Many animals exhibit pronounced shifts in ecology (e.g., habitat use, diet) as they grow. The central goal of this study was to determine whether habitat use and movement patterns of juvenile black ratsnakes (Elaphe obsoleta) differed from patterns previously documented for adult ratsnakes and to determine the conservation implications of any changes identified. We found a shift in habitat use by black ratsnakes with body size; juveniles used macrohabitats and microhabitats in proportion to their availability, unlike adult black ratsnakes that have been shown to prefer forest edges at both scales. Frequency of movement declined with body size, whereas distance traveled per move increased with body size. Habitat selection and movement patterns may be a result of ontogenetic shifts in thermoregulatory behaviour associated with changes in body size, or alternatively, may reflect size-related variation in predation risk. Home range size and fidelity, and fidelity to hibernacula all increased with body size. Despite ontogenetic changes in habitat use and movement by ratsnakes, recommendations regarding critical habitat for adults of this species should be adequate to protect juveniles.
We conducted this study at the Queen’s University Biological Station (44° 34' N, 76° 19' W), 100 km south of Ottawa, Ontario. The study area was approximately 10 km by 3 km and consisted of primarily second growth mixed deciduous forest with numerous natural edges along rocky outcrops, wetlands, and lakes and human-made edges associated with small hayfields and successional fields.
We caught snakes in funnel traps at fenced communal hibernacula during spring emergence (Blouin-Demers et al., 2000; Row and Blouin-Demers, 2006a) and opportunistically during the active season. Snakes were measured for snout–vent length (SVL), weighed, sexed, and marked with a passive integrated transponder tag.
For the purpose of this study, we defined a juvenile as an individual < 1050 mm SVL because this corresponds to the size of the smallest gravid female captured (n = 57) and the size of the smallest male observed mating (n = 15, Blouin-Demers and Weatherhead, 2002). Recently, genetic parentage analyses have shown that males as small as 875 mm SVL sire young (Blouin-Demers et al., 2005). However, only approximately 15% of identified fathers were < 1050 mm SVL. Because of the uncertainty in size at maturity and because size at maturity is likely to vary between individuals, we used SVL instead of adult/juvenile as a predictor variable wherever possible. SVL increases with age, although the relationship is much stronger prior to sexual maturation (Blouin-Demers et al., 2002).