Island Ecology

The western rattlesnake, Crotalus viridis, ranges throughout western North America. It has traditionally been divided into 8 or 9 subspecies, which are basically groups of populations that are distinct but probably not distinct enough to warrant species status. Variation is the theme for this species. Body size varies greatly between populations. For example, the average body length for a population in southwestern Wyoming is about half that of those in southern California. That is just one example. Color varies, but tends to roughly match the habitat that a snake is found in. For example, insouthwestern Wyoming (where I have done most of my research) this species is a light tan color, which often makes it difficult to see in the semi-arid desert. In contrast, snakes in cooler areas tend to be very dark, almost completely black. This happens at higher elevations and also on islands such as South Coronados and Santa Catalina Island. Venom also varies between populations. Many populations shift from feeding on elongate prey (usually lizards) as juveniles, to bulky mammalian prey as adults. Along with this dietary shift, there are shifts in the amount and types of venom components. Studies on geographic and developmental variation of venom of this species are ongoing.

As an aside, it is often said that getting bit by a juvenile snake is worse than being bitten by an adult because the venom is more toxic. In a general sense, venom from a juvenile does have a greater proportion of toxic components, so drop for drop it does tend to be more toxic. However, the amount of venom injected is positively related to the size of the snake - the bigger the snake, the more venom it can inject. So overall, the bite from an adult snake will probably be worse. But it is complicated. One other thing, I am often asked whether you can age a rattlesnake simply on the number of rattles. This assumes that snakes only add one (or a set number) segment per year. Rattlesnakes add a rattle segment each time they shed their skin. Skin is shed based on growth. The more a snake grows, the more it sheds, the more rattles it has. Juvenile rattlesnakes grow faster than adults, and accordingly they shed more frequently. Juveniles add anywhere from 1-4 rattle segments per year depending on population and resource availability, whereas adults usually only add 1-2 rattles per year. Another problem, is that rattles break which complicates the counting. So the simple answer is that you can't accurately age a rattlesnake based on the number of rattles.

RATTLESNAKES ON SANTA CATALINA ISLAND

The only rattlesnake on Santa Catalina Island is the Southern Pacific Rattlesnake, Crotalus viridis helleri. This is a subspecies of the western rattlesnake. This is different than the Catalina Island rattlesnake found only on Santa Catalina Island in the Gulf of California that has lost its rattle. The rattlesnakes on Santa Catalina Island have full rattles, although they may not use them as much as their mainland relatives (I'll get into that later). It is unclear how or when this rattlesnake came to Santa Catalina Island. Some suggest that they were brought here by humans. I am currently doing genetic analyses to see how long the rattlesnakes have been on the island. Records for rattlesnakes mainly come from near Avalon and Two Harbors. This makes sense - the more people you have, the more likely it is that they will see rattlesnakes. It appears that rattlesnakes occur throughout most of the island, however that is tentative. Rattlesnakes on Santa Catalina Island appear to be less aggressive than individuals of the same species on the mainland. This may be related to them being on the island without native terrestrial predators. I am currently doing laboratory studies of the defensive behavior of mainland vs. Santa Catalina Island rattlesnakes and hope to do field studies in 2000. The rattlesnakes on Santa Catalina Island are dwarfed relative to those on the mainland. They reach a little over half the size of those on the mainland. The largest snake that I have examined from Santa Catalina Island is about 31 inches long (from head to base of the rattle). This doesn't mean they aren't intimidating!

Kyle Ashton in the field © 1999 Kyle Ashton

I am currently a graduate student at the University of Colorado. My research focuses on the evolution of body size, behavior and life history of vertebrates. I have done a lot of research on rattlesnakes, specifically the western rattlesnake (Crotalus viridis). I am currently constructing a phylogeny, which depicts the relationships among populations of this species, from DNA sequences. This phylogeny will be very useful in addressing evolutionary questions for this species. I have done a mark-recapture study of the western rattlesnake in southwestern Wyoming for three years. This study has involved radiotelemetry to study the movements and general behavior of the snakes. Basically, I take a transmitter and surgically implant it into the belly of the snake. This allows me to track the exact movements of each snake for several months. This yield information on mating and other social behavior, movements and why those movements occur, where hibernation sites are located and so forth.

Kyle helping a rattlesnake to start breathing after surgery © 1999 Kyle Ashton

Kyle G. Ashton
Department of E. P. O. Biology
Campus Box 334
University of Colorado
Boulder, CO 80309

email: kyle.ashton@colorado.edu