Crotalus cerastes (Hallowell, 1854) is a small horned rattlesnake that ranges throughout most of the deserts of southwestern United States, and south into northern Mexico (Ernst and Ernst, 2003). This species is considered to be a psammophilous (sand-dune) specialist, typically inhabiting loose sand habitats and dune blowouts (Ernst and Ernst, 2003). Although C. cerastes is primarily a nocturnal snake, it is known to be active diurnally in the spring, and to bask in early morning or late afternoon (Ernst and Ernst, 2003). This rattlesnake species exhibits a unique style of locomotion known as sidewinding, from which it derives its common name, Sidewinder. Sidewinding is believed to be an adaptation to efficiently move in loose sand while reducing contact with the hot desert surface (Ernst and Ernst, 2003). However, during the past 12 years of surveying in the Mojave Desert at the National Training Center, Fort Irwin, California, approximately 40 km northeast of Barstow, San Bernadino County, we frequently observed C. cerastes in what is considered typical desert scrub habitat. This area primarily comprises alluvial fans and washes with gravelly soils dominated by Creosote (Larrea tridentata) and Burrobush (Ambrosia dumosa). The study area is described in more detail in Walde et al. (2009) and Harless et al. (2009). The habitat differs from the loose sandy areas where C. cerastes is typically found, as the

study area has no uninterrupted sandy areas outside of ephemeral washes, and no dune-like habitats. It is in this scrub-like habitat that we made three observations of the previously undocumented arboreal behaviour of C. cerastes.

On 7 April 2005 at 1618h, we observed an adult C. cerastes coiled in an A. dumosa shrub approximately 25 cm above the ground (Fig. 1 A). The air temperature was 21 °C and ground temperature was 27 °C. The snake did not attempt to flee at our approach, but did reposition slightly in the branches. A second observation occurred on 18 April 2005 at 1036h, when we observed another adult C. cerastes extending the anterior third of its body beyond the top of an A. dumosa shrub (Fig. 2). The air temperature was 22 °C and the ground temperature was 36 °C. Shortly after being observed, the snake returned to the ground and took shelter at the base of a L. tridentata shrub. Finally, we observed this behaviour again on 3 May, 2007 (Fig. 1B). This third adult snake was approximately 40 cm from the ground in a California Jointfir (Ephedra californica); no time or temperature data were recorded.

Rattlesnakes are not considered to be arboreal due to having relatively stout, heavy bodies and short tails. However, many species of rattlesnakes have been observed in shrubs and trees (Cunningham, 1955; Klauber, 1997), with some species like the Timber Rattlesnake (C. horridus) frequently observed in arboreal situations (Saenz et al., 1996; Coupe, 2001; Rudolph et al., 2004). Although arboreal behaviour in the genus Crotalus has not been well studied, previously published explanations for the rare behaviour include: to escape from water and flooding, to thermoregulate (Shine et al., 2005), to capture prey (as would be typical for bird nests; Martins et al., 2008), or to obtain an advantageous perch in search of prey or enemies (Klauber, 1997). Juveniles and subadults of several species more frequently use arboreal habitat than adults,

Herpetology Notes, volume 9: 55-58 (2016) (published online on 17 February 2016)

Arboreal behaviours of Crotalus cerastes (Hallowell, 1854) (Squamata, Viperidae)

Andrew D. Walde1,*, Andrea Currylow2, Angela M. Walde1 and Joel Strong3

1 Walde Research & Environmental Consulting, 8000 San Gregorio Rd., Atascadero, CA 93422

2 Department of Biological Sciences, University of Southern California, Los Angeles, California, USA

3 Kleinfelder, 3919 Riga Boulevard, Tampa, Florida 33619, USA

* Corresponding author e-mail: awalde@hotmail.com

Andrew D. Walde et al.56

with foraging proposed as the most likely purpose (C. horridus: Rudolph et al., 2004; C. oreganus helleri: Figueroa et al., 2008).

During the time of our observations, no flooding or pooled water was present that might have prompted climbing. There were no bird nests in the shrubs to suggest foraging; however, lizards frequently climb into shrubs, and the snakes might have followed them or their odours. Temperatures recorded at the time of our observations do not necessarily suggest that these snakes were attempting to escape the heat of the desert surface. In field observations followed by controlled trials of the thermoregulatory-arboreal behaviour of Red-sided Garter Snakes (Thamnophis sirtalis parietalis), Shine et al. (2005) found that the snakes would climb trees when there was a risk of heat loss

to the cooler ground. Fitzgerald et al. (2003) described a correlation between perch height, air temperature, and preferred body temperatures in Stephen’s Banded Snakes (Hoplocephalus stephensii), finding that the snakes would adjust their perch height to maintain preferred temperatures with changing air temperatures. Although C. cerastes typically bask on the ground, often in self-constructed depressions, it is possible that the snakes in the first and third observations were aerially basking in an attempt to avoid heat loss to the cooler ground. Although 2005 had a relatively wet spring (Harless et al., 2009) that might have caused the ground to be cooler and moister than normal for the time of year, 2007 was a drought year in the Mojave Desert and it was abnormally dry. Therefore, the behaviour may be more common than previously thought for the species and not correlated with moisture.

The second snake we observed in 2005 was perched high in a small A. dumosa shrub when ground temperatures were higher, so it is possible that the snake could have been using the shrub to escape heat on the ground. However, the snake could have been both thermoregulating high in the shrub while also exhibiting vigilance behaviour. Klauber (1997) observed that rattlesnakes may use vegetation to obtain an advantageous perch in order to search for enemies. Fitzgerald et al. (2003) suggested that snakes may not use open terrestrial basking sites in order to avoid predation if there are vegetative basking sites available. The second snake’s posture and behaviour suggested it was surveying the area, similar to spy-hopping in whales or scanning, anti-predator vigilance behaviours. The protection from predation offered by basking in a shrub may allow the chiefly nocturnal snake to rest in the daytime.

The only two previously reported observations of Sidewinders utilising shrubs include an individual which retreated into a low shrub after being harassed (Klauber, 1997), and a captive specimen in a laboratory which was coiled in a shrub in its enclosure (Cunningham, 1955). Cunningham (1955) attributed the behaviour, however, to the setup of the enclosure itself, as at a later date another species of rattlesnake housed in the same enclosure also coiled in the same shrub. Similarly, in a study on elapid snakes in Australia, Fitzgerald et al. (2003) found that snakes in a captive setting would often behaviourally thermoregulate arboreally, but would rarely exhibit the behaviour in the natural field setting, motivating the authors to caution against extrapolating captive results for natural wild behaviours.

The earliest reports of arboreal habitat use by

Figure 1. Adult Sidewinders (Crotalus cerastes) coiled, using arboreal resting sites in (A) a Burrobush (Ambrosia dumosa) and (B) a California Jointfir (Ephedra californica) in the Mojave Desert, California, USA.

rattlesnakes included these species: C. adamanteus, C. atrox, C. enyo, C. horridus, C. mitchellii, C. molossus, C. oreganus, C. ruber, C. tigris, C. viridis, and Sistrurus miliarus streckeri (Cunningham, 1955; Klauber, 1997). Since these early reports, others have reported observations of some of these same species in arboreal habitats, for example C. enyo cerralvensis (McGuire, 1991) and C. tigris (Pavlik, 2007); however, by far the most arboreal rattlesnake appears to be C. horridus (Saenz et al., 1996; Coupe, 2001; Rudolph et al., 2004). More recent observations of rattlesnakes using arboreal habitat include C. cataliensis (Martins et al., 2008), C. durissus (Dayrell et al., 2010), C. mitchellii (Evanhoe and Haug, 2011), C. ruber lorezoensis (Hollingsworth and Mellink, 1996), C. willardi obsurus (Holycross et al., 2002), and C. willardi willardi and C. lepidus klauberi (Rossi and Feldner, 1993). It remains unknown if these

reports document chance observations or if arboreal habitat use is a well-developed behaviour. Klauber (1997), more than 50 years ago stated that any species of rattlesnake observed long enough would, at some point, probably be observed in a tree or shrub. Here, we add the Sidewinder to this growing list of observations.

Funding for the project during which these observations were made was provided by USACERL/ERDC in Champaign, Illinois and DPW Environmental at the National Training Center, Fort Irwin, California.

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Figure 2. An adult Sidewinder (Crotalus cerastes) extending its head from a Burrobush (Ambrosia dumosa) in the Mojave Desert, California, USA.

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Accepted by Benjamin Tapley