Cantilurday viperid: Mexican mocassin

January 14, 2012 • 4:32 pm

by Greg Mayer

The cantil (Agkistrodon bilineatus), or Mexican moccasin, is a pit viper closely related to the water moccasin and copperhead of the United States. Like a number of other snakes, it moves its tail in a manner thought to attract the attention of prey, enticing them to come closer or look away from the snake’s business end, a behavior called caudal luring.

Juvenile cantil, Agkistrodon bilineatus, showing caudal luring.

In juvenile cantils, the effect is accentuated by the bright color of the tail tip; this color fades with age.

A second juvenile individual of Agkistrodon bilineatus.

There are several subspecies, distributed discontinuously from northern Mexico to northern Costa Rica. I’m not sure which subspecies the ones pictured here are.


Parkinson, C.L., K.R. Zamudio and H.W. Greene. 2000. Phylogeography of the pitviper clade Agkistrodon: historical ecology, species status, and conservation of cantils Molecular Ecology 9:411-420. pdf

20 thoughts on “Cantilurday viperid: Mexican mocassin

  1. I’ve been to El Cantil, but I’ve never seen one. But maybe one saw me and I was lucky that day.

    What sort of prey would they attract with the tail-wagging? Toads perhaps? Birds? I don’t imagine rodents would respond to that sort of thing.

    1. if they are anything like its relatives, my local Osage Copperheads, the best way to find them is to wander through the woods obliviously hiking and birdwatching,and they magically appear coiled and angry under my raised foot a split second before I step down, causing me to leap awkwardly while my heart shoots into my throat. at least this is how I always find copperheads.

      1. I’ve wandered through lots of apparently suitable cantil habitat in Mexico and Costa Rica but never found one. A large tropical rattlesnake was the “best” I ever did — and it was really troop of capuchin monkeys that found him first. They didn’t approve of his presence and made a lot of noise.

        Maybe either you have a talent that I lack, or copperheads are more common than cantil are.

  2. I’m outside my field here, but I wonder if this snake’s specialize tail could have been derived from an ancestor who also provided the rattlesnake’s tail. Both are pit vipers and both apparently move their tails in a side-to-side motion. I’m a native West Virginian, and I’ve seen plenty of rattlers. The rattle seems to be a warning to large critters to stay away rather than to lure prey, but as I understand evolution, that shouldn’t eliminate the possibility of similar derivation.

    1. It probably goes back even farther, John. A lot of young lizards also have brightly-colored, contrasting tails.

      1. our local Missouri black rat snakes, Elaphe obsoleta obsoleta, also shake their tails against dry leaves and underbrush when disturbed, making a somewhat convincing rattlesnake mimic, convincing enough to cause a momentary pause and give that lovely heart pounding rush, but I don’t think they’re known to use it for predation (young eat small lizards frogs and insects but the tails are not differently colored like Agkistrodon), just protection.

        1. Tail rattling is a pretty widespread behavior in snakes. In a few cases it may have something to do with rattlesnake mimicry (in dry leaves, it can make a similar sound), but it’s so widespread that more likely it’s just something snakes do (either for some other adaptive reason, or for no adaptive reason whatsoever), and rattlesnakes just happened to come across a morphological innovation to take advantage of a preexisting behavior.

          OTOH, I tend not to think that caudal luring in Agkistrodon or various superficially similar behaviors in lizards have any particular relation to tail rattling. They both involve tail movement, but that’s about the only similarity. Different kinds of movement, different contexts, etc.

          As a lizard example, Cophosaurus texanus will sometimes curl their tails upward (revealing strongly contrasting black & white bands) and wiggle them from side to side:

          Usually just prior to running off:

  3. According to the Shorter Oxford Dictionary, the name “moccasin” of both the slipper and the snake are from Algonquian, the slipper from early 17th C, the snake from the late 18th C (at least into English). Does anyone know why? Did they make the slippers out of the snakeskin, perhaps?

  4. Wow, another cool animal I’d never heard of. I, too, am curious about what prey this snake’s tail attracts.

    1. Yes, I’m sure you’re right. I found that site and a couple of others after I posted. But, information does seem to be scarce. My guess now is that it’s lizards that are the primary target of the tail wagging, since it’s young cantil that do this (losing the bright tail color in age) and many young snakes that dine on larger fare as adults are lizard specialists when small. Lizards are commonly alert and aggressive predators and will move quickly toward wiggling worm-like items.

  5. Many many species of snakes, including big pythons will move their tail sinuously or erratically when disturbed, while the rest of the body stays still. This may be to direct any predatory attack against the tail rather than the head – of course, lots of lizards do this as well, and can autotomise their tail as well.

  6. I’ve wondered about the relative importance of defensive vs. predatory function in the tail movements of snakes and lizards. The consensus among herpetologists is that much of it is predatory (as in the cantil), but sometimes the tail is definitely used as a distraction from the head. Rubber and rosy boas hide their head in their coils, and wave the tail around like a head. I’ve seen little tail movement in lizards, occasionally in anoles, but not often enough to generalize. In house geckos, the tail is frequently wagged just prior to rushing in to grab a prey insect in the mouth, but I don’t know that the insect is even aware of the movement, or how it could increase the gecko’s chance of a successful grab.

    It could be that both defensive and predatory functions can evolve from a common behavioral substrate. (As an example of this dual function from a common behavioral act, tongue flicking in lizards as a social signal has evolved from the use of the tongue in feeding.)


    1. “In house geckos, the tail is frequently wagged just prior to rushing in to grab a prey insect in the mouth, but I don’t know that the insect is even aware of the movement, or how it could increase the gecko’s chance of a successful grab.”

      And, of course, cats often do the same thing prior to pouncing on something. I’m not sure if anyone knows why…

    2. Skinks that show ontogenetic changes in tail colour often show tail movement to direct predatory attention to the autotomisable part. Autotomy itself, is, of course, very very cool

    3. Just to spin a different thought and data point.

      When hiking I’ve noticed a lot of lizards moving their tails as they watch me and a different species (I think) doing pushups. I wonder sometimes if it could be like a tennis player rocking which, I think, is to help in preparation for an anticipated quick response?

      Also, when trying to hand catch or swap flies, I often use my idle hand by moving it to the opposite side and closer to the fly than the hand that will be conducting the business. I wiggle my fingers in an attempt to keep the fly’s attention on the idle hand. I think I have a higher success ratio using that technique. However, I’ve never tried to formally verify it.

  7. It really looks like a copperhead snake which can be found here: except for the color. The snake’s tip on the second picture portrays a similar characteristic of copperhead snakes’ tips which are yellowish or greenish.

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