Eastern Hog-nosed Snake (Heterodon platirhinos) COSEWIC assessment and update status report: chapter 3

Species Information

Name and classification

The Eastern Hog-nosed Snake’s common name is derived from the upturned, keeled scale on the tip of its nose. Its scientific name, Heterodon platirhinos Latreille 1801 (spelled H. platyrhinos until recently, but corrected by Platt (1985)), is derived from physical aspects of the snake as well. The genus name is derived from hetero (= different) and odontos (= teeth), referring to the varying sizes of the teeth. The species name is derived from plati (= flat or broad), and rhinos (= nose), referring to – as the common name does – the upturned nose scale, which gives the snake its unique “hog-nosed” appearance (Johnson 1989).

Heterodon platirhinos is a colubrid snake and has traditionally been classified under the Subfamily Xenodontinae, a group of Central and South American snakes. However, some biologists doubt whether the Eastern Hog-nosed Snake and the Ring-necked Snake (Diadophis punctatus) – the other North American snake classified in this subfamily – are actually closely related to each other or to the other tropical, xenodontine snakes (Schueler 1997).

Morphological description

The Eastern Hog-nosed Snake is a stout-bodied snake; it has keeled scales, a divided anal plate, and a distinctive upturned, keeled scale at the tip of its nose (Harding 1997).

Adults

Individuals of this species are highly variable in colour and pattern, with phenotypes ranging from colourful and blotched to melanistic. Some individuals have a distinctive pattern of irregular blotches down the back, alternating with dark spots along the side, on a background of grey, brown, tan, olive, orange, yellow, or pinkish colour, whereas others lack all patterning and are typically plain grey, brown, olive, or black. In fact, many individuals are intermediate in pattern (Harding 1997). Dark neck blotches are visible in all but the darkest individuals. The belly is often mottled and can be yellowish, grey, cream, or pinkish (Harding 1997). The chin, throat, and subcaudal scales are usually lighter than the rest of the underside (Harding 1997). Total length of adults ranges from 50 to 115.6 cm (Harding 1997), with males being, on average, smaller than females.

Hatchlings

Hatchlings of this species have dark dorsal and smaller lateral blotches on a light grey or brown background. This pattern is present even in individuals that will be unpatterned and unicoloured as adults. The belly of hatchlings can be dark grey or black and the throat and subcaudal scales white or yellowish (Harding 1997). The total length of hatchlings ranges from 12.5 to 25.4 cm (Harding 1997).

Genetic description

There are no data available on the genetic structure of populations within the Canadian range. However, given that many populations in Ontario are spatially isolated and small, these populations are genetically isolated and therefore could experience inbreeding and/or genetic bottlenecks (Meffe and Carroll 1994; Chek et al. 2007). Because this species moves considerable distances, for a snake, populations should be less likely to be isolated in areas with continuous habitat.

There are no subspecies currently recognized. Crother (2001) and Platt (1969) concluded that variants, which are sometimes designated as subspecies, are not sufficiently discrete to warrant their subspecies recognition. 

Designatable units

The distribution of the Eastern Hog-nosed Snake in Ontario appears to comprise two disjunct regions corresponding to two Faunal Provinces (Carolinian and Great Lakes/St. Lawrence) of the Terrestrial Amphibian, Reptile and Mollusc Faunal Provinces (Figure 3 Appendix F 5 COSEWIC Operations and Procedures Manual 2007). Similarly disjunct distributions occur in several other reptile species in Ontario such as the Gray Ratsnake (Elaphe spiloides) and the Five-lined Skink (Eumeces fasciatus) (Cook 1984). It is not known whether this disjunction in hognose distribution is related to European settlement or whether it predates that time or whether it is an even older pattern based on phylogeographic history. There are no genetic data from the hog-nosed snake to test for differences between the two regions, nor is there good evidence to suggest that the conservation status between the two regions differs, although the Carolinian population is clearly more fragmented and has undergone greater loss of habitat. Given the uncertainties, it seems reasonable at present to consider the Eastern Hog-nosed Snake as a single Designatable Unit.

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