Blanding's turtle COSEWIC assessment and status report: chapter 8

Biology

Although much of the information in this section is from long-term research in  Nova Scotia and the United States, the information is pertinent to the Great Lakes/St. Lawrence population. There has been relatively little work done on this species in Ontario and Québec, so one must rely primarily on information from other localities. However, some information was also gathered from biologists and field researchers working in areas where the Blanding’s Turtle occurs in Ontario and Québec.


Life Cycle and Reproduction

Age of juvenile Blanding’s Turtles can be estimated from careful counts of growth annuli on plastral scutes (J.D. Congdon, pers. comm. 2004; Congdon et al. 1993, 2001).  However, age of sexually mature turtles is difficult to determine reliably, and cannot be inferred from body size variation. Females in Michigan with a mean minimum age of 47 years exhibited no significant difference in body size, including straight-line carapace length, when compared to a younger group with a mean age of 21 years (Congdon and van Loben Sels 1991). Sexual maturity has been estimated to occur when a minimum straight-line carapace length of 152mm has been reached (Harding 1997). This minimum straight-line carapace length corresponds with age of maturity of at least 14 years (Congdon et al. 2001; Bury and Germano 2002; Herman et al. 2003). In the northern areas of the range, including Michigan, Ontario, Québec, and Nova Scotia, maturity is estimated to be delayed up to 25 years of age (Congdon et al. 2001; Bury and Germano 2002; Herman et al. 2003; Ron Brooks pers. comm.) making it one of the latest maturing species of turtles.

Blanding’s Turtles live in excess of 75 years (Congdon et al. 1993; Power et al. 1994; Congdon et al. 2001). At maturity, one clutch of eggs is produced at a frequency of once every 1-3 years (Congdon et al. 1983).  Clutches range in size from 3-19 eggs (Congdon et al. 2001), with an average of 10-15 eggs (Ernst et al. 1994). Essentially, the extremely delayed maturity/great longevity life-history strategy of the Blanding’s Turtle represents a classic example of a trade-off between adult survival and reproductive output. This trade-off is reflected in the species’ highly iteroparous reproduction, low annual reproductive output, and very high annual survival of the adults. As noted elsewhere, populations adopting such a life-history strategy are highly vulnerable to any chronic increase in adult mortality rates, even when these increases are quite small (<5%) (Congdon et al. 1993; Samson 2003).

In Nova Scotia, mating has been observed in early spring, mid-summer, and fall (Power 1989; McNeil 2002). Mating activity appears to peak during October and November, after the turtles have aggregated in their overwintering locations (McNeil 2002).

Nesting in Nova Scotia occurs from mid -June to early July, with the majority occurring in the last two weeks of June in most years (Standing et al. 1999). In Ontario, the nesting period is slightly earlier, occurring throughout the first three weeks of June, usually peaking around the second week (Lauren Trute, pers. comm. Jan 25, 2005). Nesting activity is concentrated between 1700 and 2300hrs commencing slightly before sundown and finishing after dark (Congdon et al. 2000). During the nesting season, females may spend several days terrestrially before nesting activity is commenced (Congdon et al. 2000). The nesting period in Ontario, Quebec and Nova Scotia is notably later than in the more southern regions of the Blanding’s Turtles range. It is hypothesized that this later nesting period is a direct result of the thermal constraints of individuals living in the northern portion of their range, as temperatures in Nova Scotia, as well as in Ontario/Québec, do not reach the minimum value required for regular activity until later in the season. Hatchlings in Nova Scotia emerge from nests beginning in early September and continue to emerge as late as the last weeks of October (Standing et al. 1999; Herman et al. 2003). Hatchlings emerge during daylight hours, with 75% emerging before 13:00hrs (Congdon et al. 2000).

Female Blanding’s Turtles will preferentially choose nesting locations in relatively open areas, such as fields, or disturbed habitats such as roadways (Congdon et al. 2000). Nesting in open areas, raises the mean incubation temperature in the nest cavity, which increases the likelihood of a successful nest. Nesting in open areas may, however, lead to an increase in predation rates by mammals (see Predation section).

Rate and success of development of embryos are correlated with the temperature at which the eggs are incubated. The range for successful incubation of eggs is between 22°C and 32°C (Gutzke and Packard 1987). If the temperature falls below or rises above these limits for a significant length of time while the embryos are developing, eggs will fail to hatch or the hatchlings will have reduced viability (Ernst et al. 1994).

The size of the clutch is not dependent on the size of the female as described for other species of turtles (Congdon and van Loben Sels 1991, Congdon et al. 1993, 2001, 2003). Females over 60 years of age mate and nest more successfully than individuals under 60, particularly compared to mature turtles from the youngest age groups (less than 35 years, Congdon et al. 1993). In Nova Scotia, clutch size, as well as age and size at maturity, vary between the populations (Herman et al. 2004). The McGowan Lake turtles are smaller, have slower growth rates, mature at a later age and smaller size, and lay smaller clutches than those in Kejimkujik N.P.

Intergeneric hybridization has been observed in rare cases between the Blanding’s Turtle and the Wood Turtle (Glyptemys insculpta) (Harding and Davis 1999; Bob Knudson, pers comm. May 25, 2004). Harding and Davis noted that the hybridization event during the spring of 1997 resulted in the production of viable offspring. DNA samples were obtained from the hybrid hatchlings, and confirmed maternity and paternity. A similar event was discovered during the summer of 1998, with both events occurring in Michigan. In Ontario, a mating occurrence between a Blanding’s Turtle and a Wood Turtle was observed in the Elliot Lake area (Bob Knudson, pers. comm. May 25, 2004). In the wild, intergeneric hybridization is considered very rare (Harding and Davis 1999).


Predation

Predation on Blanding’s Turtle eggs is often extremely high. Congdon et al. (1993) reported that within the E.S. George Reserve (Michigan), nest survival ranged from 0% to 63% annually, with a mean survivorship of only 3.3% from 1985 to 1991. The nests of younger Blanding’s Turtles are depredated more frequently than those of middle aged or older turtles (Congdon et al. 2001). The major mammalian predators of turtle nests in the Great Lakes area are Raccoons ( Procyon lotor ), Striped Skunks ( Mephitis mephitis ), and Red Foxes ( Vulpes vulpes ) (Harding, 1997). Other nest predators include the Coyote (Canis latrans), and the Black Bear (Ursus americanus) and the Virginia Opossum (Didelphis virginiana ). Although predation is not the sole cause of poor nest success, it is a limiting factor in many cases (e.g. Browne 2003).

Congdon et al . (1993) noted that during their period of study (includes: 1953 to 1957 by O. Sexton, 1968 to 1973 by H. Wilbur, 1975 to 1979 by D. Tinkle and J. Congdon, and 1980 to 1991 by J. Congdon) the period of lowest nest survivorship (mean of 3.3% from 1985 to 1991) coincided with a collapse in the fur market. Trapping intensity on populations of predators was reduced due to public pressure and the decline in economic gain from trapping (Congdon et al. 1993). Gillingwater and Brooks (2001, 2002) reported that 55% of observed nests on South Point beach in Rondeau P.P. were depredated in 2000, and 99% of observed nests in 2001. In Nova Scotia, the occurrence of predation by raccoons is high, particularly along lakeshore beaches; these nests are routinely screened for protection. However, inland nests, especially away from areas of high disturbance, appear to experience lower predation in this population (Jennifer McNeil, Tom Herman, pers. comm. Jan 24, 2005).

Hatchling and small juvenile turtles are more susceptible to predation than adults due to their small size, and are eaten by small and large mammals, fish, frogs, snakes, wading birds, and crows (Harding 1997). Predation attempts do not always result in death, but may result in non-fatal amputation of limbs, tail, or claws (St-Hilaire 2003). However, turtles, especially juveniles, with missing limbs do not usually survive more than 1-2 years (Ron Brooks, pers. comm.). There are relatively few predators of adult Blanding’s Turtles, as their overall size and strong carapace prevents or deters most predation attempts. Adults could potentially fall prey to large predators such as the Black Bear, or perhaps River Otters (Lontra canadensis). During the drought on Pelee Island deceased adult turtles were observed with marks indicating predation, although it is impossible to determine if these injuries were inflicted pre or post mortem (Ben Porchuk pers. comm. April 1, 2005).

The increased predation pressure on hatchling and juvenile Blanding’s Turtles results in more cryptic behaviour. Young Blanding’s Turtles are more often observed in areas that contain a greater amount of refugia, specifically floating sphagnum mats, than their adult counterparts (Pappas and Brecke 1992).


Physiology

As with several other turtle species, the Blanding’s Turtle has a very specific thermal tolerance. Their upper maximum temperature tolerance is 39.5°C, which is one of the lowest critical thermal maxima of any turtle (Hutchinson et al. 1966). Minimal thermal tolerance for incubating eggs is 22°C, and the thermal maximum is 32°C (Ewert and Nelson 1991). The thermal tolerance range for incubating eggs in the nest results in high nest failure rates in the northern portion of the species’ range; due to fluctuating temperatures, since the eggs have a relatively high minimal thermal tolerance. Sex is determined by temperature sex determination; males are produced when the eggs are incubated at or below 28°C, and females above 30°C (Ewert and Nelson 1991). If the egg volume were to be increased, freezing tolerance in the northern populations would be extended, allowing the eggs to overwinter if required (Bleakney 1963).

Blanding’s Turtle eggs are not highly susceptible to drowning, and are able to withstand fairly dry conditions (Packard et al. 1982). However, lakeshore nests, which are common in Nova Scotia, are at risk of extended flooding during relatively wet summers. In 2003, all lakeshore nests in Kejimkujik N.P. were lost as a result of late summer flooding (Jennifer McNeil, Tom Herman, pers. comm. Jan 24, 2005). Extensive seasonal flooding of the Ottawa River may lead to nests being submerged for up to 7 days, which would likely prove fatal to the developing embryos.


Interspecific Interactions

Parasitism of turtle nests by Sarcophagid fly larvae in Rondeau P.P. was discovered by Gillingwater and Brooks (2001). They reported that in 2000, 39% of all turtle nests on South Point beach were affected by the parasite; the larvae were present in one or more of the eggs and/or hatchlings. In 2001, Gillingwater and Brooks (2002) reported that 100% of Blanding’s Turtle nests were attacked by Sarcophagid fly larvae. All infected embryos and hatchlings perished within a few days of infection. This dramatic increase in nest parasitism from 2000 to 2001 is unexplained, but creates concern, as simply employing a standard wire mesh nest protector will not protect the eggs from the Sarcophagid fly. The impact of these flies on freshwater turtles has not been measured or described elsewhere, and at present the importance of this source of mortality is unknown, although it does appear a potentially significant threat.

Blanding’s Turtle has a parasitic relationship with two different leech species. Saumure (1990) reported a Blanding’s Turtle at Big Clear Lake (Frontenac County, Ontario) with seven leeches, three of which were Placobdella parasitica, which is a well known and well documented parasite on many species of turtles. The remainder were Placobdella ornata. The leech P. ornata had not been previously known to parasitize Blanding’s Turtles.


Adaptability

Blanding’s Turtles survive in a variety of habitats, which means that a critical habitat is difficult to define. In addition to their natural habitat, Blanding’s Turtles may persist in and around major urban centres (Ruben et al. 2001; Bob Johnson, pers. comm. June 7, 2004). Unfortunately there is an extremely high nest failure rate (~100%), and an extremely low juvenile recruitment (~0%) into the sexually mature adult population in these urban areas (Congdon and van Loben Sels 1991; Congdon et al. 1993, 2001; Ruben et al. 2001). The populations that are found in urban centres are physically separated from one another by roadways, as well as urban and commercial developments (Ruben et al. 2001). Populations living close to roadways become more susceptible to being struck by vehicles, so it can be hypothesized that populations living in urban landscapes will have higher rates of adult and hatchling mortality. It appears likely that these urban populations are composed of aging cohorts, likely male-biased, with little or no recruitment, and not viable in the long term.

The long generation time of the species (exceeding 40 years) limits its ability to adapt genetically to sudden environmental changes. Populations at the extreme periphery of the species’ range are already near the limits of their physiological tolerance, andmay be particularly susceptible to climate change and extreme weather events (Herman et al. 2003). Small populations of late-maturing individuals are particularly limited in their ability to respond to small increases in adult mortality (<5%), due to the low natural rate of recruitment of juveniles into the sexually mature adult population.

However, their extreme longevity means that individuals will normally be subjected to environmental changes within their lifetime. Although individuals show high fidelity to specific locations, they may be able to shift to new areas when necessary (Herman et al. 2003). Shifts in individuals’ nesting sites, overwintering sites, and summer home ranges have been documented in the Nova Scotia population, although the cause of these shifts often remains unknown (Power 1989; McNeil et al. 2000, unpublished data). Also a shift in adult home range size and habitat selection has been observed during a drought year on Pelee Island (Ben Porchuk, pers. comm. April 1, 2005). In this instance individuals were observed to shift from inland wetlands to coastal Lake Erie areas, and begin to scrape algae from rocks as a food supply.

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