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!
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.
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!
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).
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.
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.
While the rattlesnakes continued to bask on sunny days, none of the telemetered animals moved between 29 February and 17 March. Then today, I discovered that 7 of the 9 telemetered rattlesnakes had left their shelters and moved significant distances in the past two days.
I haven’t seen courtship yet. Female 41 and Male 49 hung out together for a month (16 February through 15 March) but they were only rarely touching one another and were usually coiled a couple of feet apart with no courtship observed. She is now hunting several dozen yards away and Male 49, without a transmitter, has disappeared.
The important message, however, is that rattlesnakes are on the move and encounters between people and rattlesnakes will increase immediately. Unfortunately, I have already seen photos of a rattlesnake bite from a few days ago in the foothills. Watching carefully where you put your hands and unprotected feet and leaving snakes alone when you encounter them would prevent almost all bites. If you must walk in vegetation or rocks where you cannot be sure what you’re stepping on, wear high-top shoes or boots that cover your feet and ankles. Of course, boots that cover some of your calf are better but most accidental snakebites (where the person is not intentionally interacting with the snake) are to the ankle and below.
Remember, you can’t save all the cute animals, eat all the tasty ones, kill all the ones that scare you, and have a functional ecosystem, too!
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.
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.
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!
As sunny 70+ degree days have been warming the ground recently, body temperatures of the telemetered rattlesnakes have been creeping up… but only from the 50-60F range into the higher 60s. Then, yesterday, I found Female 41 laying in the grass next to the log where she has spent the past two winters. But just being visible doesn’t constitute emergence.
I returned late this morning and found Female 39 visible for the first time, basking in dappled sun under the end of the log where she has spent the past two winters.
Then, not only did I find that Female 41 had moved about 20 feet from her winter shelter to a smaller nearby log, but I watched as Male 49, a large non-telemetered animal with white/green paint in his rattle, arrived at the same log and joined her. Although they coiled next to each other, I saw no actual courtship behavior during the 45 minutes I watched.
Body temperatures of Males 37 and 38, both of which spent the winter high on the bluff north of the preserve, were around 90F at midday today. Because their winter locations are almost inaccessible, I don’t know if they have actually left their hibernacula but they were both definitely in the sun today. Remember that ground temperature is much higher in the sun than the air temperature, particularly on south-facing slopes.
As of today, the other five telemetered rattlesnakes remain relatively cool and out of sight and the weather forecast looks like cloudy skies, cooler temps, and more rain over the next few days.
The study animals did not begin to leave their winter shelters until the end of the first week in March last year. Although this is three weeks earlier than last year, it is certainly a limited emergence with many snakes remaining underground and inactive. But the fact remains that some rattlesnakes have left their winter shelters, so “rattlesnake season” is definitely underway.
Remember that being careful where you place your unprotected hands and feet and leaving snakes alone when you find them would prevent almost all rattlesnake bites!
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.
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.
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.
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.
The little male courting Female 47 in the video I posted on 4 April continued to court her until at least 11 April, staying with her a total of 9 days that I observed. During that time, the female made several short moves of just a few meters and I never observed any indication that she did anything but ignore the male. Presuming that she was trying to hunt (these rattlesnakes are predominantly sit-and-wait ambush predators), it is hard to imagine that she could have had much success with his nearly constant movement. Despite my best efforts, the little male evaded several attempts to catch him. On two occasions, I actually had him but was unable to get him into a bag before he wiggled loose and disappeared into thick grass. On both occasions, I was sure I had spooked him and he would abandon the female but he didn’t. He only became more wary and would vanish instantly the moment he detected my approach. Since I have enough photographs to identify him in the future, I have given him the next identification number: CROR51.
Then on 13 April, I found that Female 47 had made a 97 m (318 feet) move out into the meadow. When I located her, she was under a mat of old dry grass beneath the living grass. I did not find Male 51 with her but, because I had to dig around in the grass to find her, he certainly had time to flee unobserved. She was in the same spot with no other snake observed on 15 April, too. On 17 April, she had moved 58 m southeast, back into the edge of the forest. She was coiled in the grass, apparently hunting and alone.
Today, 19 April, she had moved only a couple of meters. But as I got close to her signal, a rattlesnake shot out of the grass near my feet and under a nearby log – Male51! When I actually found Female 47, she was about 5 m from where I had flushed Male 51 and I saw why Male 51 was not with her. She was being vigorously courted by Male 49, a much larger male that was processed and released in early March without a transmitter. In fact, Male 49 had courted this female between 22–26 March at another location 150 m away. You can see the value of marking the rattles with paint in the photo from today (below), Female 47’s rattle is marked with yellow/green paint and Male 49 is marked with white/green. Since Male 49 is not telemetered, he would not be identifiable without the paint in his rattle.
Sadly, Female 39’s radio signal disappeared on 3 April. Of the six animals implanted with that batch of refurbished transmitters, five failed early. I have found and replaced transmitters in three of them, Males 35, 38 and 40. At present, Males 36 and 37 and Female 39 are carrying transmitters with prematurely dead batteries. I am still hoping to either find them courting or being courted by telemetered snakes or to have them turn up around the buildings or Maidu Village where staff can capture them.
Now that we have accumulated some data this spring with three females that are not incubating late-term embryos, I can demonstrate more effectively a behavior I mentioned last year. During the last 2–3 months of pregnancy (generally about mid-July to October), females find shelters where they can thermoregulate to stay within a narrow body temperature range. That is, they need a shelter that keeps them warm at night but where they are protected from the midday heat. I have repeated the chart from last year below on the left, showing the body temperatures of two late-term pregnant females compared to the males. The chart on the right is corresponding data from this spring, comparing body temperatures of non-gestating females with the males.
In these charts, “frequency” is the percentage of observations where I recorded each body temperature for that sex during that period. In 2014, the average male body temp was 24C (75F) while the female average was 30C (86F). This spring, males have averaged 24C (75F) and females 22C (72F). Of course, the weather is cooler in the spring (even this spring) than in summer but there is ample opportunity for these snakes to get their body temperatures up above 30C (by late morning, ground surface temperatures are often above 40C in direct sunlight, even when air temps are relatively cool). But my point is that there is no real difference between average male and female body temperatures before the pregnant females go into their late-term thermoregulatory behavior… which is evident in the late summer data above where body temps of the pregnant females narrowly cluster around 30C (86F).
(For you statisticians that may read this, I admit that these data are not publishable in this form as they contain some significant pseudoreplication. Nonetheless, they serve to illustrate my point that gestating vs non-gestating female body temps are very different.)
Hopefully, we will get a few more females telemetered in the next month or so and, by mid summer, we should know if any are going to reproduce this season.
We now have a total of 14 rattlesnakes marked at Effie Yeaw Nature Center, including 9 males and 5 females. This spring, I have processed, marked, and released 3 males and 2 females without transmitters (mostly too small for the transmitters). With prematurely failed transmitters in Males 36 and 37, we currently have working transmitters in 4 males (35, 38, 40 & 46) and 3 females (39, 41 & 47).
You almost need a scorecard to keep track of who has been with who over the past couple of weeks. Lots of the action has occurred at a small hollow log where Female 41 spent the last month or so of her winter slumber by herself. She departed on 13/14 March and just five days later Female 47 turned up there with Males 38 and 46. The three snakes were coiled next to and touching each other on 19 March in a narrow bit of shade. The late morning sun was hot and all three body temperatures were elevated (female = 86F and males = 91F & 93F), indicating they had recently been in the sun. The female disappeared into the log when I approached but the males were more concerned about each other than me. The smaller male, 46, was still excited and head-jerking (a common part of rattlesnake courtship) a little bit. Each time he would touch Male 38, the larger male would shove him away, pushing violently by thrusting a coil sideways. Male 46 would push back, reminding me of two brothers in the back seat on a long car ride. I suspect I may have missed some male combat earlier, which was probably cut short as their body temperatures approached dangerous levels and they were forced to get out of the sun. (Click here to see a video of male combat, shot in front of the EYNC Visitor Center in 2010)
Also on 19 March, Female 41 was found (by herself, as far as I could tell) in the refuge where Female 39 delivered her kids last year.
On 21 March, Female 47 and Male 38 were were still at the small hollow log, although laying a few inches apart and not actively courting when I was there. Male 46 was by himself several dozen meters away in the grass. At 674 mm (26.5 inches) snout-vent length, Male 46 was no match for Male 38, who measured 821 mm (32.3 inches) SVL at his recent transmitter replacement surgery. As you can see from the video mentioned above, male combat is a wrestling match and larger body size is a definite advantage. Snout-vent length or SVL is the common way biologists record body length in lizards and snakes; the tail is usually measured separately.
On 21 March, Female 41 had left the birthing refuge used by 39 last year and was coiled by herself under a pile of dry live oak branches. The following day, she had been joined by Male 46 and the two were copulating at about 11:20 AM. In the photo (below), Male 46’s rattle colors are green/red and Female 41’s are white/blue, although the blue is difficult to see through the brush.
Also on 22 March, Female 47 was still at the small hollow log but Male 38 had been replaced by Male 49, who was actively head-jerking, chin-rubbing, and tongue-flicking the female. Male 49 is not telemetered (but recognizable by white/green paint in his rattle) and at 767 mm SVL, he is not quite as long as Male 38, but he outweighs Male 38 by 28 grams (377 g vs. 349 g), Of course, I have no way of knowing if 38 and 49 even crossed paths; Male 38 could have departed before Male 49 arrived.
On 23 March, Female 41 had moved back to the birthing refuge used by Female 39 last year and had apparently been followed by Male 46; they were still together there on 24 March, although they were basking about 4 feet apart when I visited on both dates.
Female 47 was laying partly in the sun on the morning of 23 March with Male 49 nowhere in sight. Of course, without a transmitter, I had no way to find him. But they were laying together in the same place again on the next two mornings, so I suspect he was there on the 23rd, just not visible.
Female 41 remained at 39’s old birthing refuge on 25 and 26 March. Male 46 was still there on the 25th but they had been joined by Male 38. On this day, Female 41 and Male 46 were again basking apart from each other and male 38 was out of sight, betrayed only by his radio signal. The next day, 26 March, the female was basking, Male 38 was there but out of sight, and Male 46 was alone in a poison oak thicket some distance away.
By 27 March, Female 47 had left the small hollow log where she had been for nine days (with 3 males at various times) but she had been replaced by Female 41, leaving Male 38 apparently alone where he had been with Female 41 for the previous couple of days. As far as I could tell, Female 41 was also alone.
Also on the 27th, Male 35 was found a few minutes after 11 AM eating a California vole (aka meadow mouse, Microtus californicus) in thick knee-high grass next to the main trail, not far from the picnic area. The snake stopped swallowing and we were lucky that he did not spit out the rodent when he was disturbed, as rattlesnakes are quite defenseless with their mouth stretched around a meal. George Nyberg and I had to remain motionless for many minutes before the snake finally decided it was safe to continue swallowing. We shot a few photos as he finished his vole.
On the 28th, all of the telemetered snakes were coiled in vegetation, alone, and apparently hunting. Both places where most of the courtship had occurred over the past two weeks were empty. Are they finished courting? I doubt it; it’s not even April yet!