The first of our four pregnant rattlesnakes has given birth since last Thursday (between September 8 and 12).
The animated GIF file above shows Female 54 in the foreground inside a hollow log and still pregnant (note the scales pulled apart on her abdomen) but with babies crawling over her on both sides. Female 39 is in the background and no longer pregnant. Since these are the only two females I have seen gestating in this hollow log this year, I believe the kids belong to Female 39. The video was recorded with our BurrowCam, actually a Ridgid Tools inspection camera that can be inserted up to three feet into burrows, logs and other narrow chambers.
As you know if you have been reading this blog for very long, mom and babies stay together for a week or a little longer, until the kids shed the corneal layer of their skin for the first time. After that, they go their separate ways to forage for food before hibernation. Thus, if my telemetered females are in sync with the larger population, baby rattlesnakes should begin appearing on the trails around the fourth week in September.
This is Female 39’s third brood in four years. Each time she has reproduced since I have been monitoring her, she has made a beeline to the same spot in the blackberry thicket on the other side of San Lorenzo Way as soon as the kids leave home. Apparently fall rodent hunting is very good there! We’ll see if she stays true to form this season.
Just a brief update to report that three or our six telemetered females (41, 39 and 53) have returned to the shelters where they have delivered broods in past years and all look quite heavy. Another, Female 66, is also stationary in a hollow log with a full abdomen but she did not reproduce last year and, since 2016 was our first year monitoring her, we don’t know where she has gestated in the past. Female 75, who did produce kids last year, is not heavy, still on the move, and looks like she will skip this year. Female 80 was on the hillside at the bluff but I have recently lost her radio signal. She was heavy and I could not palpate embryos when I replaced her transmitter in early June but she had lots of material in her GI tract which complicates the exam. Hopefully, her new transmitter has not failed…
Our females continue to often produce offspring during consecutive years, sometimes three years in a row before taking a year off. As mentioned before, this is not the norm for temperate-latitude pitvipers. While annual reproduction is common in the tropics where it never gets cold, the shorter warm season this far from the equator usually means that females need a year or two between broods to replace body fat before they can reproduce again. Conditions for our rattlesnakes are obviously quite good (even in past “drought” years), allowing them to replenish their weight quickly after losing 30%–50% of their body mass when they give birth.
Finally, I thought you’d find this photo interesting. Our big beautiful Male 37 was just in for his transmitter change two days ago and had obviously consumed a ground squirrel just before I captured him.
I’ve been getting this question a lot lately. And my response is, “It depends on where you are.”
Actually, it depends on the local weather in recent years where the question is being asked.
Many of the female rattlesnakes in the Effie Yeaw Nature Center preserve have been reproducing annually since at least 2014, including the drought years prior to this extraordinarily wet winter and spring. And, as I have written here before, that’s unusual – at least according to similar studies of other species of North American rattlesnakes. This far from the equator, most pitvipers only produce a brood every two or three years because of the time it takes females to replenish sufficient fat stores to support another pregnancy (they lose 30-50% of their body weight with each litter). But along the American River Parkway during 2014, 2015, and 2016, there was enough local rain each winter to get the annual plants growing and everything blooming, providing plenty of food for insects, squirrels, voles, and other small animals. The insects fed the abundant lizards that reproduced like crazy each year, and the rattlesnakes fed on the small mammals and lizards that were plentiful. So rattlesnakes and other mesopredators (mid-level predators in the food chain; i.e. they are eaten by bigger predators) have been thriving locally, despite the historically low reservoir levels and skimpy snowpack in recent years. And because rattlesnakes produce only one brood per year, they can’t do better than they’ve been doing.
On the other hand, when local drought reduces the availability of the rattlesnakes’ prey (i.e., reduced local rainfall significantly suppresses plant growth, which negatively affects everything else up the food chain), that changes the snakes’ behavior. Among other things, movement and courtship slows significantly and reproductive rates are almost certainly reduced.
So, in an area like the American River Parkway, this wet spring is not likely to increase the rattlesnake population because they can’t reproduce much better than before all the rain. However, in areas where reduced rainfall in recent years significantly impaired floral growth , rattlesnakes will be better fed and more active this year and they are more likely to successfully produce young.
But it is important to understand that what people interpret as “abundance” is usually just a change in behavior. I studied this phenomenon in Mohave rattlesnakes in the southern California desert for my MS thesis. In dry conditions, predators get most of their water from the body water of their prey (we’re all about 70% water). So, when rodents are abundant, rattlesnakes are well hydrated, behave normally, and males search constantly for females during spring and fall. But when their prey becomes scarce and other water sources are not available, they reduce water loss by remaining coiled and moving less, thus reducing the amount of skin exposed to the dry air. This is important because cutaneous (through the skin) evaporation accounts for about 75% of the snakes’ daily water loss.
As a result, during local drought, people encounter rattlesnakes less frequently – not because there are fewer rattlesnakes but because they remain hidden and move around a lot less (top photo, below). During non-drought times, people see more rattlesnakes because their behavior is not inhibited and they crawl around a lot more (bottom photo, below), especially the males during their courtship season.
So, if you live in an area like the American River Parkway, where there has been lush growth of grasses and other annual and perennial plants in recent years, this wet year should make little difference in the apparent abundance of rattlesnakes.
But if your neighborhood has been dry with little springtime plant growth in the last few years, you may see an increase in rattlesnakes this year for two reasons: the males may be searching for females much more than before, plus there may be more baby rattlesnakes in September and October this year (and likely next) as a result of females having plenty of rodents to eat this year.
March weather had the rattlesnakes along the American River Parkway confused! As I have mentioned before, the rattlesnakes usually emerge and begin to bask when the average temperature at ground level remains at or above about 50F. They are usually deep enough underground during the winter to escape daily high and low fluctuations, so it is the average that eventually penetrates to them and signals that winter is likely over. But not this year.
Most telemetered rattlesnakes emerged in March but remained at their hibernation sites, basking on sunny days as they usually do before leaving their winter shelters. But rather than getting drier and warmer, the weather throughout April was a constant mixture of rain and cloudy cool days and nights. Of the few rattlesnakes that left their winter hibernacula, two returned while others became inactive at other sites. The rattlesnakes do just fine in the rain. In fact, they drink rain and dew when it’s available. And cloudy weather is not a deterrent to activity if it is accompanied by warm temperatures. But nighttime air temperatures below 50F followed by overcast cool days are not good for activity in animals like reptiles that depend on the environment for their body heat.
So, in late March and throughout April, the few stretches of several sunny days with relatively warm nights in between produced some sporadic rattlesnake activity. But it was not until the end of April that more consistent sun and heat produced vigorous widespread rattlesnake activity.
As expected, when we had activity, we observed lots of courtship. This is, after all, the peak of courtship activity, followed by almost no reproductive activity from June to mid-August before an untick in courtship again in the fall.
But by far the most interesting discovery this spring was by one of my UC California Naturalist students who found a small male rattlesnake with no rattle! (See photos below) Now I hesitate to mention this, for fear of starting a panic about rattleless rattlesnakes. Please remember several things:
Rattlesnakes missing their entire rattle are extraordinarily rare. So rare, in fact, that they usually rate a note in a biological journal. This one will!
No rattlesnake has a long pointed tail like our many harmless snakes.
Rattles can be missing due to either injury or genetic mutation.
This little snake appeared to be recently missing the end of his tail and had other serious but healing injuries. Apparently a close call with a predator!
Identify rattlesnakes by looking at the tail! Yes, rattlesnakes have elliptical pupils and heat-sensitive facial pits but some harmless snakes have elliptical pupils and both these characters are too small to be clearly seen from a safe distance (two-times the length of the snake). Some harmless snakes also have rather triangular heads, especially when they have been frightened and are behaving defensively. But no rattlesnake has a long pointed tail like our harmless snakes and all of California’s dangerous snakes are rattlesnakes.
Check out the following images…
To be sure, there are dangerous snakes in other parts of the United States that have long tails without rattles (cottonmouths, copperheads and coral snakes, in southern and eastern states). But in California and other northwestern states, the only dangerous snakes are rattlesnakes.
Look at the tail to identify rattlesnakes!
And, yes, Male 87 was processed and released, just like all the others. While he has no paint in his rattle, he won’t be hard to recognize!
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.
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.
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.
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).
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.
I can then compare rattlesnake behavior, body temperatures and air temperatures. This is especially interesting when one year is compared to another.
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:
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.
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.
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.
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.
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!)
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.
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.
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!
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.
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)
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.
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.
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:
Also, compare the appearance of her abdomen in today’s video to her 3 September photo (in my last post).
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.