social behavior – EYNC Rattlesnake Study

Year end wrap-up

Well, it has been more than two months since I reported the first births of the year among the wild rattlesnakes in the Effie Yeaw Nature Center preserve. And while my hectic fall schedule prevented me from investing as much time as usual, I was able to record enough detail to compare this season’s behavior to previous years.

If the timing of the first births among our telemetered females (between 8 and 12 September) were representative of the larger population, we would expect to start encountering youngsters about the third week in September. Remember, new kids remain with their mothers in their birth shelters until their first shed, which usually occurs a little over a week after birth. Only then do they venture out into the world where people might encounter them.

Sure enough, the first young-of-the-year was encountered on 26 September, ironically on the sidewalk just outside the EYNC classroom as my UC California Naturalist class was leaving that evening. Of course, since I had just talked about baby rattlesnakes in class a few minutes before, the students were suspicious that I might have planted the little guy! The next day, I encountered another one (see photo below) stretched out next to the hollow log where Females 39 and 53 had recently given birth.

Young-of-the-year on 27 September, beside the hollow log where he was probably born.

Two shed “skins” (actually, just the outer or “corneal” layer) left by babies as they embark on their hazardous new life. Few survive to adulthood; at the size of pencils, they have lots of predators.

And sure enough, Female 39 once again made a beeline from her birth shelter to the blackberry thicket across San Lorenzo Way as soon as her kids’ natal sheds were complete (photo below). She has produced broods in three of the last four years and she has followed the same pattern each time: she used the same gestation/birth shelter, she made a direct 200+ meter move to hunt in the same blackberry thicket as soon as her kids departed, then she returned directly to her usual hibernation site after several weeks of foraging among the blackberries. Some of these individual rattlesnakes have proven to be quite predictable from one season to the next… but not all of them.

A slender postpartum Female 39, cryptically basking on the other side of San Lorenzo Way.

Female 41 spent the past three winters apparently by herself (as far as I could determine) in a ground squirrel burrow among the roots of a large living Valley Oak. However, she has apparently decided to spend this winter under the EYNC Visitor Center building! Now before getting excited, remember that she is one of only twelve rattlesnakes carrying radio transmitters at EYNC – and we have estimated that there are about 100 additional adult rattlesnakes in the preserve. We can make that assessment because we have 52 rattlesnakes marked with paint in the rattles that are not implanted with radios… and about half of our chance encounters with rattlesnakes without radios over the past year have been with unmarked animals.

Almost none of the EYNC staff or regular long-term visitors with whom I have spoken thought there were anywhere near that many rattlesnakes in the preserve. That’s a testament to just how secretive and non-aggressive rattlesnakes really are. It is highly likely that other rattlesnakes over-winter under the Visitor Center every year undetected.

It is simply prudent in rattlesnake country to be careful where you step and where you put your hands; simply look before you step or reach. Assume that a rattlesnake could be encountered anywhere – in any shed or under any wheelbarrow – and proceed with reasonable caution, not fear. Don’t walk around outside at night without a flashlight (or boots); while usually pretty timid, rattlesnakes really dislike being stepped on! That’s the same advice I have given countless people at my rattlesnake presentations.

As of 7 November, all but three of the telemetered rattlesnakes had returned to their previous hibernacula (hibernation shelters): exceptions are Female 41, who is under the Visitor Center 185 meters (a little over 600 feet) from her usual hibernaculum in the woods, Male 46 is on the hillside below the residential area instead of 265 m (about 870′) away under his usual log in the meadow, and Female 66 is under a new log 80 m (about 260′) from her usual log near the study pond.

The study comes to an end

As some of you know, 2017 will be my last year of data collection at Effie Yeaw Nature Center. We implanted the first transmitter in an EYNC rattlesnake in May 2014 and built up the group of telemetered rattlesnakes that summer. As a result, we have good data from three complete seasons during 2015, 2016 and 2017, plus many valuable observations from 2014. I will remove the transmitters from the remaining rattlesnakes in the spring and release them. Several are carrying their fourth transmitter (they must be replaced annually), so removal will be their fifth surgery. I also have blood samples in my refrigerator from nearly 70 wild EYNC rattlesnakes, as well as many dozens of shed skins collected from various locations in the preserve… and DNA can be extracted from both blood and shed skins.

Now the real work begins. Field work is great fun! Sitting at a computer for weeks or months is, of course, far less entertaining. Yet the field work means nothing if the data are not analyzed and shared. And as a favorite mentor of mine is fond of reminding me, “If you don’t publish it, it never happened!” So, while we certainly have answers to the original questions (like how many rattlesnakes are there, where do they give birth, spend the winter, etc.), the most exciting part involves the potential for unexpected discoveries. And as I have written about in this blog before, I have found the EYNC rattlesnakes associating in three loose groups without an obvious environmental reason to do so. I suspect they are socializing in family groups, which has only been documented in one previous case with Timber Rattlesnakes (Crotalus horridus) in the Appalachian Mountains. Genetic analysis of the blood and shed skins will make or break that hypothesis for the EYNC animals. I will continue to post analysis updates and eventually publications here.

So stay tuned!

I’ll leave you with one of my very favorite photos from the study: big Male 37 basking peacefully on top of a large pile of dead branches among blackberry and wild grape vines early one morning in April 2016. The third rattlesnake to receive a transmitter at EYNC, he has always been a joy to watch and has provided some of my most memorable rattlesnake encounters of the last four years. The yellow and red paint in his rattle is slightly visible in the photo. Like others of his kind, he wants nothing to do with people but is a mouse, vole or ground squirrel’s worst nightmare.

All that remains of 2016 is data analysis!

With little fanfare over the past two months, the twelve telemetered rattlesnakes have disappeared underground one-by-one and ceased surface activity for the season.

Male 71 was the first to drop out of sight and has been stationary since 7 September. This big guy was first captured and implanted with a transmitter last spring, so we don’t know where he spent past winters. But he took shelter this fall under the same large log used by Males 35 and 40 and Female 39 in the past.

He was followed into winter inactivity by Male 62 on 19 September, Male 37 on 5 October, Male 75 on 7 October, Female 39 on 11 October, Female 41 on 12 October, Female 66 on 17 October, Male 35 and Female 53 on 21 October, and Male 36 about 29 October.

Rather than implant a third transmitter (they must be replaced annually) in our old underweight Male 40 last April, I removed his second transmitter and released him without a radio. During 2014 and 2015, he was the last to leave his winter shelter and the first to return, he moved far less than other telemetered rattlesnakes of either sex, and I have never found him courting a female. He is clearly in his twilight years (I’d love to know how old he is!) and he was not producing useful behavioral data for my study. I bring this up now because he spent the past two winters under the same log with Male 35 and Female 39, as well as other non-telemetered rattlesnakes, and he is back this fall. We have sighted him at this log on 3 and 21 October, as well as yesterday, 12 November.

On 12 October, Male 46 settled into the same winter refuge used by 35, 39, 40, and 71. But between 29 October and 9 November, he moved 70 meters (77 yards) to a spot under the log he used last year. And, for the second winter in a row, he is sharing this site with Female 53. We also spotted non-telemetered Female 55 under this log on 21 October.

After spending most of the summer in a steep inaccessible area high on the bluff (and probably producing a litter of babies), Female 80 showed up at the bottom of the hill on 12 October and stayed in a small area through the end of the month (photo below). But by 9 November, her radio signal indicated she was back near the top of the bluff and will presumably spend the winter there.

Female 80 hunting in dry grass and thistle at the bottom of the bluff on 17 October 2016, after spending much of the summer out of reach high on the hillside.

Although Male 36 was the second rattlesnake we implanted with a radio in the spring of 2014, you may recall that his first transmitter failed a few months later and he remained missing for a year and a half. As a result, we have no idea where this big impressive guy spent the past two winters. This year, he was one of the last active telemetered rattlesnakes, apparently going underground around 29 October. When he disappeared, however, his radio signal became sporadic and I couldn’t locate him for days at a time. We now know where he is but his signal does not propagate very far. A large abandoned iron water pipe passes through this spot and Male 36 is likely inside that pipe. And we know from experience with telemetered snakes in the metal storm drain that runs under the visitor center that such a pipe dramatically reduces the transmitter’s signal strength.

So, as it stands now, telemetered Males 35, 40 and 71, and Female 39, are together under the same log, along with a few other marked and unmarked rattlesnakes without radios. This will be the third consecutive winter under this log for 35, 39 and 40. Telemetered Male 46 and Female 53 are spending their second straight winter together under another log, likely along with Female 55 (sighted 21 October) and other non-telemetered rattlesnakes. Female 41 is in a void under a large living oak tree for the third year in a row – by herself, so far as I can tell. Male 75 and Female 66, along with several observed unmarked rattlesnakes, are together under yet another large log. Both 75 and 66 are new this year, so I have no previous winter data for them. Male 62 is apparently by himself but I have no prior winter location for him, either.

IKG_4249 — This was all that could be seen of three different adult rattlesnakes on 17 October. Radio signals from Female 66 and Male 75 gave away their presence but only Female 66 could be visually identified by the white paint in the bottom of her rattle (above). At least one of these rattlesnakes and another a couple feet away were new unmarked animals.

It is interesting to note that we see very little basking on warm mornings in the fall, unlike spring emergence when the snakes warm themselves in the morning sun for days before finally venturing away from their winter shelters. Remember that the metabolic rate in ectotherms, who rely on their environment for body heat, slows when they are cool. And slow metabolism consumes less stored energy and water. So, in the fall, on the verge of several months of inactivity, it makes sense to simply disappear underground, cool down, and conserve stored resources for use in the spring.

In summary, all of the rattlesnakes for which we have previous winter locations have returned to the same hibernacula each year… three winters in a row for three animals and two consecutive winters for three others. Some individuals seem to spend the winter by themselves but others favor locations with certain other rattlesnakes.

Once again, we are left to contemplate why, when there are many dozens of apparently similar old logs in the Effie Yeaw Nature Preserve, do these animals return to and congregate at a tiny number of them. This would not be surprising at higher elevation (or latitude) where winters are severe and suitable shelters to escape freezing temperatures are scarce. But that is not the case at about 20 m (66 feet) above sea level along the American River Parkway where winter temperatures are mild.

As I have speculated before, it would not surprise me to learn that we are watching social behavior of mostly related animals in family groups. Sociality among family members has been shown with genetic evidence in Timber Rattlesnakes (Crotalus horridus) in the Appalachian Mountains and will undoubtedly be discovered in other species. Each of the 80 Effie Yeaw rattlesnakes we have processed has donated a blood sample and the DNA will one day shed light on the validity of this hypothesis at Effie Yeaw!

Emphasis now on hunting and shedding

Before I launch into what’s been going on with the Effie Yeaw rattlesnakes over the past few weeks, I want to pass on a link to a recent interview with Dr. Bree Putman. Bree was a grad student with Matt Holding (lead author of the journal article I linked to in my last post) in Emily Taylor’s lab at Cal Poly San Luis Obispo before she moved on to Rulon Clark’s lab at San Diego State to finish her Ph.D. In Bree’s interview (click here), she talks about ground squirrels and northern Pacific rattlesnakes and she describes some of the very intriguing behavioral questions many of us would love to answer.

Back to EYNC rattlesnakes –

This female California ground squirrel was clearly nursing pups on 26 April at Effie Yeaw Nature Center. — This female California ground squirrel was clearly nursing pups on 26 April in the Effie Yeaw Nature Center preserve.

I have not seen a courting pair of rattlesnakes since 28 April, when three rattlesnake pairs were found together in different locations. Male 37 was found with unmarked adults that were likely females on 11 and 14 May but no courtship was observed. But as the spring courtship season wound down, California ground squirrels began producing pups and the emphasis of both rattlesnake sexes turned to hunting.

After not finding a telemetered rattlesnake in or very near a ground squirrel burrow during the first 400+ observations this year, Male 46 was found in an ambush coil facing an active burrow a foot away on 9 May. In the five weeks since, several rattlesnakes have been found close to or inside squirrel burrows on several occasions.

At the same time, the snakes have also been hunting heavily in vole (aka: meadow mouse; Microtus californicus) tunnels in the grass.

Male 46 visible in a vole tunnel on the morning of 3 June 2016. Original RAW IMG_2192.CR2 — Male 46 uncovered (arrow) in a vole tunnel in the grass on the morning of 3 June. Although you cannot see it here, voles construct a maze of above-ground tunnels in thick dry grass that protects them from most predators… but not rattlesnakes. I hope you can appreciate just how impossible it would be to learn about the habits of these snakes without radiotelemetry!

And since about the end of May, the rattlesnakes – especially males – have been shedding.

Pre-shed Male 38 on 14 June 2016 Original RAW IMG_2320.CR2 — The rattle of pre-shed Male 38 under a log yesterday, 14 June. The new rattle segment forming in the base of the tail is whitish and covered by the last couple rows of scales. He is easily identified visually by red/green paint in his rattle.

Periodically shedding the corneal layer of the skin (called ecdysis; for more info, click here) takes snakes out of commission for a week or more and males seem to put it off during the spring mating season. It’s a bit like race car drivers waiting for a yellow caution flag to make a pit stop!

Even more interesting is that the rattlesnakes have favorite places where they go during this process and it is not uncommon to find several pre-shed individuals of both sexes together this time of year. Like hibernation, there seem to be many logs and burrows where they could shelter while waiting to shed but they congregate in just a few of them. Those of us who study rattlesnake behavior would love to know why. What is so special about certain locations? Or is it something else… like family ties or some other social interaction?