Tag: age

Where are the rattlesnakes?

Well, it’s March and many recent days have been sunny and quite warm. Even though daytime highs have been listed in the 50s, that’s measured in shade and the temperature in the sun near the ground is always considerably warmer. So why were rattlesnakes basking in good numbers by late February last year but not yet this year?

There are two parts to the answer. First, most of the snakes spend the winter deep enough underground that they are insulated from daily temperature fluctuations. In other words, they do not feel daytime highs or nightime lows. Deep in their winter shelters, the temperature creeps up or down gradually in response to the mean (average) outside temperature. The second part of the answer is that natural selection tends to remove animals from the gene pool that don’t spend the winter deep enough and/or that venture out too early and get caught in freezing weather. The animals that select optimal winter shelters tend to live longer, and thus produce more offspring carrying the genes for those successful behaviors. (For those of you interested in the evidence for genetic influence on behavior, Time Love and Memory (2000) by Pulitzer Prize winning author Jonathan Weiner is an entertaining and astonishing summary, written in easy-to-understand language but with references to the scholarly research.)

So how do we know the temperature in the underground shelters?

Besides having recorded hourly temperatures for nearly four years with a data logger in a burrow 1 meter (39″) below ground in the Mohave Desert during a previous study, we can routinely calculate body temperatures of the hibernating snakes from the pulse interval of their transmitters during the winter.

The transmitters come from the manufacturer (Holohil Systems Ltd.; www.holohil.com) with a calibration graph showing the correlation between pulse rate and temperature for each transmitter (they vary slightly).

A standard conversion graph supplied by the transmitter manufacturer, Holohil Systems. Put simply, the transmitters beep faster when warm and slower when cool and at a predictable rate.

I routinely check the pulse rate in a water bath at room temperature before implanting a transmitter and adjust the mathematical conversion, if it differs slightly from the factory data. Then it is a simple matter in the field to time the pulse interval with a stopwatch and convert that measurement to temperature later. And since the transmitter is implanted inside the snake’s abdominal cavity, transmitter temperature equals core body temperature.

I also record hourly shade air temperatures with a data logger in the nature preserve at Effie Yeaw Nature Center. This gives me a standard local reference against which to compare rattlesnake behavior and look for correlations with weather conditions.

My data logger is located on the north side of a large cottonwood tree near the center of the EYNC nature preserve where it is never exposed to direct sunlight.

I can then compare rattlesnake behavior, body temperatures and air temperatures. This is especially interesting when one year is compared to another.

Some early temperature data for 2017. Note that the daily mean air temperatures vary greatly, depending on weather conditions, while the mean rattlesnake body temperatures tend to be nestled within the fluctuating air temps. These snakes were still underground and not affected by daily high and low temperatures outside.

You can see about six weeks’ data from this year in the chart above. The fluctuations in mean (average) air temperature (open circles) reflect the daily changes based on cloudy verses sunny days and cold air masses moving through verses warmer southern air (the same factors discussed every day by meteorologists).  The five filled circles are the mean body temperatures of the twelve telemetered rattlesnakes on the days that I recorded their data. There are more air temperature records because the data logger records automatically around the clock while rattlesnake body temperatures are only recorded when I visit the study site (which I do less frequently when the animals are inactive).

Now take a look at the corresponding data from last year:

Early temperature data for 2016. In these data, you will see that mean body temperatures are not buried among the mean daily temperatures. Rather, especially by the middle of February, the mean body temperatures greatly exceed the mean air temperatures. That’s because the animals were out of their shelters and basking in the sun.

Checking my behavioral observations from last year, I find a few rattlesnakes were starting to bask occasionally by the end of January and most were basking regularly on sunny days by the end of February. Regular basking continued through mid-March. Then, suddenly, between 17 and 19 March, most of the rattlesnakes left their hibernation shelters, as if someone had given them the “all clear” signal. This was multiple rattlesnakes leaving multiple shelters at the same time – pretty amazing! By the way, you may also notice that I visited and recorded data more frequently in February last year. That’s because there were rattlesnakes to see and behavior to record (besides the science; it’s just more fun when there are rattlesnakes to see!).

If I combine the raw data from 2016 and 2017 for comparison, the chart gets pretty messy and trends are hard to make out. But we can create mathematical models for those same data that present a much clearer picture.

These are mathematical models derived from the data in the two charts above. Note the difference between the temperatures of the basking rattlesnakes in 2016, compared to 2017 where the snakes were still underground and their body temperatures were “chasing” the changing environmental temperature, first rising in late January, then leveling off as the weather started to cool, and finally declining as the weather turned cold again in late February.

You can see that mean February air temperatures started out much lower this year compared to 2016.  And, in 2016, with mean air temperatures exceeding 50F by early February and staying there, the rattlesnakes began to bask and their body temps took off as a result. This year, air temps started nearly ten degrees lower, rose briefly, but then took a nosedive in late February, keeping the rattlesnakes cool and underground so far.

Of course, some individuals do not behave in the same manner as most others. I have heard some reports of a few rattlesnakes being sighted in the past few weeks and one dog reportedly bitten already. While none of the telemetered rattlesnakes at Effie Yeaw have emerged from hibernation yet, one rattlesnake without a radio has been basking early – just as he has done each of the past two years. My old skinny Male 40 has been frequently laying in the sun at the entrance to his winter shelter for more than a week.

Male 40 basking in the late morning of 2 March 2017. Rattlesnakes, like other animals, have individual idiosyncrasies, and this animal has been the first to emerge and bask three years in a row. Interestingly, he is typically the last to leave the winter shelter and the first to return in the fall.

As you may recall, Male 40 was implanted with a transmitter in 2014 but I removed his second radio last spring and released him without one because he was not producing useful behavioral data. It was not that he was just behaving differently than the others (after all, unusual behavior is interesting and valuable to record), he has been chronically and significantly underweight, appears to be quite old, moves very little during the summer, and has never been found courting a female. Given his poor body condition, I was afraid that he would die during hibernation and his transmitter would not be retrievable. But, even without a radio, he continues to be sighted regularly and has now made it through another winter.

But most others seem to be waiting for some sustained warmer weather before venturing out.

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.
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.
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…