SPECIES ACCOUNT. Brown Snake / Couleuvre brune (Storeria dekayi)
Department of Biology, Queen’s University, Kingston, ON Canada K7L 3N6
Taxonomy: Class: Reptilia. Order: Squamata. Suborder: Serpentes. Family: Colubridae. Subfamily: Natricinae. Genus: Storeria. Species: Storeria dekayi.
This common small snake is also known as DeKay’s Brown Snake after the 19th Century New York naturalist James Ellsworth DeKay. The genus name Storeria is in honour of David Humphreys Storer, an 18th Century American zoologist. Brown snakes are in the family Colubridae which includes approximately 2/3 of all snake species worldwide. The subfamily Natricinae also includes water snakes, garter snakes, and red-bellied snakes among others (King 2009, Cunningham and Burghardt 1999). Brown snakes have been separated into between 5 and 8 subspecies based largely on superficial colour characters (e.g. dorsal cross bars, labial markings – see Trapido 1944, Anderson 1961, Sabath and Sabath 1969). Trapido (1944) identified 6 subspecies, 3 of which occur north of the Rio Grande: the Northern (Storeria dekayi dekayi), the Midland (S. d. wrightorum), and the Texas (S. d. texana) Brown Snake. Since 1944 the Marsh Brown Snake (S. d. limnetes), and the Florida Brown Snake (S. d. victa), both originally classified as separate species, have been added to the list of Storeria dekayi subspecies (see Anderson 1961, Sabath and Sabath 1969, Neill 1950). There is little recent work on the taxonomy of this species and much of the taxonomy remains uncertain due to the large overlapping ranges of the subspecies, particularly between S. d. dekayi and S. d. wrightorum and between S. d. wrightorum and S. d. texana. Lack of genetic work on this species makes it hard to determine the degree of interbreeding between subspecies or detect the existence of cryptic species. It can be difficult to unequivocally identify the subspecies of an individual. Most researchers simply consider specimens at the species level.
Description: Brown snakes are small, cryptic and retiring temperate zone snakes, with an average length of 23-33 cm and rarely exceed 38 cm (Harding 1997). Body colours range from light tan or gray to dark brown. Two parallel rows of black spots run down the back, and the mid-dorsal region
between these rows of spots, approximately 4 scales wide, is almost always lighter in colour than the rest of the dorsum. Spots may be obvious or inconspicuous and in some subspecies, such as the Midland and Texas Brown Snake, the spots are connected across the spine making narrow lateral bands. There may be one or two dark stripes or blotches in temporal regions (sides of the head behind the eye). These markings vary in size and shape among individuals of different subspecies (Conant and Collins 1998). Northern brown snakes have a dark bar on the anterior temporal scale and the dorsal spots are not connected by dark stripes (Fig. 1). The belly is whitish or pinkish, and may have pigmented speckling on the sides of the ventral scales (Conant and Collins 1998, Ernst and Barbour 1989).
Neonates have a light ring around the neck, thus superficially resembling ringneck snakes (Diadophis punctatus). However the two species are easily distinguished: ringnecks have yellow or orange bellies and smooth scales while brown snakes have whitish bellies and keeled scales (Conant and Collins 1998). Neonates are typically darker in colour than adults and the rows of dorsal spots are harder to see. Mean neonate length at birth was 75mm and 77mm for males and females respectively for specimens collected in Ottawa county, Ohio and Essex county, Ontario (King et al. 1999).
Brown snakes are sexually dimorphic species with detectable morphological differences between sexes even as neonates (King et al. 1998). Females are larger than males, but have a smaller relative head size. The proportionally greater head size in males results in a larger gape for their body size, so that despite being smaller, a male may feed on the same prey items as a larger female (King 1997, 2002). This convergence in head size between the two sexes is relatively rare in species that are dimorphic in body size because it may increase competition for prey items between the sexes (see Shine 1986). Indeed one of the hypotheses for the evolution of sexual size dimorphism is reduction in intraspecific intersexual competition for limited food items (see Rand 1952, Selander 1966). Not enough is known about the size or availability of prey items (e.g. earthworms, snails, slugs) to test for the effects of these factors on sexual dimorphism in body and head size in the Brown Snake (King 1997).
Males have longer tails and more subcaudal scales than females. Males’ tails have 46-73 subcaudals which corresponds to 23-27% of total body length, while females’ tails are 17-23% of the body length and are made up of 36-66 subcaudals (Harding 1997). Male-biased dimorphism in tail size occurs in many snake species and it may be used as a basis for sexing individuals. The reason for the dimorphism in brown snakes is not clearly understood. In the Water Snake (Nerodia sipedon), a related species, there is some evidence for the dimorphism being the result of the coupling of two factors. The first is a morphological constraint for a minimum tail length placed on males because of the space needed for hemipenes and retractor muscles. The second is the result of natural selection for increased reproductive output in females, which results in an increase in body capacity and thus a decrease in relative tail length (King 1989a). Whether these factors affect sex differences in brown snakes is unknown.
Distribution: Brown snakes are widely distributed in North America. Their range extends from Southern Quebec West to Wisconsin and Minnesota, covering Southern Ontario and most of the Great Lakes region. Going south, the range covers the Eastern and Midwestern US to the Gulf of Mexico, and the eastern half of Texas south to northeastern Oaxaca, Mexico. There are also tropical subspecies that occur from southeastern Mexico to central Honduras (Ernst and Barbour 1989).
All of the Canadian range is within a putative contact zone for the northern and midland subspecies. The Northern Brown Snake’s range extends from Southern Quebec and Ontario South to Virginia. The Midland Brown Snake can be found from Wisconsin east to the western Carolinas and south to the Gulf Coast (Ernst and Barbour 1989, Conant and Collins 1998).
In Ontario the species’ range is concentrated in the southern portion with northernmost records restricted to Manitoulin Island, the north shore of Georgian Bay, and eastward on a line running through the cities of Sudbury and North Bay (Fig. 2). At QUBS, brown snakes are found every year on trails and the edges of many of the forested tracts, typically under cover (rocks, boards, discarded tin).
Habitat: As habitat generalists, brown snakes can be found in a variety of habitat types within their large range. They live in damp wooded areas, marshes, old fields, and were once common in urban habitats such as gardens, parks, dump sites and golf courses. Their presence in cities, however, has declined in recent years perhaps due to pesticide use and other pollution. They seek cover objects such as flat rocks, logs, boards, and other debris and remain hidden under them most of the time (Ernst and Barbour 1989, Harding 1997).
During the winter months they hibernate below the frost line in mammal burrows, house foundations, and crevices. They often hibernate in groups of conspecifics or with members of other species such as garter snakes (Thamnophis sirtalis), redbelly snakes (Storeria occipitomaculata) and smooth green snakes (Opheodrys vernalis) (Harding 1997).
Ecology: Brown snakes feed primarily on earthworms, slugs, and snails but also eat other invertebrates such as beetles, insect larvae, sow bugs and spiders. Occasionally they have been documented to eat small amphibians (Rossman and Myer 1990, Harding 1997). They are nocturnal. Because they hunt at night and mostly underground, a popular hypothesis is that they locate their prey by olfaction (Harding 1997). An interesting adaptation of this species is their ability to extract snails form their shells, a skill which has evolved independently in several species of colubrids. Brown snakes do this by biting a snail’s exposed body, wedging the snail against a rock, and then applying a twisting force for approximately 10 minutes to fatigue the snail’s columellar muscle. Once this muscle is fatigued the snake can quickly extract and ingest the snail. This adaptation is largely behavioural; however certain aspects of the brown snake’s tooth and jaw morphology (e.g. long teeth and shallow, flexible maxillae) are likely to be specializations related to this adaptation (Rossman and Myer 1990). Self-grooming behaviour after eating has also been observed in this species. After contact with active or immobilized earthworms and earthworm mucus, Cunningham and Burghardt (1999) found that brown snakes and related species will rub their heads against the substrate in a specific way accompanied by occasional gaping of the mouth. Earthworms secrete an alarm pheromone when attacked, so head rubbing might help eliminate this chemical signal so that snakes can continue to hunt earthworms without being detected as easily by their prey. To a lesser degree, brown snakes also groomed after contact with slugs, so head rubbing is likely a general response to sticky substances on the face (Cunningham and Burghardt 1999).
Brown snakes hibernate throughout winter, typically returning to the same hibernaculum year after year, but may venture out on particularly warm winter days (Fisher et al 2007). They mate when they emerge from hibernation sites in April or May. Courtship involves female pheromone release. Males follow the chemical trail and engage in scramble competition when pursuing a female. They may attempt to push a rival off of a female but the interaction never escalates to combat. When a male reaches a female he flicks his tongue, positions his body alongside hers, rests his head on her neck and twists his tail around hers to grasp it (Noble 1937). The male then begins a series of wavelike muscle contractions that begin at the tail and move up the body to the head, still keeping his head on her neck. Eventually the female’s tail is lifted slightly and the male inserts a hemipenis into the cloaca and then allows himself to be dragged along by the female, still attached to her, for approximately 20 minutes (Noble 1937, Harding 1997).
Gestation lasts 100-130 days and females give birth to live young in late summer/early fall, usually in August. Average litter size is 10-14 but the maximum may be as high as 40 (Harding 1997). There is a trade-off between offspring number and size. King (1993) found that female body size and condition are inversely related to offspring size, a counter intuitive result that may be explained by an interaction between direct effects (e.g. female body condition on number of offspring) and indirect effects (e.g. female condition on offspring size through offspring number). There is no parental care in this species, but young snakes often stay together after birth. The neonates grow rapidly and usually reach sexual maturity in 2 years (Harding 1997).
Brown snakes are eaten by wide range of predators including other, larger snakes, ground-feeding birds, hawks, crows, small mammals, and large frogs and toads (Ernst and Barbour 1989, Harding 1997). When threatened, brown snakes will attempt to flee or exhibit stationary defence behaviours such as releasing musk and feces from the cloaca and flattening the body to appear larger (Harding 1997). When placed in water, brown snakes will feign death to deter predators, likely because they are less able to flee than when on land. Small neonates are most likely to feign death because their inferior swimming performance does not provide a good escape mechanism (Gerald 2008). However as brown snakes very rarely enter water willingly (King & Lawson 2001), this is probably a less important defence mechanism than releasing musk and fleeing.
Brown snakes are often found sharing cover objects with one or two conspecifics or with similarly sized members of other species such as redbelly and garter snakes (Noble and Clausen 1936, Gregory 2004). Neonate snakes are capable of species recognition and will preferentially share cover objects with conspecifics (Burghardt 1982). Little is known about snake aggregations not related to hibernation or breeding (Burghardt 1982). Snakes may aggregate under the same cover objects due to lack of suitable cover elements and overlapping habitat preferences between species, they may seek out social interactions, or aggregations may occur randomly (Gregory 2004). Little is known about the aggregation tendencies of brown snakes or related aspects of their ecology (e.g. territoriality, microhabitat preferences, distribution). Further research is needed in this area.
Brown snakes and other small snakes may occur at high densities and be major contributors of biomass in an ecosystem (Willson and Dorcas 2004). However, we know relatively little about the ecology of this species because they are small, nocturnal, and secretive. It is likely that there is considerable variation in life history traits (e.g. brood size, time of birth, age of sexual maturity) as well as morphological traits (e.g. length, scalation, coloration) in S. dekayi across its large range. King (1997) detected microgeographic differences in SVL, scalation, and to a lesser degree, some head dimensions (e.g. jaw length) between populations of brown snakes on the Ontario mainland, the Ohio mainland, and populations on 5 Lake Erie islands. Some of these differences may be due to demographic differences in these populations (King 1989a), but King and Lawson later (2001) estimated FST for these populations and found that there is significant population subdivision between them, and that gene flow between pairs of populations (estimated Nm) was inversely related to the distance between the populations, although this trend was not statistically significant. Studying the roles of gene flow and genetic drift in shaping brown snake populations is tricky because it is hard to get a good estimation of effective population size. To date no other work has been done to quantify gene flow in other locations, identify other distinct populations or detect geographic differences in life history traits. There are no data on the life history of this species from QUBS or from Ontario in general.
Conservation status (from the National Heritage Information Centre)
This species is not currently considered to be under threat and is common except for populations on the boundaries of its range; however its numbers in urban habitats are declining due to increasing development and presence of toxic chemicals. There are no scientific data on its population size or demography in Canada (Fisher et al 2007).
GRANK date: 10-30-1996
NRANK: N5 (nationally secure)
Ontario General Status: Secure (date: 11-1-1999)
COSEWIC (Committee on the Status of Endangered Wildlife in Canada): Not at risk
Research needs: Detailed study of the postglacial history and molecular systematics of the species is needed, the latter to evaluate whether there exist cryptic species as has been found in other taxa. Despite being one of the more common species within its core range, we know relatively little about Brown Snake spatial ecology, microhabitat selection and interactions with other snake species (notably the congeneric Red-Bellied Snake with which it is often syntopic in Ontario). By extension, we really know little of variation in key life history attributes across the range of Brown Snakes and this lack of information is certainly pronounced in Ontario.
Literature and Further Reading:
- Ernst, C.H. and R.W Barbour 1989 Snakes of Eastern North America. George Mason University Press. Fairfax, Virginia. 282 pp.
- Burghardt G.M. 1983. Aggregation and species discrimination in new born snakes. Z. Tierpsychol. 61:89-101.
- Cunningham D. S. and G. M. Burghardt. 1999. A comparative study of facial grooming after prey ingestion in colubrid snakes. Ethology 105:913-936.
- Conant, R. and J. T. Collins. 1998. A Field Guide to Reptiles and Amphibians of Eastern/Central North America. Houghton Mifflin Company. Boston, MA. 429 pp.
- Fisher, C., Joynt, A., and R. J. Brooks. 2007. Reptiles and Amphibians of Canada. Lone Pine Publishing. Edmonton, AB. 208 pp.
- Gerald G.W., and D. L. Claussen. 2004. Thermal influences on the velocity of neonate brown snakes (Storeria dekayi) during three different modes of locomotion. Int. .Comp. Biol. 44:557-557.
- Gerald G.W. 2008. Feign versus flight: influences of temperature, body size and locomotor abilities on death feigning in neonate snakes. Anim. Behav. 75:647-654.
- Gregory P.T. 2004. Analysis of patterns of aggregation under cover objects in an assemblage of six species of snakes. Herpetologica 60:178-186.
- Harding, J. 1997. Amphibians and Reptiles of the Great Lakes Region. Univ. Michigan Press. Ann Arbor, MI. 400 pp.
- King R.B. 2009. Population and Conservation Genetics. In Snakes: Ecology and Conservation edited by S. J. Mullin and R. A. Seigel. Cornell University Press. Ithaca, NY. 365 pp.
- King R.B. 2002. Predicted and observed maximum prey size – snake size allometry. Funct. Ecol. 16: 766-772.
- King R.B. 1997. Variation in brown snake (Storeria dekayi) morphology and scalation: Sex, family, and microgeographic differences. J. Herpetol. 31:335-346.
- King R.B. 1993. Determinants of offspring number and size in the brown snake, Storeria dekayi. J. Herpetol. 27: 175-185.
- King R.B. 1989a. Body size variation among island and mainland snake populations near Lake Erie. Herpetologica 45:84-88
- King R.B. 1989b. Sexual dimorphism in snake tail length – Sexual selection, natural-selection, or morphological constraint. Biol. J. Linn. Soc. 38: 133-154.
- King R.B. and R. Lawson. 2001. Patterns of population subdivision and gene flow in three sympatric natricine snakes. Copeia 2001:602-614.
- King R.B., T. D. Bittner, A. Queral-Regil, and J. H. Cline. 1999. Sexual dimorphism in neonate and adult snakes. J. Zool. 247: 19-28.
- Neill, W.T. 1950. The status of the Florida brown snake, Storeria victa. Copeia 1950:155-156
- Noble, G.K. The sense organs involved in the courtship of Storeria, Thamnophis and other snakes. Bull. Am. Mus. Nat. Hist. 73:673-725
- Oldham, M.J. and W.F. Weller. 2000. Ontario Herpetofaunal Atlas. Natural Heritage Information Centre, Ontario Ministry of Natural Resources. <http://www.mnr.gov.on.ca/MNR/nhic/herps/ohs.html> (updated 15-01-2001).
- Rand, A.L. 1952. Secondary sexual characters and ecological competition. Fieldiana Zool. 34:65-70.
- Rossman D.A. and P.A. Myer. 1990. Behavioral and morphological Adaptation for snail extraction in the North American brown snakes genus Storeria. J. Herpetol. 24:434-438.
- Sabath, M.D. and L.E. Sabath. 1969. Morphological intergradation in Gulf Coastal brown snakes, Storeria dekayi and Storeria tropica, Am. Midl. Nat. 81:148-155.
- Selander, R.K. 1966. Sexual dimorphism and differential niche utilization in birds. The Condor. 68:113-151
- Shine, R. 1986. Intersexual dietary divergence and the evolution of sexual dimorphism in snakes. Am. Nat. 138:103-122.
- Willson, J.D. and Michael E. Dorcas. 2004. Aspects of the ecology of small fossorial snakes in the western piedmont of North Carolina. Southeastern Naturalist. 3:1-12.
Reviewers: Jacqueline D. Litzgus (Laurentian Univ.) and James Bogart (Univ. Guelph)