Spring emergence has begun!

It took only a few days of moderately warmer weather to stir our sleeping rattlesnakes. I haven’t downloaded my temperature logger recently but it has been easy to notice that it’s in the 50s instead of the 30s the past few mornings, while daytime highs have been in the high 60s.

And today, we found some rattlesnakes stirring. Until now, the body temps of all of the hibernating rattlesnakes have been in the single digits Celsius (less than 50F). Earlier today, ten of twelve telemetered rattlesnakes were in double digits despite a full thick overcast. Male 37 – still inaccessible on the bluff – produced a calculated body temperature of 21C (70F). (The bluff is always warmer because it is steep and faces south, so solar energy is more concentrated per unit of surface area.) Male 37 is clearly no longer deep underground.

Next was Male 46 at 16C (61F) and, sure enough, he was peering out from under the log where he spent the winter.

Male 46 around midday today, 10 March 2017, peering out from under his log.

When we arrived at Female 41’s radio signal, we found a rattlesnake laying out in the grass and assumed it was her. The rattle was not clearly visible at the time but later examination of photographs disclosed white/green paint in the rattle. Female 41 is white/blue and the rattlesnake we observed turned out to be Male 49, marked and released without a transmitter exactly two years ago today, on 10 March 2015! Female 41 was very close but out of sight.

Close examination of photographs proved this snake to be Male 49 (white/green), not Female 41.
Male 49, as we first observed him; 10 March 2017.

We also found Male 40 laying out in the same spot as last week (see the blog post on 6 March), plus we found a new animal laying in the grass next to another large log where two telemetered adults have been napping since last fall. He (or she) was captured and, while too small for a transmitter, will be measured, marked, and will donate a blood sample before being released as CROR81.

More news to follow!

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.

Just when you think you know what to expect…

Well, we are entering that time of the season when we can expect to find baby rattlesnakes any day now. Of course, the youngsters will not be roaming around where they might encounter people for about 10-14 days after birth…so we usually don’t encounter them on trails and in yards until early September.

But our six telemetered females are displaying a variety of behaviors indicating that all may not be about to produce babies. Six weeks ago I reported that Females 39, 41, and 53 were all pregnant and in the gestation shelters they have used repeatedly to thermoregulate before giving birth in previous years. Furthermore, I could feel six fetuses in Female 80’s belly when I implanted her transmitter in June.

Since that mid-July report, Females 39 and 53 have behaved predictably for expecting moms, remaining in their gestation shelter, and both appear to be about to produce kids. In the photos below, note the abdominal girth of these females, with scales pulled widely apart.

Female 39 in her gestation shelter on 20 August 2016 - the same shelter she has used to incubate her kids for three years in a row. Her body temperature at the time of the photo was 32C (90F).
Female 39 in her gestation shelter on 20 August 2016 – the same shelter she has used to incubate her kids for three years in a row. Her body temperature at the time of the photo was 32C (90F).
Female 53 in a different part of the same hollow log as Female 39 on 20 August. Femape 53's body temp was 30C (86F).
Female 53 in a different part of the same hollow log as Female 39 on 20 August. Female 53’s body temp was 30C (86F).

This will be the third consecutive year that Female 39 has produced offspring and the second for Female 53, so far as we know. We were not monitoring them previously. Between the first of July and yesterday, 23 August, the average body temps for Females 39 and 53 have been 28C (82F) and 29C (84F), respectively.

Six weeks ago, Female 41 had just returned to the gestation shelter she used the previous two years, which led me to believe she was likely pregnant again. However, after staying only three weeks, she left and has been spending her days mostly out of sight in various ground squirrel burrows during August. I have not been able to get a good look at her in recent weeks but her behavior and average body temperature of 26C (79F) suggest that she may not reproduce this year. Although Female 41 produced kids in both 2014 and 2015, I want to remind readers that skipping a year or two between broods is far more common than annual reproduction for temperate-latitude pitvipers.

Our remaining three telemetered females are new to the study this year, so I have no data from previous years. Female 66 has behaved quite normally for a non-reproductive year: hunting continuously throughout the spring and summer. Her July-August average body temp has been 22C (72F).

Female 66 in a typical ambush coil in late May. She continues to move around and hunt during July and August.
Female 66 in a typical ambush coil in late May. She continues to move around and hunt during July and August.

Female 75, however, appears ready to produce babies, although she has been moving back and forth frequently between two ground squirrel burrows 17 meters (56 feet) apart. Her average body temperature during July and August has been 27C (81F).

Female 75 on 23 August, illuminated by sunlight reflected from a mirrow, a few inches inside one of the two ground squirrel burrows she has been occupying in recent weeks. Again, note her distended abdomen. Body temp was 27C (81F) at the time of the photo.
Female 75 on 23 August, illuminated by sunlight reflected from a mirror. She is a few inches inside one of the two ground squirrel burrows she has been occupying in recent weeks. Again, note the distended abdomen with scales pulled apart. Body temp was 27C (81F) at the time of the photo.

Finally, there is Female 80. When I captured her and implanted her transmitter in June, I could clearly feel six fetuses lined up in her abdomen. Shortly after I released her at her capture site near the base of the bluff, she climbed up the hillside and has remained near the top ever since. On three occasions, I have climbed that steep, loose, and treacherous slope but have failed to locate her. Her radio signal seems to emanate from thick ornamental ivy growing down from a residential backyard under fig and valley oak trees. She is also just above an underground wasp nest. Just to be clear, I am far more comfortable with rattlesnakes than a nest of wasps – especially where I cannot easily run away! Since I have no previous data from her, she could be in her usual gestation shelter. However, her July-August body temperatures are not consistent with a gestating female, with an average of 23C (73F). I’m not sure how this is going to play out…

Just for comparison, the seven telemetered males have been moving around a lot, hunting and occasionally hanging out with the pregnant girls for a day or two at a time. Their average July-August body temps range from 17C (63F) to 26C (79F), with the average between the boys of 23C (73F). Note how closely these data match non-reproductive Female 66’s average body temp of 22C (72F) during the same period.

A big impressive rattlesnake at over 38 inches and about 1.5 pounds (last October), Male 37 ignored us yesterday as he crossed the trail in front of me and an EYNC visitor with whom I was chatting. The visitor was quite impressed and I never tire of watching these amazing and interesting animals with such an undeserved reputation for aggressiveness!
A big impressive rattlesnake at over 38 inches and about 1.5 pounds (last October), Male 37 ignored us yesterday as he crossed the trail in front of me and an EYNC visitor with whom I was chatting. The visitor was quite impressed and I never tire of watching these amazing and interesting animals with such an undeserved reputation for aggressiveness!

That’s it for now. Next post will almost certainly be (cute!?!?) baby pics!

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.
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.


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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.CROR66 on 17 March 2016

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!

2015 Wrap-up

At the conclusion of my last post (29 October), I mentioned that it appeared that all of the rattlesnakes had settled into their winter shelters and, indeed, two days later, they had done so. Of the six telemetered rattlesnakes for which I have refuge locations from last winter, five of them returned to the same refuge this winter. The exception is Male 38, who spent last winter under a log in the flat floodplain but this winter is high on the hillside near the northern preserve boundary. On 31 October, I climbed the bluff and found Male 38’s location just a few meters away from Male 37, who had returned to his 2014/2015 location. Both animals’ radio signals were originating from under thick vegetation covered by wild grape vines high on the steep hillside. Both are undoubtedly underground, likely in ground squirrel burrows.

Of the eight telemetered rattlesnakes this winter, three are hibernating together in one refuge, two are together in another, and three are hibernating alone in different locations (including Males 37 and 38). Of course, there may be non-telemetered rattlesnakes with any or all of them but that’s hard to determine at this stage. I will do some burrow camera work soon but that is often inconclusive, as their shelters are often too deep or have too many sharp turns for the camera. Even if I reach them, I cannot visually identify the snakes without looking for paint in their rattles, which are usually covered by their coils. In warm weather I often poke them with the camera to get them to expose their rattles but I don’t want to disturb them when they are cold.

So how cold are they? The table below shows the ground surface temperature in sun and shade, as well as the coldest, warmest, and average body temperatures of the telemetered rattlesnakes over the last five weeks of 2015. I will continue to occasionally collect temperature data until spring emergence. That will be foretold by the snakes beginning to bask in the morning sun, probably sometime in March. Even if I don’t catch them basking, they won’t be able to hide their elevated body temperatures; then we’ll know they’re about to begin their 2016 season.

Brumation temp tbl 2015
Brumation vs. hibernation

Several people have asked me about the term “brumation” and the difference between brumation and hibernation. According to Harvey Lillywhite’s Dictionary of Herpetology (2008, Krieger Publishing, Malabar, FL), brumation is:

“A condition of torpor during extended periods of low temperature (winter dormancy), intended to distinguish such states of inactivity of amphibians and reptiles from the term ‘hibernation’ that is used commonly in reference to birds and mammals. Term coined by W. Mayhew (Comp. Biochem. Physiol. 16:103–119, 1965)”

When used regarding endotherms (animals that regulate their body temperature internally, like birds and mammals), “hibernation” generally describes a state of deep sleep in which an animal lowers its body temperature and metabolism to conserve stored nutrients during harsh winters, such as many bears and bats. In reality, ectotherms like rattlesnakes do essentially the same thing with two exceptions: they remain responsive and do not “sleep” although they become very sluggish as they get cold and, after all, it is not a big physiological change for ectotherms because their body temperature varies with the environment year-round. As a result, many herpetologists ignore brumation and use hibernation to describe winter inactivity in reptiles and amphibians, too (e.g., F. Harvey Pough, et al. 2016. Herpetology, 4th ed., Sinauer Associates, Sunderland, MA).

Best wishes for a safe and Happy New Year!


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.


Missing males and waiting for kids

As birthing season approaches, I have been watching intently for signs of baby rattlesnakes. While postpartum mothers usually stay inside their shelters, the neonates are typically active and easily spotted. Although they do not leave their shelters before shedding the first time, babies can usually be seen crawling or basking at the entrance. And when we introduce the BurrowCam into the shelter, the kids can be seen exploring their new surroundings and crawling around on their mother. So far, there’s been no evidence of babies yet this year. This would be a bit early but quick warming last spring has me wondering about the potential for early births this year.

Remember that pregnant female rattlesnakes in our area hangout in carefully selected thermal shelters where they can maintain consistently warm body temperatures around the clock until they give birth. This period of thermoregulation lasts several months, during which the pregnant moms do not forage for food.

All five of our telemetered females are apparently pregnant, plus Female 55, who was processed and released without a transmitter in June (no transmitter surgery due to some old but significant trauma to her abdomen; click here for details). These females settled into their gestation shelters between 8 June and 1 July and have maintained body temperatures between 28ºC and 32ºC (82º–90ºF), almost without exception, ever since.

In fact, at dawn on a recent cool morning (16 August) when the ground temperature just before sunrise was in the mid-50s F, these girls still had body temps in the high 80s F. They maintain similar body temps during hot afternoons when the ground temperature (much hotter in the sun than air temp) outside is 120ºF and more. They thermoregulate like this by selecting logs or large rocks that have just the right thickness and sun exposure to stay warm at night but not get too hot in the afternoon sun. Such places are apparently scarce because three of our telemetered females are together in one shelter, while Female 41 is with non-telemetered Female 55 in another. Female 54 is in a third location, possibly by herself, but there could be others in there without radios. Females 39 and 41, both of whom produced broods in 2014, are in the same shelters as last year.

Flash photo of Female 41, tucked into her gestation refuge at sunrise on 16 August 2015. Note in the inset how her scales are pulled apart by her developing brood. Female 55 is also in this shelter but not visible on this morning.
Flash photo of Female 41, tucked into her gestation refuge at sunrise on 16 August 2015. Note in the inset how her scales are pulled apart by her developing brood. Female 55 is also in this shelter but not visible on this morning.

On 27 August, the BurrowCam revealed Female 39’s abdomen to be greatly distended, extending all the way to the cloaca. So maybe delivery of her 2015 brood is not far off? The frame grab (below) from the BurrowCam video shows her abdominal scales pulled far apart. In the 50-second video (watch here), you’ll see what I see when we thread the BurrowCam into a passage. Female 39 is identified by red/blue (red-over-blue) paint in her rattle and the edge of another dark gray rattlesnake appears to be visible under 39’s coils. Known to be behind her in the passage (because of their radio signals) are Females 47 and 53, as well as Male 46. Additionally, in recent days, I have seen non-telemetered (and non-pregnant) Female 48 (green/green) and Male 36 (red/red; carrying a failed transmitter) in this log. It’s a popular place this time of year!

Frame grab from a 27 August BurrowCam video of the distended abdomen of telemetered Female 39, deep in her gestation refuge.
Frame grab from a 27 August BurrowCam video showing the distended abdomen of telemetered Female 39, deep in her gestation refuge.

As you may recall, Males 36 and 37 have been missing for months since their transmitters failed prematurely in September and December, respectively. Until last week, Male 36 had been last seen on the BurrowCam in a hollow log courting postpartum Female 41 on 2 October 2014, and I last saw Male 37 as his tail disappeared down a hole on 7 March 2015. There had been no sign of either of them since until a fellow photographer and herpetologist I encounter frequently at Effie Yeaw showed me a photo of Male 37 (IDed by his yellow/red rattle marking) crossing a trail on 20 August! Then, just 5 days later, while checking for babies in the shelter with Females 39, 47 and 54, and Male 46, I was surprised to see Male 36’s red/red rattle. (See photos below) So both are alive and well… but both still elude recapture.

Male36 (red/red paint in rattle) deep inside a hollow log with Female 41 on 02 October 2014.
Male36 (red/red paint in rattle) deep inside a hollow log with Female 41 on 02 October 2014.


Male 36 inside another hollow log on 25 August 2015 with at least three pregnant females and a smaller male. Compared to the 2 October photo (above), note that he has two additional rattle segments between the paint and the live black segment, indicating he has shed twice in the past ten months.
Male 36 inside another hollow log on 25 August 2015 with at least three pregnant females and a smaller male. Compared to the 2 October photo (above), note that he has two additional rattle segments between the paint and the live black segment, indicating he has shed twice in the past ten months.

Earlier today, 29 August, I found Male 46 coiled in poison oak dozens of meters away from the log where he has been hanging out with the three pregnant girls continuously for the past two weeks. It is likely he has been chased off by a larger male, so maybe Male 36 is still in there. This refuge has a narrow deep passage that is nearly impossible to thread the BurrowCam into and, even when successful, I can usually only see whichever rattlesnake is closest to the top (for example, the 50-second video of Female 39, with the link earlier in this post).

So Baby Watch continues and I still hope to recapture missing Males 36 and 37.

Pregnant females, injuries, and shedding

First a quick general update: Spring courtship seems to be over; I have not seen a courting pair since 16 May. Since the end of May, the pregnant females have taken up refuge in ideal shelters where they can thermoregulate optimally. Females 39 and 41 are now in the same shelters where they gave birth last year (but not together) and Female 47 is with 39. Female 54 is by herself and has not moved since we implanted a transmitter and released her on 23 May. Neither 47 nor 54 were telemetered last year so I have no history for them. These soon-to-be mothers are all maintaining body temperatures within a couple of degrees of 30C (86F). The males and Female 53 (not pregnant?) have been hunting, mostly hanging around California ground squirrel burrows for the past month as the squirrels produce the first pups of the season (more on hunting ground squirrels) and the body temperatures of these foraging snakes has varied widely compared to the pregnant females (more on body temps).

In my last post, I showed you a photo of an unidentified rattlesnake in the refuge with Female 41 – the same refuge where Females 41 and 43 had babies last year. (You may remember that Female 43 was found dead at the refuge last October; click here for that account) While I could only see the new snake’s nose and a small area of flank at the first encounter, I saw her twice more over the next eight days. She was shades of dark brown, while Female 41 is quite pretty with chocolate brown dorsal blotches on a gray background. During the subsequent two sightings, I could also see the new animal’s rattle, which was long and unbroken (i.e., she still had her birth button). Then a week ago, I found Female 41 and the new rattlesnake basking next to each other and was able to capture the new animal (CROR55).

The first thing I noticed was that she was pre-shed. That is, her eyes and new rattle segment were milky white (more about shedding below). The next important discovery was that she is, indeed, a female – and quite heavy…maybe pregnant. A photo of her snout (bottom photo, below), when compared to the nose in the photos of the unidentified rattlesnake on 3 June (top photo, below) confirms that she is the same animal.

unidentified Crotalus oreganus under log at Refuge 03 on 03 June 2015, Effie Yeaw Nature Center Origonal RAW IMG_7382.CR2

I have numbered some landmark scales in these photos that you can compare but also compare the size and arrangement of surrounding unnumbered scales. And while the fine pigmentation of the individual scales is obscured in the pre-shed photo, I have circled some larger pigmented areas that are visible. Keep in mind that the photos were taken from slightly different angles, making some scales that are visible in one hard or impossible to see in the other. The size, number, and arrangement of nose and crown scales on these rattlesnakes are a bit like fingerprints on primates: they are individually unique, so far as we know. Also note the whitish eyes and how the scales on her nose appear a bit swollen in the pre-shed photo.

As I examined her further, I made another interesting discovery: she has sustained a serious injury to her abdomen sometime in the past. Although well healed now, her skin is scarred on the dorsal midline 575 mm (23 in) from her nose (her body length, excluding tail [snout-vent length or SVL] is 720 mm [28 in]). Furthermore, her body is noticeably narrowed at the scar (photo below) and her abdomen is hard and dense to the touch for several inches on both sides of the scar.

Female CROR 55 Original RAW IMG_7555.CR2

Nonetheless, she looks and acts healthy and might, indeed, be pregnant. I could feel two masses in her anterior abdomen that were consistent with fetuses but could not differentiate anything posteriorly where her abdomen is apparently scarred internally. She would normally be a great transmitter candidate but I elected to release her without one because of the suspected internal scarring where the transmitter would be implanted, plus I did not want to damage her skin as she prepares to shed.

This brings up the point that life is not easy for these snakes. In addition to this healed injury to Female 55 and the death of Female 43 last year, you may remember that I processed and released a small male (CROR44) early last December that had recently sustained some significant trauma from a predator, including a deep penetrating abdominal wound that I suspected would prove fatal over the winter (more details). While processing Male 52 early last month, I removed a “foxtail” (a seed from one of the non-native Bromus grasses that blanket the preserve) from his cloaca (cloaca defined). This little floral harpoon had not yet caused much damage but I don’t know what would have prevented it from burrowing into his abdomen and causing a potentially fatal injury. My point is that these rattlesnakes, despite their formidable reputation, are susceptible to constant hazards.

Shedding (the technical term is ecdysis) is the sloughing or molting of the outer epidermal layer (the stratum corneum) in scaled reptiles. This corneal layer is a matrix of keratin (the same material as your hair and fingernails – and the rattlesnake’s rattle!) infused with lipid (fat) molecules that greatly slows the passage of water through the skin. Because this matrix is acellular (contains no cells), it cannot grow. Thus, as the snake grows, this layer must be replaced periodically. When the time comes, the snake’s body produces a new corneal layer under the old one. This creates the blue or whitish tint, most notable in the eyes. In rattlesnakes, a new segment is produced at the base of the rattle during each shed, which is also whitish at this stage. Once the new corneal layer is ready, the snake’s body secretes fluid between the old and new layers, separating them and softening the old one. When this fluid is secreted, the whitish color disappears (the eyes clear) and the snake is ready to shed. They then rub their face on any available surface and start to peel back the old layer from around the nose and mouth (photo below). They continue rubbing, eventually crawling out of the old “skin,” leaving it inside-out, usually in one piece.

A 10-day-old Northern Pacific Rattlesnake beginning his post-partum shed while being processed during my El Dorado Hills field study.
A 10-day-old Northern Pacific Rattlesnake beginning her post-partum shed while being processed during my El Dorado Hills field study. (Also note the “birth button” at the end of her tail)

I’ll leave it there until next time, when I’ll explain rattle growth and trying to estimate age from the rattle.