In Search of the Coronado Island Rattlesnake

The famous arch of Cabo San Lucas

Baja California is my new obsession. The weather is perfect, the landscape is beautiful, and the ocean is always a stone’s throw away. I just got back from two trips past the border: (1) to Cabo for my honeymoon (where the screams of spring breakers soar through the air), and (2) to Southern Coronado Island in search of the elusive Coronado Island Rattlesnake.

Rattlesnakes live on several islands off the coast of Baja. Many of these snakes are the laid-back island counterparts of their mainland selves, and like other island inhabitants, many have drastically changed in body size from their mainland ancestors. For instance, Crotalus ruber lorenzoensis of San Lorenzo Island is a tiny descendent of the larger mainland Crotalus ruber. The Santa Catalina Island Rattlesnake (C. catalinensis) is one of the more famous species as it is the only known rattlesnake to “lose” its iconic rattle, although lorenzoensis and others seem to be on their way (Radcliffe and Maslin 1975). Poor thing only has one rattle segment to its name! Most of the Baja island rattlesnakes are endemic, which means they occur in one place and nowhere else (and this would be whichever island they inhabit). Although most of these snakes are not endangered, their endemic status is still a conservation concern because it means they have extremely restricted geographical ranges.  

The Catalina Island Rattlesnake (far left) is considered rattleless with only one small segment at the base of its tail. The two island
subspecies of ruber (middle and right) are on their way to becoming rattleless. Taken from Radcliffe and Maslin 1975.

  

The tiny island of Southern Coronado. Photo: B.J. Putman.

The Coronado Islands are about 15 miles South of San Diego and 8 miles from mainland Mexico. The largest of the four is Southern Coronado Island with just 1 square mile of land. The Coronado Island Rattlesnake (Crotalus oreganus caliginis) is the only snake species on the island*. Not much is known about this species and so my adviser at San Diego State University has collaborated with Dr. J. Jesus Sigala Rodriguez of Universidad Autonoma de Aguascalientes on a new project to learn more about its ecology. Last week, I went with him to the southernmost island to search for snakes that will become part of a long-term monitoring project. We drove from San Diego to Rosarito with minor hiccups other than being stopped by border patrol agents who could not comprehend the unusual amount of wooden cover boards in our trunk. From Rosarito, we took a panga boat to the island. In about 45 minutes we arrived to our destination. The island does not have a dock so our skilled boat driver slowly maneuvered the panga to the closest rocks, which we jumped onto from the boat.
* Article correction: the night snake and gopher snake also live on Coronado Island

About to launch our boat in Rosarito, Mexico.

Although the island boasts 1 square mile of land, most of the land is inaccessible because it is so steep. We set off searching for snakes where we could and placed cover boards in areas that seemed promising. Alligator lizards (Elgaria multicarinata) were by far the most commonly encountered herp – we saw way too many to count! We also found a few skinks (Plestiodon skiltonianus). In all, our greatest find was a legless lizard (Anniella pulchra?).

We found 8 individuals of our target species, and not to brag, but this tiny girl found the most out of everyone in our group (brushes off shoulders). I found one of the rattlesnakes next to a dead alligator lizard. I assumed it was about to consume the lizard before I disturbed it. Interestingly though, the lizard’s eyes were missing which suggests it had been dead for some time (probably a day or two). We were thus unsure whether the snake had struck the lizard the day before and had just relocated it or whether the snake was scavenging it.

Left: The beautiful legless lizard.  Right: A rattlesnake found with an eyeless alligator lizard (sorry for my horrible photo editing skills)

Left: Processing a snake while enjoying the view.  Right: Drawing blood from a "tubed" individual.

We implanted a personal integrated transponder (PIT) tag into each snake we found. We also determined each snake’s sex, drew a blood sample, and recorded measurements on body size, rattle size, and mass. The Coronado Island Rattlesnake looks almost identical in pattern and coloration to the Southern Pacific Rattlesnake (C. oreganus helleri), its closest relative. In fact, the island snake is thought to have come from a mainland population of helleri. However, the island species dramatically differs from the mainland species in body size: it’s miniature in comparison. This may come as no surprise as island variants are often smaller than their mainland counterparts, a phenomenon known as Island Dwarfism.  

On top of a hill on the island. About to place down some cover boards.

Island dwarfism is an interesting phenomenon because it goes against Cope’s Rule, the prevailing trend in nature that organisms evolve toward larger body sizes. But islands provide special circumstances whereby they limit animals to a restricted area and also limit the amount of available resources. Hence, compared to giants, dwarfs are able to utilize limited resources more completely and are less likely to succumb to population crashes (Wassersug et al. 1979).

Left: A snake found coiled under a rock.  Right: Same snake tubed and ready for processing.

Island Gigantism also occurs, whereby animals grow in body size when isolated on islands. Gigantism is thought to result from many islands’ lack of large mammalian predators. Without these predators, animals are free to grow and exploit niches that were unavailable to them on the mainland. This is a phenomenon known as Predator Release.

Changes in available prey resources can also influence island dwellers’ body sizes. Across all island snake species, populations that are dwarfed tend to prey on lizards and populations that are giant tend to prey on colonies of nesting seabirds (Case 1978). For rattlesnakes that specialize on small mammal prey, islands lacking small mammals should lead to dwarfism and islands with prey equivalent to or larger than small mammals should lead to gigantism. Another interesting benefit larger-bodied viperid snakes have is their enhanced ability to fast compared to smaller snakes (Meik et al. 2010). Thus, gigantism benefits viperid snakes on islands where prey population sizes frequently fluctuate and/or where prey have relatively high extinction rates. 

Left: A specimen of the giant speckled rattlesnake of Angel de la Guarda.
Right: a specimen of the dwarf speckled rattlesnake of El Muerto Island.
Taken from Meik et al. 2010.

  

Almost all Baja California island rattlesnake populations are dwarfs. Only one, the Angel de la Guarda Island speckled rattlesnake (C. mitchelli angelensis), has undergone island gigantism. The Speckled Rattlesnake (Crotalus mitchelli) seems to love the island life boasting three descendants (C. m. angelensis, C. m. mitchelli, and C. m. muertensis) on 14 different islands. A study done by Meik et al. (2010) found that the body sizes of island populations of speckled rattlesnakes were best explained by island size (Meik et al. 2010). Smaller islands house smaller snakes and larger islands house larger snakes. The authors found that rattlesnakes typically tend toward dwarfism on islands that are smaller than about 20 square kilometers. In addition, rattlesnakes tended to dwarf on islands where the relative abundance of small lizards was greater than rodents. The authors then suggest that shifts to consuming larger prey (chuckwallas), fluctuating prey densities, and predator release likely resulted in gigantism for the speckled rattlesnakes on Angel de la Guarda Island.

Larger islands tend to house bigger snakes (SVL = snout-vent-length, a measure
of body size). Taken from Meik et al. 2010.

Dwarfism of the Coronado Island Rattlesnake is likely the result of small island size (less than 2 square kilometers!), and a greater reliance on lizard prey (pocket mice are the only known rodent inhabitant of the island). Research from the long-term monitoring program should provide quantitative data to support these theories. In all, islands are awesome (and beautiful) places that can act as “closed” ecosystems providing scientists the means to conduct unique experiments and find new discoveries.

------------ 

References:

Case, T. J. 1978. A general explanation for insular body size trends in terrestrial vertebrates. Ecology 59:1–18.

Meik,J. M., A. Michelle Lawing, and A. Pires-daSilva. 2010. Body size evolution ininsular speckled rattlesnakes (viperidae: Crotalus mitchellii). PLoS ONE 5:1–8.

Radcliffe,C. W., and T. P. Maslin. 1975. A new subspecies of the red rattlesnake,Crotalus ruber, from San Lorenzo Sur Island, Baja California Norte, Mexico.Copeia 1975:490–493.

Wassersug,R. J., H. Yang, J. J. Sepkoski, and D. M. Raup. 1979. The evolution of body size on islands: a computer simulation. The American Naturalist 114:287–295.