Wood Turtle

Wood Turtle in pristine habitat - Kiley Briggs

Species Description

The Wood Turtle is a semi-aquatic turtle that gets up to about 9 inches in length and primarily occurs in the Northeastern United States and Canada.  Striking in appearance, the Wood Turtle‚Äôs head is black, contrasting sharply against the orange on their neck and insides of their legs, which are sometimes vibrantly colored.  The front of their forelimbs are sometimes darkly colored. 

The top of a Wood Turtle‚Äôs shell (carapace) is usually brown, but sometimes has yellow flecks, spots, or stripes radiating out from the center of each scute (armored plate).  The carapace is also characterized by circular growth rings with shallow grooves separating each ring, giving their shell a sculpted appearance.  The rings appear to be stacked on top of each other, shaping each scute into a pyramid, which makes the shell very strong.  The rings can also be used to estimate the turtle‚Äôs age until roughly their 20th year.  Over time, the growth rings wear down, giving the shell a polished appearance.  The plastron (bottom of the shell) is mostly yellow and has black markings in the outside corner of each scute, with shapes that are unique to each turtle, a fingerprint.

Males are somewhat larger than females, and have a concave plastron.  Due to the agricultural settings that surround many Wood Turtle streams, their shells often have scars from past injuries, such as impacts from mower blades.  Although Wood Turtles sometimes survive such injuries, their shells will bear those scars for the rest of the turtle‚Äôs life, which may exceed 80 years.  It is also common to find Wood Turtles missing one or more legs, usually the result of predation by raccoons or mustelid predators such as otters.

 

Taxonomy

The Wood Turtle‚Äôs scientific name, Glyptemys insculpta, refers to the shell‚Äôs sculptured appearance.  Although their name is sometimes assumed to mean the species lives in woodlands, it too is inspired by the shell‚Äôs color and texture.  Wood Turtles were formerly placed with Spotted Turtles in the genus Clemmys but were moved to Glyptemys, which they shares with Bog Turtles, to better represent evolutionary relationships.  They are within the family Emydidae, which includes the majority of turtles in the United States including pond turtles and box turtles. 

 

Distribution

Wood Turtles are found in the northeastern United States and southeastern Canada.  The Wood Turtle‚Äôs range is almost contiguous east of the Great Lakes from northern Virginia, up through New England, and into southern Quebec and Nova Scotia.  Their range extends to the west north of the Great Lakes to eastern Minnesota, with the turtles in the northwestern portion of their range distinct from those in the northeast, both genetically and in having paler coloration.

 

Habitat

Wood Turtles are a river valley specialist that overwinters in streams, but spends considerable time foraging on land, sometimes spending weeks on end away from water in floodplains and upland habitats more than 1000 feet away from their streams.  Suitable streams for Wood Turtles have a meandering form, with sharp twists and turns as they wind back and forth through valley floors.  These rivers are also characterized by having prominent sand and gravel bars on the inside of sharp bends where Wood Turtles will nest.  

During the winter, water quality is important to Wood Turtles as they lay dormant in their streams, typically the bottom of shallow pools, in undercut banks or logjams, or inside leaf packs.  Predicting exactly where Wood Turtles will overwinter is difficult, as any part of the river with a continuous supply of dissolved oxygen and protection from strong currents will suffice.  

On land, Wood Turtles are habitat generalists, but spend most of their time within 300 feet of the streams where they overwinter.  While foraging, they seem to favor early successional habitat such as meadows and farm fields, but they also venture into forested settings.  Regardless of habitat type, Wood Turtles seek out microhabitats where they can conceal themselves from predators.  Dense vegetation, the branches of fallen trees, brambles, and leaflitter are some of the more common hiding places Wood Turtles will use.  Wood Turtles favor edge habitat, and while they may venture into the middle of meadows and fields, tend to stay close to transitional habitat between woods or shrublands and more open settings. 

 

Boreal Wood Turtle habitat – Kiley Briggs

Movement and Home Range

Creatures of habit, Wood Turtles have very high site fidelity and adults are known to forage and shelter in the same places throughout their home range from year to year, usually overwintering in the same locations as well.  The size of their home ranges vary greatly, but tend to be between 25 and 50 acres, with males tending to stay closer to streams, but covering much greater distances up and down river valleys. 

Wood Turtles emerge in the spring shortly after river ice breaks up and most snow is melted in northern regions.  In Northern New England, that can be anywhere from late March to early May.  Early in the season, the turtles will bask on riverbanks on sunny days, partially concealed in vegetation, and retuning to the river most nights.  Once the threat of hard frosts has passed, the turtles venture farther from river to forage on land.  While most will spent the majority of their time within 300 feet of streams, many will travel up to 1000 from water in the summer, with a small percentage moving much greater distances. 

Females nest from mid-May to early July, usually on prominent sand and gravel bars on the banks of the streams where they overwinter.  If instream nesting habitat is limited, or degraded by invasive plants, they are likely to travel into upland environments to lay their eggs.  Some females make lengthy migrations to nest sites more than a mile from the rest of their home range.  

As temperatures cools in the fall, Wood Turtles cease foraging in upland settings and return to their rivers, and settle into their overwintering areas by mid to late October.  Rarely, on warm sunny days, Wood Turtles can be found basking in mid-winter, sometimes on top of snow, but the vast majority will remain underwater until very early spring.  During that time, some Wood Turtles do move around in the rivers, and they are sometimes observed walking around under ice. 

 

Diet

Wood Turtles feed on both land and in water, and are opportunistic omnivores.  They eat a variety of plant and animal matter, but their diet changes as they age.  The protein-rich diets of juveniles include a higher proportion of insects, invertebrates, and carrion compared to adults, whose diets include more vegetation.  Slugs and snails make up a large part of the Wood Turtle‚Äôs diet, and the fragments of snail shells are often observed stuck to the outside of a Wood Turtle‚Äôs beak.  Overall, their diet is highly varied, with the precise species of plant, insect, and other animal matter limited largely by what is available in their home ranges.

A subset of the plants consumed by Wood Turtles includes grasses, mosses, algae, fruits such as elderberries, raspberries, grapes, blackberries, dewberries, and the leaves of many shrubs and flowering plants.  Animal matter in the Wood Turtle‚Äôs diet includes mollusks, amphibian eggs and larvae, dead fish, bird carrion, newborn mice, beetles, and other insects.  Fungi are also regularly consumed when available.  

 

Wood Turtle with foraged snail – Kiley Briggs

Reproduction

Wood Turtles reach sexual maturity in approximately 12-20 years depending largely on geographic location, with females taking longer to reach breeding age than males.  Mating usually occurs in the water, but can take place on land as well, and peak breeding season occurs in the early spring and fall.  Like other turtles, females can store and use sperm from males for at least a couple years, and it is common for a clutch of eggs to have mixed paternal parentage. 

Females lay up to one clutch of eggs every year, and may occasionally skip a year.  Eggs are laid in the mid to late spring through early summer, typically from late May into early July.  Females nest primarily on sand and gravel bars on the banks of a valley stream, but if instream nesting habitat is limited, may travel into upland habitat to nest in lawns, gardens, roadsides, and sand or gravel pits.  Some females move great distances up or downstream to nest more than a mile from their home ranges, even when ideal nesting habitat is abundant.  Wherever a female nests, she is likely to return to the same site year after year, but may choose a new location if better habitat becomes available.  Nests typically contain between 7 and 11 eggs, which usually hatch between mid-August and mid-September.  Unlike the vast majority of other turtles, which have temperature dependent sex determination, the sexes of Wood Turtles are determined genetically.  

The courtship and territoriality of Wood Turtles is rather complex.  In a given stretch of Wood Turtle habitat with dozens of mature males and females, the vast majority of eggs will be fathered by a small number of larger dominant males.  The dominant males maintain their hierarchy by aggressively bullying other males, biting at their tail, feet, and shells, and sometimes mounting them as they would during copulation with a female.  When mating, males often mount the female, arch their head downward to face the female, and bob their head back and forth, nipping at the shell and legs of the female.  Occasionally, plastron-to-plastron mating is observed. 

 

Conservation Concerns

Many factors threaten the habitat and survival of Wood Turtles, ranging from direct habitat loss, invasive plants, degraded water quality, roadkill, mortality in farm fields, and collection, both incidental and deliberately through an international black market.  In most areas, the driving factor behind Wood Turtle population decline is human land use in river valleys.  Because river valleys carve relatively flat paths through hills and mountains, and have rich soils, many Wood Turtle streams are paralleled by roads and used heavily for agriculture. 

When Wood Turtles forage on land, they regularly travel 300 or more feet from streams, and agricultural lands such as hay fields closely resemble natural meadows where food is abundant.  If Wood Turtles are present in farm fields when those fields are harvested by heavy equipment, they can be crushed by tires or struck by mowers, tillers, diskers, or other equipment.  When roads occur within 300 to 1000 feet of the streams where Wood Turtles overwinter, turtles are likely to cross the road or nest along its margins, putting them at great risk of being hit by cars.  Where streams cross under roads through perched culverts (which have a drop off on the downstream side), turtles determined to move upstream are forced over the road. 

Invasive plants, especially Japanese knotweed, can degrade the nesting habitat of Wood Turtles by overrunning sand and gravel bars, forcing females to travel greater distances to find a suitable place to lay eggs.  Near urban setting, residential neighborhoods, and farms, the populations of predators such as raccoons and skunks can be much greater than in remote environments and are significant predators of turtle eggs and hatchlings, especially where nesting habitat is concentrated.  Raccoons and otters, although unlikely to kill and consume an entire adult Wood Turtle, frequently gnaw off one or more limbs, after which the turtles may perish depending on the severity of their wounds (roughly 12% of Wood Turtles are missing one or more limbs).

Due to the longevity of Wood Turtles, which may live 80 or more years, their low reproductive rates, and slow maturation, their populations are very sensitive to any threat that kills adults or removes them from the population.  In some cases, only the loss of one or two Wood Turtles per year due to human causes leads to significant population decline or extirpation (localized extinction).  Apart from mortality on roads and fields, that also includes incidental and deliberate unlawful collection.  When people encounter Wood Turtles on trails or crossing roads, they are sometimes collected to keep as pets, and if they are ever released, may be brought to unsuitable habitat far removed from other Wood Turtles.  Demand for wild-caught Wood Turtles through an international black market also leads to high poaching pressure in some areas.  The best way to prevent poaching is to be discrete about the location of good Wood Turtle sites, but due through social media and the internet at large, finding good locations to collect Wood Turtles is becoming easier for poachers.

 

Our mini-documentary, “The Great Northern Turtle.”