Reptiles
Rattlesnake
Crotalus and Sistrurus · also called rattler
A rattlesnake is a pit viper that carries its own alarm. On the tip of its tail is a stack of loosely linked buttons of dead skin, added one at a time at each shed, and when the snake vibrates that tail dozens of times a second they buzz — a signal that means the same thing to a bison, a coyote, or a hiker: come no closer.
Overview
Rattlesnakes are around 40 species in two genera, Crotalus and Sistrurus, found only in the Americas and most diverse in the deserts and dry uplands of the southwestern US and northern Mexico. All are pit vipers — venomous, with folding front fangs and heat-sensing facial pits — and all are ambush hunters of warm-blooded prey, mainly rodents.
The rattle
Each shed leaves behind one hollow keratin segment at the tail tip, and these knock together when the tail vibrates up to 90 times a second. Segments also break off, so you cannot read a snake’s age from the count. The rattle is thought to have evolved from the tail-buzzing many nervous snakes do, enlarged until it became loud.
Senses and the strike
A pit on each side of the face reads infrared, letting the snake build a thermal picture and strike accurately in the dark. The strike itself — head out, fangs forward, venom in, recoil — takes about 50 to 90 milliseconds. Large prey is bitten, released, and then tracked by scent as the venom works.
Venom that varies
Most rattlesnake venom destroys tissue and blocks clotting, but some populations — certain Mojave rattlesnakes especially — add a strong neurotoxin, so bites from the same species can differ sharply by region.
Live young, shared dens
Rattlesnakes give birth to live young, and in cool regions many return each autumn to a traditional communal den used for generations.
Status
Rattlesnakes as a group are Not Evaluated, trend decreasing. Several are common, but many are falling to habitat loss, roads, deliberate killing, and roundups, and a few — the Aruba rattlesnake, the New Mexico ridge-nosed rattlesnake, the eastern massasauga — are seriously threatened.
Rattlesnake: common questions
Can you tell a rattlesnake's age from its rattle?
No, that is a myth. A rattlesnake adds one new segment to its rattle every time it sheds its skin, but it sheds two to four times a year depending on age, food, and growth rate, not once a year, so the segment count does not equal years. On top of that, the end segments regularly wear out and break off, especially in older snakes, so most wild rattlesnakes carry only a partial rattle of five to ten segments regardless of how old they are. A very long, complete rattle is unusual and is usually seen only in captive snakes that never damage it.
What should you do if you encounter a rattlesnake?
Stop, locate the snake, and back away slowly to at least a few metres, giving it a clear route to leave. A rattlesnake that is rattling is asking for space, not threatening to chase you, and it cannot strike further than about half to two thirds of its body length. Do not try to move, catch, or kill it, since a large share of bites happen during exactly those attempts, and a severed rattlesnake head can still bite by reflex for up to an hour. Keep pets leashed in snake country, watch where you put your hands and feet on rocky ground, and step onto logs rather than over them.
What does a rattlesnake bite do, and how is it treated?
Most rattlesnake venom is dominated by enzymes that break down tissue and interfere with blood clotting, so a serious bite causes rapid pain and swelling that spreads up the limb, bruising, blistering, oozing from the wound, and sometimes bleeding from the gums or in the urine, along with nausea and a metallic taste. Some populations add neurotoxic effects such as tingling and muscle twitching. Treatment is antivenom, given as early as possible, along with pain control and monitoring of clotting and kidney function. With prompt hospital care, deaths in the United States are rare, around five or six a year out of several thousand bites, but tissue damage can be lasting.
Why do rattlesnakes have a rattle at all?
The rattle is a warning device that benefits both the snake and large animals such as bison, deer, and people that might otherwise tread on it. For the snake, being stepped on or attacked risks fatal injury, and biting a big animal wastes venom that took days to produce and cannot be eaten, so a loud, unmistakable signal that says stay back is a good investment. The leading idea is that the rattle evolved from the simple tail-vibrating that many snakes do when alarmed, with the keratin segments gradually enlarging to make that vibration audible. It is found only in the New World pit vipers and is one of the clearest examples of a warning signal in reptiles.