By Michael DeLea
If
you’re anything like me, you have spent many a night lying wide-awake in bed,
racking your brain for the answer to the following question: How in the name of
evolution did the rattlesnake get its rattle? Well, you’re in luck because that
is exactly the topic I intend to tackle with this blog post!
Rattlesnakes
are unique among vertebrates in possessing the unusual structure that we
commonly refer to as a rattle. This innovative structure is the result of
numerous anatomical, physiological, and behavioral specializations and has been
the object of interest and speculation for decades. Additionally, it occurs in
less than 30 of the approximately 2,700 known species of snakes in the world.
The structure and complexity of the rattle suggests that it arose only once in
a shared common ancestor as opposed to evolving independently in each of the
different species of New World pitvipers.
Rattlesnake
neonates are born with what is referred to as a “button” at the tip of the tail
that is bulbous with two annular constrictions that prevent it from being shed.
This is the site of actual rattle segment formation and consists of the bony,
club-like style that is attached to the terminal region of the caudal vertebrae,
the fibrous connective tissue that forms the rattle segment mold and the epithelial
tissues. Every time a rattlesnake sheds, a segment is added to the base of the
rattle. A juvenile snake may add three to four segments each year where an
adult may only add one segment annually. This notion, and the fact that rattles
may break if they become too large, disprove the myth that you can determine the
age of a rattlesnake by counting the number of segments.
Rattles with varying numbers of segments |
Rattling
in these snakes is accomplished by shaking the rattle structure at very high
frequencies with the use of specialized muscles that are attached to the
modified caudal vertebrae. Three pairs of muscles that contract out of phase causes
the segments to rub against one another, producing the unmistakable sound. What
is interesting about the “tailshaker” muscle group is that it contains no
apparent unique structures. Instead, the proportions of the structures that are
common to all skeletal muscles are suited to minimize activation, contraction,
and relaxation times and to maximize ATP production.
Now
that you are familiar with the mechanism behind the rattling behavior from
which the name is derived, let’s talk about the potential adaptive
significance of the structure in these organisms. While the rattle itself may
be exclusive to the New World pitvipers, tail vibration is not. In fact,
defensive tail vibration is quite common in snakes, including many non-venomous
species. So why, then, did the common ancestor evolve this unique morphological
structure? There have been many hypotheses attempting to explain the origin of
the rattle. These potential explanations range from outlandish (i.e. snakes use
their rattle to “charm” and immobilize prey) to plausible. I will discuss the
three main hypotheses that are currently the most appealing amongst the
herpetological community.
The
first, and most popular hypothesis is that rattles evolved to serve as an
aposematic warning of the snake’s venomous nature. It is thought that snakes
evolved in the montane regions of Mexico, where they would have encountered
small predators such as the White-nosed Coati (Nasua narica) and the Ringtail (Bassaricus
astutus). It is therefore reasonable to think that the rattle was used to
warn these predators of the danger that would follow if they were to engage the
rattlesnake.
Another
hypothesis is that the rattle could be used to attract prey into the strike
range of the rattlesnake, as demonstrated in the feeding strategy known as
caudal luring. This behavior has been documented in many species of extant
snakes, including rattlesnakes. In this sense, the rattle could have evolved to
enhance the attractiveness of the “lure,” perhaps mimicking the insect prey of
frogs or lizards.
Caudal luring + Camouflage = Deadly Combination |
The
final hypothesis is that the tail vibration behavior (in venomous and
non-venomous snakes alike) is used to bring attention to the snake’s tail
instead of its more vulnerable head. If this were the case, then having a
rattle would only increase the effectiveness of the distraction when confronted
with a predator. This hypothesis may be the most difficult to believe as a
result of the recent discovery of at least two populations of rattle-less
rattlesnakes. The Santa Catalina Island Rattlesnake (Crotalus catalinensis) and the San Lorenzo Island Rattlesnake (Crotalus ruber lorenzoensis) populations
are characterized by a high frequency of individuals lacking a noise-producing
rattle. A possible explanation for this occurrence is the unusual arboreal
habits of these snakes. It is not difficult to imagine how having a noisy rattle
might be a hindrance while hunting prey in trees.
There
has not been conclusive evidence supporting any of these theories so the topic
continues to be hotly debated. New evidence, such as the loss of rattles in
certain species and subspecies, will be helpful in the attempts of biologists
to piece together the evolution of the rattle and the phylogenetic
relationships can be determined as a result.
References:
Meik, J.M., and A. Pires-daSilva. 2009. Evolutionary morphology of the rattlesnake style. BMC Evolutionary Biology 9: 35-43.
Moon, B.R. 2001. Muscle physiology and the evolution of the rattling system in rattlesnakes. Journal of Herpetology 35: 497-500.
Rowe, M.P., T.M. Farrell, and P.G. May. 2002. Rattle loss in pygmy rattlesnakes (Sistrurus milliarius): causes, consequences, and implications for rattle function and evolution. Biology of the Vipers: 385-404
Schaeffer, P.J., K.E. Conley, and S.L. Lindstedt. 1996. Structural correlates of speed and endurance in skeletal muscle: the rattlesnake tailshaker muscle. The Journal of Experimental Biology 199: 351-358
Photo Credit:
http://www.theradzoo.com/wp-content/uploads/2012/04/800px-Crotalus_atrox_USFWS.jpg
http://sdsnake.com/Snake/rattle_2X.jpg
http://28.media.tumblr.com/tumblr_luvsp5NKKy1r3jmtqo1_500.jpg
References:
Meik, J.M., and A. Pires-daSilva. 2009. Evolutionary morphology of the rattlesnake style. BMC Evolutionary Biology 9: 35-43.
Moon, B.R. 2001. Muscle physiology and the evolution of the rattling system in rattlesnakes. Journal of Herpetology 35: 497-500.
Rowe, M.P., T.M. Farrell, and P.G. May. 2002. Rattle loss in pygmy rattlesnakes (Sistrurus milliarius): causes, consequences, and implications for rattle function and evolution. Biology of the Vipers: 385-404
Schaeffer, P.J., K.E. Conley, and S.L. Lindstedt. 1996. Structural correlates of speed and endurance in skeletal muscle: the rattlesnake tailshaker muscle. The Journal of Experimental Biology 199: 351-358
Photo Credit:
http://www.theradzoo.com/wp-content/uploads/2012/04/800px-Crotalus_atrox_USFWS.jpg
http://sdsnake.com/Snake/rattle_2X.jpg
http://28.media.tumblr.com/tumblr_luvsp5NKKy1r3jmtqo1_500.jpg
Well, here's why I'm hunting this topic. The death-throes of creationism seem to consist of identifying various adaptations which have no function until they are fully, and NOT partially, formed. So how on earth could they evolve by Natural Selection? In Philosophy this kind of trick is called "the God of the Gaps" - you take the diminishing number of things that science cannot currently explain, and you announce, "Well, of course: that's where God resides." It involves twisting on its head a very useful credo of Samuel Butler's: "Whenever we are able to explain something, we remove it from the sphere of God's action."
ReplyDeleteUntil recently they were still trying this on with the feathered wings of birds - until it emerged that the feathers of late dinosaurs were plumes for display or the conservation of heat, and only later did it turn out that they were also useful for flight. Still the argument is being adduced against the alarmingly rapid (or so it seems) emergence of flowering plants and the insects that pollinate them.
So here is the Creationists' challenge: the rattle of rattlesnakes has no function until it has multiple segments, in themselves ineffectual, and is fully developed. I can find no discussion of rattlesnake evolution which gets beyond impenetrable morphological and zoological jargon that neither I nor other members of the public can understand. Please: in plain and real English: anybody care to rise to the challenge?