Ecology of the rock rattlesnake, Crotalus lepidus, in the northern Chihuahuan desert

The Rock Rattlesnake, Crotalus lepidus is a small species that is found from southern Arizona, southern New Mexico, and southern Texas, in the U.S., into northern Mexico. To date, little is known about the ecology of this species. Ecological information is becoming desperately needed for supporting the conservation and protection of species living in fragile environments such as the Chihuahuan Desert amid current local and global threats (e.g., habitat destruction and modification, urban development, and climate change). Although rattlesnakes spend a significant amount of time underground while overwintering, little is known about the physiology and behavior of these organisms during winter, and the same applies to the Rock Rattlesnake, Crotalus lepidus. The goal of this study was to investigate home range, movement patterns, habitats and microhabitats, and overwintering characteristics of the Rock Rattlesnake, C. lepidus, in an arid Chihuahuan Desert landscape on the Indio Mountains Research Station (IMRS), located in Hudspeth County, in far west Texas. From summers 2007-2010, 12 adult rattlesnakes were captured and monitored during the winter season (November-March), and from those, eight (two females and six males) were radiotracked during the active period (April-October). Average home range for all individuals was (13.69 ± 3.06 ha). Rattlesnakes leave their winter shelters primarily in early April and returned to shelters in early November. Average home range was 7.29 ± 0.89 ha (n = 2) for females and 15.82 ± 3.71 ha (n = 6) for males. The 50% kernel for all individuals averaged 2.83 ha, and varied from 1.6 to 6.1 ha. However, the average kernel area was smaller for females than for males (1.5 ± 0.10 ha and 3.26 ± 0.75 ha, respectively). Daily movement of all rattlesnakes averaged 8.46 ± 1.45 m/d, and individual daily mean movement ranged from 3.6 to 15.8 m/d. By month, rattlesnakes showed the highest average movement rates in September, followed by June, July and August. Conversely, snakes showed the average lowest movement rates in April, followed by May, and October. Monthly movement rates were not statistically significant. Movement rate increased during the monsoon months, however; the relationship between movement and precipitation was not statistically significant. Crotalus lepidus was observed in five habitats on IMRS, with arroyo habitat being the most frequented (407, 55%), followed by alluvial rocky slopes (128, 17%), rocky slopes (110, 15%), and alluvial slopes (77, 10%). Alluvial flats (18, 3%) had by far the lowest number of observations. Rattlesnakes were observed in 13 identified microhabitats, mostly from “under shrubs” (43%), followed by “under rocks” (19%). Most microhabitats utilized by C. lepidus recorded in a square meter at observation site were characterized by the regular presence of vegetation as the main ground cover component (% average = 54.8 ± 1.157 %), followed by rock, gravel and plant litter (19.9 ± 1.092, 12.0 ± 0.695, and 8.7 ± 0.568 %, respectively). A comparison of selected sites (n = 263) with random sites (n = 263) indicated that Rock Rattlesnakes in the study area preferred microhabitats with specific characteristics. Miscellaneous comments on rain harvesting, reproductive behavior, and predation are also discussed with respect to activities observed within the home ranges of C. lepidus. Rattlesnakes overwintered at single sites from early November until late-March or early-April. Mean ingress date was 8 November during all winters. There was no significant difference between ingress dates for winters 2007-2008 and 2009-2010. Mean egress date was 31 March during all winters. There was no significant difference between egress date for winters 2007- 2008 and 2009-2010. Overwintering period for all rattlesnakes had a mean of 143 days, ranging from 134 to 170 days. There was no significant difference in number of days spent overwintering between winter seasons 2007-2008 and 2009-2010. Average overwintering days for females (n = 2) was 140 days and 145 days for males (n = 10)