Gray ratsnake (Elaphe spiloides) COSEWIC assessment and status report: chapter 8

Biology

Most of the biological information used in this report is from long-term published research on Gray Ratsnakes at the Queen’s University Biological Station (QUBS). Research conducted on other populations in the United States is also included; however, this research is less extensive and less applicable to the Canadian populations.

The Queen’s University Biological Station is close to the geographic centre of the Frontenac Axis and the biology of this population should be fairly indicative of ratsnakes across the Frontenac Axis. Although there are differences in habitat and climate between the Frontenac Axis and the Carolinian region, the general biology of the two populations should be similar.


Life Cycle and Reproduction

Because of the extreme climate in Ontario, Gray Ratsnakes are only active approximately 6 months (mid-April – mid-October) of the year (S. Thompson, pers. comm.), leading to slow growth rates and delayed sexual maturity (Blouin-Demers et al. 2002). Using growth models to predict the age of individuals based on their size, Blouin-Demers etal. (2002) estimated the maximum life span of ratsnakes in the Great Lakes/St. Lawrence population to be 25-30 years, and the age of sexual maturity for males and females to be 9.1 and 9.7 years, respectively. Recently, these estimates have been modified to an age of maturity of approximately 7 years (G. Blouin-Demers pers. comm., 2005), and the average age of reproducing individuals in the Great Lakes/ St. Lawrence population is approximately 10 years (unpublished raw data used in Blouin-Demers and Weatherhead, 2006).

Gray Ratsnakes are oviparous, and once sexually mature, females will produce a clutch every 2-3 years, but occasionally females will produce clutches 2 or 3 years in a row (Blouin-Demers et al. 2004). In Ontario, the mating season typically spans from late May to mid-June, well after individuals have dispersed from their hibernacula. Because of the low density of Gray Ratsnakes during the active season, females are likely courted by only one male at a time (Blouin-Demers and Weatherhead, 2002a). If males encounter each other while courting, they will compete for access to the female in a ritualized physical combat (Rigley, 1971; Gillingham, 1980). Despite the low density, females will usually mate more than once and produce clutches that are sired by two or more males (88% of clutches have multiple paternity; Blouin-Demers et al. 2005).

After mating, there is typically a gestation period of approximately 30 - 50 days before females will oviposit a clutch of approximately 10 - 15 eggs (N = 84 clutches; mean = 13 eggs; range 7-23; Blouin-Demers et al. 2005) in late June to early August. The incubation period depends on incubation temperature with means ranging from 52 (incubation temperature = 30°C) to 62 days (incubation temperature = 25°C). The average temperature within natural communal nests in the field is 28°C and highly variable (Blouin-Demers et al. 2004) and, therefore, it is likely that incubation times will be closer to 60 or more days in the wild, translating to hatching dates ranging from late August to early October. There is genetic sex determination for embryos, which results in an even sex ratio (Blouin-Demers et al. 2004). Neonates are approximately 285 – 300 mm SVL and there is no significant difference in SVL between male and female neonates (Blouin-Demers et al. 2002).

After hatching, very little is known about the neonatal life stage until the young snakes join communal hibernacula, which is close to the time of sexual maturity. It is therefore impossible to estimate survival rates for juvenile life stages. Based on unpublished raw data used in Weatherhead et al. (2002), annual adult survivorship was estimated to be approximately 0.68. This estimate was generated from the long-term monitoring of 4 hibernacula from 2 different populations (2 in each population). Size specific survival rates do not differ significantly between males and females (Blouin-Demers et al. 2002), but males grow faster than females and survival significantly increases with size, resulting in a male-biased sex ratio in the larger size classes (Blouin-Demers et al. 2002).


Predation

Known predators of adult Gray Ratsnakes include a number of large birds of prey (e.g. red shouldered hawk (Buteo lineatus), osprey (Pandion haliaetus), red-tailed hawk (Buteo jamaicensis)) and medium-sized mammals (e.g. fisher (Martes pennanti), mink (Mustela vison) and raccoon (Procyon lotor)). Potentially, young and sub-adults would be susceptible to the same predators, as well as a number of smaller predators such as American crows (Corvus brachyrhynchos). In some areas, increased contact with humans can be a large source of mortality either by the direct intentional killing of individuals, or indirectly as a result of human activities (e.g. road mortality).

Some nests, especially those in compost piles and open stumps, would seem to be susceptible to a wide variety of typical nest predators such as raccoons or skunks (Mephitis mephitis), but these sources of predation have rarely been observed at several communal nests at QUBS (G. Blouin-Demers pers. comm. 2005). Evidence of the burying beetle, Nicrophorus pustulatus, however, has been found at most communal nests at QUBS and in other populations and can cause significant mortality (see Biology - Interspecific Interactions) (Blouin-Demers and Weatherhead, 2000). Several Eastern Milksnakes (Lampropeltis triangulum) were also radio-tracked to communal ratsnake nests after the laying season, and on one occasion, a milksnake was observed preying on ratsnake eggs (pers. obs.). 


Physiology

Gray Ratsnakes are ectotherms and, therefore, thermoregulate mainly through behavioural mechanisms. Because most physiological processes are temperature dependent (Peterson et al. 1993), thermoregulation can have important implications for survival and, therefore, fitness (Christian and Tracey, 1981; Huey and Kingsolver, 1989). Within Canada, ratsnakes are at the northern extreme of their range in a thermally challenging environment, which makes thermoregulation particularly important (Blouin-Demers and Weatherhead, 2001b).

In a thermal gradient set up in the laboratory, ratsnakes demonstrated preference for body temperatures between 27°C and 30°C (Blouin-Demers and Weatherhead, 2001b). The optimal temperatures for tongue flicking (30°C), striking speed (29°C), and swimming speed (27°C) all fell within, or close to this range (Blouin-Demers et al. 2003). Within the Canadian populations, environmental temperatures are often well outside this range (Blouin-Demers and Weatherhead, 2001b) forcing ratsnakes to invest a lot of time and energy into maintaining body temperatures, especially in low thermal quality habitats (Blouin-Demers and Weatherhead, 2001b). Ratsnakes maintain body temperatures through habitat selection and choice of microclimate; gravid females and recently fed individuals maintain more preferable body temperatures by selecting higher thermal quality habitats, such as edges and retreat sites (Blouin-Demers and Weatherhead 2001b, Blouin-Demers and Weatherhead 2001c). Ratsnakes cannot tolerate temperatures below freezing and, therefore, hibernate underground during the winter months (see Habitat – Hibernation).


Dispersal/Migration

Although there is large individual variation, the size of an average adult ratsnake’s home range (Minimum Convex polygon (MCP)) is approximately 18.5 ha (Blouin-Demers and Weatherhead, 2002). Ratsnakes will often over-winter in hibernacula not located within their home range and commute (mean distance = 454 m; range 0 – 4km) (Blouin-Demers and Weatherhead, 2002) to their home range shortly (3-7 days) after emerging from hibernation, and return shortly before hibernation (~mid-September). The emergence period lasts about 5 weeks starting in late April (Blouin-Demers et al. 2000) and most snakes are within their home range by early June (Blouin-Demers and Weatherhead, 2002).

Adult ratsnakes demonstrate strong fidelity to both their hibernacula (see Habitat – Hibernation) and general home ranges (Weatherhead and Hoysak, 1989) each year, limiting the dispersal potential and rescue effect from other populations for this life stage. Juvenile ratsnakes, however, frequently do not join communal hibernacula until they reach maturity and show a lower fidelity to both their hibernacula and home ranges (Bjorgan, 2005), demonstrating a greater potential for dispersal. Because of the numerous gaps in understanding the neonatal life stage, actual dispersal distances from hatching until maturity have not been estimated, and home range data are lacking for this age class. At the QUBS study site, over 1800 neonates have been hatched in the lab since 1996. All of these individuals have been marked with a passive integrative transponder (PIT tag) and eventually may provide greater insight into neonatal dispersal distances (pers. obs.).


Interspecific Interactions

Blouin-Demers and Weatherhead (2000) discovered that the burying beetle, Nicrophorus pustulatus, parasitizes ratsnake eggs on the Frontenac Axis and could be a significant cause of mortality. Evidence of N. pustulatus was found in 6 of 7 nests and caused close to 100% mortality of clutches when present. Similar reports of beetle larvae in snake eggs from Illinois and Pelee Island (not ratsnakes) (Blouin-Demers and Weatherhead, 2000) suggest that this problem is not unique to the Frontenac Axis.

Gray Ratsnakes are generalist foragers mainly feeding on small mammals and birds (Fitch, 1963; Weatherhead et al. 2003). Weatherhead et al. (2003) analyzed the scat of ratsnakes on the Frontenac Axis and found that mammals made up approximately 65% of the diet, while birds made up about 30%. Ratsnakes are efficient avian nest predators, and the proportion of avian prey in the diet increased to a maximum of 45% in June; the height of the bird nesting season on the Frontenac Axis. Fitch (1963) found similar prey ratios for a population in Kansas and, therefore, it is likely that foraging behaviour is similar for all Canadian populations. 


Adaptability

The Gray Ratsnake is a relatively common snake throughout the eastern United States and can be found in a variety of woodland habitats (Ernst and Ernst 2003), suggesting that they are adaptable to a wide variety of environments. Although ratsnakes can readily be found in open fields and abandoned buildings (pers. obs.), they will rarely be found far from woodlands and prefer edges between woodlands and fields, even in more disturbed habitats (Durner and Gates, 1993). These results suggest that they do not adapt particularly well to high levels of human disturbance where intense land clearing has taken place. This is evident from their virtual disappearance in the intense agricultural landscapes of southwestern Ontario.

Canadian populations of ratsnakes are at the northern extreme of their range and in a thermally challenging environment. This results in slow growth rates and late maturity, significantly increasing their generation time (Blouin-Demers et al. 2002) (see Biology - Life cycle) and making them significantly more vulnerable to disturbances than populations in less challenging environments. This will also significantly reduce their ability to adapt to a rapidly changing environment.

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