Hibernating Garter Snakes: a must or an option?
By Steven Bol (published in The Garter Snake 2004, 2, 2004 – EGSA)
The original article as it appeared in “the garter Snake” back in 2004 can also be found on the website of Fons Sleijpen (www.thamnophis.eu). I have rewritten my article and adjusted it with my experiences from the past years.
Timing of hibernation…
When you should start with hibernation? I partly let 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. But the better your room is isolated and the more terraria are in the room the less the room temperatures are influenced by outside temperatures. Years ago my snake room was far 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.
During the summer months of June – August the night temperatures almost never drop below the 21 – 23 °C at night. But when the nights start to cool in September room temperatures can drop to 18 – 19 °C at night (terrariums are only heated during daytime), and later in October and November the temperature drops further to 15 – 18 °C or even less. I believe that this cooling off during night triggers the snakes to prepare for hibernation..
Other signs which show the snakes sense that winter is coming are:
- Retreating to the coldest part of the terrarium under stones or leaves and not or hardly showing themselves to sun during daytime.
- Crawling seemingly restless through the terrarium, rubbing the snouts against the glass. Apparently looking for a good place to hibernate.
- Higher mating activity during days which are a bit warmer then usual for the time of the year. Normally little mating behavior can be observed in the summer months, but as soon as the nights start to cool down mating behavior gets more frequent and intense.
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. I also try the spread the timing when the different species enter and leave hibernation in order to prevent that all the females have their offspring simultaneously in spring and early summer.
After the last feeding activity I keep the lights (and heat) on (during daytime) for another 7 – 14 days, in order to give the snakes the opportunity 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 had any problems with snakes that stopped feeding several weeks earlier before entering hibernation: in captivity even the less well-nourished snakes are in general much fatter than many of the garter snakes I encountered during my trips to the US and Canada in fall.
How to hibernate..?
At the moment I use three different methods for hibernation.
- For the first method I use an old refrigerator or my garage. The temperature inside the fridge is 4 – 8 °C during the entire time. In my garage the temperatures in the winter vary from 0,5 – 14 °C depending on the fluctuations outside. Only if the temperatures are starting to drop below the 0°C I take the snakes out of the garage and put it on the coldest spot in my house at that time for a few days until the cold spell is over. The snakes go into relatively small containers, which are either of plastic or of metal (for instance old cookie jars). For juveniles I use small jars, with a diameter of 9 – 11 cm and a height of 7 cm. For my adult snakes (mostly hibernated in groups of 2 – 6 snakes) I have been using in the past 10 years plastic square buckets of 27 x 17 x 15 cm (lxwxh). Previously (as you can read in my original article) I used smaller containers, but I prefer a little more space now. The lids or sides are perforated with a few small holes, just enough for some air exchange but not so much that the content dries out too quickly! Preventing the “drying out” is extra important when in the fridge.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 or have used for all garter snakes from the US and Canada. All subspecies of Thamnophis sirtalis I have kept, Thamnophis ordinoides, Thamnophis radix, Thamnophis elegans vagrans, T.e.arizonae, T.e.terrestris, T. atratus, T.rufipunctatus etc. Of the Mexican species I only hibernate T. eques virgatenuis with this method. But also the european Viperine water snake (Natrix maura), the Northern Water Snake & the Lake Erie Water Snake (Nerodia sipedon sipedon & N.s.insularum) are hibernated this way.
- The second method is hibernating the snakes simply inside their own terrarium in the unheated snake room with the lights and heating switched off. Temperatures fluctuate of course based on the outside temperatures. But beside the fact that the temperatures are always much higher than method nr 1 (and rarely below the 10 – 14 °C) this method also differs from method nr 1 in the fact that the snakes are not exposed to high humidities. The snakes remain in a completely dry terrarium with a bowl of water to drink from. I do use this method sometimes for some Mexican Garter Snakes but always for the 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. I just mentioned that I use this method for some of the Mexican Garter Snakes (Thamnophis eques ssp); usually I give them the possibility to bask for some hours under a cooler lamp during the winter months (which they sometimes ignore and not use for weeks) but sometimes I switch of the heating and lights for 2 – 4 weeks. During the cooler winter months, either with or without a heat source, Thamnophis eques sometimes uses the water bowl to remain practically completely submersed for days or weeks in a row.
- A new third method I have started using only in the past 2 years in an attempt to mimic the natural circumstances in which some rare highland Garter Snake species live on high altitudes in Mexico. For this I build some terraria in my unheated shed. A part of the terrarium is filled with a 10 cm thick layer of slightly humid cocopeat. These terraria are heated during daytime by a strong reflector bulb that is switched on for about 8 hours a day. This way the snakes get the change to bask during daytime for a few hours at local temps of 25 – 30 °C. But in the thick layer of cocopeat the temps usually stay below the 10-15 °C during daytime and cool down to 2 – 10 °C during the cold nights. I keep them in this cold shed for 1 – 3 months and then they go to my snake room inside the house. So far I have gained some experience with this third method with Thamnophis scalaris, T. scaliger, T. conanti and T.pulchrilatus. Sometimes the snakes remain hidden for a few days or even a week in a row, but then they bask for a couple of hours.
After switching out the lights and heating in the winter or fall, when using method 1, I put the snakes sometimes directly, sometimes after a few days but sometimes even after several (up to 6) weeks in their hibernation buckets or containers. This way all the snakes experience a shorter or longer period of hibernation method 2 (in the unheated terrarium), before they are moved to the fridge or garage.
Survival rate during hibernation…
I have actually 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 not fit for some time.
Over the last 10 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 than 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 quite substantial.
But when analyzing 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 quite 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 10 years or so the mortality rates of the juveniles born inside has not been much higher than 5 %!
Heating up the snakes in spring…
The procedure to end 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 (especially in the nights) usually quit low (14 – 17 ° C) so the transition is not so big. But local temperatures in the terrarium can be as high as 30 ° C or even a little more.
A gradual change from low temperatures towards higher temperatures is not considered as essential to me, and in the wild it is usually vary abrupt as well.
In the wild the den temperatures can easily be as low as 1 – 8 °C 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 °C easily within minutes… And the same late afternoon when the sun is going down and they retreat in the den they cool down to 1 – 8 °C, so big fluctuations in body temperature are very normal. And in some areas these huge fluctuations in body temperature occur daily in early spring.
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).