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.

Interesting rattlesnake news

How well do rattlesnakes tolerate surgically-implanted transmitters?

As I have discussed before, there is a long (20+ years) history of telemetry studies of rattlesnakes in which individual animals tolerate the transmitters for years, enduring periodic surgeries to replace the radios. The animals thrive, repeatedly producing offspring and growing at the same rate as rattlesnakes without transmitters.

I bring this up because of a phone call last weekend from the landowner where I conducted my El Dorado County field study. He had just encountered the first Northern Pacific Rattlesnake I ever marked and telemetered, still identifiable by the yellow-over-yellow paint remaining in his rattle. He is now an exceptionally large male with twelve rattle segments – but in 2009, he was a young animal with a tapered unbroken rattle. He eventually endured four annual surgeries to implant and replace transmitters, followed by a fifth surgery in 2013 to remove his last radio.

Here is a PowerPoint slide of Male 01
Here is a PowerPoint slide I use to illustrate how marking the rattles helps to judge growth and shedding frequency. The photos are of my Male #1. It also demonstrates how the rattle breaks over time. According to my friend, the 2011 paint is now just two segments from being lost but the snake is big,  healthy, and thriving. You can see that, once the early tapered segments are gone, the rattle offers little insight into the age of the snake.

Male 01 being sighted alive and healthy is just more evidence that the surgical protocol and other study methods used by me and many of my rattlesnake-researcher colleagues is well tolerated by the animals we seek to learn more about.

Rattlesnake intelligence?

Despite my frequent admonition that we often tend to give rattlesnakes and similar animals too much credit for cognitive thought, friends at San Diego State University recently published some compelling evidence that rattlesnakes may learn from experience and apply those lessons to anticipate and mitigate problems during future similar circumstances. Bree Putman and Rulon Clark have spent years studying rattlesnake predation tactics by setting up video cameras on hunting rattlesnakes and recording their predatory encounters with small mammals. (This works because rattlesnakes are ambush hunters that sit still for long periods of time, waiting for prey to wander by.)

While reviewing 2000 hours of video, Bree and Rulon discovered two examples of rattlesnakes using their heads and necks to move foliage out of the way that might otherwise interfere with a strike when prey wanders close (click here for video). The animals involved were Northern Pacific Rattlesnakes – the same species as we have in the Sacramento area. Similar behavior has been reported a couple of times in the past, once involving a Prairie Rattlesnake (Crotalus viridis) and once involving an Arizona Blacktail Rattlesnake (Crotalus molossus); both these incidents were witnessed by observers but not recorded.

Thus, evidence continues to accumulate that rattlesnakes are likely more social and maybe more intelligent than previously thought – although many habits are undoubtedly genetically programmed by natural selection. The new report by Putman and Clark is contained in the current issue of The Southwestern Naturalist (volume 60, number 4; December 2015).

For more interesting videos of natural predatory behavior by rattlesnakes, go to Rulon’s YouTube page.

Courtship has begun

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

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

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

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

 

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.

The 2016 rattlesnake season is underway

After some limited basking by a couple of telemetered rattlesnakes and short movements by Female 41 and Male 49 two weeks ago, the return of cool cloudy weather sent these animals underground again. Then, despite several days of sunny weather last week, the rattlesnakes remained cool and out of sight – with the exception of Male 49. While Female 41 stayed underground and cold, he napped most afternoons in the sun just a couple feet away.

But yesterday, everything changed. Males 37 and 38 who spent the winter near the top of the bluff had moved down; one was out of sight under a log and the other was in an ambush coil in the grass under a small fig tree.

CROR38, first sighting of the year; 29 February 2016 Original RAW IMG_0195.CR2
My first look at Male 38 this year. He appeared to be hunting at the base of the bluff, just a few dozen yards from where he spent the winter on the hillside.

A rapidly pulsing transmitter told me before I arrived that my underweight geriatric Male 40 had survived another winter and was in the sun at his winter shelter.

CROR60 at RFG02, first 2016 sighting; 29 February 2016; Effie Yeaw Nature Center Original RAW IMG_0177.CR2
My old friend, Male 40, made it through another winter. Over two seasons, he has moved very little and I have never found him courting a female. This is my first look at him this year. He had a body temperature of 88F when this photo was taken. Although he may have had more surface area exposed to the sun before I arrived, it takes very little exposed skin to dramatically raise their body temperature on a sunny windless day.
Non-telemetered Male 49 (foreground) and telemetered Female 41 (background)
Male 49 (foreground, no transmitter) and telemetered Female 41 (background). Female 41 was underground with a body temperature of 52F on Sunday but had a body temp of 86F when this photo was taken yesterday (24 hours later).

Female 41 was in the sun with Male 49 laying partly on top of her, although I saw no active courtship during my brief visit.

Other telemetered snakes had moved short distances but were out of sight when I was there yesterday. But in my travels, I also sighted five unmarked rattlesnakes basking, including a beautiful little female sporting a food bolus about the size of a vole – so she has already fed successfully.

So for the next three months or so, females will be trying to eat as much as they can to nourish their next brood while males will be wandering all over the place (and hunting less) in search of receptive females. It is during this time that male rattlesnakes tend to turn up in yards and on trails, producing interactions with people.

Remember that rattlesnakes want nothing to do with something the size of a person. Leave them alone and they will be happy to avoid you, too!

Life and death among the rattlesnakes

You may remember from my last post that pregnant Female 53 had made a surprising move of more than 220 yards near the end of August and was discovered, apparently by herself, in a small burrow at the edge of the river bottom. Interestingly, after monitoring her there for a week, she turned up back in the original refuge on 8 September. Although I have not been able to get a look at her with the BurrowCam, the burrow she was in for a week is empty and I have no reason to believe that she’s not still pregnant.

Then, last Thursday (September 10), I found three significant developments when I visited this same birthing refuge occupied by expecting Females 39, 47 and 53. First, Female 39 was gone, with a distant weak radio signal. Second, I finally got a direct look at some babies – either three of them or the same one three times, crawling around inside the refuge (photo below)! Interestingly, the one(s) I saw had shed; they were brightly marked and their little two-lobed rattle buttons were uncovered. That, of course, means that they were around 10–14 days old and ready to leave. And, third, one of the visible adults was jerking and chin-rubbing on Female 47 – sure signs of a courting male (click here for a courtship video). This guy had no paint in his rattle, so he’s new but he was inaccessible inside the shelter. But the fall portion of the courtship season has definitely begun.

A post-shed young-of-the-year crawling over an unidentified adult inside RFG01EY on 10 September 2015. Original RAW IMG_8590.CR2.
A post-shed youngster crawling over an unidentified adult rattlesnake in its birth shelter on 10 September 2015.

Female 39 was already postpartum, so her departure was not surprising. But her offspring should have left (or be leaving) at the same time. The post-shed kid(s) I saw could have been her’s or Female 47’s. When I tracked down Female 39’s radio signal, she was in the blackberry thicket on the other side of San Lorenzo Way, laying in diffuse sunlight and sporting a very recent food bulge (just behind the U-shaped bend in her neck in the photo, below). Interestingly, this annually-reproducing female had her babies in the same refuge last year and, when she departed, she immediately made the same long move to the same spot in the same berry thicket – on 15 September 2014. Apparently this is the best place to find a good meal when you finally get the kids out of the house!

Recently postpartum female Northern Pacific Rattlesnake (CROR39) with recent food bolus, basking on 10 September 2015 near Effie Yeaw Nature Center. Original RAW IMG_8606.CR2.
Recently postpartum female Female 39 with recent food bolus, basking on 10 September 2015.

 

As of the morning of 14 September, all females except 53 had left their gestation shelters. I came across Female 47 crawling in the grass, so I maneuvered in front of her and shot some video as she crawled toward me. Although I remained motionless, I think she detected me as she got within two or three feet, because she started carrying a slight “S” bend in her neck, which I interpret as a defensive precaution in case she needed to strike and bite (when not feeling threatened, they usually extend the neck when crawling, as in the first half of the video). I was accidentally kneeling almost on top of a ground squirrel burrow and she dove into it when she found it. Click here to see the 28-second video.

A short time later, I found that Female 54 had also departed from her gestation refuge but had made it only a few dozen meters. Her partly eaten carcass was laying in the edge of a trail just a few feet from where an adult turkey had been killed and eaten a couple of weeks ago. Her blood had not yet coagulated and the exposed tissue was still moist and glistening. In recent weeks, I’ve frequently seen one or two of the now almost grown coyote pups in this area and there has been coyote scat everywhere. I have no doubt that one of them got her.

Female CROR54 partly eaten, morning of 14 September 2015. Blood is not coagulated and exposed tissue is moist and glistening. Original Droid IMG_20150914_094119031.jpg
Female 54, partly eaten on the morning of 14 September. I suspect a coyote was having her for breakfast when I approached and interrupted his meal.

Then, at the shelter used for the past couple of months by Females 39, 47 and 53 – and still occupied by 53, I came across a freshly shed youngster a couple of feet outside of his birth refuge.

Two-week-old youngster just emerged from his birth refuge, 1 m behind him. Original IMG_20150914_164416116.jpg
A two-week-old rattlesnake, less than a foot long, that had just emerged from his birth refuge. He (or she) was contemplating the big unknown and dangerous world for the first time.

 

Assuming that Female 53 delivers a brood soon, our six monitored females will probably have contributed nearly 50 baby rattlesnakes to Effie Yeaw’s Nature Preserve (average litter size is 8). But remember that, on average over time in a stable population, a female rattlesnake (or any other species) only produces a replacement for herself and a mate in her lifetime that survive to reproduce themselves. Otherwise, the population increases or decreases.

A kingsnake visits a birthing refuge occupied by two pregnant rattlesnakes
Captured by a time-lapse camera, this kingsnake was checking out a gestation refuge occupied by two pregnant rattlesnakes in September 2011, during my El Dorado Hills study. Both females gave birth within the following week. Baby rattlesnakes are just hors d’oeuvres for kingsnakes, which probably know very well where the rattlesnakes give birth and they may well eat a large percentage of newborn rattlesnakes.

 

The vast majority of offspring never live long enough to pass on their genes. Of course, there are cycles between predator and prey species. When predator numbers are up, they soon knock the prey population down, which eventually results in a reduction of predators as food becomes scarce. Then, as predator numbers decline, the prey population begins to increase again… and the saga continues. Remember what I’ve said before: it’s a violent world out there and Nature is a cruel mother; most wild creatures’ lives end in the jaws of another!

 

Pregnant females, injuries, and shedding

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

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

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

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

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

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

Female CROR 55 Original RAW IMG_7555.CR2

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

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

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

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

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

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.

unidentified Crotalus oreganus under log at Refuge 03 on 03 June 2015, Effie Yeaw Nature Center Origonal RAW IMG_7387.CR2

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.

adult female Crotalus oreganus (CROR41, in situ) swallowing an unidentified rodent (Mus or Microtis) at Effie Yeaw Nature Center on 13 May 2015 Original RAW ING_7074.CR2

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).

Female Northern Pacific Rattlesnake (Crotalus oreganus; CROR47; in situ) eating Southern Alligator Lizard (Elgaria multicarinata) at Effie Yeaw Nature Center on 19 May 2015. Original RAW IMG_7254.CR2.

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).

CROR53 in situ at capture site. Original RAW IMG_7320.CR2

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.

29 March: Lots of courting pairs and a vole goes down!

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.IMG_5745

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.

CROR35 eating vole 27Mar15A  CROR35 eating vole 27Mar15B

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!