Friday, November 7, 2014

Many Tales of the Snake Tail

Upon first glance, snakes’ bodies may appear to be one giant tail (or one long body depending on how you look at it). However, snakes actually have a defined tail region which is separate from their main body. The tail starts at the cloaca, the magic hole where defecation, fluid release, and reproduction take place. We have multiple holes for these functions, but snakes only have one.  


The snake's tail is just after the cloaca


So what’s so special about a snake’s tail? Well, because snakes are limbless, their tails fill many of the roles that limbs play in other animals. For instance, the tail is used to grasp onto things, in defense against predators, and as a communication device. Because snakes use their tails for a variety of functions, their tails often look different than the rest of their bodies.

Specialized tail movements are exhibited in more than 70 snake species (Greene 1973). Tail movements usually consist of conspicuous motions of waiving the tail back and forth. Although many species differ in the ways in which they move their tails (slow undulatory motions compared to fast jerky movements), all tail displays probably serve an adaptive function (they benefit the snakes in some way). Many proposed functions for this behavior exist. They are detailed below.  





Many juvenile vipers, including rattlesnakes, use their tails to attract prey in what’s called a caudal lure. Their tails are often brightly colored and mimic insect larvae. The movement of their tails attracts animals that eat insects such as lizards and amphibians. Usually these snakes abandon caudal luring behavior (and their tail coloration fades) once they reach adulthood because their diet switches to mammals which are not attracted to insect larvae (Rabatsky and Waterman 2005b, Reiserer and Schuett 2008). 


Can you tell the Yellow-Lipped Sea Krait's head apart from its tail (left)?  The Spider-Tailed 
Horned Viper from Iran has a lure that looks suspiciously like a spider (right). 
(http://www.telegraph.co.uk/earth/wildlife/5979242/Sea-snake-fools-predators-by-making-tail-look-like-head.html)
(http://beta.ar15.com/archive/topic.html?b=1&f=5&t=1613157)


Other snakes use their tails in defense against predators. When attacked, many of these snakes will hide their heads under their bodies and waive their tails in the air. Some snakes, such as the Malaysian Pipe Snake (Cylindrophis rufus), do not just waive their tail at random, but violently strike it from side to side as if it were a head. The idea is that predators will aim for the tail thinking it is the snake’s head and this is beneficial to the snake because injuries to the tail are far less serious than injuries to the head. Many snake’s tails, such as those of the Indian Sand Boa (Eryx johnii.), actually resemble their heads in an effort to further confuse predators. Evidence of more scaring on some snakes’ tails compared to other parts of their bodies supports the notion that their tails deflect attacks away from the head (Greene 1973).

(Taken from Greene 1973)


What’s so fascinating about these tail displays is that they may hold the key to the evolution of the rattlesnake’s rattle. We know that today the rattle is used in defense and serves as a warning to predators. However, debate continues as to why the rattle evolved in the first place. There are two camps, those who believe the rattle first evolved to attract prey then switched to a defensive function and those who believe the rattle has always been for defense.  In support of the “prey -attractant-first hypothesis”, Schuett et al. (1984) state that the rattle pre-cursor must have started out small (1-2 segments) so it would have been incapable of making sufficient noise to warn others of the snake’s dangerousness. In support of the “function-has-never-changed hypothesis”, others point out that no other snake lineages that use their tails to attract prey have ever evolved anything similar to a rattle. 
The Dusky Pigmy Rattlesnake has a yellow tail and a small rattle. 
This tiny snake only has one rattle segment! Photo by Mark Herse.

The only rattlesnake we know of to use its tail (and not its rattle) for both prey capture and for defense in adulthood is the Dusky Pigmy Rattlesnake (Sistrurus miliarius barbouri). This species has the smallest rattle compared to its body size of all rattlesnakes (Cook et al. 1994), and 50% of adults in a typical population cannot produce sufficient rattling sounds because of the smallness of their rattles (Rabatsky and Waterman 2005a)! So these pigmy rattlesnakes may be similar to what rattlesnake ancestors may have looked and acted like. However, we don’t know for sure and debate continues on how and why the rattle evolved. 

Over the many years of remotely filming wild rattlesnakes, I have recorded three individuals exhibiting non-rattling tail displays (all adult Northern Pacific Rattlesnakes, one female and two males). This display consists of slowly flopping the raised tail from side-to-side (see video below).




Strimple (1992) emphasized the importance of collecting precise descriptions of the contextual stimuli that elicit tail displays to better understand their function. I have noticed some common themes among the three incidents I recorded. All snakes were in a loosely-coiled body position. This is different from an ambush coil which snakes employ when hunting prey. Snakes are typically loosely-coiled when shedding, digesting, or recovering from surgery (one snake was recovering from surgery, the other two could have been digesting but I am unsure). Snakes exhibited the display intermittently over several minutes (approximately 2-6 minutes), then left their sites almost immediately after. The cloaca of the snakes appeared swollen when they were displaying. Two snakes were alone when they displayed while the third was with another rattlesnake (both were males) and their tail displays can be viewed in the YouTube video above.

What could be the function of this non-rattling tail display? Although I lack enough evidence to definitively determine its function, I can speculate on the options.

Is it for prey capture?
  • This is unlikely because all snakes were not in hunting body positions when they exhibited this behavior (they were loosely-coiled).

Does it defend against predators? 
  • No predatory threat was visible on camera (although predators could have been close by) when this behavior was recorded. All defensive displays reported in other snake species are elicited by touching or severely harassing the snake (their first line of defense is camouflage). Thus, I remain skeptical that this is a defensive display given that the snakes were not physically disturbed.

Does it communicate with others of the same species?
  • This is possible. The cloaca appeared swollen and could have been discharging scented fluids (which have been shown to affect conspecifics). Perhaps the tail movements laid down the scent? Also in support of this, I recorded the tail display when two adult males were interacting with each other. The presence of one male appears to have caused the other to exhibit the display. Schuett (1997) found that tail writhing is displayed by defeated males after male-male combat in Copperheads (Agkistrodon contortrix), and is assumed to advertise the submission of the defeated male. However, it was not the breeding season and these males were never observed to behave aggressively toward each other. Thus, I remain doubtful that this display was to advertise submission.

I hope to converse with other naturalists and scientists who have seen similar tail behaviors in adult rattlesnakes. With enough anecdotal evidence we may be able to parse out the contexts in which this behavior occurs to generate hypotheses to test its function. 

Please contact me if you have observed this behavior! 


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References:

Cook, P. M., M. P. Rowe, and R. W. Van Devender. 1994. Allometric scaling and interspecific differences in the rattling sounds of rattlesnakes. Herpetologica 50:358–368.

Greene, H. W. 1973. Defensive tail display by snakes and amphisbaenians. Journal of Herpetology 7:143–161.

Rabatsky, A. M., and J. M. Waterman. 2005a. Non-rattling defensive tail display in the Dusky Pygmy Rattlesnake, Sistrurus miliarius barbouri: a previously undescribed behavior. Herpetological Review 36:236–238.

Rabatsky, A. M., and J. M. Waterman. 2005b. Ontogenetic shifts and sex differences in caudal luring in the dusky pigmy rattlesnake, Sistrurus miliarius barbouri. Herpetologica 61:87–91.

Reiserer, R. S., and G. W. Schuett. 2008. Aggressive mimicry in neonates of the sidewinder rattlesnake, Crotalus cerastes (Serpentes: Viperidae): stimulus control and visual perception of prey luring. Biological Journal of the Linnean Society 95:81–91.

Schuett, G. W. 1997. Body size and agonistic experience affect dominance and mating success in male copperheads. Animal Behaviour 54:213–24.

Schuett, G. W., D. L. Clark, and F. Kraus. 1982. Feeding mimicry in the rattlesnake Sistrurus catenatus, with comments on the evolution of the rattle. Animal Behaviour 32:625–626.

Strimple P. D. 1992. Caudal-luring: a discussion on definition and application of the term. In: Strimple PD, Strimple JL, eds. Contributions in herpetology. Cincinnati, OH: Greater Cincinnati Herpetological Society, 49–54.


1 comment:

  1. I've also heard the cloaca called the 'USB port' of orifices, because of its all-purpose nature.

    Fascinating observations. I do not doubt that these snakes are more social than is commonly believed, and that pheromones play a big role. You can be certain I'll be on the lookout for this behavior in the future.

    More on Pseudocerastes: http://snakesarelong.blogspot.com/2013/04/spider-tailed-adders.html

    ReplyDelete