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…

 

 

11 thoughts on “Rattle Growth, Shedding & Estimating Age”

  1. We have property in Hawk Creek Canyon, Lincoln County, WA. In July 2016, we found a western rattlesnake between our cabin and the creek that we (carefully) estimated to be between 45 and 48 inches long, about 2 inches in diameter. It had the button, seven hollow rattles, and the dark new rattle. My son-in-law, a biologist, thought it was in its third or fourth year, but was not certain. We try not to bother the rattlers, they kill gophers and mice, and we appreciate them. …But a big one, just 20 feet from the cabin, was a bit much.

    We captured it, put it into a garbage can, and transported it a few miles up the road, where we released it. Knowing how far they can travel, he may be back next year. I have a few pictures of it, but don’t know how to post them here.

    1. Hi Jim. Wow, rattlesnakes must grow fast in Lincoln County! You have the same species of rattlesnake as we have here in northern California (Crotalus oreganus) and a nearly four-foot animal would be the largest I have seen. Yet your son-in-law’s estimate of age is probably right in the ballpark for a rattlesnake with an unbroken rattle of nine total segments. The good news is that, at that size, the snake is almost certainly a male and males tend to wander widely, rather than hang around in a small area… like around your cabin! Here and farther south, the males’ wandering is usually greatest during their courtship season which is in spring and late summer/fall, not in July. But I’m not sure how the courtship season might change at greater latitudes with shorter summers. In any event, thanks for carefully moving him rather than killing him. There’s an excellent chance that you’ll never see him again.

    1. You really cannot tell a rattlesnake’s age from its rattle. The exception is young rattlesnakes with a tapered unbroken rattle, which allows for a good estimate of how old they are. They produce a new rattle segment each time they shed the outer layer of their skin and the frequency depends on how fast they are growing: more frequently when young and growing fast and less often when old, large, and not growing much. They almost always shed more than once per year, so the old “one rattle segment per year” idea is not accurate. The rattle grows from the base and big rattlesnakes invariably have broken off the tapered part of their rattle, which means we have no idea of how many segments have been lost when we encounter these big guys. By the way, nine feet is a record-length rattlesnake of any species!

  2. Hi there, asking if there is a ‘rattle skin’ left behind when the snake sheds. In Terlingua, Texas we recently found a 3′ rattlesnake skin and it looked like there was a part for the rattle.

    I do have a pic but not sure how to attach.

    Thanks!

    Julie z.

    1. Hi Julie,

      Except for the first natal shed that occurs about 10 days after birth (when the snakes are the size of pencils), the tail end of the shed “skin” from a rattlesnake is just an open tube that slips off over the rattle. If an adult rattlesnake’s rattle is damaged or deformed, I suppose old rattle segments could come off with the shed skin as the snake crawls out of it. I have never seen that but the old dry rattle segments routinely get brittle and cracked, allowing them to fall off or be broken off. I don’t know why this could not happen during the shedding process but the rattle segments would not be actually attached to the shed skin.

      Mike

    1. The paint in the rattle allows me to easily identify a rattlesnake visually that I have captured before. A rattle without paint signals an animal I have never captured and processed. Each animal that I process for the study gets a unique color combination of acrylic paint harmlessly injected into a hollow rattle segment.

      But to be able to repeatedly find rattlesnakes to study them, I surgically implant a small radio transmitter into their abdominal cavities. The battery lasts a year and the radio must be replaced annually but the signal allows me to find the radio-tagged animals any time I wish. I only have radios in twelve rattlesnakes currently, due to the time it takes to find the that many snakes and record data.

      In short, the rattlesnakes are “tagged” in different ways. I now have about 50 Effie Yeaw rattlesnakes marked for visual identification with paint in the rattles and twelve of those are also implanted with radios that allow me to find them frequently.

      Mike

  3. I saw a wee snake at a park near our home–about 12″ in length–and it had the patterning of a western rattlesnake, as well as the triangular head shape, but could see no evidence of a rattle. When I looked at a photo I took of it, on the shadow of its tail I did see a slight bulb-shape at the tip. I would post the photo here but I guess this comment section isn’t set up for that.

  4. I was taught (and have observed) that c. Atrox (western diamondbacks’) rattles were straight and c. Viridis (prairie rattlers’) rattles were tapered. Is this a myth or is the tapered rattle = young snake only true for the species you study?

    1. Hi Jerry! Sorry for the late reply… too much spam to sort through. Tapered rattles indicate young snakes in all rattlesnake species and straight rattles are older snakes. The reason is that a new rattle segment forms at the end of the tail each time the snake sheds the outer layer of its skin. Thus, each new segment is the same width as the tail. So small rattlesnakes produce narrow segments and larger rattlesnakes produce wider ones (because the tail is bigger). And like other vertebrates, young rattlesnakes grow rapidly so each rattle segment is a little wider that the one before. Big older rattlesnakes grow very slowly or not at all, so rattle segments are the same width. The rattle is made of keratin, the same stuff as our fingernails, and the older segments get dry and brittle and eventually break off. Over a few years, the snake loses the tapered part of the rattle it produced when it was small and growing rapidly, leaving only the more recent larger untapered segments. I hope that makes sense… it’s really a simple process and it’s the same across the various species. Best wishes, Mike

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