The San Francisco Garter Snake in the wild!

The San Francisco Garter Snake is severely jeopardized and becoming very rare in the wild.
Not many people have been lucky enough to observe this beautiful subspecies in the wild.

On February 21st 2011 I was in the opportunity to check out the natural habitat of this rare garter snake. The location is in the coastal regions of southern San Mateo County. The week prior the weather had been relatively rainy with daytime temperatures not exceeding the 10 ºC. February 21st started promising for herpetological observations. It was quite cool (appr. 4 – 5 ºC ) but clear blue skies in the morning.
By 10.30 AM temperatures had risen well above the 10 ºC by the warm sun and the first Garter Snake was observed near the border of a pool. It disappeared before I could have a close look but likely it was a Coast Garter Snake. The air was filled with singing tree frogs and possibly other amphibians.

One of the pools had a extraordinary high population of garter snakes. The pool and it’s borders were overgrown with blackberries, reeds and weeds. The most abundant species was Thamnophis elegans terrestris. They were basking in between the blackberries and weeds, their colors largely invisible since they were covered in dried mud and clay. The combination of rains and being hidden in their subterraneous hiding places will probably have been the cause of them being covered in clay. At least 20 adults Coast Garter Snakes (probably more) were observed. Some of them were large adult females of 70 – 85 cm total length(TL) but the majority (more then 75%) were adult males of appr. 50 – 60 cm TL. Beside that some juveniles (probably young born in the previous summer) were seen, measuring appr. 30 cm. The large females were surrounded by 3 – 4 males, but I did not observe actual mating behavior. Many specimens of T.e.terrestris in this population  had extreme amounts of red coloration.

A second sympatric Garter Snake I did observe was a couple  of Thamnophis atratus, the Santa Cruz Garter Snake. The snakes  had characteristics of both T.a.atratus and T.a.zaxanthus . Very likely this population is part of a wide intergradation zone between the 2 subspecies. The female was quite large and skinny  and had a total length of 81 cm.

The absolute highlight of this fantastic day in February was a large adult female Thamnophis sirtalis tetrataenia, The San Francisco Garter Snake.  Like many  of the Coast Garter Snakes her striking coloration was masked by the dried up clay. At 1.30 PM she was basking close to the edge of the pool. She had a TL of 86 cm with a snout-vent length of 66,5 cm. Maximum temperatures that day were appr. 15 – 17 ºC. By  cleaning the snake a bit in the water it’s striking colorations became visible.

None of the Garter Snakes I observed that day had recently eaten and they all appeared to have come out of hibernation very recently.

Año Nuevo State Reserve, California. The San Franscisco Garter Snake.


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