CROTALUS MITCHELLII

 

Despite its relative abundance in appropriate habitats (rock outcrops, rocky hillsides),  we had very little information on the Speckled Rattlesnake (Crotalus mitchellii). To my knowledge, prior to my study there was only one ecological study published on this snake (Moore, 1978, Copeia 1978:439-442). The lack of information on this species was likely due (in part) to its cryptic nature (see right), and to the extensive amount of time this snake spends under cover (see below).

Right: Crotalus mitchellii exhibits extensive variation in color and pattern, even within a single population. Some of the individuals I monitored were whitish-beige, others had pronounced brownish-blackish bands, while others were almost brick-red. The latter variant nicely blends in the purplish volcanic rocks found on some parts of my field site. Can you see him?

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As_part of my Ph.D. dissertation, I conducted a 3-year radiotelemetric study (2006-2009) of the Southwestern Speckled Rattlesnake (C. m. pyrrhus) in the Eldorado Mountains, just outside of Las Vegas, Nevada. One of the objectives of my dissertation was to use radiotelemetry as a tool to gain a better understanding of the general ecology (spatial, foraging, and thermal) of this ectothermic organism living in the harsh Mojave Desert. In addition to the field study, I dissected approximately 900 C. mitchellii museum specimens to conduct range-wide analyses of variation in life history traits, reproductive and feeding ecology. Hopefully, my study will provide a relatively comprehensive picture of the ecology of this snake species, and fill in the knowledge gap about this poorly-known animal. The field component of my Ph.D. research was recently published in 2 articles (see PDF, PDF), and I plan on publishing the museum data in the near future.

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Right: An adult male C. mitchellii prospecting around the nest of a Desert Woodrat (Neotoma lepida) in the early morning during the summer. Woodrat nests are an important resource (i.e., food and cover) for C. mitchellii, as well as many other snake species, such as Gopher Snake (Pituophis catenifer), Patch-nosed Snake (Salvadora hexalepis), and Coachwhip (Masticophis flagellum).

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Crotalus mitchellii has a reputation of being a vicious snake that would rattle and strike furiously at anything approaching. I studied them in the field for 3 years, and I have caught approximately 40 of these snakes. I found these individuals relatively mild-mannered. As many other rattlers, if they are coiled they typically rely on camouflage, and will not advertise their presence until danger is relatively close. However, one pattern I have observed is that C. mitchellii will rattle furiously from a long distance if they detect something coming while they are moving (and likely feel relatively unsafe.

Left: A male C. mitchellii displaying rattling behavior.

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Well…Why would they be unsafe? First, some humans unfortunately love to kill rattlesnakes, and as a matter of fact all kinds of snakes. Second, something ate the rattlesnakes on my field site in a way that makes me think that predation pressure may be a strong selective force driving the secretive behavior of these snakes. Even more surprising to me is that the largest male rattlesnakes were eaten. So who dares eating these large venomous snakes? Well I can only speculate…. Great Horned Owls (Bubo virginianus) are fairly common on my field site, and I believe that this bird may be responsible for the death of at least 2 of my rattlesnakes. Coyotes (Canis latrans) and Bobcats (Lynx rufus) are likely predators too. But I think that something smaller has developed a taste for C. mitchellii’s flesh….possibly the Ring-tailed Cat (Bassariscus astutus).

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Above: I found the carcass of this large male (total length: 95.3 cm; mass: 608 g) partially eaten. What struck me is that only the head and a small part of the body was eaten. The carcass was lying at the entrance of a medium-sized burrow underneath a rock outcrop. On several instances I have found partially eaten carcasses. This is what lends me to think that the unknown predator is of relatively small size. Ironically, I had not monitored any movements in this snake for the last 3 months (from mid-April to mid-July).

Left: An adult male under a rock in early May shortly after emergence from hibernation. As stated above, C. mitchellii does not spend too much time exposed on the surface. They occasionally (but rarely) spend time basking in the sun. I think they rely on conduction rather than radiation to thermoregulate when they emerge from hibernation.

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Right: Could the pinkish color of this rattlesnake be the result of natural selection for blending in the environment? A plausible hypothesis….

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Below: A male rattlesnake coiled at the entrance of a Woodrat nest. Thermoregulating, resting, or waiting for a Desert Woodrat (Neotoma lepida) to return from foraging? Maybe all at once? He sure is the only one to know. Most rodent activity occurs at night though except for the diurnal sciurid rodents (Ground Squirrels and allies). However, I have come across several Woodrats that were active in early mornings so we can not discard the Woodrat ambush hypothesis. Further, this guy may also be ambushing diurnally-active lizards, which C. mitchellii occasionally feeds on, White-tailed Antelope Squirrels (Ammospermophilus leucurus) and/or Canyon Mice (Peromyscus crinitus) which also use these nests.

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One scarce resource in the desert is obviously water. Animals inhabiting desert regions have evolved various traits to cope with the lack of water. Crotalus mitchellii, like some other rattlesnake species (i.e., the Mojave Rattlesnake [Crotalus scutulatus]; Cardwell, 2006, Herpetological Review 37:142-144), has evolved a behavioral trait that allows it to collect and drink the water that falls onto its scales during raining events.

Pictures below: I monitored rain-harvesting behavior in C.  mitchellii during my study. This is the first report of this behavior in this species (see PDF). On a rainy day in July, I observed a C. mitchellii going erratically from rock to rock in order to collect the droplets fallen onto rocks. After a little while, the snake coiled upon itself and started to collect the droplets. Immediately after coiling, the snake started to drink the droplets collected onto its body. Could there be selection for morphological traits (i.e., scale texture) enhancing this water-collecting technique?

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