Autor: Steven BOL

Introduction

In an earlier article I described my observation on Garter Snakes in Colorado. The last 1,5 year I have visited Texas on an almost monthly schedule, and when I had a weekend off I was able to do some observation on the local herpetofauna. In this article I will describe observation done in the Big Bend National Park, the Davis Mountains and the Guadalupe Mountains. The Guadalupe Mountains lye on the border of Texas and New Mexico, and on both sides I have been able to do some observations.

The part of Texas I have visited is the western portion of this huge state. It consists mostly of desert highlands and mountains. It is a very dry region, and open water is quit rare. Normally they get most of the rain in the summer, but the last 7 years it has hardly rained in the summer. Due to the El Ninjo effect according the local people. This year it rained a lot in July and August for the 1st time since years! A large surface of this part of Texas is private land, owned by farmers which have really huge farms with some cows. There is one farm in the area which is as big as the Netherlands! Not a lot of people live here. You can sometimes drive for hours without seeing any houses or people, and when another car passes you by they normally wave at you.

Big Bend National Park: The Chisos Mountains

Habitat

The Chisos Mountains are located in the Big Bend National Park, where the Rio Grand makes a big bend. The river "Rio Grande" forms the natural border between Mexico and the USA. In literature I had read about observations done in the late fifties in a small canyon in the Chisos Mountains, where in late July a lot of Blackneck Garter snakes could be found.

In order to reach the canyon you have to walk a 2 hours into the mountains. Starting in "The Basin" (elevation 1.646 m) you have to walk through desert mountainland with no water in sight. When you get higher and further in the mountains more and more trees are growing (the lower part of The Big bend National park consist of a desert landscape (The Chihuahuan Desert) with only low bushes and cactusses). Finally you reach a canyon which is dry most of the year. Elevation approximately 2.150 m. It can be very hot in summer up to 45-50o C. The ground can reach temperatures of 82o C at midday! The winter is normally very dry with night temperatures around and below 0o C from November through February, but temperature can easily rise up to 25o C or more!

First visit

On April 22nd I visited the little canyon for the first time. I did not have a lot of time, and I arrived in the Big Bend national Park in the middle of the afternoon. Armed with a lot of water, my camera, pen and paper and a measuring tape I started the heavy climb. At high speed, since I had to go back to Holland the following day, I climbed the steep mountain slopes. Sweating heavily I finally reached the canyon. It was a beautiful day with a blue sky, 30o C and I really thought I would find myself some Blackneck Garter Snakes in no time. ROSSMAN et al. (1996) mention March or April as probable time of emergence out of hibernation at most localities.

The canyon was almost dry, it had hardly rained for a long time. The "river" was not more then a few small pools with water which did not contain any sign of fish, tadpoles or Garter Snakes. Only some aquatic invertebrates could be found.

On the way back I met a hiker who had walked this trails already many times and he said that the spring had dried up many years ago so I figured that both the tadpoles and the Garter Snakes had become extinct. Disappointed I descended and left for El Paso ( a four hour drive) to fly back to Holland.

Second visit

In July and August I spend my summer Holidays in the USA and since the trail in the Chisos Mountains was very beautiful I decided to walk the same trail again and show my girlfriend how beautiful it was in The Big Bend. Of course I had some silent hope that the rain had awakened the garter snakes. It had rained already a lot for several weeks in Texas. The weather was reasonably good; 25o C, a lot of clouds and from time to time the sun shone.

On August 1st the canyon contained more water then in my April visit. Now there was a small river flowing from pool to pool over a length of at least 1-2 km. The "river" was not more then 50 cm width and 5-10 cm deep, but some of the pools were fairly large: 5 x 2 m and up to 50 cm deep (most of them were smaller). In some parts of the canyon there was no shade by trees, at other parts there was a lot of shade.

The pools contained a lot of tadpoles of the Canyon Tree frog (Hyla arenicolor) which were already 2,5 - 3 cm long!! I did not have a lot of time to search because we had to go back. But within 10 minutes I spotted 2 Blackneck Garter Snakes ( Thamnophis cyrtopsis) swimming in the water. The first one crawled into the rocks so I could not catch it. The second one I could catch: a male of 48 cm (No.1 in table 1) , quit skinny and he had not eaten as far as I could see. I was very excited about this. So they had not become extinct at all!

On the way back I found a Texas Alligator Lizard (Gerrhonotus liocephalus infernalis). At the parking lot a big skink was sunning itself: a Great Plains Skink (Eumeces obsoletus).

Third visit

On Sunday August 22nd I visited the little canyon for the 3rd time. The weather was very good, sunny and 30o C. At 2 p.m. I reached the canyon and this time I had more time to search the canyon. Water levels were already lower. The water was not flowing anymore and the "river" consisted of pools with a maximum size of 5 x 2 m, and mostly not deeper then 30-40 cm. Large numbers of tadpoles of the Canyon Treefrog were still present. Some of them had already undergone a metamorphosis.

Between 2.20 and 4.45 p.m. I could observe about 15 Blackneck Garter Snakes, most of them swimming in the pools and hunting underwater (table 1). Several times I could observe a Blackneck resting with its head above the water and breathing heavily. Then they dived under the water and swam under water over the bottom of the pool and searching between the rocks. As soon as a tadpole swam away they chased the tadpoles. It was obvious that they can see underwater. Several times I saw a blackneck eating a tadpole, and 3 Blackneck Garter Snakes had recently fed; several tadpoles were regurgitated (4 and 1) by 2 of them. All the others had not eaten recently. Note that soft food like tadpoles could be digested easily so maybe a day after feeding the remains are not palpable anymore.

In general all the observed snakes were quit skinny. One female showed signs of a coming shedding, the others had already shed their skins or still had to do so.

Table 1 shows all the length in cm of the snakes which were caught. 3 of them got away, 1 of them was quit small (estimated 40 cm). Except 2 of them all the Blacknecks were found in the part of the canyon were there was no shade of trees.

Later in the afternoon I had the impression that the activity had gone down since the sun could not shine in the narrow canyon anymore. But at 4.35 p.m. I found No 12 hunting in a shaded pool.

Fourth visit

On October 16th I wanted to have a look if the literature is right about timing of hibernation. Especially since the start of the active seasons seems to be even later then stated (later the April) I could hardly imagine that the snakes would enter their dens already in September (ROSSMAN et al., 1996).

The weather had been quit warm the last 2 weeks (25-33o C). On October 16th the night temperature will have been around the 15o C, but during the day it became 25-27o C. From time to time the sun shone, especially after 1 p.m. At 1.30 p.m. I already arrived in the canyon. There were still several pools present, but a lot of them dried out. In the lower part of the canyon I could not find any tadpoles anymore so I thought that I would not find any garter snakes.

In the higher part of the canyon I discovered pools with tadpoles and young treefrogs on and under the surrounding rocks. And at 2 p.m. I discovered a male (No. 14) crawling on land in between 2 pools (5 m distance to both pools) in a heavily shaded area with only small spots were the sun could shine. The snake felt warm and had obviously been basking on a spot with sunshine. It had shed recently and looked very good except being very skinny. It showed no signs of feeding and a large amount of water/mucous came out of its mouth. It had either pneumonia or had drank a lot recently and regurgitated the water.

At 2.50 p.m. I could observe a female swimming in a pool. It had eaten recently, probably a (1) young treefrog. Activity of T. cyrtopsis in the middle of October has not been described in literature as far as I know! Other reptiles encountered were the Big Bend Tree Lizard (Urosaurus ornatus schmidti), the Nortern Fence Lizard (Sceloporus undulatus hyacinthinus) and the Crevice Spiny Lizard (Sceloporus poinsettii poinsettii). Especially the later was very common in the canyon.

While driving out of the "Basin" a Sonoran Gopher Snake (Pituophis catenifer affinis) (1,3 m) crawled over the road. The next day it became quit cold all of a sudden, 8o C during the day and around freezing in the night.

Description

The Chisos Mountains form a zone of intergradiation between 2 subspecies of T. cyrtopsis: T. c. cyrtopsis and T. c. ocellatus. The major difference between the 2 subspecies is the size and shape of the 2 rows of alternating black spots in the neck region. In typical cyrtopsis there are 2 rows of (bigger or smaller) black spots between the stripes starting already in the neck. In occelatus the 2 rows of black spots are fused into single large blocks on either side of the neck

In the Chisos mountains both typical T. c. cyrtopsis as specimens which more resemble T. c. occelatus can be found. 7 snakes (50 %) had some larger blocks on the side of the neck, but not as spectacular as on the photo in ROSSMAN et al. (1996)

Remarks on life history and ecology

Some short remarks will be made on the life history and ecology will be given. All snakes range from 42,5 - 62,5 cm. Probably all the males will have been adult. At what length the females become adult is hardly known. The 2 females with a SVL smaller then 40 cm will probably not have been adult yet. But maybe even the biggest female (No. 4: 60,5 cm) is not adult yet. Not much is known about the reproductive biology of this species, although ROSSMAN (1996) mention June and July as the month in which these snakes give young. In August none of the females was pregnant (maybe none of them was adult) , and all were far to skinny to be pregnant. Also no young snakes were observed. The smallest one was a female of 42,5 cm. She will be at least 2 years old, maybe older.

When you combine the fact that all snakes were very skinny, that it has been dry for several years in row and no young snakes or pregnant females were found in August you could think that this population has not been very successful . It looks like no reproduction has taken place in 1998 and 1999. Sexratio was 57% in favor of the males ( male 1 and 3 are very probably the same).

It is difficult to say anything about the size of the population, but I have the impression that I have seen the majority of the population which inhabited the lower 800 m of the canyon. I do not think that the population will have consisted of more then 30-50 specimens.

Other Garter Snakes in the Big Bend Park

In literature (TENNANT, 1998) I have read that both the Checkered Garter T. marcianus as the Arid Land Ribbon Snake T. proximus orarius occurs in this area beside T. cyrtopsis. The Checklist of amphibiens and Reptiles of the Big Bend national Park only mentions T. cyrtopsis and T. marcianus. T. cyrtopsis is said to occur throughout the park ("common") but mainly in the Chisos Mountains (in the rest of the park there is hardly any water beside maybe some small springs) , the highest part of the park.

T. marcianus is said to be uncommon and only occurring in the Rio Grande floodplain and its major tributaries. October 17th I check one of those tributaries (Tornillo Creek) but I could not even find any water. T. proximus obviously does not occur in the park. T. cyrtopsis is found on the higher spots in the park and T. marcianus on the lower spots. So their will not be much competition between the two of them.

Zusammenfassung

Bei insgasamt 4 Besuchen der Chisos Mountains im Big Bend Nationalpark (Texas, USA) wurden zahlreiche Beobachtungen an Thamnophis cyrtopsis gemacht. Es werden Daten zur Biotopbeschreibung und zum Klima gegeben. 15 Schlangen konnten gefangen und vermessen werden (Tabelle 1). Es werden Angaben zu Beobachtungen anderer Schlangenarten gemacht.