Wood turtle (Glyptemys insculpta) COSEWIC assessment and status report: chapter 6
Biology
Since the original COSEWIC Report (Litzgus and Brooks 1996), there have been many studies completed on various aspects of the biology, demography, and ecology of Wood Turtles across the range of the species, so most of the information in this section will be referenced from this published work (see Ernst et al. 1994; for review up to the time of the original COSEWIC report). There are also many unpublished studies since 1996, some of which are ongoing.
Life cycle and reproduction
Wood Turtles emerge from hibernation in late March to early April. They mate throughout the active season (April to September), but most commonly in spring and fall (Kaufmann, 1992b; Foscarini, 1994; Walde et al., 2003; Trochu, 2004). Mating usually occurs in shallow water (DeGraaf and Rudis, 1983), although Wood Turtles occasionally have been observed copulating on land (S. Gillingwater, pers. comm. 2006). There has been at least one account of Wood Turtles and Blanding’s Turtles (Emydoidea blandingii) mating and at least one account of Wood-Blanding’s hybrids (Harding and Davis, 1999).
Female Wood Turtles nest in late May to mid-June (Schaffer, 1991; Smith, 2002, R. Brooks, pers. comm. 2004), and hatching, when it occurs successfully, is in late August to September or early October (Schaffer, 1991; Foscarini, 1994; Smith, 2002). There are few accounts of hatchlings overwintering in the nest (DeGraaf and Rudis, 1983; Schaffer, 1991; Parren and Rice, 2004), and it is unlikely this happens often, if at all, in Canadian populations (R. Brooks, pers. comm. 2004). Female Wood Turtles lay only one clutch per year (Powell, 1967; Farrell and Graham, 1991; Brooks et al., 1992), although individual females may not nest every year (R. Brooks, pers. comm. 2005). In late May to early June, females migrate to nesting areas and dig nests, usually in the evening, often after several “test” digs (Bishop, 1927; Thomas, 1983; Schaffer, 1991; Brooks et al., 1992; Kaufmann, 1992b; Foscarini, 1994; Walde, 1998; Smith, 2002; Brooks et al., 2003; Trochu, 2004). However, nesting can occur throughout the day or night depending primarily on climatic conditions (Walde, 1998; R. Brooks, pers. comm. 2005). Typically, nests are dug in sand or gravel beaches, riverbanks or other open areas near water (Thomas, 1983; Smith, 2002, Wesley et al. 2004). Wood Turtles also nest in gravel pits (Foscarini, 1994; Walde, 1998), along roads and railways (Brooks et al., 1992; Trochu, 2004), utility rights-of-way, agricultural fields, pastures and old fields (Saumure, 1997, 2004; Saumure and Bider, 1998; Trute et al., 2004), areas that are exposed to sunlight and amenable to digging.
Sex determination is independent of incubation temperature in Wood Turtles (Ernst, 2001). Successful embryonic development and hatching requires sufficiently warm thermal conditions and there are several accounts of unsuccessful nesting in years when the summer has been too cool for incubation to be completed, and eggs/hatchlings do not survive over winter in the nest (Brooks et al., 1992; Compton, 1999). This constraint likely determines the northern limit of distribution of the species (Compton, 1999). Females lay clutches of 1 to 20 eggs, but the average is from 8-12 (Powell, 1967; Harding and Bloomer, 1979; Brooks et al. 1992; Walde, 1998; Peiman and Brooks, 2003). Hatching success can be high, but is often low due to cool summers or to nests being destroyed by predators (Brooks and Brown, 1992; Brooks et al., 1992; Walde, 1998; Cameron et al., 2002). In addition, larvae of a Sarcophagid fly may attack and kill embryos and newly hatched turtles in the nest (Smith, 2002), but it is possible that the larvae may only feed on dead embryos/hatchlings in most cases (Bolton 2007). Mortality of embryos is generally 20-80%, but often as high as 100% (Brooks and Brown, 1992; Brooks et al., 1992; Foscarini, 1994).
Hatchling Wood Turtles are uncommon and, because of their small size, are difficult to find or study (Peiman and Brooks, 2003) and, therefore, there is little information on habitat, survivorship or diet during this stage of life. Wood Turtles do not reach maturity until 11 to 22 years of age (Brooks et al., 1992; Walde et al., 2003). Sexual maturity seems to be related more to body size rather than to a specific age, and size at maturity is greater in northern populations than in southern ones (Brooks et al., 1992; Daigle, 1997; Cameron et al., 2002; Smith 2002; Peiman and Brooks, 2003; Walde et al., 2003). Maximum ages for Wood Turtles in the wild are difficult to estimate due to the turtles’ longevity and to wear on the carapace, which limits the possibility of counting annuli in older turtles (Harding and Bloomer, 1979). Also, growth slows dramatically after the turtles reach maturity, so that after maturity, the growth lines either are not deposited or become difficult to detect. Nevertheless, some researchers have managed to count as many as 30-50 growth lines on some turtles suggesting that that these turtles reach ages of at least 50 years in the wild (Cameron et al., 2002; D. Coulson, pers. comm. 2004). One female captured as an adult on the New Jersey Turnpike has survived over 40 years in captivity (R. Brooks, pers. comm. 2005). Generation time (GT) (average age of adults) has not been calculated in the literature, but an estimation based on published values for age at maturity (AM) and adult rates of mortality (MR) and using the IUCN formula would be: GT=AM +1/MR= 15 + 1/0.05=35 years.
Herbivory/predation
Wood Turtles are opportunistic omnivores at all stages of life (Bishop, 1927; Breckenridge, 1944; Harding and Bloomer, 1979; DeGraaf and Rudis, 1983; Schaffer, 1991; Walde et al., 2003). The main predators of Wood Turtles are raccoons (Procyon lotor), skunks (Mephitis mephitis), and foxes (Vulpes vulpes) (Ernst et al., 1994; Peiman and Brooks, 2003; Bourgeois et al., 2004), though large fish such as largemouth bass (Micropterus salmoides) (Breckenridge, 1944) and northern pike (Esox lucius) (Seburn, 1996) and birds such as great blue herons will include hatchlings in their diet (Seburn, 1996). Raccoon, skunks and foxes dig up and eat eggs (Brooks et al., 1992), resulting in the high levels of nest failure previously mentioned. Some species, particularly raccoons and coyotes (Canis latrans) will also attack adult Wood Turtles, resulting in the high numbers of amputated limbs and truncated tails seen in most Wood Turtle populations (Saumure and Bider, 1998; Cameron et al., 2002; Smith, 2002; Peiman and Brooks, 2003). For example, raccoons killed seven of the 37 (19%) female Wood Turtles on a Quebec nesting site in 2004 (D. Masse, pers. comm. 2005). During the spring survey in 2005, eight more dead females were found near the main nesting site. These females had been marked and had nested at the site in previous years. It is estimated that predators have killed 40% of the nesting females at this site in the past few years (J-C.Bourgeois, pers. comm. 2005). In forested areas in Quebec, the mink (Mustela vison) is another important predator (D. Masse, pers. comm. 2005).
Physiology
There have been few physiological investigations of Wood Turtles, although there have been some recent studies of thermoregulation in freeliving populations (Y. Dubois, pers. comm. 2005; Dubois, 2006). Wood Turtles hibernate underwater (Schaffer, 1991; Smith, 2002), and they are adapted for anaerobic respiration during this period (Graham and Forsberg, 1991). It is also suspected that aquatic pollutants may cause hatchling deformities or other reproductive problems (Ernst, 2001).
Dispersal/migration
Wood Turtles are philopatric using the same general area (home range) both during a year and over many years, with males being territorial (Thomas, 1983; Ross et al. 1991; Quinn and Tate, 1991; Brooks and Brown 1992; Kauffman, 1992b; Foscarini, 1994; Walde 1998; Cameron et al., 2002; Smith, 2002; Arvisais et al., 2002; Peiman and Brooks, 2003; Wesley et al., 2004). Wood Turtles hibernate in underwater “hibernacula” over winter (October to April, depending on location) (Harding and Bloomer, 1979; Ernst et al., 1994; Smith, 2002). This species may hibernate alone, communally with other members of the species or with other species of turtles (Breckenridge, 1944; Harding and Bloomer, 1979; Foscarini, 1994). Hibernacula are usually just the bottom of deep pools in streams. Wood Turtles remain close to water after emerging from hibernation (Arvisais et al., 2002; Arvisais et al., 2004), then become more terrestrial as summer progresses (Bishop, 1927; Breckenridge, 1944; Arvisais et al., 2002; Peiman and Brooks, 2003; Arvisais et al., 2004; Trochu, 2004).
Home range sizes vary in response to many factors, including distance to nesting and hibernation sites and habitat productivity (Daigle, 1997). Home range sizes of 0.25 ha up to 70+ ha have been reported (Quinn and Tate, 1991; Ross et al., 1991; Brooks and Brown, 1992; Arvisais et al. 2002; Smith, 2002; Trochu, 2004). There is great variability in size of home ranges not only among study sites but among individual turtles within sites. The reasons for these differences within sites remain obscure. Wood Turtles can home reliably over 2 km, but there are accounts of them homing greater distances as well (8 km: Harding and Bloomer, 1979); straight-line distances travelled have been recorded up to 8.3 km (Daigle, 1997; Cameron et al., 2002; Smith, 2002; Adams, 2003; Wesley et al., 2004), and 23 km over 5 years (Brooks and Brown, 1992).
Interspecific interactions
Wood Turtles actively “stomp” to attract earthworms, which are then eaten (Kaufman, 1989); display “anting behaviour” (use of ants to remove epibionts) (McCurdy and Herman, 1997); and remain still while being cleaned by blacknose dace (Rhinichthyes spp.) (Kaufmann, 1991).
There are many accounts of Wood Turtles with leeches, Placobdella parasitica and P. ornata, on their legs, necks and carapaces (Brewster and Brewster, 1986; Farrell and Graham, 1991; Foscarini, 1994; Saumure and Bider, 1996; Smith, 2002), but it is not clear how this ectoparasite affects the Wood Turtle (Kaufmann, 1991). Other parasites of the Wood Turtle include trematodes, an acanthocephalan, caddisfly larvae (an epibiont), and the flesh fly, Sarcophagus spp., which may parasitize eggs and hatchlings (Foscarini, 1994; Walde, 1998; Smith, 2002).
Adaptability
The Wood Turtle’s longevity, late age of maturity, low reproductive success and inability to respond to increases in adult mortality with compensatory reproduction makes them slow to recover from population declines (Litzgus and Brooks, 1996; Oldham, 1998; Compton, 1999; Brooks et al. 1991; Cameron and Brooks, 2002). Headstarting of hatchlings is being attempted in Ontario, but it will take several years for enough appropriately headstarted turtles to have an impact on the population (Cameron and Brooks, 2002; M. Malhiot, pers. comm. 2004). La Mauricie National Park in Quebec is also considering a headstarting program to sustain its declining population (J-C Bourgeois, pers. comm. 2005).
Page details
- Date modified: