Exciting observations on two sympatric Garter Snakes in “La Laguna de Chapala”, Mexico.

Steven Bol (Author…)


Laguna de Chapala“La Laguna de Chapala” or lake Chapala is the largest lake in Mexico. It is one of the many endorheic lakes (lakes without an outlet) in the transvolcanic belt of Mexico Roger Conant (2003) visited during the sixties of the last century.

His analysis of the large series of snakes (40 years after visiting the area) he collected resulted in the description of 7 new subspecies of the Mexican Garter Snake (Thamnophis eques). One of the new subspecies Conant (2003) described occurs in lake Chapala: Thamnophis eques obscurus, the “Chapala Mexican Garter Snake”.

State Jalisco, MexicoIn 2007,  2008 and 2009 I visited the lake in the hope to observe T.e.obscurus.
I was able to do some observations that revealed some exciting and partly new facts about the two sympatric garter snakes species (T.e.obscurus and T.melanogaster canescens, the “Mexican Black-bellied Garter Snake”) inhabiting this huge lake.

In total 30 T.m.canescens and 55 T.e.obscurus were observed. Since little information is available in English literature on garter snakes from Mexico.
I felt these observations should be documented.

In short the new facts are: mating, recent shedding and feeding of T.e.obscurus on June 1st 2008 suggest that the activity season of Thamnophis eques starts earlier then Conant (2003) suspected. Mating behaviour in November of T.eques had not been reported before. Evidence of possible hybridization between the 2 separate species was found.
Several completely melanistic T.m.canescens were found, this colour morph was not previously reported for this species.

La laguna de Chapala

La laguna de ChapalaLake Chapala is the largest of Mexico’s endorheic lakes in transvolcanic belt. It measures 80 km east-west and 20 km in width. Many of the lakes in the transvolcanic belt are formed because of volcanic activity that changes the flow of rivers. Rio (river) Lerma is flowing into lake Chapala but there is no outlet.

Because many of these lakes have become isolated from other water systems (rivers and lakes) many organism had ample time to differentiate and there is a lot of endemism described (Conant, 2003) for this area, especially for water organisms.

The lake is located in the state of Jalisco (and partly Michoacán) at 1525 m above sea level. Most of the rain falls in the summer months (June September) and water levels in the lake can vary throughout the year.

The maximum depth is 8 – 9,8 meters (Conant, 2003) and there are several islands in this huge lake. Several smaller and larger towns are build on the borders of the lakes and it is popular place for (mostly Mexican and American) tourists.

Based on the amount of little boats often with nets is seems that fishing is an important income for many of the local inhabitants.

Visits to the lake

Habitat of T.e.obscurus and T.m.canescens. Lake Chapala in November 2007, site 1. (see text)

Habitat of T.e.obscurus and T.m.canescens. Lake Chapala in November 2007, site 1. (see text)

On November 10th 2007 I visited the lake for the first time. I arrived at the north shore of the lake west of the town of Chapala at 10.45 AM. The night had been cool (10 º C appr.)  but it was a clear and sunny day and temperature had already risen to appr. 23 º C at this time.  Maximum temperatures that day must have been around 26 º C (estimation).  It felt like a beautiful warm summer day as we experience them occasionally in Holland.

Between Chapala and Jocotopec there are small settlements everywhere. Many houses border the lake and have gardens that end in the lake. Because of this  there are not so many places where you can walk along the border of the lake.

Female T.m.canescens of 50 cm TL found November 2007. The snake showed signs of recent parturition.Fishermen, their boats and nets can be seen at many places. The shores are very open with hardly any cover for snakes to hide, other then clumps of water hyacinths and reeds washed ashore.
And all sort of garbage left by men; this habitat was obviously strongly influenced by human activity.
My first impression was that this place did not look very promising for observing garter snakes, and the local fishermen said that they hardly ever saw snakes.

Despite this I saw some rocks which look big enough for a snake to hide under and underneath rock  number 4 (right at the water’s edge) I discovered a fairly large female T.m. canescens.  She was greyish, 72 cm total length (TL) and in good condition. It was snake habitat after all!

I decided to search the 100 meters of shore surrounding this spot (before I hit the fences of a garden) intensely.
Juvenile of T.e.obscurus of 26,9 cm TL found in November 2007 at site 1.I witnessed a group of sparrows busy killing what looked like a juvenile snake. Within half an hour (11.45 AM) I found a juvenile T.e. obscurus (26.9 cm TL) hiding in a clump of reed. In total I found in that 100 meters of shoreline habitat (hereby called “location 1”) within an hour 3 T.e.obscurus and 3 T.m.canescens. The other 2 T.e.obscurus were a juvenile male (27 cm TL) hiding under a rock in the water and a large male that was sunning on decaying vegetation (85 TL, looked old, thin and “damaged”). The other 2 T.m.canescens were females; 1 female (50 cm TL) looked like she had recently given birth and she was crawling in the sun over decaying water hyacinths and the other female (36.8 cm TL) was hiding under a stone 1 meter from the water’s edge.

At certain other places in between the villages and houses I tried to reach the water’s edge but most paces were poorly accessible.

At one location (hereby called “location 2”) I did find however 2 more T.m.canescens (both males of 32 and 36 cm TL respectively) under rocks and decaying water hyacinths around 1 PM.
Before I had to leave the lake I tried to search at another location (hereby “location 3”).

Habitat of T.e.obscurus and T.m.canescens. Lake Chapala in November 2007, site 3. The small ditch where 4 juvenile T.e.obscurus were found (see text).

Habitat of T.e.obscurus and T.m.canescens. Lake Chapala in November 2007, site 3. The small ditch where 4 juvenile T.e.obscurus were found (see text).

Although this location was also near a small town the borders were less disturbed by human interference. It was definitely more quiet. Here I found a small and shallow ditch cut of from the lake were small fish could be observed. I saw a little snake hunting for fish under water. I could not catch this one to make sure which species it was but I am quit sure it was T.e.obscurus.  By turning every stone in the neighbourhood of the small ditch I could find 4 juvenile T.e.obscurus (24, 31.5, 35 and 37.5 cm) and 1 juvenile T.m.canescens (28.6 cm). Except 1 of the T.e.obscurus that was hunting in the water all were found under rocks or under a fallen tree.
1 of the juvenile T.e.obscurus had just been crushed under the rock when someone had stepped on it.

Habitat of T.e.obscurus and T.m.canescens. Lake Chapala in June 2008, site 3. Note the low water level and the willow trees in the water.So in total 6 melanogaster and 7 eques were found on November 10th 2007 within 7.5 hours.

A second visit was planned at  June 1st  2008. I arrived  at the lake around 9.30 AM and decided to spent all my time at “location 3”.

Night temperature averages in that time of the year around 15 ºC. Upon arrival air temperature probably was something like 20 ºC. The water level was much and much  lower then during the first visit and the ditch were I had found several juveniles in November 2007 had completely dried.

Laguna de ChapalaThe water’s edge looked quit open to me and I wondered were the snakes would live and hide. Not many places where snakes could hide. Hardly any growth, just bald rocks.  Most of the rocks too small to hide under. In Arizona T.eques (megalops) is described as a secretive snake always hiding in grasses and weeds bordering the waterline. None were available for the snakes. It was going to be a hot and sunny day (30 – 35 ºC), the water levels were much lower then in November and the habitat looked poor. Conant (2003) expected the snakes to remain quiescent until the end of July when the rainy season starts.

Adult female of T.e.obscurus (77 cm TL) found in June 2008 at site 3 at 9.50 AM. Note how extremely thin this female is.

Adult female of T.e.obscurus (77 cm TL) found in June 2008 at site 3 at 9.50 AM. Note how extremely thin this female is.

With hardly any expectations I started walking along the water’s edge and within 15 minutes I saw a large T.e.obscurus sunning in some branches washed ashore at the water’s edge. Because the night had been cold the snake still felt cold and was too slow to escape. It was an extremely thin female with a total length of 77 cm.

Ten minutes later (10:00 AM) I saw a fairly large garter snake (no doubt T.e.obscurus) swimming in the lake 20 meters away from the shore. After a few minutes I saw the snake climb into a dead tree standing in the lake to bask.

10.15 AM I saw a third T.e.obscurus lying in the water close to the shore. It was a large female of 102.6 cm TL and she still felt cold. Her condition was slightly better but also she was very skinny.

At 10.25 AM I found 2 female T.m.canescens (42 and 49.5 cm TL) coiled up under some rocks and a piece of bark 50 cm from the waterline. They were well nourished (one had fed recently) and both were not pregnant. One had milky white eyes, the other had shed it’s skin probably recently.

Adult female of T.e.obscurus (92 cm TL) found in June 2008 at site 3 at 10.40 AM. She had recently eaten a big prey (probably a fish) and she had recently shed her skin.I decided to check out a lonely willow tree standing in the lake, after seeing a snake climb into one of them half an hour earlier. I was lucky. A large female T.e.obscurus (92 cm TL) was sunning in it’s branches at 10.40 AM. This snake had recently eaten a big prey (probably a fish) and had recently shed it’s skin. This T.e.obscurus was in a good condition (although still quit skinny and not pregnant). Probably a second one got away.

Almost at the same time I saw an adult T.e.obscurus swimming in the lake 10 meters from the shore.

Couple of T.e.obscurus (female 94 cm TL: male 75 cm TL) found in coitus in June 2008 at site 3 at 11 AM. Note the obvious sexual dimorphism.Ten minutes later an adult male T.e.obscurus (75 cm TL)  was observed sunning on a pile of rocks near the shore line. This one was very skinny and had not shed it’s skin recently.
At 11:00 AM I arrived at a group of willows standing in the lake. Since the temperatures were rising and the snake were becoming warmer and thus quicker to escape I had to sneak up on them otherwise they would drop in the water and disappear. I saw a large T.e.obscurus sunning on one of the branches. I caught the snake and walked back to the shore to observe and measure the snake.
Then I realised I was holding 2 snakes: a copulating couple.
While taking pictures the snakes stayed attached. They both were in good condition and both had shed their skins recently. The female was 94 cm TL, the male 75 cm TL.

Mating couple of T.e.obscurus (female 94 cm TL, male 75 cm TL) found June 2008 at location 3 at 11 AM.Careful examining the group of three willows revealed  approximately 12 adult T.e.obscurus plus approximately 8 adult T.m.canescens sunning in the branches 10-200 cm above the water level. I had to walk  slowly through the 100-120 cm deep water of the lake to observe this large group of snakes basking and mating in the willow trees. Careful not to fall and spoil my digital camera, since I wanted to capture this once in a lifetime experience on film.
The group of T.e.obscurus consisted of 9 males and 3 females, gender identified visually on basis of the obvious sexual dimorphism. I only caught and measured one of the larger females (104 cm TL). Some had shed their skins recently, others not.


Basking T.e.obscurus (lower group of 3-4 snakes) and T.m.canescens (melanistic specimen) in a willow tree in Lake Chapala in June 2008 at 11 AM, site 3.Since sexual dimorphism is not as obvious in the much smaller T.m.canescens I could only establish gender of 3 specimens I caught and measured. I measured 1 male and 2 female T.m.canescens (61.5, 65.5 and 69.8 cm).  All 3 were well nourished and looked healthy. 2 had shed their skin recently. One female (69.8 cm) was pregnant with approximately 12 developing embryo’s (palpation) in her body. Of the 8 adult melanogaster 3 were completely melanistic. This colour morph has never been described before as far as I know.
After spending about an hour observing this group of snakes I moved on.

“Mating ball” of T.e.obscurus (7.1) found in June 2008 at site 3 at 11.55 AM in a willow tree.

“Mating ball” of T.e.obscurus (7.1) found in June 2008 at site 3 at 11.55 AM in a willow tree.

Maybe 100 meters further stood another willow tree (a single one this time) and I tried to approach as quiet as possible wading through the water. I observed approximately 1 T.m.canescens and 8 T.e.obscurus at 11.55 AM.
Seven of then were males, several of them trying to mate with the female. I read much about the so called mating balls and I was thrilled to witness this.
What a perfect day! I measured 5 of the males: 64.9, 77.0, 82.7, 90.2 and 91.5 cm. Except one they had not shed their skins yet. 1 was skinny and 2 were very skinny. The other 2 were ok.

About 40 minutes later, at 12.40 PM I saw a very heavy T.e.obscurus female (102 cm) sunning on a primitive fence made of willow branches above the water.  She was very thick, either bloated or pregnant. She had something that looked like a big tumour in her neck, so I think she must have been sick.

Unfortunately my batteries of my digital camera died, so I could not take any pictures anymore that day.

Almost at the same spot in the fence a T.m.canescens was basking: a pregnant female (minimum of 6 embryos could be felt) of 59 cm TL.

10 minutes later I saw a dead large male  (92.5 cm TL) T.e.obscurus floating in the water.
The snake must have died very recent and I could not see any obvious cause of death.

At 12.50 and 12.51 I saw 2 T.m.canesceens; one basking on the tules (reed) and one resting in the water  (hunting?) with head above the surface. One was melanistic, the other regularly coloured.

At 12.53 I caught and measured a T.m.canescens that was hiding underneath a fishermen’s boat. It was a female, 65.5 cm long and clearly pregnant with 10 developing embryos in her belly.

Habitat of T.e.obscurus and T.m.canescens. Lake Chapala in June 2008, site 3. Note the low water level and the willow trees in the water.I had to head back, and when I arrived at the lonely willow again where at 11.55 AM, 8 T.e.obscurus and 1 T.m.canescens were observed I could only find 3 T.m.canescens basking at 1,2 and 3 meters above the water. 2 of them were measured, both females 62 and 63 cm long. One of them pregnant with 10 embryos, the other well nourished but not pregnant visually.

At 1.05 PM I arrived at the group of willows where I had seen the largest cluster of obscurus and melanogaster. Now I saw 5 male and 1 female T.e.obscurus and 2 T.m.canescens.

Between 1.07 and 1.11 PM I saw 5 additional snakes, 4 of them belonged to T.e.obscurus and were swimming close to the water’s edge and one far (10 meters) in  the lake.
Number 5 was a beautiful orange- red T.m.canescens that managed to escape under water before I could catch it.

I rescued 1 male T.e.obscurus who got stuck in the remains of fishing nets hanging around a tree. The net had cut into its cloacal area and his  hemipenis was extruding and looked inflamed. Probably the snake died anyway despite my efforts, but I will never know.

Another snake had not survived it’s encounter with a fishermen’s net. And was hanging dead.

While exploring the shores of the lake I had seen at least 10-20 dead adult snakes (1 or 2 T.m.canescens, but most of them were large T.e.obscurus) lying on the shore.
Some of them with head smashed in, so probably killed by the local fishermen. 5 of those had been killed in the last 1-2 days.

At 1.15 PM I found a male T.e.obscurus (77.5 cm) resting in the water under a stone lying at the waterline.

At 1.20 and 1.21 PM I saw 2 more T.m.canescens, both resting under stones along the shore in the water.

One of them was a very unusual female of 51.5 cm. It was heavily blotched, clearly different then most the T.m.canescens I saw that day. The head was bigger then a normal T.m.canescens. The head scales not as smooth and there was a pattern on the upper side. I am convinced this was a hybrid between T.e.obscurus and T.m.canescens. While looking at this snake I realised I had seen another “melanogaster” hanging dead in a net with the same untypical heavy blotching. Unfortunately I have no proof (picture) since my batteries of my camera were empty.
It was 1.30 Pm and I had to leave the lake in order not to miss my flight.

In total 21 melanogaster(including one possible hybrid)  and 35 eques (45-55 if I include the dead ones) were found within 3.5 hours of searching.

Habitat of T.e.obscurus and T.m.canescens. Lake Chapala in November 2009, site 3. Note the high water level and the willow trees in the water.November 29th 2008 I visited Lake Chapala for a third time. I arrived quit late (4 PM) at location 3 and the sun was low. The wind was fierce and it was cooling down rapidly. It felt cold. The water level was extremely high (the small ditch was “swallowed” by the lake) and the lake was hardly accessible. I searched for half an hour but no snakes were found.

November 22nd 2009 I visited Lake Chapala for a fourth time. I arrived early at location 3 where  again I would focus all my attention. I had been here already 3 times previously which gives you the advantage of knowing the precise habitat.

When I left Guadalajara at 7 AM it was 10 ºC but upon arrival at the shores of the lake at 08.30 AM it was already 15 ºC. This may be the effect of the large lake which does not cool down as much as over land.
Water levels were comparable to 2007. The little ditch I described in the 2007 observations contained water again but superficial inspection did not reveal any snakes. I did not turn much stones though.
A local fishermen told me that he saw “culebras de agua” (water snakes) year round and according to him there is no period where they are absent. During previous visits local boys told me the same. He also told me he had seen huge ones of 1,5 meter, but that may be exaggerated since stout bodied (pregnant) snakes seem bigger then they really are.

Since in June 2008 the willow trees in the water were very successful I tried to reach them, although due to the high water level I got wet up onto my belt. None of the willow trees revealed any snakes.
Maybe it was still too early (9AM)?

On the shores I discovered at 9 AM a fairly large T.m.canescens female (68 cm TL) basking in some green shrubs 120 cm above the ground.

Adult male of T.e.obscurus (70.2 cm TL) found in November 2009 at site 3 at 9.45 AM hiding under a boat.At 9.45 AM I discovered a T.e.obscurus underneath a small rowing boat that was pulled ashore. It was a male
(70.2 cm) with nice blue lateral stripes and a reddish brown dorsal colour and the double row of spots was clearly visible. Well fed and it looked like it had shed it’s skin recently.

Around 10.15 AM I was just about to head back to the car (I had to leave the lake at 11 AM) but I decided to check out  a habitat I had not searched yet: small patches of tules  which where growing here and there (not so much though at location 3).

This specific patch was about 20 m² in surface and indeed a (small) snake (likely a melanogaster based on the size and the speed with which it disappeared) dropped in the water before I could take a good look at it. A second one I saw before it tried to escape: a fairly large female T.m.canescens (70.5 cm TL).
The snake was full and well fed just like the previous female melanogaster I caught (both not pregnant) and it was sunning curled around the horizontal leaves of the tules.

On the other side of the patch of tules, not visible from the shore several of the tules were bend and dead forming a horizontal floating basking place for snakes. And yes, there were several T.e.obscurus basking at 10.25 AM! A recently shed skin was lying on the basking spot.  I could see 4 males trying to copulate 1female. 1 meter further there was 1 male lying on top of a female. 2 meters further there was a large female that needed to shed it’s skin somewhere in the coming 10 days and a large male (88 cm TL) lying by her side. 2 – 3 other males were basking in the same area. So in total appr. 9 males and 3 females were seen, all large adults of appr. 75-90 cm (estimation, did not catch them except one large male mentioned above). The males were focussed on mating. When I came to close while filming (I got get as close as 1 meter) and the females escaped and dove into the water the males were reacting by looking around and searching where the female went. They did not try to escape very fast as they usually do. My impression was that the large male (that was lying close to the female that needed to shed it’s skin) was waiting until the female would shed it’s skin and then he would start mating her, since also he reacted very sluggish and slow when the female escaped and merely kept on smelling where she had been basking instead of escaping.

One (of the many) questions that popped into my head while exploring this habitat (and remain unanswered) is where the adults spent the night. This time of the year I expect them to be exclusively diurnal (although I did not check of course). Do the snakes spent the night in / under water and then start basking in the morning or do they retreat to hiding places on the shore.
The male I found under the boat probably spent the night there.

Air temperature at 11 AM (car thermometer) was 23 ºC but due to the intense sun it felt much warmer. So in total 3 melanogaster and 13 eques were found within 2.5 hours on November 22nd 2009.

Once again these observations show that you have to be at the right time at the right place, as many other herpetologists (for instance Conant, 2003) have concluded before. Three visits in November in 3 consecutive years generated different information. If I had not checked the tules in November 2009 for instance I would not have found the large group of mating adults and I would not have know that they also mate late in November.

Description of the snakes.

Adult male of T.e.obscurus ( appr. 70 cm TL) from Lake Chapala.Thamnophis eques obscurus appears (especially in the wild where they are often a bit dirty) to be relatively dark brown and unpatterned. Hence the Latin name obscurus, referring to the suppression or complete lack of stripes; Conant (2003) describes them as “a virtually stripeless  race of Thamnophis eques. Adults in life lacked pale longitudinal stripes.  A few preserved for many years show faint traces of lateral stripes that now appear bluish. There is no indication of a middorsal stripe in any adult of the sample available”. After reading this description I had expected to find a unpatterned and dark garter snake.

How different were the snakes that I observed in November 2007, May 2008 and November 2009. The lateral stripes are clearly visible in all adult snakes I observed and they have a beautiful blue or greyish blue colour.  Also the supralabial, neck and ventral scales have this typical blue-greyish blue colour.   Traces of the dorsal stripe are visible in almost all adult specimens, but indeed sometimes difficult to notice.

In the juveniles all 3 stripes are clearly visible but the bluish colour of the lateral stripes is less prominent.

The two lower scale rows (between the ventral scales and the lateral stripes)  have a light nucleus with a more or less golden or bronze colour and shine. They are clearly lighter then the dorsal scales between the stripes in most specimens.
The general colour of the scales between the lateral stripes varies from lighter brown to dark brown. Sometime warm, chestnut brown. The double row of spots between the lateral and dorsal stripe are sometimes prominent in the juveniles. With aging these rows tend to become obscure and only visible in pregnant females or just after feeding. There are some white spots on the skin between the scales with the dark spots but this is definitely only visible when the snakes are pregnant or after eating a big prey.

The ventral scales have the typical bluish-gray colour with black edges and the chin , throat and infralabial scales as well as the underside of the tail are  a  pale yellow. As in the other subspecies of Thamnophis eques coloration is difficult to describe exactly due to the shiny and fluorescent nature of the scales.

Adult melanistic male of T..m.canescens (61,5 cm TL) from Lake Chapala.

Adult melanistic male of T..m.canescens (61,5 cm TL) from Lake Chapala.

Thamnophis melanogaster canescens is highly variable. The snakes can be reddish, brown, green, grey and all colour tones in between. I only saw one snake with a conspicuous red coloration. In some specimens the lateral stripes are visible but certainly not prominent.
I have not seen any snakes in lake Chapala with a middorsal stripe. Two rows of spots above the lateral stripes are present in some snakes but never very prominent.

I also discovered a colour morph not described previously in literature, as far as I know.
A small proportion of the T.melanogaster canescens were almost completely melanistic. Also the chin scales were entirely black, unlike most  melanistic Thamnophis sirtalis sirtalis. 16 % of the population in location 3 was melanistic. In the other locations I have not observed melanistic specimens, but of course the number of observations at location 1 and 2 is limited.
Including the 5 specimens found in November 2007 at location 1 and 2 along the shores of lake Chapala 13 % of T.m.canescens was melanistic.


Adult male of T.e.obscurus ( appr. 70 cm TL) from Lake Chapala.

Conant (2003) suspected that Thamnophis eques obscurus remains quiescent during the dry season. He was not able to find any eques until the end of July when the summer rains have started and water levels start to rise. Thamnophis melanogaster canescens could be found each night when he explored the lake after sunset. He expected canescens to be active year-round.

Most rainfall occurs between July and September.  The driest part of the year were the lake is at its lowest levels will be April-June.

So when I visited lake Chapala June 1st 2008 I had expected that obscurus would be dormant.
It was very hot and dry and water levels were very low. As described above it is obvious that obscurus was far from quiescent.

Many obscurus were active June 1st  and the high concentration of snakes in combination with a sex ratio favouring the males are typical for spring mating season for Thamnophis. Mating attempts and coitus were observed so it was clear that it was mating season (assuming that mating is confined to a specific period of the year in this area). Observations in November 2009 also revealed mating behaviour. This could be described as fall mating, which is common in probably all species of (at least American and Canadian) garter snakes.

About 30 % of the obscurus had recently shed their skins on June 1st, the other portion looked like it had been a while. Not more then 10 % had eaten recently. Many of them were thin or extremely skinny, suggested that they had not been feeding for a while. None of them looked well nourished with some fat reserves (possibly except the very large female specimen, but I suspect she was sick and bloated). None of the females (except  possibly that single large female) showed any signs of pregnancy.

Also Conant (2003) found no pregnant females in July and August. He did find some very small young (24.4 and 21.9+ cm) in July 1965 in the town of Chapala, suggesting recent parturition.

The most plausible conclusion seems to be that the snakes had become active somewhere in May after a period of quiescence where they had not been feeding. This could explain the fact that many males and females were extremely skinny (almost emaciated). The high mortality of the large adult snakes could be the combination of poor health and killing by fisher men (the latter is an assumption).

If activity starts as early as May it is strange Conant did not find any T.e.obscurus during his exploration in the sixties in early July. He did explore a different portion of the lake and timing of activity may differ at different habitats of this huge lake. Activity  may also very from year to year of course.

The small young Conant found in July 1965 may be young born very late in 1964 which had not started feeding before the quiescence period and thus had hardly grown half a year later.

The obvious mating on June 1st  2008 could have resulted in parturition somewhere in August/September or a little later.. If this was also the case in the years Conant explored Chapala he should have found pregnant females. Most of his observations were apparently done during the month of July but also mentioned collecting a snake on August 19th. Perhaps the snakes show obvious signs of pregnancy a little later in the season, say from the end of August with parturition between late September through October.

Another conclusion may be that also T.e.obscurus is active year-round as T.m.canescens probably is (Conant, 2003). The local people confirm seeing snakes year round and T.e.obscurus is the most likely one to see due to it’s size and it’s more conspicuous behaviour (swimming with a large part of the body partly above the water level).  So if obscurus is active year-round  the very skinny females found in May could have been so skinny due to very recent parturition. And not so much due to hibernation as I suggested above. Of course this does not explain why many of the T.e.obscurus males were also very skinny and I would have expected to find many recently born juvenile obscurus under rocks etcetera if all these large female had given birth recently.

Research in Arizona (USA) showed that Thamnophis eques (megalops) has its young as early as June and July (Rossman, Ford & Siegel, 1996). The authors mention the unusual pattern of follicle enlargement in fall and ovulation in late March and early April. Clearly my observations shed some light on the life cycle of obscurus (and melanogaster) but still much is unknown.

It seems safe to assume that the snakes will remain active from May onwards  based on my observation and Conant’s observations (who found them from late July onwards). Finding juveniles in November hunting and feeding and the recent observations November 22nd 2009 of mating adults, evidence of shedding and snakes that still need to shed their skins combined with the nice weather so late in fall do not suggest that the activity season is already ending.

The young obscurus found in November 2007 were 24, 26,9, 27, 31.5, 35 and 37.5. Conant (2003) found juvenile in July with a length of 24.4 and 21.9+ cm. Neonates of obscurus measure between 21 and 29 cm TL (personal measurements on 3 litters born in captivity) directly after birth. The juveniles can grow in captivity (personal observations) under ideal circumstances quit fast (3,9 cm per month).

If we look at the size of the young Conant found in July these could very well be neonates born very recent, although young can grow much and much slower under circumstances with lower temperatures and low food availability. Especially if they would go into a stage of quiescence.
So parturition late in November for instance is also possible (although not very likely).

The young I found in November do suggest parturition in the previous months. Neonates in captivity can easily grow from 29 cm to 60-80 cm within a year in captivity (personal observations). So 24-37.5 cm do not suggest the snakes are already a little over 1 year old.
The smaller ones could have been born in October/November but the biggest one (37,5 cm) should at least have been 2-3 months old (assuming a size of 25-29 cm at birth and a maximum growth speed of 3,9 cm per month).

To be sure when parturition occurs in lake Chapala more observations are needed in other times of the year. But based on Conant’s observations and my personal ones I would estimate September/ October.

Adult melanistic male of T..m.canescens (61,5 cm TL) from Lake Chapala.I have found T.m.canescens in June 2008, November 2007 and November 2009.
67% of the females found June 1st 2008 above the 50 cm TL were clearly pregnant.
Parturition may have followed anywhere in the following 1 – 2 months (June – early August).
The small snakes found in November 2007 (26.8 – 31.5 cm) are very likely young born in June-August. Neonates of canescens average around 19 – 22 cm (personal measurements in captivity). They can grow 1 – 2 cm/month (not as fast as obscurus, personal observations) so the young of 27 – 31 cm could have been born June-July.

The large females found in November 2007 and 2009 all were well fed but not pregnant.
No mating behaviour of canescens is observed. My observations are in line with Conant’s suggestion that canescens is year-round active.


At least one of snakes ( a female of 51,5 cm TL) observed June 1st 2008 along the shores of lake Chapala showed characteristics of both T.m.canescens and T.e.obscurus. I have no doubt that this was a hybrid between the 2 species.

The general appearance of this female was of a T.m.canescens. The dorsal pattern showed a heavy and very regular blotching which is not typical for T.m.canescens. Normally the head of T.m.canescens appears pointed and elongated but this snake had a bigger head and a more rounded snout (like obscurus). Third clear difference was that it lacked the smooth, shiny and unpatterned head scales. The head scales were more like T.eques, rougher and with some pattern.

A snake I saw earlier that day (a “T.m.canescens” hanging dead in a fishing  net)  was also heavily blotched, just like the presumed hybrid I found. I found this quit unusual at that moment, but since this snake was already dead for a while and smelled quit bad I did not have a good look.

Unfortunately the batteries of my camera died so I could not photograph this snake.

Hybrids between distinct species of Thamnophis have been mentioned in literature (Rossman, Ford and Siegel, 1996). If this snake (or these 2) indeed are hybrids they would represent 1 – 2% of the total population. But maybe I was just lucky to accidentally find these 2 (or 1) snakes and is the real % much lower.

The fact that the two species occur together in high numbers in the same habitat and they remain 2 distinct species suggest that there are strong mechanisms in place keep them separated, and that hybrids are “accidents”. If melanogaster mates in the willow trees and in the tule fields at the same time and both species are all crawling together a “mistake” is easily made by either the male or female. For instance a male melanogaster rubbing it’s chin on the back of a female melanogaster but with his rear body he is trying to lift the tail of a female obscurus.

Health and demography of the population

Finding both eques and melanogaster active in high numbers just like Conant (2003) observed in the sixties of the previous century suggests that the population is not in jeopardy. Finding juveniles of both species also show they are successfully reproducing.

Grafiek 1

Grafiek 1

When we look at the size distribution (see graph 1) a clear difference seems obvious.
T.m.canescens is represented in all size classes from 20 – 30 cm (juveniles) to 70 – 80 cm (maximum size for adult females). Finding all size classes is often seen as a sign of a healthy, undisturbed population (Rossman, Ford and Siegel, 1996). The size group of 60.0 – 69.9 cm is over represented. Closer examination reveals that only 1 of the adult melanogaster is a male (61.5 cm). So of the 12 snakes larger then 50 cm ( minimum size for adult females) only 1 is a male (8%). So the females between 60 and 79.9 cm all are large and full grown females. The largest female I observed was 72 cm.

Size distribution of T.e.obscurus looks very different. First of all it is very obvious  that obscurus grows to much larger size then melanogaster. 6 specimens fall in the smallest 2 size classes, representing young of the year or possibly young of a little over 1 year old. The rest of the specimens are old and large adult snakes of 70.2 – 104 cm. No snakes of the middle 3 size classes (40 – 70 cm) are found. This would suggest low survival rate of the juveniles and it is typical for disturbed populations. The sexratio of the adult obscurus is strongly favouring the males. Of the caught and measured snakes 10 of the 18 are males(56 %), but if I include the snakes I did not catch (but of which I am pretty sure they were males or females based on the obvious sexual dimorphism) 30 of the 42 snakes are males (72%).

So melanogaster and canescens show an almost totally opposite demography. Both in size distribution and of sex ratio of the adults.

Graph 2

Graph 2

It is likely that the difference in sexratio (see graph 2) will vary over the year. In mating season males often dominate the population, and they become less conspicuous when the females are pregnant and developing their embryos. So in June most adults melanogaster were females (92 %) and 67% of them were pregnant.

Both November and in June most obscurus were males (72%) and mating behaviour was observed in both months. No pregnant females were found.
So the opposite sexratio may very well be the result of the a-synchronised reproductive cycle.
This a-synchronizing of the reproductive cycle may limit the chance of hybridization significantly.

Adult female of T.e.obscurus ( appr. 107 cm TL) from Lake Chapala.The much larger T.e.obscurus will be able feed on much larger prey then the much smaller T.m.canescens. Especially if you realize that relative width and size of head is very different in both species. This may limit the competition for food. It would be interesting whether the success of melanogaster on feeding on the smaller fish species (or size classes of fish) is the reason that the middle size classes of obscurus are absent. In that case melanogaster out competes obscurus.

Previously I already mentioned finding very skinny T.e.obscurus in November 2007 (the large male) and in June 2009 (several specimens). Only 1 of the adults had fed recently in June 2008.
1 adult male and 2 of the 6 juveniles were well nourished. All the other were either skinny or very skinny but none were what I would call well nourished.

I also mentioned finding many (10 – 20) dead ones in June. These numbers seem quit high to me.

Some of the dead ones looked like their heads were smashed so this suggests being killed by fishermen. But others showed no external signs.

Possible reasons for finding very skinny (and dead) snakes are lack of food, pollution or recent parturition (females only).

Conant (2003) mentioned that the Rio Lerma is draining a very large agricultural surface (and thus also chemicals used in the fields, but also pollution caused by industries) and that the eastern end of lake Chapala is highly polluted.

Only a few melanogaster were found dead. Since melanogaster is less conspicuous in it’s behaviour it is less likely to get killed by fishermen. But none of the melanogaster were skinny.
2 or 3 had recently eaten, but al but maybe one were classified as well nourished or even fat. So whatever the cause is for the high numbers of obscurus that looked either ill or merely thin, it does not seem to effect the melanogaster.


Rossman, Ford and Siegel  (1996) mention that in Arizona T.eques is strongly associated  with permanent water with vegetation.  They also mention that eques is more secretive then other Thamnophis and that they remain in dense vegetation instead of basking in open areas. They do mention willow trees.

“La Laguna de Chapala”, Mexico.At Laguna de Chapala dense vegetation was not readily available on the shores of the large lake, at least at the sites I explored. As described above obscurus was found specifically in the willow trees standing in the lake (and not so much in willow trees growing on the shores) where they could easily drop into the water to escape. Also the fields of tules provided perfect habitat where they could bask in the open and escape from predators by diving into the water.
A third habitat was beneath all sorts of objects: rocks, boats and plant masses washed ashore.

Rossman, Ford and Siegel (1996) mention that T.m.canescens is very aquaticand apparently fills the “Nerodia niche” in the area. I observed T.m.canescens in exactly the same habitat as obscurus, although they were found more often underneath objects like rocks, boats and plant masses. Due to their smaller size they can more easily hide underneath those objects at Laguna de Chapala, especially if you take into consideration that large rocks (that would be suitable for adult obscurus) are for a big part absent at Laguna de Chapala.

My impression is that both obscurus and melanogaster are equally aquatic and that they divide the “space within the Nerodia-niche” amongst themselves. The much larger obscurus with wide heads preys on relatively very large fish and the melanogaster with narrow and pointed heads preys on  much smaller fish. It would be interesting to compare the diet of the 2 snakes and correlate this with the species of fish, their size and their behaviour.

Discussion and summary

During visits to La Laguna de Chapala in Jalisco, Mexico in May 2008 and November 2007, 2008 and 2009 in total 30 T.m.canescens and 55 T.e.obscurus were observed.
In June 2008 the T.melanogaster population was dominated by large pregnant females.
On the contrary population T.e.obscurus was dominated both in November and in June by adult males.

Mating behaviour of T.e.obscurus was observed in both months.
The demography of melanogaster looks more healthy then the one of obscurus. Whether this is a result of competition for food, especially for the smaller size classes of obscurus (which are absent in my sample) needs further research.

The high amount of dead obscurus found in June and the apparent relative poor fitness of the large and old adults observed in both June and November suggests a clear cause like pollution, lack of food or internal parasites. But since both species inhabit the same habitat and both feed on aquatic prey like fish and frogs it is to be expected that both species are equally vulnerable for both pollution and internal parasites.

A possible case of hybridization is observed and documented, but at the same time it seems there is a clear a-synchronising of the reproductive cycle reducing the chance of hybridisation.

The current observations plus the comments of local boys and fishermen contradict with Conants (2003) observations made in the sixties of the previous century that obscurus goes into a quiescence during the dry season (spring/early summer).
The description of T.e.obscurus by Conant (2003) seems to contradict slightly with my observations. Especially the fact that he specifically mentions the total absence of a middorsal stripe (which was visible to a certain degree in all adults specimens). Also the fact that he specifically mentions that the lateral stripe is also absent and that only long preserved specimens show faint traces of lateral stripes that now appear bluish are in contradiction since all specimens show a clear lateral stripe that is bluish or bluish grey in all specimens (except the new born babies in which the lateral stripes are more whitish).

Laguna de ChapalaIt seems unlikely that the snakes have been evolving so rapidly over a course of appr. 40 years (4 – 10 generations maximum). Or were most if his specimens collected at other places and is there a lot of variation within La Laguna de Chapala. Some of my specimens were collected (location 1 and 2) between Chapala and Jocotepec were Conant (2003) has collected as well. And I have not seen any significant differences between location 1, location 2 and location 3.

Several melanistic specimens of T.m.canescens were found at one location in June 2008.

Conant, R., 2003. Observations on Garter Snakes of the Thamnophis eques complex in the Lakes   of Mexico’s Transvolcanic Belt, with descriptions of New Taxa.  American museum novitates   3406: 1-64.
Rossman, D.A., N.B.Ford & R.A.Siegel (1996). The Garter Snakes. Evolution and ecology. University of Oklahoma Press, Norman
During visits to lake Chapala in highlands of central Mexico in 3 consecutive years (2007, 2008 and 2009) the author has done exciting observation that reveals some new facts about the biology of Thamnophis eques obscurus and Thamnophis melanogaster canescens. Mating behaviour of obscurus observed in June and November suggest a longer seasonal activity then suggested by Conant (2003). A case of possible hybridization between the 2 species is reported, as well as a obvious a-synchronised reproductive cycle limiting the chance of hybridisation. Melanistic specimens of canescens are reported for the first time in literature.