Observations on the year-round communal use of an artificial structure by Northern Pacific rattlesnakes Crotalus oreganus oreganus in coastal Central California

Abstract

C gatherings of snakes have been recorded in temperate regions of the Northern Hemisphere, and species documented in such congregations include several species of Crotalus (Fitch, 1949; Hirth, 1966; Brown & Parker, 1976; Prior & Weatherhead, 1996). These congregations are typically observed in and around communal hibernacula, usually in high altitude or high latitude environments (Hamilton & Nowak, 2009). In these locations, seasonal climatic conditions are unfavourable for year-round snake activity so that hibernation is essential for some months of the year. Suitable hibernation sites are limited and as a consequence high densities of snakes may be observed in small areas (Drda, 1968; Klauber, 1997; Gienger & Beck, 2011). Such congregations dissipate during the active season as individuals disperse into different habitats to forage and reproduce (Fitch, 1949; Gannon & Secoy, 1985; Putman et al., 2013). While congregations during the active season are known for several species of snakes, these have been of gravid females in communal breeding sites (Graves & Duvall, 1995). Evidence of year-round communal living in snakes appears to be lacking. Here, we report observations of a small group of Northern Pacific rattlesnakes (Crotalus oreganus oreganus) living communally throughout the year in an artificial pit in Central California. Our observations were made on the University of California Fort Ord Natural Reserve (FONR), located in Monterey County, California. The FONR is a protected research reserve built around the former Fort Ord Army Base and it harbours remnants of artificial structures related to past military activities. These wood and concrete structures are now broken down and covered in vegetation; they provide shelter for small animals. While conducting herpetological surveys on the FONR on 8th April 2018, we observed two adult C. o. oreganus basking around the edges of a degraded concrete pit (36.6856° N, 121.7755° W; WGS 84). The entrance of the pit was covered by poison oak (Toxicodendron diversilobum). Subsequent observations in the following weeks found several individuals using the pit communally. From May, we conducted visual encounter surveys (VES) around the pit on a daily basis between 11:00 h and 14:00 h. We considered marking individuals using a method such as painting of rattle segments (Putman et al., 2013) to document the number of rattlesnakes using the structure, but to avoid unnecessary handling we instead photographed the dorsum of all individuals encountered around the pit. By slowly approaching basking individuals from behind with a camera, we were able to photograph rattlesnakes from a safe distance. Photographs of dorsal patterns were sufficient to recognise individuals, as dorsal patterns of rattlesnakes have unique combinations of blotches varying in size and shape. We also recorded approximate snout-to-vent length (approx. SVL) for all identified individuals. In July, we installed a trail camera (Bushnell Corporation, USA) approximately four meters away from the pit entrance to observe rattlesnake behaviours in and around the pit with minimum disturbance. The camera was set to activate with the detection of movement and was retrieved after 10 days. Throughout mid-April to late September, we identified at least eight different individuals using the pit through our VES (Fig. 1). These individuals varied in growth stage from neonate (~ 17 cm approx. SVL) to fully-grown adult (~ 85 cm approx. SVL). Both our VES and video recordings showed that rattlesnakes came out from the pit to bask from approximately 10:00 h to 13:00 h. During this time, the snakes were frequently observed basking on concrete surfaces of the pit with their bodies extended. No rattlesnakes were observed around the pit after 14:00 h. Although regular observations of this group ended by December 2018, the same snakes were observed again basking in April 2019, suggesting that this group is not an ephemeral congregation. One possible explanation for this congregation is security of the pit. The pit had numerous cracks and holes, and we observed snakes readily retreating into these places. The poison oak brush covering the broad entrance of the pit from above may provide additional protection from aerial predators and other carnivores of the reserve. Snakes may potentially also use internal structures of the pit as a hibernaculum. Snakes often travel long distances to reach suitable hibernacula (Hirth et al., 1969; Brown & Parker, 1976; Brown et al., 1982) and can be exposed to predation during migration (Kingsbury & Coppola, 2000; Rudolph et al., 2007). The Herpetological Bulletin 155, 2021: 25-27