ground squirrel – EYNC Rattlesnake Study

Year end wrap-up

Well, it has been more than two months since I reported the first births of the year among the wild rattlesnakes in the Effie Yeaw Nature Center preserve. And while my hectic fall schedule prevented me from investing as much time as usual, I was able to record enough detail to compare this season’s behavior to previous years.

If the timing of the first births among our telemetered females (between 8 and 12 September) were representative of the larger population, we would expect to start encountering youngsters about the third week in September. Remember, new kids remain with their mothers in their birth shelters until their first shed, which usually occurs a little over a week after birth. Only then do they venture out into the world where people might encounter them.

Sure enough, the first young-of-the-year was encountered on 26 September, ironically on the sidewalk just outside the EYNC classroom as my UC California Naturalist class was leaving that evening. Of course, since I had just talked about baby rattlesnakes in class a few minutes before, the students were suspicious that I might have planted the little guy! The next day, I encountered another one (see photo below) stretched out next to the hollow log where Females 39 and 53 had recently given birth.

Young-of-the-year on 27 September, beside the hollow log where he was probably born.

Two shed “skins” (actually, just the outer or “corneal” layer) left by babies as they embark on their hazardous new life. Few survive to adulthood; at the size of pencils, they have lots of predators.

And sure enough, Female 39 once again made a beeline from her birth shelter to the blackberry thicket across San Lorenzo Way as soon as her kids’ natal sheds were complete (photo below). She has produced broods in three of the last four years and she has followed the same pattern each time: she used the same gestation/birth shelter, she made a direct 200+ meter move to hunt in the same blackberry thicket as soon as her kids departed, then she returned directly to her usual hibernation site after several weeks of foraging among the blackberries. Some of these individual rattlesnakes have proven to be quite predictable from one season to the next… but not all of them.

A slender postpartum Female 39, cryptically basking on the other side of San Lorenzo Way.

Female 41 spent the past three winters apparently by herself (as far as I could determine) in a ground squirrel burrow among the roots of a large living Valley Oak. However, she has apparently decided to spend this winter under the EYNC Visitor Center building! Now before getting excited, remember that she is one of only twelve rattlesnakes carrying radio transmitters at EYNC – and we have estimated that there are about 100 additional adult rattlesnakes in the preserve. We can make that assessment because we have 52 rattlesnakes marked with paint in the rattles that are not implanted with radios… and about half of our chance encounters with rattlesnakes without radios over the past year have been with unmarked animals.

Almost none of the EYNC staff or regular long-term visitors with whom I have spoken thought there were anywhere near that many rattlesnakes in the preserve. That’s a testament to just how secretive and non-aggressive rattlesnakes really are. It is highly likely that other rattlesnakes over-winter under the Visitor Center every year undetected.

It is simply prudent in rattlesnake country to be careful where you step and where you put your hands; simply look before you step or reach. Assume that a rattlesnake could be encountered anywhere – in any shed or under any wheelbarrow – and proceed with reasonable caution, not fear. Don’t walk around outside at night without a flashlight (or boots); while usually pretty timid, rattlesnakes really dislike being stepped on! That’s the same advice I have given countless people at my rattlesnake presentations.

As of 7 November, all but three of the telemetered rattlesnakes had returned to their previous hibernacula (hibernation shelters): exceptions are Female 41, who is under the Visitor Center 185 meters (a little over 600 feet) from her usual hibernaculum in the woods, Male 46 is on the hillside below the residential area instead of 265 m (about 870′) away under his usual log in the meadow, and Female 66 is under a new log 80 m (about 260′) from her usual log near the study pond.

The study comes to an end

As some of you know, 2017 will be my last year of data collection at Effie Yeaw Nature Center. We implanted the first transmitter in an EYNC rattlesnake in May 2014 and built up the group of telemetered rattlesnakes that summer. As a result, we have good data from three complete seasons during 2015, 2016 and 2017, plus many valuable observations from 2014. I will remove the transmitters from the remaining rattlesnakes in the spring and release them. Several are carrying their fourth transmitter (they must be replaced annually), so removal will be their fifth surgery. I also have blood samples in my refrigerator from nearly 70 wild EYNC rattlesnakes, as well as many dozens of shed skins collected from various locations in the preserve… and DNA can be extracted from both blood and shed skins.

Now the real work begins. Field work is great fun! Sitting at a computer for weeks or months is, of course, far less entertaining. Yet the field work means nothing if the data are not analyzed and shared. And as a favorite mentor of mine is fond of reminding me, “If you don’t publish it, it never happened!” So, while we certainly have answers to the original questions (like how many rattlesnakes are there, where do they give birth, spend the winter, etc.), the most exciting part involves the potential for unexpected discoveries. And as I have written about in this blog before, I have found the EYNC rattlesnakes associating in three loose groups without an obvious environmental reason to do so. I suspect they are socializing in family groups, which has only been documented in one previous case with Timber Rattlesnakes (Crotalus horridus) in the Appalachian Mountains. Genetic analysis of the blood and shed skins will make or break that hypothesis for the EYNC animals. I will continue to post analysis updates and eventually publications here.

So stay tuned!

I’ll leave you with one of my very favorite photos from the study: big Male 37 basking peacefully on top of a large pile of dead branches among blackberry and wild grape vines early one morning in April 2016. The third rattlesnake to receive a transmitter at EYNC, he has always been a joy to watch and has provided some of my most memorable rattlesnake encounters of the last four years. The yellow and red paint in his rattle is slightly visible in the photo. Like others of his kind, he wants nothing to do with people but is a mouse, vole or ground squirrel’s worst nightmare.

Gestation has begun for pregnant females

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…

Female 41 in the same hollow log where she gestated and delivered broods in 2014 and 2015. She took last year off and hunted throughout the summer. These images are made with our BurrowCam (actually an inspection camera made by Rigid Tools).

Radio signals from Females 39 and 53 are currently coming from this hollow log. Female 39 produced broods here in 2014 and 2015. She hung around here last year but we are not sure she produced kids; she was never very large and we never spotted babies but she behaved like she reproduced. We’re just not sure. Female 53 produced offspring here in 2015 and gestated here for a while in 2016 before moving to a shelter in the dry riverbed to give birth. Since we cannot see the rattles on these two, it is impossible to tell if these are Females 39 and 53, since other pregnant females often share this shelter. Photo with the Rigid BurrowCam.

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.

Male 37 anesthetized for surgery yesterday morning. I was surprised at how much of his meal he digested overnight… the squirrel was better defined and the bulge was even larger just a day before. This was probably a half-grown ground squirrel pup. These pups are everywhere right now, are easier for the rattlesnakes to kill, and an adult squirrel would have made a really enormous bulge in the snake. Just for reference, the rule is a “meter stick” and the snake is about 40 inches long. The small bump on his left side behind the squirrel is a bit of shed skin (actually just the corneal layer of the skin) stuck in the scar from his last transmitter surgery. Shortly after this photo, he received his fourth transmitter and was released yesterday back into the EYNC preserve.

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?

New research on ground squirrels’ resistance to rattlesnake venom

I have mentioned before that much research has been done on the interactions, both behavioral and biochemical, between Northern Pacific Rattlesnakes (Crotalus oreganus) and California Ground Squirrels (Otospermophilus beecheyi). And that research continues.

It started, so far as I know, with studies by UC Davis psychology professors Donald Owings and Richard Coss in the 1970’s, when they became interested in how California Ground Squirrels behaved when confronted by Northern Pacific Rattlesnakes. Several researchers have since spun off various aspects of the relationship between these two species, including Dr. Rulon Clark and his students at San Diego State University, who study the phenomenon from the rattlesnakes’ perspective. A common thread among these studies is that the adult squirrels are largely resistant to the rattlesnakes’ venom, often surviving with nothing but a nasty wound that eventually heals (although adult squirrels occasionally succumb, vividly illustrated by the photo accompanying the Washington Post article linked below).

But while adult ground squirrels seldom die from rattlesnake bites, their pups are much more vulnerable and the rattlesnakes hunt them intensely, starting about this time of year. I have linked a 60-second video made by Denise and I in July 2014 of our Male 36 (yes, the same one just recaptured after 20 months) preying on a ground squirrel pup while the pup’s mother tries to defend her offspring (Read original account here).

Tail-flagging and pushing grass at the snake are common behaviors by adult California Ground Squirrels when confronted by rattlesnakes. In this one-minute clip, the snake had already bitten a pup, which is laying in the grass and out of the frame at the start. The adult squirrel soon retreats to the stricken pup, which appears as a dark area in the grass. The adult squirrel’s attempts to deter the rattlesnake appear to work momentarily a couple of times as the snake turns away but almost immediately comes back toward the bitten pup. Near the end of the clip, the snake reaches the pup and bites it again. Although the pup runs out of the frame, it only makes it a few feet. The rattlesnake follows and swallows it a few minutes later. Excuse the background helicopter noise, as the fire department was conducting an operation in the river nearby. View the video here.

I bring this up now because my friend, videographer George Nyberg (who produced the very nice 2015 video of my rattlesnake study), has alerted me to a new Washington Post article on the biochemical “arms race” between Northern Pacific Rattlesnakes and California Ground Squirrels (view article). Thanks, George!

Matt Holding, whose research is the focus of the WP piece, is a former graduate student of another friend, Dr. Emily Taylor at Cal Poly San Luis Obispo. Jim Biardi, second author on the new study, is a former member of the UC Davis group that originally studied ground squirrels and rattlesnakes.

The Washington Post does a nice job of describing how natural selection works: in short, there is always variation among individuals and some are better adapted than others to feed themselves (or avoid being eaten!) and those individuals tend to survive longer and produce more offspring, which carry the genes for those successful traits. Less successful traits are passed on less frequently (i.e., fewer offspring are produced). The peer-reviewed paper upon which the WP article is based was published in the journal Proceedings of the Royal Society B (volume 283, issue 1829, April 2016). However, since this is not an open source journal, access to the complete manuscript is not easily available to the general public right away.

Courtship has begun

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

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

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

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

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

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

Pregnant females, injuries, and shedding

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

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

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

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

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

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

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

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

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

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

Mike

The effect of drought on rattlesnakes

Field studies of rattlesnakes indicate that they cease most movement when water stressed and remain in their established home ranges, rather than migrating into developed areas in search of water, despite frequent claims to the contrary.

In my last post, I mentioned witnessing Females 41 and 47 feeding, as well as finding new Female 53, who was very heavy and likely pregnant. Since then, I have come across Female 41 eating another vole, found another new female (#54) that is heavy and definitely pregnant, and come across a fat but unidentified rattlesnake in the refuge where Females 41 and 43 had babies last year. I could only see the face and a bit of a flank of the unidentified animal (photo below) so I couldn’t even determine sex.

It could be a male that has just eaten a ground squirrel pup – but it is more likely another pregnant female. We now have five telemetered females (39, 41, 47, 53 and 54) and all are in great shape, with three either confirmed or likely pregnant and the others in good shape to reproduce although I have not yet had my hands on them this year to palpate for fetuses.

This brings up a timely point: This will obviously be a good year for rattlesnake reproduction in our area, despite being in the midst of an historic drought. Since the news media often quotes “experts” claiming drought “drives rattlesnakes out of the hills and into yards looking for water,” this is a great opportunity to set the record straight about how drought affects rattlesnake movement.

We live in a Mediterranean climate, historically characterized by warm dry summers and cool wet winters. Even during years with “normal” precipitation, vast tracks of mountains, foothills and many valley areas have no surface water between late spring and the return of winter rains in November or December – yet they support healthy populations of rattlesnakes. Herbivores (insects, rodents, etc.) get most of their water from the plants they eat and rattlesnakes get water from eating the herbivores. The bodies of terrestrial vertebrates are usually composed of 65–75% water, so eating a 100 gram (3.5 ounce) rodent is like drinking about 70 grams (2.5 ounces) of water for a rattlesnake (plus the nutrients and energy gained). Make no mistake, rattlesnakes suck droplets from various surfaces, including their own skin, deposited by rain and dew (photo, below) and they will certainly drink from standing water when it’s available. But especially during summer and fall, these other sources are not available and virtually all of the water a rattlesnake needs is obtained from its prey.

adult Crotalus s. scutulatus (Mohave Rattlesnake) drinking rain water near Apple Valley, San Bernardino County, California, USA [wild animal, in situ] — Mohave Rattlesnake drinking rain water from its own body. [Cardwell, M. D. 2006. Rain-harvesting in a wild population of *Crotalus s. scutulatus* (Serpentes: Viperidae). *Herpetological Review* 37:142-144.]

Rattlesnakes are models of low energy physiology. As ambush predators, they move comparatively little and rely largely on anaerobic metabolism. Their sedentary lifestyle combined with the corneal layer of their skin (full of water-blocking lipids) dramatically lowers the amount of water that passes into and out of their bodies – known as “water flux.” Nonetheless, multiple studies have shown that the most significant mechanism for water loss in terrestrial snakes is evaporation, with about 75% being lost through the skin and the remainder via exhaled breath.

During my four-year field study (2001–2004) of Mohave Rattlesnakes in southern California, I was able to compare behavior, including average daily movement and reproductive effort, between the severe drought year 2002 and 2003–2004, when rainfall returned to average or above average. I found that average daily movement during 2002 was less than one third of 2003–2004 averages. And while I encountered dozens of courting pairs during the two non-drought years, I observed a male courting a female on only one occasion in 2002. Yet these rattlesnakes continued to eat at a rate indistinguishable from the non-drought years, based on scats deposited in holding containers and later analyzed. These snakes were reducing exposed surface area (and, therefore, evaporative water loss) by remaining coiled and immobile, covering much of their skin within their coils. They even buried their coils partially in loose soil at times, covering additional skin area. Remaining stationary eliminated their ability to find and court mates but, as sit-and-wait ambush predators, it allowed them to continue to hunt – and obtain the body water of their herbivorous prey. They also positioned themselves behind vegetation and ground contours in 2002 to avoid wind and sun, both of which increase evaporation rates. You can find more details in my MS thesis.

We have seen similar behavior in Northern Pacific Rattlesnakes in recent years at my El Dorado Hills study site, where the rattlesnakes remained tightly coiled and stationary in deep chaparral on north-facing slopes during particularly hot dry summer weather. At Effie Yeaw, all of the rattlesnakes caught by staff around the ponds during the past year, as well as the telemetered rattlesnakes I have found there, have been males found during the courtship season. The females have remained in the woods, away from water sources. While the snakes will drink when they find the ponds (or other water sources), that’s not why the males are there… they’re wandering around looking for females! And, yet, the females are now fat and pregnant.

Drought probably does not affect rattlesnake movement until it becomes locally severe, as it did at my Mohave Rattlesnake study site in 2002. When the snakes start to become water stressed, they don’t set out into uncharted territory looking for surface water. Rather, they stop moving and hunker down where they can best reduce evaporative water loss while still striking any prey that wanders by. Currently, if the preserve at Effie Yeaw Nature Center is any indication, there is lots of annual plant growth and the vole and ground squirrel populations are thriving – and so are the rattlesnakes.

So when people find a rattlesnake in their yard during a drought, the most likely explanation is that it is a male looking for receptive females and the drought is not severe enough locally to stress the rattlesnakes. When they are truly water stressed, rattlesnakes move less – not more – than usual. Unlike most large mammals that have much higher metabolic and water flux rates and require standing water to drink, there is no evidence that rattlesnakes leave their established home ranges looking for water, despite the popular belief to the contrary. They do just the opposite.

More feeding, courting, and a new female

The first thing I encountered early in the afternoon of 13 May was a dead adult rattlesnake in the grass behind the Visitor Center. The carcass had completely putrefied and had been eviscerated by insects. It was an average-sized adult and had no paint in its rattle (i.e., it was not one of our study animals). I could not determine the sex. The head and rattle were intact, indicating it was unlikely a human had killed it. There was no obvious evidence of trauma elsewhere, either, but it was in such bad shape that it was difficult to tell. People often ask about the life expectancy of rattlesnakes and the answer is that, in captivity, they have been known to live more than 30 years. But in the wild, if they make it to adulthood (which few do), they are constantly threatened by raptors, coyotes, kingsnakes, people and their cars, temperature extremes, and other hazards. My guess is that few make it to ten years.

About 3 PM on the same day (13 May), I found the radio signal from Female 41 coming from dry grass on the hillside near the stairs on the trail behind the amphitheater. When I refined her location and parted the grass, I found her swallowing a small rodent – which she immediately spit out (this is common defensive behavior, as rattlesnakes are defenseless when swallowing prey). I had to remain completely still for five minutes or more before she decided it was safe to eat again. Because she was so deep in the grass and I had only a small opening through which to view her (photo below), I could not tell exactly what kind of mammal she was eating. It was uniformly gray with very fine fur, a gray belly, and was about mouse-sized. It was too large for a shrew and the lack of a light-colored belly ruled out most native mice. The dorsal fur looked too fine for a vole and I could not see the tail. What I could see looked like a house mouse, Mus musculus, but we will never know for sure as I did not want to bother her further and be the cause of her abandoning her kill.

When I next visited her two days later, she had moved only a few feet and was coiled next to a California Ground Squirrel burrow, which she retreated into when I arrived. This was the first time this season I have found a rattlesnake close to or in a ground squirrel burrow. I haven’t seen ground squirrel pups yet but I saw a pregnant female on 11 May, so I suspect there are pups in burrows by now and the rattlesnakes hunt the pups heavily in spring and early summer. (Click here and scroll to the bottom of the first page for more info on the fascinating interaction between Northern Pacific Rattlesnakes and California Ground Squirrels)

With all of our telemetered rattlesnakes alone and apparently hunting (and not having found a courting pair since 20 April), I was beginning to wonder if courtship might have concluded early this spring. But on 16 May, I found Male 38 on top of an unmarked female next to the large log in the meadow. She was heavy, healthy, and had no paint in her rattle – a beautiful rattlesnake, but she was also quick! As so often happens, the courting male was far less afraid of me (too much testosterone?!?!) than the female, who fled immediately and made it under the log before I could capture her.

I wish that people who fear rattlesnakes and think they are so malevolent could see how these animals really react to being approached by a person. If I had been able to catch all the unmarked rattlesnakes that have escaped from me this spring, our quota of seven males and seven females with transmitters would be full… but they are shy and very quick to flee into the grass, wanting nothing to do with something as big as a person. Remember, in their tiny primitive brains, rattlesnakes react to encounters based on three criteria: Can it eat me? Can I eat it? Can I mate with it? Clearly, we fall into the first category!

On 19 May, I found Female 47 in the meadow a little after 10 AM, crawling slowly through the grass, carefully tongue-flicking as she moved. I took a few photos and she crawled out of sight while I recorded my standard data. But when I started to depart, I came across her a couple of meters away with an alligator lizard, Elgaria multicarinata, in her mouth. She retreated a short distance to a small shrub with the lizard still in her mouth and, after I stood motionless once again for several minutes, she began to swallowed it (photo below).

Two days later, while searching for Male 35 on 21 May, his signal led me to a large and very dense thicket of armpit-high Milk Thistle (Silybum marianum) in the meadow. I have avoided penetrating this thicket when various males have occasionally visited it this spring because the spines go right through all clothing and there is just no way to avoid them. But on this cool morning, when I peered over the thistle into a little clearing in the thicket, I saw a nice unmarked rattlesnake laying in the grass, probably hoping the clouds would clear and allow some sun to shine through (photo below).

Hoping that she was a “she” and probably the reason that Male 35 was also in the thicket, I carefully retreated without spooking her and dropped all my gear in the grass except a cloth bag and snake hook. Stepping into the sea of spines, I slipped the snake hook under her and began to gently lift her before she reacted. I almost got her clear of the thistles before she wriggled off the hook but I was able to quickly catch her again before she completed her escape. Once clear of the thistles and in the dry yellow grass, she could no longer hide and I had her in the bag shortly thereafter.

Her sex was later confirmed and she became CROR53. (Click here for an explanation of CROR). She was very heavy for her length and full of shiny white abdominal fat when I implanted the transmitter. I could not be sure of her reproductive condition because of so much material in her bowel; she clearly had been feeding very successfully. Reproductive condition is determined by palpating her belly through her belly scales, feeling for yolk masses and, later in the season, for embryos. Because the transmitter incision is less than 3/4 of an inch long and made on the side, two scale rows up from the end of the belly scutes, it doesn’t help in determining reproductive condition. When she was released the morning after her surgery, she became our fourth telemetered female, along with four telemetered males (plus two additional males with failed transmitters I am still hoping to recapture). I am up to 413 recorded encounters with EYNC rattlesnakes this year… we are off to a great start!

Soon to come: an explanation of shedding and rattle growth.

16 March: General emergence has occurred!

The day after I saw Male 37 in the flat woodland (7 March), down from his winter shelter on the hillside, Male 38 was found about 10 meters (33 ft) from his hibernaculum, where he remained coiled in the grass for several days while basking when possible. Although other telemetered rattlesnakes remained at their winter shelters for most of last week, they were often found basking in the sun, as were several unmarked rattlesnakes.

I caught and marked several of those new basking rattlers and, as it appeared that another cold snap was unlikely, I implanted transmitters in CROR46, captured on 6 March, and in a new female, CROR47, captured a few days later. I could not palpate any fetuses in Female 47 and I don’t think she is pregnant but it’s possible, as it is very early in the year.

As of today, Female 41, who produced kids last season and apparently hibernated by herself, was coiled in deep green grass about 30 yards from where she spent the winter (below), apparently hunting. Male 35 was coiled in deep grass under a live oak tree he frequented last year. He’s the one who spent so much time in the vegetation near the bike rack, silently welcoming guests much of last summer. We’ll see where he goes next.

You may remember that Male 40’s transmitter was the most recent to fail prematurely. Well, I was able to recapture him today. I also recaptured Male 38, who’s transmitter is still working but is part of the batch that have been failing early. Both will be released with new transmitters in the next day or two. Males 36 and 37 remain on the loose with failed transmitters.

Finally, Male 46 and Female 47 have found each other and were laying in the sun together this afternoon (below). There was no active courtship when I saw them (they were very still) but I’m not buying that this is a platonic relationship this time of year!

As you can see from the photos, these rattlesnakes are almost impossible to see in the thick carpet of green grass that covers the park now. And when approached too closely, they disappear silently into the grass – as Female 47 did as soon as I tried to get closer after the photo above. Of course, the green grass makes it very easy to step on a rattlesnake!

As of today, Female 39 is the only telemetered animal that has not left her winter shelter, although her body temp has been warm (indicating she has been basking, although I haven’t seen her) and, several days ago, she moved to the other end of the log where the location of her radio signal was identical to that of Male 35 for a couple of days before he left. Hmmm…!

I also heard two different California ground squirrels incessantly chirping their alarm call this afternoon. When they are signaling a hawk or coyote, they usually chirp once or twice and dive for cover. When they continue to chirp over and over, it is often a snake (unlike a hawk or coyote, once they discover a snake, they can avoid it without going underground). Unfortunately, I didn’t have time to investigate today.