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.

How are the rattlesnakes doing in the flood?

Lots of you have been asking me how the rattlesnakes are doing with all the high water coming down the American River from Folsom and Nimbus Dams.

Water level on 17 January, looking out at the spot where Female 53 delivered her young a few months ago. Water flow on this day was reported to be 30,000 cubic feet per second (cfs). Days earlier, it had been much deeper when flowing at 60,000 cfs!

Well, the telemetered rattlesnakes are all in the oak forest and well above the high water. You may recall that Females 53 and 75 both gave birth to their broods a few months ago out in the flood plain near the usual river channel. And since December, both of their birth sites have been covered periodically by lots of fast-moving water.

The good news is that these females returned to higher ground shortly after their kids completed their neonatal sheds and dispersed from their birth sites in the fall and both have been tucked into their usual winter shelters since cold weather set in. We know that, of course, because of the radio transmitter each carries in her abdomen.

The fate of their babies, on the other hand, is not known. As I have mentioned before, we cannot effectively radio-track the little ones because we do not have tiny transmitters that will last long enough to make surgeries worthwhile. In other words, it is not practical to surgically replace transmitters every few weeks, which would be necessary due to the tiny batteries such transmitters use.

One thing we do know about baby rattlesnakes, however, is that few of them survive their first six months of life. They are frequently encountered in September and October, shortly after birth, but they’re scarce by spring. We know they have many predators when they’re small but it is also not hard to imagine the little ones born in the riverbed remaining there for the winter and perishing in the flood. If so, that is just natural selection in action: those rattlesnakes, adult or baby, that fail to seek higher ground for the winter are less likely to survive and pass their genes on to future generations.

Keep in mind that, on average, if a pair of rattlesnakes – or any other kind of animal – produces more than replacements (i.e., kids that reach maturity and reproduce) for themselves in their lives, we would be swimming in rattlesnakes! Put another way, on average over time in a stable population, animals are just replacing themselves… which means that most offspring never live to adulthood.

If you have been reading my blog for very long, you will have read this before: Nature is a cruel mother! Most wild animals’ lives end in the jaws of another…or sometimes in a flood. That’s just the way it is.

On a brighter note, we started seeing some basking rattlesnakes on sunny days by the end of February last year. So we may well be within a few weeks of the kickoff of the 2017 rattlesnake season! (Remember, watch where you put your hands and feet once the weather turns warm and leave rattlesnakes alone when you encounter them and your chance of being bitten is very near zero!)

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.

IMG 4208
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!

 

 

 

 

 

More baby news

First, I have added a link on the main menu to a new video (UC Santa Cruz Video). Bethany Augliere and Brendan Bane from the UC Santa Cruz Science Communication Program visited the EYNC Rattlesnake Study last May and recently posted the resulting video, which satisfies one of the requirements for their graduate degree program. I hope you enjoy it!

Back to baby rattlesnakes

As of 22 September, all three telemetered reproductive females (39, 53 and 75) had left their birthing shelters. Two are clearly hunting and the third is just a few feet away, being courted by a male.

Female 39 produced a brood in the same hollow log for the third year in a row. Then, immediately following the kids’ neonatal sheds on 14–15 September, she made a long move to the same place in the blackberry thicket on the other side of San Lorenzo Way – also for the third year in a row. As I observed last year, she apparently knows where to find a reliable meal after the kids leave the house!

Female 75 abandoned the ground squirrel burrow she had been in for weeks between 17 and 19 September and moved to a blackberry thicket near the Duck Pond. Although I never observed babies in the burrow with her, the tunnel was deep and she was sometimes out of sight of my three-foot-long Burrow Camera. After she left, however, a single neonatal shed “skin” was visible in the burrow and I recovered it yesterday. Hopefully, DNA from it will confirm that Female 75 produced a litter and reveal who the father was. Since multiple paternity is common in rattlesnake broods, the DNA from this skin will not identify the paternity of any other siblings.

Recovered neonatal exuvium from ground squirrel burrow occupied for weeks by Female 75; 23 September 2016 Original DROID IMG_20160923_125147783.jpg
Recovered neonatal exuvium (shed corneal skin layer) from the ground squirrel burrow occupied for weeks by Female 75. Tying a tool designed to retrieve dropped screws to the Burrow Camera allowed the “skin” to be fished out of the burrow.

Female 53 left her shelter in the stream bed between 19 and 22 September, moving only about 4 meters to another shelter where she is accompanied by a non-telemetered male, CROR 72 (green/yellow paint in his rattle). Note the postpartum skin fold in the frame shot (below) and then watch the brief video of the two rattlesnakes together. (click here)

Postpartum Female 53. Note the long lateral fold of empty skin along her abdomen, typical after loosing 30-50% of her body mass during birth.
Postpartum Female 53. The long lateral fold of empty skin (yellow arrows) along her abdomen is typical after loosing 30-50% of her body mass during birth.

The underground void that Female 53 just left remains occupied by an unmarked female with babies. You can see fresh neonate skins on top of and stuck to this female and it is difficult to tell if the kids in the video have shed yet. I suspect the recently shed skins belong to Female 53’s litter which has already departed and the remaining babies belong to the unmarked female. If I am right, these babies will remain with mom for a few days more. However, if the fresh sheds belong to them, they and their mother will be gone the next time I visit. It is important to remember that maternal accompaniment of neonate rattlesnakes has only been known since radiotelemetry has been used to study these animals. Watch the video here.

So it appears that the 2016 birthing season is nearly complete. People around the American River Parkway and other places where rattlesnakes live will be encountering baby rattlesnakes with some frequency between now and the onset of cold weather. But, at the size of pencils, the little guys have many predators and few of them survive until spring.

 

Baby season update

Just a quick update on our reproductive females:

On Monday, 7 September, I found Female 53 missing from the hollow log where she had been gestating with Female 39, who was still present. This is the hollow log where I photographed a new baby on 6 September (see my last post) and I could glimpse youngsters deep inside the log again on the 7th. After considerable searching, I detected 53’s radio signal and followed it to a sycamore tree near the edge of the American River, 368 meters (402 yards) from where she had been. Since that day, she has moved a hundred meters or so back toward the oak woodland but has settled into a small cavity in the rocky riverbed.

This morning, 15 September, I found multiple shed “skins” from babies back at the hollow log and Female 39 was gone. I had checked the log yesterday and found no neonatal sheds.

Fresh neonatal shed "skin" (actually, the shed outer or "corneal" layer of the skin) in the log where Female 39 has been gestating since 14 June.
One of the neonatal shed “skins” (actually, just the outer or “corneal” layer of the skin) found this morning in the log where Female 39 has been gestating since 14 June.

 

Female 39’s radio signal led me to her 195 meters away, where she was coiled in dappled sun with lots of loose skin hanging on her. The babies shedding over the past 20 hours and the departure of Female 39 confirms that the kids were hers and were only a day or two old when discovered on 6 September (about 11 days between birth and the postpartum shed). Inspection of the inside of the log today with the BurrowCam revealed no rattlesnakes.

Then, when I checked Female 53 in her rocky riverbed hole this morning, she no longer appeared pregnant. In the BurrowCam video (link after the still photos below), look closely beyond her, just right of the center of the frame (next to the snail), beginning about 41 seconds into the clip. For the next 8 seconds, you can see a shiny wet baby moving behind her! I have circled the place to look in this still frame:

Female CROR 53 in streambed burrow Frame grab at 47 seconds into original BurrowCam video; 15 September 2016
The place to look for the shiny moving baby in the video is circled in this still frame grab.

Also, compare the appearance of her abdomen in today’s video to her 3 September photo (in my last post).

Female CROR 53 in streambed burrow Frame grab at 57 seconds into original BurrowCam video; 15 September 2016
Female CROR 53 in her streambed burrow at 52 seconds into the video clip. The violet sutures remaining from her May transmitter surgery are circled, indicating that we are looking at her posterior abdomen, which is no longer swollen.

Today’s 60-second BurrowCam video can be viewed on YouTube (click here).

Meanwhile, Female 75 remains in her burrow, still without kids, while Female 80 is still high on the bluff and inaccessible.

First baby rattlesnakes of the season at EYNC

We have kids!

Just a quick post to let you know that as of last Saturday, 3 September, Females 39, 53 and 75 were all still visibly pregnant.

Females 39 (red/blue) and 53 (blue/yellow) laying together in their gestation shelter on Saturday, 3 September, illuminated by sunluight reflected from a mirror. Both were still pregnant. They are laying on top of a recently shed skin from another large rattlesnake.
Females 39 (red/blue) and 53 (blue/yellow) laying together in their gestation shelter on Saturday, 3 September, illuminated by sunlight reflected from a mirror. Both were still pregnant. They are laying on top of a recently shed skin from another large rattlesnake.

 

But yesterday afternoon, 6 September, a newborn baby was coiled in the hollow log where 39 and 53 had been on Saturday. I could not see the adults well enough to tell which one had given birth. There were undoubtedly other kids that were not visible. Since the neonates start a shed (ecdysis) cycle almost immediately after birth, which turns their eyes bluish-white, this one’s clear eyes indicate he is not very old.

A newborn northern Pacific rattlesnake in the hollow log with females 39 and 53 on 6 September. His eyes are still clear, indicating he is no more than a day or two old.
A newborn northern Pacific rattlesnake in the hollow log with females 39 and 53 on 6 September. His eyes are still clear, indicating he is no more than a day or two old.

 

As of Saturday, I could not see neonates in the burrow with Female 75.

Since my last post, I also came across the first Fall courtship. Early on 29 August, I came across an unmarked male courting Female 66, who is not pregnant this year and has been hunting all summer. A couple hours later, the apparently happy pair were copulating! Remember, these rattlesnakes have a bimodal courtship season: they court in the Spring, lay low during the hot months, and resume courtship in late Summer/Fall.

Female 66 is almost hidden by an unmarked male courting her on 29 August 2016
Female 66 is almost hidden by an unmarked male on top of her on 29 August 2016. Note his tail wrapped around and under hers.
Female 66 copulating with an unmarked male on 29 August 2016
The same pair copulating two hours after the photo above was made. The marked and telemetered female has white paint in the top half of her rattle; the male’s rattle is not marked with paint.

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!

Pregnancy, growth and drought

We currently have transmitters implanted in six female rattlesnakes. We have been tracking Female 39 since 2014 and Females 41 and 53 since 2015 and all are currently pregnant and in their gestation shelters where they will likely thermoregulate for six weeks or more. Furthermore, they have all produced a brood during each season we have been following them, so this will be the third year in a row for Female 39 and the second in a row for 41 and 53. And we don’t know how many consecutive years they might have reproduced before that! Each has returned to the same gestation refuge each year, although 41 uses a different location than 39 and 53, who gestate together. We have also found pregnant unmarked females in both places in past seasons with these girls.

That’s not all. We have three new females this season, numbers 66, 75 and 80. These snakes do not appear to have settled into gestation shelters yet (and I don’t know where they were in previous years) but I just implanted a transmitter in Female 80 a few weeks ago and could feel six fetuses in her belly…it felt like she had swallowed six soft ping pong balls! I’m not sure about the reproductive status of 66 and 75, since their surgeries were earlier in the year and both had so much material in their intestines that it made identifying small embryos with confidence difficult.

Another opportunity to assess the health and growth of the rattlesnakes at Effie Yeaw Nature Center occurred day-before-yesterday (13 July) when Kelly came across an adult rattlesnake at the end of the Visitor Center building early in the morning. Per protocol, she expertly maneuvered it into the capture bag and deposited it in the holding barrel for me. It turned out to be Male 52, a rattlesnake previously captured, processed, and released without a transmitter early in May 2015. At that time, he was 30.7 inches in total length and weighed 11.6 ounces. He now measures 33.5 inches and weighs 18.9 ounces. While increasing 9% in length and 63% in mass in 14 months, he has shed three times (see photo below).

For those of you new to the blog, I inject acrylic paint into the first hollow segment of the rattle
For those of you new to the blog, I inject acrylic paint into the first hollow segment of the rattle, next to the black live segment at the end of the tail. With a different color combination for each snake, it allows me to visually identify them. As the snake grows and sheds the corneal layer of its skin periodically, it produces a new rattle segment with each shed. The segment with the paint is thus moved away from the tail until it eventually breaks off. When this seems imminent, as in Male 52 above, I inject paint into another segment to preserve the marking.

 

The constant growth of all the rattlesnakes being sampled and the annual reproduction of many of the females attests to the health of not just the rattlesnake population but the overall small animal community in the riparian habitat at Effie Yeaw Nature Center. While the region is undeniably in a severe long-term drought, enough local rainfall has occurred to keep the annual plants, shrubs, some trees, and the food web they support healthy.

Finally, I want to share with you a little bit about a presentation I made at the annual Joint Meeting of Ichthyologists and Herpetologists last week in New Orleans. I have reproduced the published abstract (summary) of my talk below. Please excuse the Latin names. Crotalus scutulatus is the scientific term for the Mohave rattlesnake and Crotalus oreganus is our own northern Pacific rattlesnake. While recent drought has not yet affected the rattlesnakes or their prey in the areas of northern California I have sampled, 2002 was a rainless year in the Mohave Desert, with no plant growth and a dramatic reduction in the availability of kangaroo rats and other small mammals that make up the great majority of the rattlesnakes’ diet there. During 2002, Mohave rattlesnakes changed their behavior very significantly, staying tightly coiled and avoiding wind and sun while moving very little and not courting or mating.

Cardwell abstract

The take-home message I delivered in New Orleans was that (1) these animals are used to hot dry summers and get most of their water from their prey; (2) regional drought does not necessarily equal local drought; (3) rattlesnake behavior is probably not affected by drought until prey availability is affected; (4) water-stressed rattlesnakes minimize exposed skin by remaining coiled most of the time; (5) when water-stressed, they don’t move more, they move a lot less than usual; and (6) there is no evidence that they leave their normal home range during a drought.

In other words, there is zero evidence to support the frequent news media claims that drought drives rattlesnakes into yards.

Emphasis now on hunting and shedding

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

Back to EYNC rattlesnakes –

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

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

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

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

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

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

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

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

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