Spring 2015 – EYNC Rattlesnake Study

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!

Copulation & a malformed rattle

Just a quick entry regarding two very interesting recent observations:

First, just in case you were wondering what rattlesnakes do when they first warm up after a long winter’s nap… I came across this copulating pair early this afternoon. What you see in the photo (below) is almost all I could see. Most of their bodies were hidden in the deep grass. The elevated tail is the female’s and the male’s tail is wrapped under her cloaca. You’ll notice that neither rattle contains any paint; they were (and remain) unmarked snakes. I didn’t disturb them!

Also, the most recent animal to be processed and released at EYNC was a small (554 mm/22 inch) female with a deformed rattle (below). These are pretty rare. It looks like a congenital deformity rather than being the result of trauma. One additional segment clings to the terminal one and the snake can make a little buzz but I am sure she is incapable of accumulating a string of segments. The lesson here is that even in the rare event of a deformity or amputation, no rattlesnake has a long tapered tail like California’s harmless snakes.

16 March: General emergence has occurred!

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

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

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

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

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

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

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

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