Some observations on…

Some observations on Garter Snakes (Thamnophis spp.) in Colorado, USA

Steven Bol (Author…)


In 1997 I have visited Colorado several times for my work.
When I had some spare time I tried to do some observations on Garter Snakes.

For +15 years keeping and breeding Garter Snakes and other Natricinae species has been a hobby of mine. Since I had never been in the USA before I was very curious about their behaviour in the wild. So visiting the USA for my work gave me the perfect opportunity to observe Garter Snakes in their natural habitat.
Articles like the one of Udo Strathemann in our last issue (1997/4) of the “Garter Snake” I find very interesting and if there are any members who have been able to do some observations on Garter Snakes (in the wild) I would like to ask them to put something on paper.

Colorado Grand Mesa and Mt. Garfield

I have observed Garter Snakes at several sites in Colorado.
I will give some short descriptions of the state of Colorado and the habitats.
After that the observations at each site will be discussed.
Some notes on the looks of the observed Garter Snakes are made.

The distribution of the observed species in Colorado will be discussed.


The state of Colorado lies in the mid-western part of the USA. The Rocky Mountains run through the middle of the state and the highest peaks reach an altitude of 4400 m.

National Atlas ColoradoThe winters can be very long and cold, the summers are very hot. In July, August and September maximum temperatures vary from 26 to 35 °C. From November through March temperatures drop below zero regularly and in December-Februari -7 to -10 °C is normal.
In autumn and spring the difference between day and night temperature can be more than 25 degrees: because of the high altitude the sun is very intens (20 % more UV radiation than at sea level) and it can heat up very quick when the sun is shining. But in the night time temperature can drop very fast again.
The visited areas lie 20 – 30 km North of Denver (near the cities Brighton, Ft.Lupton and Longmont) and 50 – 60 km North-West of Denver (Rocky Mountains National Park).

Observation Areas

Longmont areaNear the city Longmont there is a small lake (+ 500 m long) surrounded by grassy slopes. The whole area is slightly hilly. The south-facing slope north of the lake is almost treeless and covered with grass (farmland). The surface is on some places very moist because of the seepage. Just in front of the lake flows a small creek (2 m width and maximum 50 cm deep) Just beside the creek lie some piles of rock, wood and iron (some parts of an old car). Elevation will be + 1600 m above sealevel.

North of Longmont lies the city of Berthoud. A small creek (2m width and maximum 50 cm deep) flows southward of this city in western direction through a westfacing slope. This slope is covered with weeds and grass (no trees) and looks like abandoned farmland. There is also a small marshy area (fed by seepage??) with some reeds. Elevation + 1600 m above sealevel.

Ft.Lupton and BrightonThe Platte River flows beside the cities of Ft.Lupton and Brighton. This river is up to 20 m width and is relatively fast flowing. It will have also quite deep areas. The banks of the river are quite wild and they are covered with several weeds and trees. On most places the banks are more than 1 – 3 meters elevated above the waterlevel.
There are several spots where the sun can reach the ground. The strip with trees and weeds is at most places not wider than 5 to 20 m and it is surrounded by an interstate and city on one side. The other side is mostly farmland with cows. Elevation + 1500 m above sealevel.

The above mentioned citys all lie in the slightly hilly area in front of the Rocky Mountains.
The National Park really lies in the Rocky Mountains.
The Rocky Mountains National Park is a beautiful park with a lot of wild life like deer, Moose, Elk, Bear and Mountain Lion. Along one of the trails (Fern Lake Trail) made for visitors of the park flows a fast-flowing creek, probably fed by melting snow (and rainfall in the summer). The Creek is 2 – 4 m width and not very deep .
The southfacing bank of the Creek is at lower elevation covered with grasses, rocks and trees. On a lot of spots there are open areas where the sun can reach the ground, also because the canyon is still quit broad.
Following the trail up into the mountains the canyon gets very narrow and covered with trees and their few spots where the sun can reach the ground.


Habitat of Thamnophis elegans vagrans and T.radix near Longmont, Colorado.On 31 May the Longmont habitat was visited late in the afternoon. The temperature was somewhere between 24 and 28 °C.

Underneath some piles of rocks, wood and iron (+ 2 m from the creek) I could discover three Wandering Garter Snakes Thamnophis elegans vagrans by turning around these materials. The first one was a male of 61 cm (46 cm snoutvent Thamnophis elegans vagrans from Longmont, Colorado.length(SVL)) who had milky white eyes as a sign of shedding. A second male had a total length (TL) of 56.5 cm (42 cm SVL). The third one was a female: 57 cm TL (44 cm SVL). They were all found more or less together (less than 5 m apart).

The second day (1 June) I stopped around noon at the same spot and underneath a piece of wood I found a female Wandering Garter Snake of 37.5 cm TL (29 cm SVL).

It was Thamnophis radix: male of 58 cm from Longmont, Colorado.again quite warm. The snake had shed here skin and had fed recently.
Underneath a small piece of plastic, + 100 m from the creek and the lake, in the midst of the grassy slope I found a male Plains Garter Snake Thamnophis radix. It had shed its skin recently and was very brightly coloured. Total length was 58 cm (44 cm SVL). It was a quite unusual place to find a snake: the only cover for the snake in a circle of 25 m was the tiny piece of plastic.


Habitat nabij BerthoHabitat nearby Berthoud, Colorado of Thamnophis elegans vagrans, T. sirtalis parietalis and T. radixud, Colorado van Thamnophis elegans vagrans, T. sirtalis parietalis en T. radixOn 1 June this site was visited between one and two o’clock in the afternoon. On the banks of the creek and around the “bridge”, where the small creek flows underneath the road, several Garter Snakes were discovered. They were all hiding underneath pieces of rock, wood, cardboard or iron. None of the snakes were basking; the temperature was between the 25 – 30 °C and the sunshine feld very intens: they could probably reach their optimum temperature easily while hiding underneath the material in the middle of the day.
Some snakes where found underneath pieces of rock in the open field in the sun on a high and dry underground. These snakes feld quit warm. Others however where underneath pieces of material which were still in the shade at 1 in the afternoon on a wet underground: these snakes still feld very cold.

Thamnophis sirtalis parietalis from Berthoud, Colorado. Male of 60 cm.In total 9 Wandering Garter Snakes and 1 Red-sided Garter Snake Thamnophis sirtalis parietalis were found within one hour of searching and they were less then 50 m apart from each other.
One female Wandering garter Snake (68.5 cm TL, 59 cm SVL) had milky white eyes and several blisters on her body and belly. Because of the blisters I suspect that this was her first shedding after hibernation. I would not be happy if one of my snakes looked this bad after hibernation. No follicles could be feld.
Thamnophis sirtalis parietalis from Berthoud, Colorado. Female of 74 cm.

A second female Wandering Garter Snake (62 cm TL, 48 cm SVL) also was in a stage of shedding. She was quite thick and follicles/developping embryos could be feld.

The third snake I caught was a quite large female Red-sided Garter Snake (74 cm TL, 66 cm SVL).
A large part of her tail was missing. Also this snake was in a stage of shedding.
No embryos or follicles could be found.

Thamnophis radix from Colorado Female of 68 cm.The next snake was a female Wandering Garter Snake (67.5 cm TL, 52 cm SVL). She was in a stage of shedding and follicles/embryos could be feld.
A male Wandering Garter Snake (60 cm TL, 44.5 cm SVL) was in a stage of shedding as was another male of 45 cm TL (35 cm SVL).
Four more Wandering Garter Snakes were found. Three females: 60 cm TL/ 46 cm SVL; 68 cm TL/ 61 cm SVL (part of tail missing)and 73 cm TL/ 57 cm SVL. In the first two females 8 and 13 developping embryos could be feld. The first female was in a stage of shedding, the letter two had recently shed their skin. The fourth snake was a male: 75 cm TL (56 SVL). This one was in the last stage prior to shedding its skin.

A large number of the above mentioned Wandering Garter Snakes were found in a big pile of rocks.
It is very probable that this was also their hibernaculum, since they could crawl away quite deep into the pile of rocks. The other snakes were found under scattered material in the field were they could hardly crawl deep into the ground to escape the frost in the winter. Burrows made by mice where present however and they could also have served as hibernacula.

On 3 August I visited the same spot again. It was very hot (30 °C) and the weeds were high so it was very difficult to walk through and search for garter snakes. Underneath the piece of wood where I had found a female Red-sided Garter Snake in June there was a male Red-sided Garter Snake hiding. It had an estimated total length of 60 cm.
Underneath a rock I saw a Wandering Garter Snake which quickly dissappeared into the weeds.

Fort Lupton

On 1 June I searched along the banks of the Platte River near Ft.Lupton from 8 to 12 am. I found two Red-sided Garter Snakes. The first one was a female with a total length of 43 cm (34 cm SVL). It was hiding under a rock near a bridge on a very dry spot.
The second one was a large snake: probably a female. It was crawling on the bank of the river in the early morning sun. As soon as the snake saw me she “jumped” into the Platte River and disappeared. She had an estimated total length of 1 m and she looked very bright as if she had recently shed her skin.
Bull snake (120 cm) from Colorado. I found a fresh skin of a garter snake + 2 m further. It measured 117 cm, so it could very well have been her skin.
Despite the intensive search of 4 hours at a time when most snakes can be found basking only 2 garter snakes were found which was a dissappointing number.

Other reptiles which could be observed where a large Bull Snake with a total length of 117 cm which was basking in the morning sun and 3 Yellow-bellied Racers of which one male measured 89 cm. All three Yellow-bellied Racers were crawling through the high grass and were very fast.


On 5 August I had the change to look in a small park on the banks of Platte River near Brighton. Temperature was around 25 °C with heavy clouds. The sun could hardly shine through the clouds.
Around noon I saw a male Red-sided Garter Snakes crawling out of the bush through the wet grass 5 m away from the river. It was very large specimen for a male: 91 cm TL ;69 cm SVL.
Brighton, Platte riverOn the rocky bank of the river I noticed an adult couple Plains Garter Snakes 1 m apart from each other. The male was busy shedding his skin and had an estimated total length 85 cm. The female was slightly larger (estimated total length was 90 cm) and was crawling along the river. She fled into the river. She was very thick and it was very clear that she was pregnant. I think she would have given birth within two or three weeks at the most.
Other reptiles I encountered during a two hour search were: A large female Bull Snake (120 cm total length) which was hiding under a rock and 2 Yellow-bellied Racers of which 1 male was basking with milky blue eyes (in a stage of shedding) with an estimated total length of 90 cm. The other one was hiding under a big rock.

Rocky Mountains National Park

Rocky Mountain National Park, Fern Lake TrailOn 11 October I visited the Rocky mountains National Park.
It had frozen at night and in the morning some snow had fallen.
Around noon the sun started shining and it was little above zero.
Just after the starting point of the Fern Lake Trail the canyon is quit broad and the sun was able to shine on the south-facing slope of the creek. In the canyon there was hardly any wind and the dry grass and dead leaves on the slopes of the canyon felt relatively warm.
To my big surprise I observed 2 female Wandering Garter Snakes basking. They were still able to move pretty fast and fled with big speed when I approached them.
They had not fed for some time and I suspect that this was one of the last basking possibilities prior to hibernation.


Thamnophis radix van Berthoud, Colorado.The observed Plains Garter Snakes of Colorado look very much like the “light morph” in Rossman et al (1996). The middorsal stripe is bright yellow and broad and the spots between the stripes are very prominent.

The Wandering Garter Snakes of Colorado are light coloured; grey to brown. The middorsal stripe can be relatively bright (yellowish to whitish) in the neck region and gradually fadens in posterior direction.The two rows of alternating spots between the stripes are relatively well developped, especially the upper row. I observed several specimens with a prominent black “crown” in the neck region divided by the middorsal stripe combined with blackening of the parietal scales. In some specimens some pinkish colours could be observed in the supralabial scales.
They differ from the specimens which were imported in 1995 and now have found their new home amongst some members (the author included)of our association.

Thamnophis elegans vagrans uit Berthoud, Colorado. Vrouwtje van 68 cm.The Red-sided Garter Snakes of Colorado look more or less exactly like T.s.dorsalis in Rossman et al (1996). The yellow middorsal and lateral stripes are well developped. The amount of red varies greatly amongst the several specimens I observed. In some specimens it is subdued to the skin between the scales and some specimens have lot of red, but the red spots are always quit obscure: the division between the red and black – brown tones is never very sharp. The ground color is more brown than real black. In some specimens there is a black line bordering the middorsal stripe, which should be typical for T.s.dorsalis.
This shows how difficult it is to identify the several subspecies of the Common Garter Snake without knowing the exact place they come from. These specimens I would probably have determinated as T.s.dorsalis , when I had not known they come from Colorado.
Recently there is a new investigation going on (this is mentioned in Rossman et al (1996)) to determine the distribution and variance of the different subspecies. Maybe the populations of Thamnophis sirtalis in Colorado will turn out to be T.s.dorsalis.

Concluding Remarks

In only a couple of days of searching you can observe quite a lot of
Garter Snakes when you are lucky and learn more about their ecology

The total numbers of observed Garter Snakes are:

Longmont:  4 Wandering Garter Snakes (2.2) and 1 Plains Garter Snake (0.1)
Berthoud:  10 Wandering Garter Snakes (6.3 + ?) and 2 Red-sided Garter Snakes (1.1).
Ft.Lupton:  2 Red-sided Garter Snakes (2.0)
Brighton:    1 Red-sided Garter Snake (0.1) and 2 Plains Garter Snakes (1.1).
Rocky Mountains National Park: 2 Wandering Garter Snakes (2.0).

It seems for Colorado that the first shedding occurs around end of May/beginning of June. It is also noteworthy that several specimens, which had not shed yet, looked quite bad wit a lot of brownish blisters on belly and back. So one should not be too scared when one of your one Garter Snakes develobs a blister during hibernation.
Around this date also the first signs of feeding have been observed, but this could of course have happened earlier.

Some of the female Wandering Garter Snakes already had enlarged follicles on 1 June as a sign of pregnancy: in total 3 of the 8 females found on 31 May/ 1 June.
Two of the females who showed no signs of pregnancy might have been immature: 29 cm SVL and 44 cm SVL. The first 1 will almost certainly have been immature, but the 44 cm SVL female could have been mature.
The smallest “pregnant” female measured 46 cm SVL. Rossman (1996) state that females from British Columbia can reach sexual maturity at 45 cm SVL. It has to be stated however that a female which does not show any signs of pregnancy end of May still can produce young that year. The latter happened to the big female Red-sided Garter Snake from Berthoud, which appeared to be pregnant after all.
Around which date parturition normally occurs in Colorado is hard to tell which my limited data. The only thing I can mention is the pregnant female Plains Garter Snake I found on 5 August which should have given birth within the coming 2 – 3 weeks.

The 2 female Wandering Garter Snakes which were found on 11 October suggest that the hibernation does not start before October, but of course that can vary from year to year. But the winters in Colorado start quite early and I do not expect that there is a lot of activity after the middle of October.
The distribution charts in Rossman et al (1996) shows that the visited area is quite special: it lies near the eastern border of the range of the Wandering Garter Snake in Colorado and near the western border of the Red-sided Garter Snake and the Plains Garter Snake in Colorado. The sites of Ft. Lupton and Brighton lie within the range Plains Garter Snake and the Red-sided Garter Snake and almost exactly on the border of the range of the third species, which I have not observed at these sites. The sites near Longmont and Berthoud lie in the range of all three species. The Rocky Mountains National Park lies in the range of the Wandering Garter Snake and almost exactly on the border of the range of the Red-sided Garter Snake, which I did not observe there.
All I can say is that my observation agree with the distribution charts in Rossman et al (1996).

Rossman, D.A., N.B. Ford & R.A. Siegel, (1996). The garter snakes. Evolution and ecology. University of Oklahoma Press, USA.
Strathemann, U., 1997. Some observations on populations of Thamnophis atratus atratus near San Francisco. The Garter Snake 1997/4: 23-26.

The author has in his free hours during working visits to America some observations done to Gartersnakes in Colorado.
The visited areas are 20 – 30 km north of Denver and 50 – 60 km northwest of Denver. The Gartersnakes have been observed in the habitats visited where be briefly described. The observed species are named with mention of sex and length. There are also what made comments about nutrition, Ecdysis, reproduction and dissemination of observed species. In total there are 16 Western terrestrial Garter Snake Thamnophis radix and Thamnophis sirtalis vagrans, 3 5 subniger parietalis observed on five dates in 1997 (31 May, 1 June, 3 & 5 August and 11 October). There are also in short a few comments made about the appearance of the species in Colorado; T.s.parietalis of Colorado has many away from T.s.dorsalis.