their metabolisms a little lower. And this is what she saw – they were active in the morning and evening. My big takeaway from her study is that sometimes tarantulas may need to be a little cooler rather than warmer, and their behavior may change based on their life stage or simply time of day. Note that there’s more work she did, and this is just my interpretation of the more relevant points.

Stewart, D. M., & Martin, A. W. (1970). Blood and Fluid Balance of the Common Tarantula, Dugesiella hentzi*. Zeitschrift Für Vergleichende Physiologie, 70, 223–246.

Note that this paper is very old, and these spiders are now called Aphonopelma hentzi. Their sample sizes were a little small, but Stewart and Martin (1970) did some very important work here on blood, body composition, and water loss. I am going to focus on evaporative water loss (what happens when you sweat) because that is very important for safely providing heat to tarantulas. They took three spiders and put them in a device where they could control temperature and airflow. They weighed the spiders at intervals and took small blood samples to measure water loss. Additionally, they measured water loss if the air mixture had more CO2 in it. CO2 causes spiders to open their spiracles (apparently, tarantulas do have them in addition to their book lungs). The tarantulas lost water weight under all conditions. The lower temperature was 20C (68 F) and the highest was 40C (104F). For each temperature, the tarantulas lost water weight over the five hour experiment. The highest rate of loss occurred un the first 2-3 hours and began to level off, with the exception of the 40C experiment. At 40C their largest spider (15g) lost .282g or 1.88% of their total body weight. They noted that high CO2 can double this rate of water loss. Their conclusions mention that most of this water loss probably came from the exoskeleton of the spiders . Once that water was lost, the rate of water loss leveled off because their exoskeleton protects them from losing water from their body. However, the exoskeleton was not effective at protecting the tarantulas if temperatures got too hot. Lastly, they observed drinking behavior over time. Their spiders drank approximately once per week – so they were actively replacing water that they lost naturally. One other experiment they did involved observing spiders that were starved for 50 days. There were only three individuals but the results are striking (figure 3). Sudden weight gains were purely from drinking, and one tarantula gained weight over the course of the experiment. Another scientist. William Baerg similarly reported that starving tarantulas could maintain their weight, at least for one season, by drinking. While less relevant for this blog post, it might be that keepers don’t know their tarantula is sick until it’s very near death because they drink and maintain weight during the early stages of illness.

There are more studies about thermoregulation that I can add, but these four get the point across pretty well. Tarantulas can behaviorally thermoregulate, they have preferred temperatures, and they don’t seem to cook themselves. In fact, they have many physiological and behavior adaptations to avoid that.

What do I think keepers should take away from this?

Consider providing a species appropriate thermal gradient so your tarantula can have the freedom to regulate its own temperature

*Safety precautions*

• Tarantula enclosures tend to be small. If you put a high wattage bulb on your tarantula enclosure, and you spider cannot move away from the high temperatures…YOU CAN COOK YOUR TARANTULA.
• Your temperature gradient does not need to be extreme! Read about your species of interest and look at the ranges I have posted here.
• Most tarantulas in these studies had a preferred temp well below 40C (104 F). So, if you are getting close to 40C in your enclosure at any point, that is way too hot!
• If you increase your temperatures (even in gradient), your humidity is going to go down. Even desert fossorial tarantulas have higher humidity levels in their burrows. Make sure your keep your humidity up to prevent dehydration.
• Tarantulas in dry enclosures with poor airflow (high CO2) may be at increased risk of dehydration.
• Always provide fresh water.
• If you use a heating mat, you should use a thermostat so that your mat does not cook your tarantula. In small tanks with only single burrows, tarantulas may not be able to escape a faulty heat mat.
• If you use a heat mat, are your really providing a gradient? Do you have a cool burrow and warm burrow? Can your spider actually regulate their temperature?

I know this is contentious in the tarantula hobby and it increases cost, time, and mental overhead while caring for tarantulas. It increases risk if you aren’t sure what temperature and humidity gradients you need to provide or how to provide them to your t’s. We know that tarantulas can survive without extra heat, so why even consider adding small, species appropriate, temperature gradients?

1. I think more environmental choice will produce better long-term welfare outcomes, though no one has studied this yet!
   • If you are an undergraduate studying zoology, biology, ecology, animal science, veterinary science, or something similar this could make a good project. Many universities have funding for undergraduate research projects. I recommend asking your professors about opportunities.
2. There are more studies out there about thermoregulation and growth that I need to add. These four are not the end all, be all, thoughts on thermoregulation.
   • I highly recommend, New World Tarantulas: Taxonomy, Biogeography and Evolutionary Biology of Theraphosidae (Zoological Monographs, 6) for further reading.

Very last. I’m not here to shame. I have some tarantulas that are in a room with much better environmental control that I do not provide extra heat. I would like to in the future, but that’s a long term goal and there are mitigating factors with those individuals. However, the tarantulas I keep as pets all have multiple burrows and hiding places along a thermal gradient. Generally speaking, we need to do much more scientific research about husbandry. It would benefit the thousands of tarantulas in captivity and provide a scientifically valid backbone for keeping and breeding practices. It could also make a very cool citizen science project. While scientists should use their expertise to design studies and analyze data, there’s no reason that data needs to come from a lab. It could absolutely come from keepers.