basking – 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.

What a confusing spring!

March weather had the rattlesnakes along the American River Parkway confused! As I have mentioned before, the rattlesnakes usually emerge and begin to bask when the average temperature at ground level remains at or above about 50F. They are usually deep enough underground during the winter to escape daily high and low fluctuations, so it is the average that eventually penetrates to them and signals that winter is likely over. But not this year.

Most telemetered rattlesnakes emerged in March but remained at their hibernation sites, basking on sunny days as they usually do before leaving their winter shelters. But rather than getting drier and warmer, the weather throughout April was a constant mixture of rain and cloudy cool days and nights. Of the few rattlesnakes that left their winter hibernacula, two returned while others became inactive at other sites. The rattlesnakes do just fine in the rain. In fact, they drink rain and dew when it’s available. And cloudy weather is not a deterrent to activity if it is accompanied by warm temperatures. But nighttime air temperatures below 50F followed by overcast cool days are not good for activity in animals like reptiles that depend on the environment for their body heat.

So, in late March and throughout April, the few stretches of several sunny days with relatively warm nights in between produced some sporadic rattlesnake activity. But it was not until the end of April that more consistent sun and heat produced vigorous widespread rattlesnake activity.

A courting pair of rattlesnakes on 28 March, next to the log under which they spent the winter. The male is on top of and mostly obscuring the female, which is paying no attention to him.

As expected, when we had activity, we observed lots of courtship. This is, after all, the peak of courtship activity, followed by almost no reproductive activity from June to mid-August before an untick in courtship again in the fall.

But by far the most interesting discovery this spring was by one of my UC California Naturalist students who found a small male rattlesnake with no rattle! (See photos below) Now I hesitate to mention this, for fear of starting a panic about rattleless rattlesnakes. Please remember several things:

Rattlesnakes missing their entire rattle are extraordinarily rare. So rare, in fact, that they usually rate a note in a biological journal. This one will!
No rattlesnake has a long pointed tail like our many harmless snakes.
Rattles can be missing due to either injury or genetic mutation.
This little snake appeared to be recently missing the end of his tail and had other serious but healing injuries. Apparently a close call with a predator!
Identify rattlesnakes by looking at the tail! Yes, rattlesnakes have elliptical pupils and heat-sensitive facial pits but some harmless snakes have elliptical pupils and both these characters are too small to be clearly seen from a safe distance (two-times the length of the snake). Some harmless snakes also have rather triangular heads, especially when they have been frightened and are behaving defensively. But no rattlesnake has a long pointed tail like our harmless snakes and all of California’s dangerous snakes are rattlesnakes.

Check out the following images…

Our rattleless Male 87, on the day of his capture, 09 April 2017. He is only about two feet long. Note that his appearance is “rattlesnake” in every way except for the stump of a tail without a rattle.

Dorsal x-rays of Male 87’s tail (right), compared to a normal rattlesnake’s tail (left). The large opaque “club” in the normal tail is the calcified stylus inside the tail to which the shaker muscles attach (the hollow keratinized rattle on the normal tail is present but invisible to the x-rays). Many thanks to Hazel Ridge Veterinary Clinic for the radiograph.

One of several other healing injuries on Male 87. (The scale is centimeters; 2.54 cm = 1 in)

This is a harmless but badly frightened gopher snake that has just been stepped on by a hiker. Note how it has flattened its head into a triangle. It was also vibrating its tail in the grass, producing a good imitation of a rattlesnake. But a look at its tail reveals that it is not a rattlesnake (see next photo) and, therefore, a harmless California snake.

No rattlesnake has a long pointed tail like this gopher snake.

To be sure, there are dangerous snakes in other parts of the United States that have long tails without rattles (cottonmouths, copperheads and coral snakes, in southern and eastern states). But in California and other northwestern states, the only dangerous snakes are rattlesnakes.

Look at the tail to identify rattlesnakes!

And, yes, Male 87 was processed and released, just like all the others. While he has no paint in his rattle, he won’t be hard to recognize!

Where are the rattlesnakes?

Well, it’s March and many recent days have been sunny and quite warm. Even though daytime highs have been listed in the 50s, that’s measured in shade and the temperature in the sun near the ground is always considerably warmer. So why were rattlesnakes basking in good numbers by late February last year but not yet this year?

There are two parts to the answer. First, most of the snakes spend the winter deep enough underground that they are insulated from daily temperature fluctuations. In other words, they do not feel daytime highs or nightime lows. Deep in their winter shelters, the temperature creeps up or down gradually in response to the mean (average) outside temperature. The second part of the answer is that natural selection tends to remove animals from the gene pool that don’t spend the winter deep enough and/or that venture out too early and get caught in freezing weather. The animals that select optimal winter shelters tend to live longer, and thus produce more offspring carrying the genes for those successful behaviors. (For those of you interested in the evidence for genetic influence on behavior, Time Love and Memory (2000) by Pulitzer Prize winning author Jonathan Weiner is an entertaining and astonishing summary, written in easy-to-understand language but with references to the scholarly research.)

So how do we know the temperature in the underground shelters?

Besides having recorded hourly temperatures for nearly four years with a data logger in a burrow 1 meter (39″) below ground in the Mohave Desert during a previous study, we can routinely calculate body temperatures of the hibernating snakes from the pulse interval of their transmitters during the winter.

The transmitters come from the manufacturer (Holohil Systems Ltd.; www.holohil.com) with a calibration graph showing the correlation between pulse rate and temperature for each transmitter (they vary slightly).

A standard conversion graph supplied by the transmitter manufacturer, Holohil Systems. Put simply, the transmitters beep faster when warm and slower when cool and at a predictable rate.

I routinely check the pulse rate in a water bath at room temperature before implanting a transmitter and adjust the mathematical conversion, if it differs slightly from the factory data. Then it is a simple matter in the field to time the pulse interval with a stopwatch and convert that measurement to temperature later. And since the transmitter is implanted inside the snake’s abdominal cavity, transmitter temperature equals core body temperature.

I also record hourly shade air temperatures with a data logger in the nature preserve at Effie Yeaw Nature Center. This gives me a standard local reference against which to compare rattlesnake behavior and look for correlations with weather conditions.

My data logger is located on the north side of a large cottonwood tree near the center of the EYNC nature preserve where it is never exposed to direct sunlight.

I can then compare rattlesnake behavior, body temperatures and air temperatures. This is especially interesting when one year is compared to another.

Some early temperature data for 2017. Note that the daily mean air temperatures vary greatly, depending on weather conditions, while the mean rattlesnake body temperatures tend to be *nestled within the fluctuating air temps*. These snakes were still underground and not affected by daily high and low temperatures outside.

You can see about six weeks’ data from this year in the chart above. The fluctuations in mean (average) air temperature (open circles) reflect the daily changes based on cloudy verses sunny days and cold air masses moving through verses warmer southern air (the same factors discussed every day by meteorologists). The five filled circles are the mean body temperatures of the twelve telemetered rattlesnakes on the days that I recorded their data. There are more air temperature records because the data logger records automatically around the clock while rattlesnake body temperatures are only recorded when I visit the study site (which I do less frequently when the animals are inactive).

Now take a look at the corresponding data from last year:

Early temperature data for 2016. In these data, you will see that mean body temperatures are not buried among the mean daily temperatures. Rather, especially by the middle of February, the mean body temperatures greatly exceed the mean air temperatures. That’s because the animals were out of their shelters and basking in the sun.

Checking my behavioral observations from last year, I find a few rattlesnakes were starting to bask occasionally by the end of January and most were basking regularly on sunny days by the end of February. Regular basking continued through mid-March. Then, suddenly, between 17 and 19 March, most of the rattlesnakes left their hibernation shelters, as if someone had given them the “all clear” signal. This was multiple rattlesnakes leaving multiple shelters at the same time – pretty amazing! By the way, you may also notice that I visited and recorded data more frequently in February last year. That’s because there were rattlesnakes to see and behavior to record (besides the science; it’s just more fun when there are rattlesnakes to see!).

If I combine the raw data from 2016 and 2017 for comparison, the chart gets pretty messy and trends are hard to make out. But we can create mathematical models for those same data that present a much clearer picture.

These are mathematical models derived from the data in the two charts above. Note the difference between the temperatures of the basking rattlesnakes in 2016, compared to 2017 where the snakes were still underground and their body temperatures were “chasing” the changing environmental temperature, first rising in late January, then leveling off as the weather started to cool, and finally declining as the weather turned cold again in late February.

You can see that mean February air temperatures started out much lower this year compared to 2016. And, in 2016, with mean air temperatures exceeding 50F by early February and staying there, the rattlesnakes began to bask and their body temps took off as a result. This year, air temps started nearly ten degrees lower, rose briefly, but then took a nosedive in late February, keeping the rattlesnakes cool and underground so far.

Of course, some individuals do not behave in the same manner as most others. I have heard some reports of a few rattlesnakes being sighted in the past few weeks and one dog reportedly bitten already. While none of the telemetered rattlesnakes at Effie Yeaw have emerged from hibernation yet, one rattlesnake without a radio has been basking early – just as he has done each of the past two years. My old skinny Male 40 has been frequently laying in the sun at the entrance to his winter shelter for more than a week.

Male 40 basking in the late morning of 2 March 2017. Rattlesnakes, like other animals, have individual idiosyncrasies, and this animal has been the first to emerge and bask three years in a row. Interestingly, he is typically the last to leave the winter shelter and the first to return in the fall.

As you may recall, Male 40 was implanted with a transmitter in 2014 but I removed his second radio last spring and released him without one because he was not producing useful behavioral data. It was not that he was just behaving differently than the others (after all, unusual behavior is interesting and valuable to record), he has been chronically and significantly underweight, appears to be quite old, moves very little during the summer, and has never been found courting a female. Given his poor body condition, I was afraid that he would die during hibernation and his transmitter would not be retrievable. But, even without a radio, he continues to be sighted regularly and has now made it through another winter.

But most others seem to be waiting for some sustained warmer weather before venturing out.

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!

Courtship has begun

Despite persistent cool mornings and alternating sunny and cloudy days, nine of eleven telemetered rattlesnakes have left their winter shelters and the remaining two have been basking regularly and will undoubtedly be on the move soon. We have also been finding plenty of new rattlesnakes basking and now have 27 animals processed and marked with colored paint in the rattles, in addition to the eleven with radios.

As I have already described, Female 41 was one of the first to leave her hibernation site and was soon joined by Male 49, a snake marked last season but not telemetered. Male 49 and Female 41 remained in close proximity at the same location from 16 February until 20 March and, although they were occasionally coiled in contact with one another, I never saw the male courting her. Male 49 followed when Female 41 moved more than 50 meters between 20 and 24 March but there has still been no witnessed courtship. However, the grass is very thick at their new location, making her hard to see and him impossible to spot if he’s not with her (and I don’t want to step on him… they’re very fragile!).

On 22 March, I came across Female 55, another animal processed last year and released without a transmitter, basking alone at the edge of a large log. But when I returned on 24 March, she had company: an unmarked (no paint in the rattle) male on top of her, jerking, chin-rubbing, and tongue-flicking – typical courtship behavior (you can view a 2015 clip of Female 41 with another male here).

Female 55 basking alone in the afternoon sun on 22 March 2016. Note her rattle with red over yellow paint, visible through the grass on the left side of the photo. Without a radio, she had not been seen since June of last year.

type here — Then two days later, on 24 March, Female 55 is almost completely hidden by a new male on top of her; you can see his unmarked rattle in the middle foreground of the photo. Although the male is distracted by me in this picture, he had been vigorously courting the female just moments before. Unfortunately, the video camera auto-focused on the grass, making the resulting image unusable. Courtship continued at this location for at least three days but they were gone on 29 March, replaced by Male 35, who was enjoying the sun by himself yesterday afternoon. Although he would retreat under the log when I approached closely, he would re-emerge within minutes as soon as I backed away.

We can expect the females to hunt for the next two-to-three months, after which pregnant females will retreat to their favorite gestation shelters to thermoregulate until their kids are born around the first of September. Non-pregnant females will continue to hunt through the summer. Males will spend most of their time looking for receptive females until late May/early June, after which they, too, will hunt full-time for voles and ground squirrel pups until courtship resumes in late summer.

General emergence appears to be underway (after a couple of false starts)

After a couple of false starts during warm periods in mid and late February, a new series of cool storms kept the Effie Yeaw Nature Center rattlesnakes immobile through the first half of March. On days with a little sunshine, both telemetered and other rattlesnakes were frequently found basking at the entrance to their shelters but they did not venture away from them. Natural selection tends to remove animals from the gene pool that wander too early and get caught in a late-season freeze. As if to demonstrate that it was cool temperature and not the rain that was keeping them inactive, several rattlesnakes were found lying out in the rain on days when the temperature was in the high 60s, despite the clouds.

Male 49 lying in the rain on 4 March 2016. Rattlesnakes are very adept at harvesting rain and dew, being very capable of sucking individual droplets off of vegetation or their own bodies. While that will drink from standing water when available, they don't seem to need such water sources and we have not detected tham traveling to either the ponds near the Nature Center or to the river. — Male 49 lying in the rain on 4 March 2016. Rattlesnakes are very adept at harvesting rain and dew, being able to suck individual droplets off of vegetation and their own bodies. While they will drink from standing water when available, they don’t seem to need such water sources and we have not detected them traveling to either the ponds near the Visitor Center or to the river. The only rattlesnakes discovered over the past two seasons around the Visitor Center (and its water sources) have been males during the breeding season – presumably looking for females, not water.

While the rattlesnakes continued to bask on sunny days, none of the telemetered animals moved between 29 February and 17 March. Then today, I discovered that 7 of the 9 telemetered rattlesnakes had left their shelters and moved significant distances in the past two days.

I haven’t seen courtship yet. Female 41 and Male 49 hung out together for a month (16 February through 15 March) but they were only rarely touching one another and were usually coiled a couple of feet apart with no courtship observed. She is now hunting several dozen yards away and Male 49, without a transmitter, has disappeared.

The important message, however, is that rattlesnakes are on the move and encounters between people and rattlesnakes will increase immediately. Unfortunately, I have already seen photos of a rattlesnake bite from a few days ago in the foothills. Watching carefully where you put your hands and unprotected feet and leaving snakes alone when you encounter them would prevent almost all bites. If you must walk in vegetation or rocks where you cannot be sure what you’re stepping on, wear high-top shoes or boots that cover your feet and ankles. Of course, boots that cover some of your calf are better but most accidental snakebites (where the person is not intentionally interacting with the snake) are to the ankle and below.

Remember, you can’t save all the cute animals, eat all the tasty ones, kill all the ones that scare you, and have a functional ecosystem, too!

The 2016 rattlesnake season is underway

After some limited basking by a couple of telemetered rattlesnakes and short movements by Female 41 and Male 49 two weeks ago, the return of cool cloudy weather sent these animals underground again. Then, despite several days of sunny weather last week, the rattlesnakes remained cool and out of sight – with the exception of Male 49. While Female 41 stayed underground and cold, he napped most afternoons in the sun just a couple feet away.

But yesterday, everything changed. Males 37 and 38 who spent the winter near the top of the bluff had moved down; one was out of sight under a log and the other was in an ambush coil in the grass under a small fig tree.

CROR38, first sighting of the year; 29 February 2016 Original RAW IMG_0195.CR2 — My first look at Male 38 this year. He appeared to be hunting at the base of the bluff, just a few dozen yards from where he spent the winter on the hillside.

A rapidly pulsing transmitter told me before I arrived that my underweight geriatric Male 40 had survived another winter and was in the sun at his winter shelter.

CROR60 at RFG02, first 2016 sighting; 29 February 2016; Effie Yeaw Nature Center Original RAW IMG_0177.CR2 — My old friend, Male 40, made it through another winter. Over two seasons, he has moved very little and I have never found him courting a female. This is my first look at him this year. He had a body temperature of 88F when this photo was taken. Although he may have had more surface area exposed to the sun before I arrived, it takes very little exposed skin to dramatically raise their body temperature on a sunny windless day.

Non-telemetered Male 49 (foreground) and telemetered Female 41 (background) — Male 49 (foreground, no transmitter) and telemetered Female 41 (background). Female 41 was underground with a body temperature of 52F on Sunday but had a body temp of 86F when this photo was taken yesterday (24 hours later).

Female 41 was in the sun with Male 49 laying partly on top of her, although I saw no active courtship during my brief visit.

Other telemetered snakes had moved short distances but were out of sight when I was there yesterday. But in my travels, I also sighted five unmarked rattlesnakes basking, including a beautiful little female sporting a food bolus about the size of a vole – so she has already fed successfully.

So for the next three months or so, females will be trying to eat as much as they can to nourish their next brood while males will be wandering all over the place (and hunting less) in search of receptive females. It is during this time that male rattlesnakes tend to turn up in yards and on trails, producing interactions with people.

Remember that rattlesnakes want nothing to do with something the size of a person. Leave them alone and they will be happy to avoid you, too!

Early emergence has begun!

As sunny 70+ degree days have been warming the ground recently, body temperatures of the telemetered rattlesnakes have been creeping up… but only from the 50-60F range into the higher 60s. Then, yesterday, I found Female 41 laying in the grass next to the log where she has spent the past two winters. But just being visible doesn’t constitute emergence.

I returned late this morning and found Female 39 visible for the first time, basking in dappled sun under the end of the log where she has spent the past two winters.

IMG_0091; 16 February 2016 — This was all that was visible of Female 39 this morning but it takes only a small surface area in the sun to act as a radiator and warm the entire snake.

Then, not only did I find that Female 41 had moved about 20 feet from her winter shelter to a smaller nearby log, but I watched as Male 49, a large non-telemetered animal with white/green paint in his rattle, arrived at the same log and joined her. Although they coiled next to each other, I saw no actual courtship behavior during the 45 minutes I watched.

IMG_0117.CR2; 16 Feb 2016 — Male 49, just a few feet from joining Female 41 this morning. Without a transmitter, I have no way of determining his body temperature or knowing where he spent the winter. An encounter with him is purely chance but white/green paint in his rattle makes him easily recognizable.

Body temperatures of Males 37 and 38, both of which spent the winter high on the bluff north of the preserve, were around 90F at midday today. Because their winter locations are almost inaccessible, I don’t know if they have actually left their hibernacula but they were both definitely in the sun today. Remember that ground temperature is much higher in the sun than the air temperature, particularly on south-facing slopes.

As of today, the other five telemetered rattlesnakes remain relatively cool and out of sight and the weather forecast looks like cloudy skies, cooler temps, and more rain over the next few days.

The study animals did not begin to leave their winter shelters until the end of the first week in March last year. Although this is three weeks earlier than last year, it is certainly a limited emergence with many snakes remaining underground and inactive. But the fact remains that some rattlesnakes have left their winter shelters, so “rattlesnake season” is definitely underway.

Remember that being careful where you place your unprotected hands and feet and leaving snakes alone when you find them would prevent almost all rattlesnake bites!

A death mystery and rattlesnake season winding down

Male 37

Our wayward male shed on the morning of 7 October, five days after being recaptured. His faulty transmitter was surgically replaced that afternoon and he was released the following morning.

CROR37 recovering from anesthesia on 07 October 2015. Original Droid IMG_20151007_162411334.jpg — Male 37 recovering from anesthesia on a heated pad after his transmitter surgery on 7 October. He now measures 969 mm (just over 38″) total length, making him the largest rattlesnake I have measured at Effie Yeaw. Besides shedding four times in the past 16 months, his body mass has increased 26% (now 668 g or 1.5 lbs.), while his length increased by almost 5%. This is a very healthy rattlesnake!

CROR38 at release on 08 October 2015 Original RAW IMG_9094.CR2 — Male 37 on the morning of 8 October, disappearing into the hollow log where he was captured. Note that I marked his rattle again, since his original paint is only two segments away from being lost.

I took advantage of the necessity to allow Male 37 to shed before his surgery to collect some useful data. His pre-shed body mass was 693.5 g. By weighing him again and subtracting his post-shed mass of 668.4 g, we find that he lost 25.1 g (0.9 oz.), which was 3.6% of his body mass. Then, by drying his shed “skin” on a hot plate to drive off ambient moisture and subtracting that mass (5.1 g) from 25.1g, we can calculate that 80% of the mass he lost was water. Although these rattlesnakes are unaffected by California’s drought so far (more details here and here), that will eventually change and such data will be invaluable for calculating water loss and gain over time (water “flux”) and predicting when the snakes will become water stressed.

More Mortality

Another healthy postpartum female, CROR 47, was killed on the morning of 21 September. I may have been premature in assuming that a coyote killed Female 54 on 14 September (details). Female 47 was also very fresh when I found her, killed in the early morning in the meadow by something that did not eat her.

Death scene: CROR47 in meadow of Effie Yeaw Nature Preserve, morning of 21 September 2015 Original RAW IMG_8736.CR2 — Death scene: Female 47 in the meadow at Effie Yeaw Nature Preserve on the morning of 21 September. The posterior end of her abdomen was laying a few feet away. The edge of the bright red transmitter can be seen in the grass near the middle of the photo. Clearly, the carnivorous yellow jacket wasps found her quickly.

It was obvious that Female 47 had been pulled apart, with her body in two large pieces and internal organs and strips of skin pulled away. But she was all there. Maybe most interesting was the condition of her head: The top of her head was intact and unmarked but her lower jaw was mostly gone and the roof of her mouth was mangled, especially where the fangs had been (caution: graphic close-up).

In reconsidering the death of Female 54 a week before, I assumed that her missing head and neck had been eaten but it is entirely possible that I just didn’t find them. And I rationalized that a coyote dropped the snake as it probably fled (unseen) from me. But I see coyotes around the meadow frequently and, while they give people a wide birth, they are not panicked by our presence. If I interrupted a coyote with Female 54, why wouldn’t it have carried the rattlesnake away to finish the meal?

So what would encounter both rattlesnakes in the meadow early in the morning and pull them apart before leaving them uneaten? I have had coyotes kill Mohave rattlesnakes in southern California in the past but they eat the entire snake and chew the transmitters; these transmitters were undamaged. And what would remove the jaw and mangle the inside of the mouth?

My best idea is turkeys. Interestingly, I have found nothing in the scientific literature about wild turkeys killing adult rattlesnakes, although there are several anecdotal accounts in books, including Laurence Klauber’s Rattlesnakes (1972, Univ. California Press) of turkeys mobbing adult rattlers. Inquiries of my rattlesnake researcher colleagues has produced one witnessed account of a wild turkey killing and eating a two-foot timber rattlesnake (Crotalus horridus) in Minnesota. And there is a video on YouTube of two turkeys apparently killing what appears to be a large gopher snake (Pituophis) on a golf course.

If anyone reading this has first-hand information about wild turkeys interacting with rattlesnakes, I would love to hear about it.

2015 Season almost over?

Well, it’s the last week in October and our eight telemetered rattlesnakes appear to be mostly settled into their winter shelters, despite relatively warm weather. Last year, we still had three telemetered rattlesnakes roaming around well into November (see my “Rattlesnake Update” from 26 November 2014).

This year, one of our telemetered animals has been stationary since 16 September and others have stopped moving since 6 October, 12 October (2 animals), and potentially three more in just the past couple of days. Most interesting of all is that five of them are in the same shelters they used last year – and a sixth may be on his way. This is a departure from behavior observed during my field work in El Dorado County, where the rattlesnakes did not return to the same locations winter-after-winter, although some returned to previously used sites after spending a winter or two in other locations.

This year at Effie Yeaw, our skinny geriatric Male 40 went immobile on 16 September under the same log he shared with Males 35 and 38 and Female 39 last winter. Then Male 35 and Female 39 joined him, arriving on 6 October and 12 October, respectively – although their radio signals indicate that they are not together under the large log. Male 38 is still on the move, even visiting the residents along Edgehill Lane three days ago (26 October). I was unable to gain access to his location due to the necessary residents apparently being away and, when I returned yesterday, I found that he had returned to the hillside within the nature preserve. Male 37 spent a few days in the same area on Edgehill Lane in August 2014 and also returned to the preserve before I could gain access to him. These are the only two times (in two full seasons) that telemetered animals have entered the residential area on the bluff. When this occurs, I try to contact the property owners and would remove the snake, if that’s the property owner’s desire and the snake is accessible. In both cases, however, the rattlesnakes returned to the preserve before I could make contact with the necessary residents. But back to Male 38, he was on the hillside yesterday morning and may be headed back to last year’s log with the others.

Male 37 went directly to the hillside after I released him with a new transmitter on 8 October and I have not detected a change in the location of his radio signal since 12 October – which sounds like the same place he spent last winter (again, refer to my “Rattlesnake Update” from 26 November 2014). Once I’m sure he’s down for the winter, I’ll make the treacherous climb and verify his exact location.

Female 41 arrived on 20 October at the log where she spent last winter. She moved a few meters away over a couple of days but has been back for the past three days, so she may also be done for this season. As far as I could determine last winter, she was alone. Male 46 and Female 53, neither of which were telemetered last winter (i.e., I don’t know where they spent last winter), are very close together under another large log and have been there for at least three days.

It is interesting to note that the rattlesnakes that are already in their winter shelters are not basking, preferring to remain out of sight with body temperatures mostly in the 60–65F range. Earlier in the activity season, when cool nights are followed by warm morning sun, the rattlesnakes come out and warm up by exposing some or all of their skin area, depending on ambient temperature and the intensity of the solar radiation. But in ectotherms (animals that get their body heat from the environment) like rattlesnakes, body temperature varies with the environment and both metabolic rate and water flux increase with higher body temperature, burning more stored energy and using more water. So, on the verge of four or five months of hibernation, the rattlesnakes are likely programmed by evolution to cool off and slow their metabolism, thereby conserving energy and water for use in the spring.

Once I’m sure the snakes are done moving for the year, I’ll summarize the season’s findings in the context of last year’s data and our original questions and study goals.

19 April: Things heat up and more courtship

The little male courting Female 47 in the video I posted on 4 April continued to court her until at least 11 April, staying with her a total of 9 days that I observed. During that time, the female made several short moves of just a few meters and I never observed any indication that she did anything but ignore the male. Presuming that she was trying to hunt (these rattlesnakes are predominantly sit-and-wait ambush predators), it is hard to imagine that she could have had much success with his nearly constant movement. Despite my best efforts, the little male evaded several attempts to catch him. On two occasions, I actually had him but was unable to get him into a bag before he wiggled loose and disappeared into thick grass. On both occasions, I was sure I had spooked him and he would abandon the female but he didn’t. He only became more wary and would vanish instantly the moment he detected my approach. Since I have enough photographs to identify him in the future, I have given him the next identification number: CROR51.

Then on 13 April, I found that Female 47 had made a 97 m (318 feet) move out into the meadow. When I located her, she was under a mat of old dry grass beneath the living grass. I did not find Male 51 with her but, because I had to dig around in the grass to find her, he certainly had time to flee unobserved. She was in the same spot with no other snake observed on 15 April, too. On 17 April, she had moved 58 m southeast, back into the edge of the forest. She was coiled in the grass, apparently hunting and alone.

Today, 19 April, she had moved only a couple of meters. But as I got close to her signal, a rattlesnake shot out of the grass near my feet and under a nearby log – Male51! When I actually found Female 47, she was about 5 m from where I had flushed Male 51 and I saw why Male 51 was not with her. She was being vigorously courted by Male 49, a much larger male that was processed and released in early March without a transmitter. In fact, Male 49 had courted this female between 22–26 March at another location 150 m away. You can see the value of marking the rattles with paint in the photo from today (below), Female 47’s rattle is marked with yellow/green paint and Male 49 is marked with white/green. Since Male 49 is not telemetered, he would not be identifiable without the paint in his rattle.

Sadly, Female 39’s radio signal disappeared on 3 April. Of the six animals implanted with that batch of refurbished transmitters, five failed early. I have found and replaced transmitters in three of them, Males 35, 38 and 40. At present, Males 36 and 37 and Female 39 are carrying transmitters with prematurely dead batteries. I am still hoping to either find them courting or being courted by telemetered snakes or to have them turn up around the buildings or Maidu Village where staff can capture them.

Now that we have accumulated some data this spring with three females that are not incubating late-term embryos, I can demonstrate more effectively a behavior I mentioned last year. During the last 2–3 months of pregnancy (generally about mid-July to October), females find shelters where they can thermoregulate to stay within a narrow body temperature range. That is, they need a shelter that keeps them warm at night but where they are protected from the midday heat. I have repeated the chart from last year below on the left, showing the body temperatures of two late-term pregnant females compared to the males. The chart on the right is corresponding data from this spring, comparing body temperatures of non-gestating females with the males.

In these charts, “frequency” is the percentage of observations where I recorded each body temperature for that sex during that period. In 2014, the average male body temp was 24C (75F) while the female average was 30C (86F). This spring, males have averaged 24C (75F) and females 22C (72F). Of course, the weather is cooler in the spring (even this spring) than in summer but there is ample opportunity for these snakes to get their body temperatures up above 30C (by late morning, ground surface temperatures are often above 40C in direct sunlight, even when air temps are relatively cool). But my point is that there is no real difference between average male and female body temperatures before the pregnant females go into their late-term thermoregulatory behavior… which is evident in the late summer data above where body temps of the pregnant females narrowly cluster around 30C (86F).

(For you statisticians that may read this, I admit that these data are not publishable in this form as they contain some significant pseudoreplication. Nonetheless, they serve to illustrate my point that gestating vs non-gestating female body temps are very different.)

Hopefully, we will get a few more females telemetered in the next month or so and, by mid summer, we should know if any are going to reproduce this season.

Stay tuned!