Gray ratsnake (Elaphe spiloides) COSEWIC assessment and status report: chapter 5

Update
COSEWIC Status Report
on the
Gray Ratsnake
Elaphe spiloides

Great Lakes/St. Lawrence Population
Carolinian Population

in Canada
2007

Species Information

Name and Classification

The Eastern Ratsnake was originally named and classified by Say (1823), and since that time has been divided into 5 subspecies: Elaphe obsoleta obsoleta (Say, 1823), Elaphe obsoleta lindheimeri (Baird and Girard, 1853), Elaphe obsoleta quadrivittata (Holbrook, 1836), Elaphe obsoleta rossalleni (Neill, 1949), and Elaphe obsoleta spiloides (Duméril et al. 1854). This classification was based mainly on the colour pattern variation observed across the range of this species. All Canadian populations were classified as Black Ratsnakes (Elaphe obsoleta obsoleta).

Recently the classification and nomenclature of the Eastern Ratsnakes (Elaphe obsoleta) has come into question (Burbrink et al. 2000; Burbrink, 2001). Using two mitochondrial gene sequences and 67 morphological characteristics, Burbrink (2001) determined that the previously recognized 5 subspecies did not represent separate evolutionary lineages and instead suggested that Eastern Ratsnakes comprised three distinct clades, and proposed separating them into three species: 1) Elaphe obsoleta (western clade), 2) Elaphe spiloides (central clade), 3) Elaphe alleghaniensis (eastern clade), and included the Canadian populations as part of the central clade (Burbrink, 2001) (Fig. 1). The Canadian populations were classified based on geographic trends, however, and no samples were collected from individuals in any Canadian population. Recent morphological and genetic evidence (Gibbs et al. 2006) suggests that the ratsnake populations in southwestern Ontario are part of the central clade (Elaphe spiloides), whereas the Great Lakes/ St. Lawrence populations are hybrids between the central (Elaphe spiloides) and eastern clade (Elaphe alleghaniensis). If the two Ontario populations are considered separate species from each other, it would have important implications for conservation efforts in Canada. Gibbs et al. (2006) suggest, however, that these E. spiloides and E. alleghaniensis are not separate “species” because of the hybridization between the two proposed species. Given these taxonomic uncertainties, this report will retain the current name for the central clade, Elaphe spiloides, for all Ontario ratsnakes, recognizing that there are genetic differences between the ratsnakes in southwestern versus southeastern Ontario (see section on Designatable Units). The common name for E. spiloides is Gray Ratsnake (Crother et al. 2003).


Morphological Description

Throughout all populations in Ontario, adult Gray Ratsnakes are typically a plain, shiny black snake with white, yellow, orange or red colouration on the skin between the scales. The ventral surface is typically white or yellowish with a clouded grey or brown pattern, often resulting in a checkerboard appearance. They can often be distinguished from other snakes by their throat, which has a plain white or cream colour (Conant and Collins, 1998) and the anal plate, which is usually divided or semi-divided (Ernst and Barbour, 1989). In contrast to the adults, juveniles are dorsally patterned with dark grey or brown blotches on a pale grey background.


Figure 1: North American Distribution of Gray Ratsnakes (Elaphe spiloides) and Other Putative Species of the Eastern Ratsnake Complex

Figure 1. North American distribution of Gray Ratsnakes (Elaphe spiloides) and other putative species of the Eastern Ratsnake complex.

Adapted from Burbrink 2001.

The Gray Ratsnake is the largest snake in Canada, often exceeding 130 cm in snout-vent length (SVL) and reaching a maximum SVL of approximately 190 cm. Sexual dimorphism is present in adults; males have longer tails relative to their total body length (males 16 – 19%; females 14 – 18%; Ernst and Barbour, 1989), and attain greater maximum lengths. The size dimorphism is the result of faster growth rates and larger asymptotic sizes in males (Blouin-Demers et al. 2002).

Similar species to adult Gray Ratsnakes in Ontario are: Northern Watersnakes (Nerodia s. sipedon), melanistic Eastern Gartersnakes (Thamnophis s. sirtalis), and Blue Racers (Coluber constrictor foxii), but as adults all of these snakes can be relatively easily distinguished from Gray Ratsnakes. Northern Watersnakes have strongly keeled scales and dark brown bars on their neck. Melanistic gartersnakes are jet black, have keeled scales and the anal plate is not divided. Blue Racers have no pattern as adults and smooth scales with blue-green to dark-blue on the dorsal surface and light grey to white on the ventral surface. Blue Racers are now only found on Pelee Island, Ontario, where ratsnakes no longer occur. Juvenile Gray Ratsnakes can be readily confused with juvenile Eastern Foxsnakes (Elaphe gloydi) and juvenile Eastern Milksnakes (Lampropeltis triangulum). Eastern Milksnakes, however, have single anal plates and foxsnakes have 216 or fewer ventral scales whereas Gray Ratsnakes will have 221 or more (Conant and Collins, 1998).


Genetic Description

Genetic Structure

Adult Gray Ratsnakes exhibit home range fidelity for several years (Weatherhead and Hoysak, 1989) and overwinter in communal hibernacula (Blouin-Demers et al. 2000), to which they exhibit strong fidelity (Prior et al. 2001). These life-history traits have the potential to lead to genetic structuring at fine geographic scales (Gannon, 1978). The genetic structure of Gray Ratsnakes has been examined at regional (> 400 km), subpopulation (15-50 km), and local scales (1 – 5 km) using 2 different genetic markers.

Prior et al. (1997) used 7 RAPD markers to analyze blood samples from southeastern Ontario, southwestern Ontario, Maryland, and Arkansas (see Prior et al. 1997 for specific information and the map of population locations). The distances between these regional populations ranged from 500 – 1500 km and the researchers found significant genetic variation at this scale (FST = 0.266 ±0.062 SE). They did not, however, find significant differentiation between the two Canadian populations (FST = 0.019). At the subpopulation scale, they analyzed five subpopulations in southeastern Ontario (Distance: mean = 34.4 km; range = 15 – 50 km) and found significant variation (FST = 0.130, p < 0.001). Finally at the local scale, there was no significant genetic differentiation (FST = 0.006) between 2 hibernacula (distance = 1.6 km) within one population in southeastern Ontario. This was expected given that home ranges of individuals from different local hibernacula regularly overlap (Weatherhead and Hoysak, 1989; Blouin-Demers and Weatherhead, 2002a).

Lougheed et al. (1999) generally came to similar conclusions analyzing blood samples from the same populations using 6 microsatellite loci (40 alleles). At both the regional (mean FST = 0.168) and subpopulation (mean FST = 0.06) scales, Lougheed et al. (1999) found significant genetic differentiation. This included a significant divergence between the Carolinian and Great Lakes/St. Lawrence populations, although these populations were not significantly different according to RAPD markers (Prior et al. 1997). Lougheed et al. (1999) also analyzed samples from 11 hibernacula within 3 different subpopulations in southeastern Ontario (mean distance = 2.46 km, range = 0.5 – 4.9 km) and found no differentiation (mean FST = 0.01) at this scale, supporting the results from the RAPD markers.

Genetic Diversity

Using 7 RAPD markers, Prior et al. (1997) determined that the Great Lakes/ St. Lawrence (0.144±0.51 SE) and Carolinian (0.104 ±0.055 SE) populations were less heterozygous than the populations from Maryland (0.213 ±0.069 SE) and Arkansas (0.162 ±0.069 SE). These differences, however, were non-significant. Similarly, Lougheed et al. (1999) also looked for heterozygote deficiencies using 6 microsatellite loci. Heterozygote deficiencies can be a result of genetic inbreeding, which can lead to a reduction in fitness and increase local extinction risk. Mean heterozygosity ranged from 0.56 to 0.75 for 9 hibernacula in southeastern Ontario, and none tested significant for deficiencies using the exact test of GENEPOP.


Designatable Units

The Canadian population of Gray Ratsnakes consists of populations in 2 geographically disjunct regions in southwestern and southeastern Ontario. These populations are separated by approximately 300 km, and show some genetic differentiation (Lougheed et al. 1999). Evidence suggests that this disjunction likely preceded European settlement (See Distribution - Canadian Range). Because of the large separation, and the amount of unsuitable habitat between these populations, they will almost certainly remain isolated. Based on genetic differences, spatial separation and different conservation status these populations should be considered as separate designatable units. The southeastern Ontario ratsnakes are the Great Lakes/St.Lawrence population and the southwestern Ontario snakes are the Carolinian population.

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