Rubber boa (Charina bobttae) COSEWIC assessment and status report: chapter 6
Biology
Reproduction
Courtship and copulation begin immediately following emergence from hibernation in March and April, and usually continue into early or mid-May. Female Rubber Boas produce clutches of 2-8 offspring, 180 to 280 mm long, in late August and early September (St. Clair 1999; Dorcas and Peterson 1998; Hoyer and Stewart 2000a). The relative clutch mass for Oregon boas was 0.360, which is within the normal range for other small North American snakes (Hoyer and Stewart 2000a). Clutch frequency is likely less than annual (perhaps as infrequently as every 4 years; Hoyer and Stewart 2000a), and females may be unable to successfully reproduce in years where low temperatures reduce the rate of embryonic development (Dorcas and Peterson 1998). In one unseasonably cool summer, female Rubber Boas in Idaho were unable to stay as warm as in previous years, even using the same microhabitat; consequently, embryos did not complete development by the end of the normal hatching period. Two pregnant females captured and brought into captivity aborted their young and one female died the following spring (Dorcas and Peterson 1998). In the Creston Valley population, two live births of 4 and 6 offspring and one stillbirth of 4 offspring have been recorded (St. Clair 1999).
Physiology
Rubber boas are frequently cited as one of Canada’s more cold tolerant snakes, although this claim must be taken with caution. While the Rubber Boa is active at temperatures lower than the majority of reptiles studied to date (range 6°C to 28°C), the preferred daily body temperature of Rubber Boas in the Creston Valley population was 30°C (St. Clair 1999) and 31°C in Idaho (Dorcas and Peterson 1998). Also, data on the relationship between embryonic development and temperature suggest that it may be cool temperatures that limit the reproductive potential and distribution of the species (Dorcas and Peterson 1998). Low temperature activity in the Rubber Boa may be the result of a tradeoff between the benefit of feeding at night when fewer snake predators are active and the cost of being active at less than optimal temperatures (Dorcas and Peterson 1998).
Hibernating Rubber Boas in Idaho had body temperatures (Tb) between 4 and 9°C and Tb did not vary more than 0.3°C during any 24h period. Pregnant females maintained Tb between 27 and 34°C by moving toward the surface to warm during the day and then retreating deep into rock crevices or gathering together at night, although such high Tb’s could not be achieved when environmental temperatures were low (Dorcas and Peterson 1998). Rubber Boas also regulate their head temperature, generally maintaining temperatures 2-3°C above Tb unless Tb exceeds the thermal preference of 30-31°C (Dorcas and Peterson 1997).
Movements/dispersal
Little is known of movements or dispersal of Rubber Boas. In the Creston Valley population, the hibernation site was contiguous with areas where Rubber Boas were found in summer. One individual traveled a distance of 1.5 km in one week to reach the hibernaculum, perhaps indicating an intentional return to a favourable den site (R. St. Clair, unpublished data).
Nutrition and interspecific interactions
Throughout its range, the Rubber Boa feeds primarily on nestling rodents and insectivores, although it will occasionally eat lizards and their eggs, bats, baby rabbits, and small birds (Hoyer 1974; Hoyer and Stewart 2000b). Smaller boas (144-268 mm) favour squamate eggs and lizards, whereas larger boas (352-711 mm) for go eggs and add mammals and birds to their diet (Rodriguez-Robles et al. 2001). The blunt and hardened tail of the Rubber Boa is used as a diversion or a defensive weapon against small mammal parents attempting to defend their litter from being consumed (Hoyer and Stewart 2000b). Known predators of the Rubber Boa in the United States include the common raven (Corvus corax), red-tailed hawk (Buteo jamaicensis), raccoon (Procyon lotor) and the ringneck snake (Diadophis punctatus) (Hoyer and Stewart 2000b).
Survival
Rubber Boas are long-lived, frequently surviving 20 to 30 years in the wild and longer in captivity (Hoyer 2001). Fewer than 10% of boas captured in Oregon were juveniles, indicating that reproductive potential is low, or that juvenile survivorship is low, or perhaps that juveniles have behavioural differences from adults that make them unlikely to be captured (Hoyer 1974). Females are more subject to severe injury (heavy scarring or shortened tail tip) than males, but are also larger in size. The nutritional requirements of females for growth and reproduction probably require that the female spend more time foraging and, consequently, must more frequently ward off attacks from the prey species’ parent.
Behaviour/adaptability
Due to their nocturnal feeding habits and preference for covering debris when at the surface, Rubber Boas are rarely seen. When handled, they are docile and have never been known to bite, although they will excrete a smelly musk if handled too roughly (Hoyer 2001). This species may sustain populations within short distances of human habitation and disturbance; however, the presence of coarse woody debris remains a major habitat requirement.
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