THE MYSTERY OF THE RATTLESNAKE 1

The Mystery of the RattleSnake

Geri Glinsek

Special Program: Project Dragonfly

Miami University of Ohio THE MYSTERY OF THE RATTLESNAKE 2

Communication is used in all living things. Animals, plants, bacteria, and even an organism's cells use communication, but each of them express it differently. In animal communication there is a signaler (sender) and a receiver (Rosenthal & Ryan, 2000). The sender's signal was shaped into an evolved trait because of the effect it had on the receiver's behavior. Most animals use visual or auditory signals in communication. The auditory signals are produced by sound from an animal's organ. The visual signals are considered traits which are continuously expressed despite the signaler's determined state or environmental condition. The signaler and receiver interaction happens between species (interspecies communication), but it is more common within a species

(intraspecies communication) (Roger & Kaplan, 2002).

Pursuit-deterrent communication is an example of interspecies communication (Clark, 2005).

This type is between predators and prey. The prey are the signalers and the predators are the receivers. For example, many pursuit-deterrent communication happens between rodents and rattlesnakes. Most rodents will signal by mobbing and by using visual cues to warn other prey that snakes have been spotted. This form of communication mutually benefits both predators and prey because predators will avoid a difficult costly pursuit, and prey increase their survival rate by living another day. In only studying animals there are many different ways communication can happen.

A prime example of pursuit-deterrent communication is a rattlesnake's rattle. The rattle is only seen in Crotalus and Sistrurus (Allf, Durst & Pfennig, 2016). The rattle is made up of keratin which are loosely scaly segments located at the rattlesnake's tail tip. What makes the rattle vibrate is the loosely scaly segments rubbing against each other at 50 cycles per second

(Zug & Erst, 2004). THE MYSTERY OF THE RATTLESNAKE 3

One of the most complicated and unique acoustic-producing structures in science is the rattle

(Reisner & Schuett, 2016). There is a lot of controversy on how the rattlesnake rattle evolved.

Did it evolve because snakes were feeling threatened or was it due to Caudal Luring (CL) where baby lizards mistakenly identify the rattlesnake's rattle for dinner? One scientist held the idea that behavioral plasticity took a part in the evolution of the rattlesnake's rattle.

Another alternative to the rattle's beginning

Many different hypotheses are on the rattle controversy, but there is one hypothesis that many researchers and scientists agree on. They proposed the hypothesis that the rattlesnake's rattle evolved to warn predators to stay away (Zug & Ernst, 2004). However, not everyone agrees with this popular hypothesis. According to Reiserer and Schuett (2016), the popular hypothesis does not look at selection playing a part in the initial stages of the rattle which include anatomy, physiology, and behavior. Their study suggested two potential behaviors that formed the early selection on the rattle. The first one is Defensive Tail Vibration and Thrashing (DTVT). This behavior is similar to the popular hypothesis which the rattle evolved to warn predators, but also using it to cause distraction from a predator attacking it's head. Caudal luring (CL) is the other potential behavior that may have helped in the evolution of the rattlesnake's rattle. CL can happen when juvenile lizards are feeding. They are attracted by moving tails that resemble their prey (worms, insects), and end up biting snake tails. CL might have led to the early phenotypic change of the rattlesnake's tail so it mimicked a cephalized region of a moth or beetle larva.

Behavioral plasticity and the rattle

Behavioral plasticity is another possibility of how the rattle evolved. Allf and his colleagues

(2016), believed that behavioral plasticity could give rise to morphological evolution. They THE MYSTERY OF THE RATTLESNAKE 4 thought the best way to examine this is by looking at animal communication signals. In the natural world communication signals are combined with both complex behavioral and morphological traits which most scientists have yearned to understand. Communication signals begin when the receiver uses simple behaviors to guess the actor's next actions which expand over time. This leads to simple behaviors preceding to more complex signaling. Allf and his colleagues (2016) decided to study their predictions by testing the evolution of the unique behavior of the rattlesnake and the rattle.

Behavioral plasticity study

The researchers captured 155 individual snakes from 56 species and measured tail vibration behaviors (Allf, Durst, & Phennig, 2016). They filmed captive snakes so they could control the different environmental factors that could have affected tail vibration. Defensive behavior was expressed by snakes when researchers waved a stuffed animal head in front of them. Then the researchers assessed the strength of the phylogenetic signal, the species phylogenetic distance from rattlesnakes, and the relationship between each response assessed. The assessment helped to determine if tail vibration behavior came before the evolution of the rattle. Their results supported that tail vibration may have given rise to the rattlesnake's rattling behavior.

Conclusion

Snakes comprise more than 2,800 species, and yet we know little of their physiology (Zug &

Ernst, 2004). Humans only represent one species, but much more is known about us compared to snakes. Some species of snakes like the Santa Catalina Island Rattlesnake and the San Lorenzo

Island Rattlesnake have a high rate of losing their rattle segments (Rowe, et al., 2002). Why? It could be poor development, direct selection, or mutations. What does that mean for rattlesnakes? THE MYSTERY OF THE RATTLESNAKE 5

References

Allf, B. C., Durst, P. A., & Pfennig, D. W. (2016). Behavioral plasticity and the origins of novelty: the evolution of the rattlesnake rattle. The American Naturalist, 188(4), 475-483.

Clark, R. W. (2005). Pursuit-deterrent communication between prey animals and timber rattlesnakes (Crotalus horridus): the response of snakes to harassment displays. Behavioral Ecology and Sociobiology, 59(2), 258-261.

Meik, J. M., & Pires-daSilva, A. (2009). Evolutionary morphology of the rattlesnake style. BMC Evolutionary Biology, 9(1), 1-9.

Reiserer, R. S., & Schuett, G. W. (2016). The origin and evolution of the rattlesnake rattle: misdirection, clarification, theory, and process. Rattlesnakes of Arizona, 2, 247-274.

Rogers, L. J., & Kaplan, G. T. (2002). Songs, roars, and rituals: Communication in birds, mammals, and other animals. Harvard University Press.

Rowe, M. P., Farrell, T. M., & May, P. G. (2002). Rattle loss in Pigmy Rattlesnakes (Sistrurus miliarius): causes, consequences, and implications for rattle function and evolution. Biology of the Vipers, 385, 404.

Ernst, C. H., & Zug, G. R. (1996). Snakes in question: The Smithsonian answer book. Washington, DC: Smithsonian Institution Press.