Rattlesnakes without the rattle?

My crappy cell phone photo of Crotolus molossus in the Rincon Mountains a few weeks ago.
My crappy cell phone photo of Crotolus molossus in the Rincon Mountains a few weeks ago.

I dream relatively often of being bit by a snake. Probably because 3-5 days a week, I am tromping off trail through Saguaro National Park to my research sites, and I try to stay hyper alert to the threat of venomous booby traps. I have been buzzed off my intended path by rattlesnakes several times a summer, both last year and this year. A few encounters, however, have involved a snake lying stretched out on a road or wash that peacefully watched me approach, notice it, and move around. Then there was the one my boyfriend stepped on as he jumped down a pile of volcanic rocks in the dark, which rattled angrily after the encounter, but was fortunately too busy falling off the opposite side of the rock as him to strike.

Have you heard the rumor that rattlesnakes are less and less likely to rattle? The hypothesis goes that because humans (or in some versions, human introduced livestock) are likely to kill rattlesnakes they discover, those that avoid rattling have a survival advantage. Whether the decreased propensity to rattle is a genetic defect in the tail muscles or a behavioral predisposition to silence, selection favoring that morph is increasing its frequency.

I had asked herpetologists at the Arizona-Sonora Desert Museum last summer about this hypothesis, and been told there was little or no evidence for it. I received a similar answer from herpetology students at University of Arizona.

So imagine my surprise to hear NPR feature herpetologists discussing anecdotal evidence (personal observations in an unstructured setup) supporting this hypothesis in South Dakota this week. A quick search on Google Scholar and Web of Science turned up no papers by them or anyone else providing solid evidence of such a trend. A broader web search only yielded a site categorizing that as a disputed urban legend.

When you think about this, it is a difficult hypothesis to test. Quantifying the strength of selection for rattling ability and behavior across a landscape would be difficult, at best. A herpetologist could perhaps find as many rattlesnakes as possible, and quantify the proportion inclined to rattle. But beyond the question of whether they could conduct a search that is unbiased by rattling propensity (which would make those snakes easier to detect), we have no reliable long term datasets against which to compare this metric to tell if the proportion of rattling rattlers has changed over time. A massive survey of land managers regarding the number of snakes they kill every year, and how those snakes were found, could provide a metric of risk to the snakes in those areas related to rattling. But quantifying the presumed benefits of rattling with regard to the rest of the world besides humans would be an important metric to include in determining whether rattling has a net benefit or cost to survival, and that would be harder to measure.

Based on my small encounter rate, last summer, only 1 of 4 snakes rattled at me. This summer, however, 4 of the 6 snakes encountered have sounded a warning (excluding the one my boyfriend stepped on, which  I’m not sure whether or not to count or not). So in my limited experience, snakes are rattling MORE frequently.

Herpetologists, do you know anyone who is testing this? Or have better ideas of how to test it? Also, feel free to tell me why it is not even a useful question for moving the field forward, and not particularly worthwhile to investigate.

One thought on “Rattlesnakes without the rattle?

  1. I have been conducting formal studies on the Timber Rattlesnake (Crotalus horridus) since 1973. Sample size is about 15000 individuals, primarily in the central Appalachian region (Virginia, West Virginia, Maryland, Pennsylvania. I have some limited experience with the Eastern Diamondback–about 150 individuals. Unfortunately I did not record rattling. However, I have been convinced for a number of years that rattling is being bred out of many populations. In the wilder areas where rattlesnakes are likely to be accidentally trod upon by porcupines or bears they tend to rattle at the approach of a large animal. On the other hand in areas that get a lot of human use but have few or no bears or porcupines, there is a tendency to just freeze when approached and then when the danger has passed to quietly slip away. W. H. Martin whmartin@crotalus.org

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