Mike – Page 4 – EYNC Rattlesnake Study

Baby rattlesnakes!

This is just a quick post to announce the first births that we know of this year among our Effie Yeaw Nature Center rattlesnakes. When we checked on non-telemetered Female 55 this evening (Tuesday, 1 September) with the BurrowCam, she looked pretty normal (see photo below). She certainly did not look like she had lost a lot of weight. And there were no babies on or around her.

Frame grab from BurrowCam video of Female 55, not looking like she's lost much weight. No kids in sight just inside her refuge Tuesday evening - or anytime previously. — Frame grab from BurrowCam video of Female 55, not looking like she’s lost much weight. No kids in sight just inside her refuge Tuesday evening – or anytime previously.

But when I bent the BurrowCam around and pushed it deeper into the cavity behind her, at least two babies were spotted. What was surprising was that they are not brand new. Note the bluish eyes and rattle caps; these kids are several days old – maybe close to a week.

One of at least two babies in the refuge with Female 55 on Tuesday evening. Note the bluish eyes and rattle button. These little snakes are several days old and already preparing for their postpartum shed.

These little guys have never been near the opening to the refuge when I have looked. That’s very different behavior than what I have seen in the past. Click here to see the 1.75 minute video of the kids.

Female 41 from the same shelter is still pregnant and there’s no evidence of kids around the other pregnant females…but I missed these babies for several days! More to follow soon…

Missing males and waiting for kids

As birthing season approaches, I have been watching intently for signs of baby rattlesnakes. While postpartum mothers usually stay inside their shelters, the neonates are typically active and easily spotted. Although they do not leave their shelters before shedding the first time, babies can usually be seen crawling or basking at the entrance. And when we introduce the BurrowCam into the shelter, the kids can be seen exploring their new surroundings and crawling around on their mother. So far, there’s been no evidence of babies yet this year. This would be a bit early but quick warming last spring has me wondering about the potential for early births this year.

Remember that pregnant female rattlesnakes in our area hangout in carefully selected thermal shelters where they can maintain consistently warm body temperatures around the clock until they give birth. This period of thermoregulation lasts several months, during which the pregnant moms do not forage for food.

All five of our telemetered females are apparently pregnant, plus Female 55, who was processed and released without a transmitter in June (no transmitter surgery due to some old but significant trauma to her abdomen; click here for details). These females settled into their gestation shelters between 8 June and 1 July and have maintained body temperatures between 28ºC and 32ºC (82º–90ºF), almost without exception, ever since.

In fact, at dawn on a recent cool morning (16 August) when the ground temperature just before sunrise was in the mid-50s F, these girls still had body temps in the high 80s F. They maintain similar body temps during hot afternoons when the ground temperature (much hotter in the sun than air temp) outside is 120ºF and more. They thermoregulate like this by selecting logs or large rocks that have just the right thickness and sun exposure to stay warm at night but not get too hot in the afternoon sun. Such places are apparently scarce because three of our telemetered females are together in one shelter, while Female 41 is with non-telemetered Female 55 in another. Female 54 is in a third location, possibly by herself, but there could be others in there without radios. Females 39 and 41, both of whom produced broods in 2014, are in the same shelters as last year.

Flash photo of Female 41, tucked into her gestation refuge at sunrise on 16 August 2015. Note in the inset how her scales are pulled apart by her developing brood. Female 55 is also in this shelter but not visible on this morning.

On 27 August, the BurrowCam revealed Female 39’s abdomen to be greatly distended, extending all the way to the cloaca. So maybe delivery of her 2015 brood is not far off? The frame grab (below) from the BurrowCam video shows her abdominal scales pulled far apart. In the 50-second video (watch here), you’ll see what I see when we thread the BurrowCam into a passage. Female 39 is identified by red/blue (red-over-blue) paint in her rattle and the edge of another dark gray rattlesnake appears to be visible under 39’s coils. Known to be behind her in the passage (because of their radio signals) are Females 47 and 53, as well as Male 46. Additionally, in recent days, I have seen non-telemetered (and non-pregnant) Female 48 (green/green) and Male 36 (red/red; carrying a failed transmitter) in this log. It’s a popular place this time of year!

Frame grab from a 27 August BurrowCam video of the distended abdomen of telemetered Female 39, deep in her gestation refuge. — Frame grab from a 27 August BurrowCam video showing the distended abdomen of telemetered Female 39, deep in her gestation refuge.

As you may recall, Males 36 and 37 have been missing for months since their transmitters failed prematurely in September and December, respectively. Until last week, Male 36 had been last seen on the BurrowCam in a hollow log courting postpartum Female 41 on 2 October 2014, and I last saw Male 37 as his tail disappeared down a hole on 7 March 2015. There had been no sign of either of them since until a fellow photographer and herpetologist I encounter frequently at Effie Yeaw showed me a photo of Male 37 (IDed by his yellow/red rattle marking) crossing a trail on 20 August! Then, just 5 days later, while checking for babies in the shelter with Females 39, 47 and 54, and Male 46, I was surprised to see Male 36’s red/red rattle. (See photos below) So both are alive and well… but both still elude recapture.

Male36 (red/red paint in rattle) deep inside a hollow log with Female 41 on 02 October 2014.

Male 36 inside another hollow log on 25 August 2015 with at least three pregnant females and a smaller male. Compared to the 2 October photo (above), note that he has two additional rattle segments between the paint and the live black segment, indicating he has shed twice in the past ten months.

Earlier today, 29 August, I found Male 46 coiled in poison oak dozens of meters away from the log where he has been hanging out with the three pregnant girls continuously for the past two weeks. It is likely he has been chased off by a larger male, so maybe Male 36 is still in there. This refuge has a narrow deep passage that is nearly impossible to thread the BurrowCam into and, even when successful, I can usually only see whichever rattlesnake is closest to the top (for example, the 50-second video of Female 39, with the link earlier in this post).

So Baby Watch continues and I still hope to recapture missing Males 36 and 37.

Rattle Growth, Shedding & Estimating Age

Now is the perfect time for this discussion because the Effie Yeaw rattlesnakes have been shedding like crazy. Since the end of spring courtship about two months ago, the pregnant females have hunkered down to thermoregulate at optimum gestation temperature while the males and non-reproductive females have been hunting with far less attention to temperature control. All have been shedding over the past few weeks.

If you haven’t yet read my description of the shedding process near the end of my last post, do so now; the following discussion will make much more sense with that background.

Because the corneal layer of the skin does not grow, shedding (or ecdysis) is a recurring process throughout a scaled reptile’s life. Like other animals, baby snakes grow rapidly, so they must replace the corneal layer frequently to accommodate rapidly expanding bodies. Since shedding frequency is highly correlated with growth rate, juveniles usually shed several times per year. Growth rate and shedding frequency slows with size and age, with large adult rattlesnakes sometimes shedding less than once per year.

Rattle growth is a fundamental part of the shedding process for rattlesnakes. The rattle is made of keratin, the same stuff as the acellular matrix of the corneal skin layer – and your fingernails. Each time the snake generates a new corneal layer and prepares to shed the old one, it also produces a new rattle segment. Thus the newest segment is always at the base of the tail and contains live tissue – much like the base of your fingernail. And because each new segment is the width of the tail, young rapidly-growing rattlesnakes produce a tapered rattle (photo below).

Male northern Pacific rattlesnake, Crotalus oreganus (CROR46), 09 March 2015 Original RAW IMG_5583.CR2 — Our Male 46 on 09 March 2015, just before his rattle was marked with paint. All segments except the newest are hollow and dry.

The rattle is made up of loose, hollow, interlocking segments. Except for the first segment (or “birth button”), each segment consists of three lobes, yet only one is visible. The two hidden lobes fit loosely inside the older adjacent segments (photo, below). There is nothing loose inside that makes noise, the rattlesnake’s buzz comes from the loose segments vibrating together.

A single rattle segment from a mature rattlesnake, showing the three-lobed structure (left). On the right is a back-lit rattle from an old rattlesnake (note the lack of taper) showing how the segments interconnect. Older segments are at the top of the photo.

Rattlesnakes are born with a tiny hard cap on the end of the tail. I like to use the analogy of an eraser on a pencil – which is the approximate size of the newborn snake and the cap. During the first ten days or so, the first rattle segment or “birth button” is produced, which is uncovered by the postpartum shed. This is the only time that a part of the rattle comes off with the shed “skin.”

Rattle cap of a neonatal Southern Pacific Rattlesnake (Crotalus oreganus helleri), about a minute old and still in its amniotic sac. — Rattle cap of a neonatal Southern Pacific Rattlesnake (*Crotalus oreganus helleri*), about a minute old and still in its amniotic sac.

Rattle of a one-week-old Southwestern Speckled Rattlesnake (Crotalus mitchelli pyrrhus) showing the newly-formed birth button inside the rattle cap, which will be lost with the post-partum shed in a few days. — Rattle of a one-week-old Southwestern Speckled Rattlesnake (*Crotalus mitchelli pyrrhus*) showing the newly-formed birth button inside the rattle cap, which will be lost with the postpartum shed in a few days.

"Skins" from post-partum sheds showing the neonatal rattle caps. The one on the right is in its natural condition (inside-out); the one on the left has been manually everted to expose the cap. — “Skins” from postpartum sheds showing the neonatal rattle caps. The one on the right is in its natural condition (inside-out, with the cap hidden inside); the one on the left has been manually everted to expose the cap.

Birth button of two-week-old Northern Pacific Rattlesnake after post-partum shed. This is the first rattle segment and will stay with this animal until it gets too dry and brittle, which will allow it to break off while being dragged through the brush in a few years. — Birth button of a two-week-old Northern Pacific Rattlesnake after its postpartum shed. This is the first rattle segment and will stay with this animal until it gets dry and brittle, which will allow it to break off while being dragged through the brush in a few years.

During every subsequent shed, the old corneal layer will simply be an open tube that slips over the rattle. When a shedding cycle begins, formation of the new rattle segment is usually noticeable before the eyes turn blue (photo, below).

On 24 June, the tail of Female 54 showed early tell-tale evidence of a shedding cycle beginning. The black segment is the live one but notice how the scales next to it look almost transparent and the tissue beneath is much lighter... that is a new segment beginning to form. — On 24 June, the tail of Female 54 showed early tell-tale evidence of a shedding cycle beginning. The black segment is the live one but notice how the scales next to it look almost transparent and the tissue beneath is much lighter… that is a new segment beginning to form.

A week or so further along in the shedding cycle than Female 54 (above), the tail and rattle of Male 46 last May illustrate the progression of growth of a new segment in the end of the tail. During the process, live tissue receeds from the last new segment (next to the paint), leaving it hollow and dry after the old corneal layer is shed. — A week or so further along in the shedding cycle than Female 54 (above), the tail and rattle of Male 46 last May illustrate the progression of growth of a new segment in the end of the tail. During the process, live tissue recedes from the last new segment (next to the paint), leaving it hollow and dry after the old corneal layer is shed.

Injecting acrylic paint into the first hollow segment allows me to identify rattlesnakes visually, with each animal receiving a unique color combination. The paint also allows me to record how often they shed.

This is the rattle of my Female 05 from El Dorado Hills in October 2013. The paint in the middle of the rattle is from her initial capture in June 2010. I marked her rattle with new paint in 2013 (next to the black live segment) because the older segments will eventually dry, crack, and break away, taking the original paint with them.

In Female 05 (above), note that her birth button and the next two or three segments are already missing. The location of her original paint tells us that she has shed four times in the 40 months since she was first marked. The slight taper at the end of her rattle provides a hint of her age: considering the number of segments I think are missing and that snakes grow much faster and shed more frequently when young, I estimate that I originally captured her in her third or fourth year, making her about seven years old in this photo. Once all of the tapered segments at the end are lost, we have no way of estimating how old she is from the rattle.

You can also see that the rattle segments produced by Fem 05 (photo, above) as a young rattlesnake are noticeably larger than the more recent thin segments. This happens in the middle of rattles, too, with some segments being wide and robust while others are thin. I think rattle segments are a bit like tree rings in that good conditions with lots of food produce wider thicker segments. In the case of adult females like 05 above, it likely reflects the shift in resource allocation (more below) when she became sexually mature. It is worth mentioning that this snake had reproduced three years in a row when this photo was made (and she produced the four recent thin rattle segments) and her body condition was very poor after three consecutive litters.

In almost all rattlesnake species, adult males are larger than adult females. Yet the growth rate of baby males and females is indistinguishable until they reach sexual maturity. Once they start reproducing, however, female growth slows. We believe this happens because females start diverting most nutritional resources to the production of offspring, leaving much less available for their own growth. Males, on the other hand, are free to continue devoting their resources to increased body size – which is advantageous for fending off predators while searching for females and for battling other males for access to receptive females (click here for video of males fighting).

Finally, can we tell a snake’s age from the rattle? The one sure thing is that each rattle segment does not represent one year. The addition of rattle segments is well correlated with growth rate which, in turn, depends on age and food intake. We can make a pretty good estimate of age from a complete unbroken rattle and even when a few segments are missing, so long as some significant taper remains at the end. But for older snakes with broken rattles having no taper, there is just no way to know how many segments are missing.

So how long can rattlesnakes live? Decades! I have personally kept some southern California species in captivity over twenty years. The Splash Education Center at Mather Field has a healthy Northern Pacific Rattlesnake that has been in captivity for a well-documented 32 years! But how long they survive in the wild is a much different question. Captive snakes do not have to contend with coyotes, hawks, owls, king snakes, temperature extremes, and all the other hazards of a natural existence – not to mention humans and their cars. While references like Robert Stebbins’ Field Guide to Western Reptiles and Amphibians (2003; Houghton Mifflin Co.) list the maximum size of our northern California species as over five feet, the fact is that three-footers are now uncommon. Large rattlesnakes have become very rare all over the United States, with individuals approaching old record lengths almost never found. Of course, in some remote wilderness where people rarely visit (if there is such a place today!), maybe there are still a few very large wild rattlesnakes…

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.

Early home ranges and other news

Good news: On 25 April, I came across our Female 39, who’s transmitter had failed prematurely 19 days before. She was captured, her transmitter was surgically replaced, and she was released the next day. At her initial capture last July, she weighed 365 grams (12.9 oz) and she produced a litter of kids a couple of months later. When recaptured last weekend, she weighed 404 grams! She clearly has been hunting successfully and is in great shape to reproduce again this year. While tropical pit vipers often reproduce annually, pit vipers in temperate climates like ours often require a year or two to replenish body fat before they can sustain another pregnancy.

But annual births are not unheard of this far north. In fact, we had a female rattlesnake produce litters three years in a row in El Dorado County a few years ago. After her third litter, however, she was skin and bones and we didn’t think she would survive the winter. But she did and might have pulled through if she had not been nailed by a raptor the following spring.

One day we found her radio signal to be weak and coming from far down a canyon, well outside her typical home range. Because thick manzanita and chamise made getting to her signal very difficult, I didn’t investigate right away. Eventually, when her signal didn’t change, we burrowed through the chaparral until we got to a large live oak that stood by itself on a hillside. The snake’s transmitter was laying in the leaves under the tree, completely clean and undamaged – as if it had been surgically removed and washed! I have had telemetered rattlesnakes eaten by coyotes before but coyotes chew the transmitters. The undamaged transmitter under a lone large tree far from the snake’s last location just screamed raptor. The area was full of red-tailed hawks and there were certainly owls at night.

Back to our current study, just two males, 36 and 37, remain loose with failed transmitters. I recently shipped the other five faulty transmitters back to the manufacturer for evaluation and repair.

Last week I found one of the hollow logs frequently used by our telemetered rattlesnakes freshly ripped apart (photo below).

I’m not sure what kind of animal, besides a person or a black bear, might have ripped a log apart like that. While it is certainly not impossible for a bear to stray this far downstream, it would also be attracting attention in more obvious ways. Having encountered a visitor with a snake hook off-trail a few weeks ago, I thought this might be a good time to reiterate why I am no longer publishing plots of the study animals’ travels (as I did last year) or providing more details about where they are hanging out. Too many rattlesnake researchers have had study animals captured or killed after they disclosed their locations.

But I do want to share how far the telemetered rattlesnakes have been roaming during the first six weeks of the season. The illustration below shows the home ranges used by the seven rattlesnakes with working transmitters so far this year. The solid lines are the males and dashed lines are the females. The Effie Yeaw visitor center is in the top center and the EYNC parking lot is in the top left corner of the Google Earth photo.

The most interesting finding to me is that Females 41 and 47 have been moving as much as any of the males and much farther than some. Also remember that we lost almost three weeks’ movement on Female 39 when her transmitter failed. Male 35 had a small home range last year, primarily because he hung around in the elderberry and redbud thicket next to the bike rack for so much of the summer. He spent most of the rest of his time in the meadow – and that’s where he is again. Male 40 was the last to leave his winter shelter this spring, has not moved as frequently as the other males, and I have not found him courting any females. He has always been quite under weight for his length. He is an old guy, based on his untapered rattle and an impressive collection of scars, but he seems to be getting by.

Courtship seems to have slowed a bit in the past week, with most animals by themselves and apparently hunting, at least when I have visited. Nonetheless, there should be another month or so of courtship before the summer hiatus.

19 April: Things heat up and more courtship

The little male courting Female 47 in the video I posted on 4 April continued to court her until at least 11 April, staying with her a total of 9 days that I observed. During that time, the female made several short moves of just a few meters and I never observed any indication that she did anything but ignore the male. Presuming that she was trying to hunt (these rattlesnakes are predominantly sit-and-wait ambush predators), it is hard to imagine that she could have had much success with his nearly constant movement. Despite my best efforts, the little male evaded several attempts to catch him. On two occasions, I actually had him but was unable to get him into a bag before he wiggled loose and disappeared into thick grass. On both occasions, I was sure I had spooked him and he would abandon the female but he didn’t. He only became more wary and would vanish instantly the moment he detected my approach. Since I have enough photographs to identify him in the future, I have given him the next identification number: CROR51.

Then on 13 April, I found that Female 47 had made a 97 m (318 feet) move out into the meadow. When I located her, she was under a mat of old dry grass beneath the living grass. I did not find Male 51 with her but, because I had to dig around in the grass to find her, he certainly had time to flee unobserved. She was in the same spot with no other snake observed on 15 April, too. On 17 April, she had moved 58 m southeast, back into the edge of the forest. She was coiled in the grass, apparently hunting and alone.

Today, 19 April, she had moved only a couple of meters. But as I got close to her signal, a rattlesnake shot out of the grass near my feet and under a nearby log – Male51! When I actually found Female 47, she was about 5 m from where I had flushed Male 51 and I saw why Male 51 was not with her. She was being vigorously courted by Male 49, a much larger male that was processed and released in early March without a transmitter. In fact, Male 49 had courted this female between 22–26 March at another location 150 m away. You can see the value of marking the rattles with paint in the photo from today (below), Female 47’s rattle is marked with yellow/green paint and Male 49 is marked with white/green. Since Male 49 is not telemetered, he would not be identifiable without the paint in his rattle.

Sadly, Female 39’s radio signal disappeared on 3 April. Of the six animals implanted with that batch of refurbished transmitters, five failed early. I have found and replaced transmitters in three of them, Males 35, 38 and 40. At present, Males 36 and 37 and Female 39 are carrying transmitters with prematurely dead batteries. I am still hoping to either find them courting or being courted by telemetered snakes or to have them turn up around the buildings or Maidu Village where staff can capture them.

Now that we have accumulated some data this spring with three females that are not incubating late-term embryos, I can demonstrate more effectively a behavior I mentioned last year. During the last 2–3 months of pregnancy (generally about mid-July to October), females find shelters where they can thermoregulate to stay within a narrow body temperature range. That is, they need a shelter that keeps them warm at night but where they are protected from the midday heat. I have repeated the chart from last year below on the left, showing the body temperatures of two late-term pregnant females compared to the males. The chart on the right is corresponding data from this spring, comparing body temperatures of non-gestating females with the males.

In these charts, “frequency” is the percentage of observations where I recorded each body temperature for that sex during that period. In 2014, the average male body temp was 24C (75F) while the female average was 30C (86F). This spring, males have averaged 24C (75F) and females 22C (72F). Of course, the weather is cooler in the spring (even this spring) than in summer but there is ample opportunity for these snakes to get their body temperatures up above 30C (by late morning, ground surface temperatures are often above 40C in direct sunlight, even when air temps are relatively cool). But my point is that there is no real difference between average male and female body temperatures before the pregnant females go into their late-term thermoregulatory behavior… which is evident in the late summer data above where body temps of the pregnant females narrowly cluster around 30C (86F).

(For you statisticians that may read this, I admit that these data are not publishable in this form as they contain some significant pseudoreplication. Nonetheless, they serve to illustrate my point that gestating vs non-gestating female body temps are very different.)

Hopefully, we will get a few more females telemetered in the next month or so and, by mid summer, we should know if any are going to reproduce this season.

Stay tuned!

Courtship video

Yesterday, 3 April, I found an unmarked rattlesnake with a very short broken rattle on top of our Female 47. The unmarked snake fled immediately but I was able to see that he was a small adult with only two rattle segments (the live proximal segment plus one hollow one). As there is no reason for another female to be accompanying Female 47, I almost certainly interrupted a courting male.

When I returned today, Female 47 had moved about 6 m and the same unmarked snake was on top of her. Since I approached with more caution today, I was able to watch and video tape the courtship. The female appears to have been in a typical round “pancake” coil and looks to be completely unresponsive; she does not move except for being pushed around by the male. The male, on the other hand, goes through a cycle of 30-90 seconds of rest followed by a short period of head and body jerking, short rapid tongue-flicks and chin-rubbing on the female, culminating with pushing his cloaca around under her. During this process, you will see the male wag his tail. The male then goes back into a rest phase before repeating the effort.

This is very typical rattlesnake courtship behavior. Since the cloaca (on both sexes) is sealed by a flat tight-fitting belly scale, courtship by the male is almost certainly wasted effort unless the female is receptive. What makes her receptive? Can he woo her into being receptive? We are not sure. Head-jerking, tongue-flicking and chin-rubbing is energetically expensive for the male and the movement exposes both snakes to potential predation, so it must be productive or the behavior would not endure. And the female is almost certainly selective, maybe selecting mates based on size, health, courtship performance, or other criteria. How long the male can keep up the courtship may be an indication to the female of stamina and health.

Female health, particularly replenishing body fat since delivering her last litter, may affect the female’s receptivity but fertilization of ova does not necessarily coincide with copulation. Long-term sperm storage is well established in many pit vipers, including rattlesnakes. Thus, it is common for a female rattlesnake to store viable sperm for multiple seasons, allowing her to fertilize ova without having recently mated. But does that mean that a female will mate when she is not physically ready to produce young? At this point, we just don’t know how all of this works.

In the video, the male’s head is at the top of the frame and his tail is in the lower left. The female’s head is in the lower right. So, click here for a 45-second clip of today’s courtship.

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.

Also on 22 March, Female 47 was still at the small hollow log but Male 38 had been replaced by Male 49, who was actively head-jerking, chin-rubbing, and tongue-flicking the female. Male 49 is not telemetered (but recognizable by white/green paint in his rattle) and at 767 mm SVL, he is not quite as long as Male 38, but he outweighs Male 38 by 28 grams (377 g vs. 349 g), Of course, I have no way of knowing if 38 and 49 even crossed paths; Male 38 could have departed before Male 49 arrived.

On 23 March, Female 41 had moved back to the birthing refuge used by Female 39 last year and had apparently been followed by Male 46; they were still together there on 24 March, although they were basking about 4 feet apart when I visited on both dates.

Female 47 was laying partly in the sun on the morning of 23 March with Male 49 nowhere in sight. Of course, without a transmitter, I had no way to find him. But they were laying together in the same place again on the next two mornings, so I suspect he was there on the 23rd, just not visible.

Female 41 remained at 39’s old birthing refuge on 25 and 26 March. Male 46 was still there on the 25th but they had been joined by Male 38. On this day, Female 41 and Male 46 were again basking apart from each other and male 38 was out of sight, betrayed only by his radio signal. The next day, 26 March, the female was basking, Male 38 was there but out of sight, and Male 46 was alone in a poison oak thicket some distance away.

By 27 March, Female 47 had left the small hollow log where she had been for nine days (with 3 males at various times) but she had been replaced by Female 41, leaving Male 38 apparently alone where he had been with Female 41 for the previous couple of days. As far as I could tell, Female 41 was also alone.

Also on the 27th, Male 35 was found a few minutes after 11 AM eating a California vole (aka meadow mouse, Microtus californicus) in thick knee-high grass next to the main trail, not far from the picnic area. The snake stopped swallowing and we were lucky that he did not spit out the rodent when he was disturbed, as rattlesnakes are quite defenseless with their mouth stretched around a meal. George Nyberg and I had to remain motionless for many minutes before the snake finally decided it was safe to continue swallowing. We shot a few photos as he finished his vole.

On the 28th, all of the telemetered snakes were coiled in vegetation, alone, and apparently hunting. Both places where most of the courtship had occurred over the past two weeks were empty. Are they finished courting? I doubt it; it’s not even April yet!