Brumation by Steven Bol

Steven Bol gave me permission to place his article about brumation on my website.
It was originally published in The Garter Snake.

Hibernating Garter Snakes: a must or an option?

By Steven Bol (published in The Garter Snake 2004, 2, 2004 – EGSA)


In this article I describe the method I have used for the past 18 years to hibernate garter snakes successfully.

In  The Garter Snake I have read several times that people use completely different methods of hibernating their garter snakes , usually much shorter (see for instance Kreyerhoff (2002) and  Buchholz (2002)). In general many people put drinking bowls in the hibernation terrarium  and keep the substrate relatively dry. Many garter snake keepers consider the hibernation period as a difficult period, and I do not know many who hibernate the very small juveniles in their first winter.

I hope my experience can help other garter snake keepers with hibernating their favourite snakes in a successful way without losses..

In my opinion, partly based on several journeys to the US visiting the different garter snake habitats in different seasons, hibernation is such a crucial part in the life of garter snakes that keeping them warm year round seems very unnatural to me. Just like being awake 24 hours a day without sleeping. This also counts for garter snakes from regions which are considered as southern without a cold winter like Thamnophis sauritus sackenii from Southern Florida or Thamnophis sirtalis tetrataenia from California. Visiting their natural habitats during winter time reveals no or hardly any activity, except on some of the warmer winter days.

Hibernating garter snakes is very important when you want to breed them consistently, even though there are plenty of examples of successful breeding attempts without a hibernation.

Timing of hibernation

Thamnophis atratus atratus...
Thamnophis atratus atratus…

I partly led the snakes themselves decide when to start with hibernation. Studying  their behaviour can give some clues.

Somewhere in the fall it is noticeable that the snakes slow down their feeding frequency,   eating only small amounts or that they stop feeding altogether. This often happens somewhere in October or November in my current snake room.  Because I keep my snakes in an unheated room the dropping temperatures outside influence the temperatures in the room and in the terrarium. During the summer months of June- August the night temperatures almost never drop below the 20 degrees at night. But when the nights start to cool in September room temperatures can drop to 15 oC at night (terrariums are only heated during daytime), and later in October and November the temperature drops further to 10 degrees. I believe that this cooling off during night triggers the snakes to prepare for hibernation.

In my former snake room, which was less isolated and hence much colder in September/October then my current snake room, the snakes stopped feeding often as soon as end of September.

Other signs which show the snakes sense that winter is coming are:

  • Retreating to the coldest part of the terrarium under stones or debris and not or hardly showing themselves to sun during daytime.
  • Crawling seemingly restless through the terrarium, rubbing the snouts against the glass in order to find a good hibernation den.
  • Higher mating activity during days which are a bit warmer then usual for the time of the year.

Not all snakes stop feeding out of their own, and at a set date I stop feeding altogether. This is usually somewhere between the beginning of October and the beginning of November.  The exact timing can be influenced by what suits me best. If I have a busy fall I stop feeding a couple of weeks sooner for instance.

After the last feeding activity I keep the lights on (during daytime) for another 7-14 days, in order for the snakes to fully digest their last meal and empty their intestines. If the snakes go into hibernation with food in their intestines it is likely to start rotting which can be fatal for the snakes. It is better to let the snakes go without food for some extra weeks then to run the risk of food remaining in their intestines. Since some snakes already stop feeding before I offer their last meal of the year, it can easily happen that some snakes have not eaten for 4-6 weeks before the lights are switched off. I have never seen any problems with this: in captivity even the less well-nourished snakes are much fatter then many of the garter snakes I encountered during my trips to the US in fall.

Which snakes go into hibernation?

All  my Garter Snakes go into hibernation.

When snakes are in their shedding period I keep them warm until they have shed their skin.

I have never had any pregnant snakes in this time of the year. This is probably because of the fact that I try to mimic the natural seasons as much as possible. So pregnancy has never been a reason for me to skip hibernation. But of course, if you have a pregnant snake I would always advice to keep her warm until she gave birth.

Young of the year all go into hibernation. Even when they are born in August or when they are very thin. I know many people are afraid to put young born snakes in hibernation during the first winter.  It is often considered as a big risk.  I like to put it the other way round. When young snakes are very thin and when they are not feeding very well I like to put them into hibernation. My experience is that when they survive the hibernation they will start feeding very well and do much better. I do however shorten the duration of the hibernation time for young snakes, especially when they are thin.

How to hibernate?

I use 3 different methods for hibernation.

The first method is an old refrigerator. The temperature inside the fridge is 4-6 oC during the entire time.

The snakes go into small containers, which or either of plastic or of metal (old cookie jars). For juveniles I use small jars, with a diameter of 9-11 cm and a height of 7 cm.  For a group of 2-4 adult snakes I use containers with a height of 6-15 cm and a diameter of 13-20 cm.

Thamnophis hammondii...
Thamnophis hammondii…

The lids are perforated with a few small holes, just enough for some air exchange but not so much that the content dries out too quickly! The containers are filled with a mixture of sawdust and fallen leaves (collected in my garden). When putting the snakes in the container I moisturise the content by sprinkling some water in the container. I normally check on  the snakes every 2-4 weeks, and I like to see the inside of the lid wet from condense water. It this is not the case I spray more water in the container: too wet is never a problem, but when the containers are too dry snakes tend to dehydrate (especially in the fridge) and loose a lot of weight. Some times the snakes are totally wet from the condense water. Inside the containers I never put a bowl with drinking water.

This method I use for garter snakes which originate from areas with cold winters: either from northern regions ( like Thamnophis sirtalis parietalis; T.s. sirtalis melanistic; T.s.semifasciatus, T.s.pickeringii; Thamnophis ordinoides; Thamnophis radix) or from high altitude areas (Thamnophis elegans vagrans, T.e.arizonae, T.rufipunctatus). The european Viperine water snake (Natrix maura) and the american Nortern Water Snake (Nerodia sipedon sipedon) are also  hibernated this way.

The second method I use is hibernating in the unheated snake room in the same jars as described above. The temperature in this room fluctuates more then in the fridge ( see graph 1). The range normally is between the 6 and 14 degrees Celcius. Night temperatures are generally lower then day temperatures. During weeks with very cold weather (below zero) the room temperature (and also inside the containers) can drop to temperatures as low as 1-2 degrees C. If it gets any colder then that, or it stays this cold for more then a couple of days I put on the central heating which gives temperatures of 10 degrees C. at night and about 20 degrees C. during day time. Also during occasional sunny winter days with mild weather day time snake room temperatures can get between the 15 and 20 degrees C.

This method I use for Garter Snake species originating from regions where the winters are not constantly cold, but where winter temperatures at least during night can drop to below zero. Snakes which have been successfully hibernated this way are Thamnophis atratus, T.cyrtopsis cyrtopsis, T.c.occellatus,  T.elegans terrestris, T.hammondi, T.marcianus, T.proximus rubrilineatus and the famous T.s.tetrataenia. The funny thing is that the minimum temperatures for the 2nd group can be lower (1-2 degrees C.) then in the 1st group (4 degrees C.), while the snakes in the second group are considered to be more sensitive for low temperatures.

The third method is hibernating the snakes simply inside their own terrarium in the unheated snake room. Temperatures fluctuate of course in the same way as the second method. The big difference is that the snakes remain in a completely dry terrarium with a bowl of water to drink from. I do not use this method for garter snakes but for the closely related north american water snakes originating from southern locations (Nerodia erythrogaster transversa and N. rhombifera rhombifera). These snakes are more sensitive for moisture, and if they are hibernated in the same containers as the garter snakes they tend to get blisters, eye inflammations and other skin problems.
After I switched off the lights (heating) in the fall the snakes are put into containers (in case of the first 2 methods) normally within 4-6 weeks, so all snakes experience a shorter or longer period of hibernation method three (in the unheated terrarium).

How long?

In table 1 the duration of hibernation for the different species are noted. I start counting the weeks as soon as the lights are switched of in October/November. So if for example a snake stays in the unheated terrarium for 5 weeks and then for 15 weeks in a moist container in the fridge the hibernation period as marked in table 1  as 20 weeks.

On average the juveniles (YOY = Young Of the Year) are hibernated for 2,3 – 3 months, where 8 weeks is the absolute minimum when the snakes are considered very weak.

For the adults the hibernation period is much longer, 3,5 to 4,5 months average.

The maximum I have done so far is for Thamnophis marcianus: 4,8 months.

The reason I have hibernated marcianus this long is twofold: I often see that the adults Checkered Garter Snakes tend to get very well nourished (fat), and I have never come across such well fed snakes in the wild. Beside this I know that some species like T.marcianus and T.cyrtopsis are especially in western Texas living in habitats with food available for a short period, and they are adapted for surviving long periods without food.

tabel_winteslaap_steven_bolSurvival rate during hibernation

Over the last 5+ years I have never seen big losses during hibernation. Sometimes an occasional snake dies during hibernation.

For example in the winter of 2002/2003 of the 50 adult and subadult garter and water snakes only 1 died (2 %). This was an old male (8-10 years) of the wandering Garter Snakes who had been ill for some time. Over the last 5 years I estimate that 5 % was the highest mortality rate, but on average it was probably close to 1-2 %.

Of the juvenile snakes (YOY; less the 6 months old) the mortality rate was higher! In the winter of 2002/2003 9 of the 41 juveniles died during hibernation (21 %). This is quit substantial.

But when analysing the data something has to be said. 13 of the 41 juveniles were belonging to the species Thamnophis sirtalis semifasciatus and T. radix which were born in my outdoor terrarium somewhere in August/September but they were caught and put inside quit late (beginning of October). The weather had been bad in fall and the young snakes had hardly or not eaten after birth and were in poor shape when caught. Then they were offered some food for a couple of weeks in a heated terrarium but their biorhythm was already in hibernating stage (combined with the cold nights temperatures in my unheated room) and most refused to eat. Of this group (which were very, very thin and tiny) 7 died (53 %).

Of the remaining 28 juveniles which were all born much earlier inside (june-august) and which had all eaten for at least 2-4 months only 1 died (4%). Over the last 5 years or so the mortality rates of the juveniles born inside has not been much higher then 5 %!

Loss of weight

When the garter snakes are hibernated the way described above the loss of weight is often minimal, and the snakes appear to be in good shape after hibernation. Normally I weigh the snakes before they are put in the hibernation jars and also afterwards when they are put into the warm terrarium again. Although the “kitchenweigher”is not super accurate (accurate at 1 gram level) I give some measurements as an example.

Three subadult Santa Gruz Garter snakes, 1 female and 2 males lost in 2002/2003 after a hibernation period of 3.2 months (method 2) 16, 5.9 and 6.7 % of their bodyweight. These numbers are considered as pretty high, normally it is lower.

Four adult Blackneck garter snakes, 2 females and 2 males, lost in 2002/2003 after a hibernation period of  4.2 month (method 2) 4, 1.1, 0 and 4.4 % of their body weight. These numbers are considered average.

When snakes loose a lot of weight during hibernation it is often a matter of dehydration. Then the humidity inside the jar has been too low.

Heating up the snakes in spring!

The procedure to enter the hibernation is usually quite abrupt. After the period which the snakes were suppose to hibernate is over I put them right into the terrarium, and turn on the light (= heat) or sometimes the lights have already been on during a couple of hours.

When the snakes end their hibernation already in February the snake room temperatures are usually quit low (6-12 degrees C.) so the transition is not so big. But local temperatures in the terrarium can be as high as 30 degrees C. or even a little more.

A gradual change from low temperatures towards higher temperatures is not considered as essential to me. In the wild the den temperatures can easily be as low as 8 degrees C. or less when snakes leave the den to sun themselves. When the snakes leave the den around noon and crawl towards a sunny spot to heat up they can reach the 25-30 degrees C. easily… And the same late afternoon when the sun is going under and they retreat in the den they cool of towards 8 degrees C., so big fluctuations in body temperature are very normal. A nice graph of these daily body temperature fluctuations in early spring can be seen on page 81 of the magnificent book of  Rossman et all (1996).


Garter snakes can be hibernated in small containers without many ventilation holes. A high humidity is important to prevent dehydration. Before hibernation the intestines should not contain any food items.

Garter snakes of all ages, young and old can be hibernated successfully with the method described above. Duration of hibernation and temperatures during hibernation are mentioned.

For me, hibernating garter snakes is a must. Why? Because it is natural, because it enhances the chance of reproduction and because it is convenient (a period without feeding and taking care of the snakes).

One of the results is a synchronised biorhythm: the snakes within the same terrarium shed their skins for the 1st time within a narrow time frame (often the second shedding is already more spread out) and females within the same terrarium also give birth closely after each other (sometimes within days).

So I would like to recommend a lengthy hibernation period for garter snakes; Don’t be too scared: the snakes have adjusted to a hibernation period in the wild for thousands of generations. Natural selections selected those snakes who could survive the winter period, so hibernating comes natural to them.


Buchholz, T, 2002. Bei mir zu Hause. The Garter Snake  3 (02): 53-55.

Kreyerhoff, H, 2002. Verluste bei der Uberwinterung. The Garter Snake  4(02): 35.

Rossman, D., N. Ford & R. Seigel, 1996. The Garter Snakes. Evolution and ecology. University of Oklahoma press, Norman, Publishing division, USA.

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