Pacific pond turtle (Clemmys marmorata) COSEWIC assessment and status report: chapter 6
Biology
As the Pacific pond turtle has been so infrequently recorded in British Columbia, and not at all since 1959, there are no studies on its biology in Canada.
Reproduction
Very little information is currently available on the life history of the Pacific pond turtle. There are inconsistencies in the literature regarding age of maturity, time of copulation and time of nesting. These may be related to environmental differences, particularly temperature, throughout the turtle’s range; however, some of the major discrepancies are from populations within Washington state. The Pacific pond turtle reaches maturity at approximately 8 to 10 years, or at a carapace length of 135 to 140 mm (Ernst et al. 1994). Information specific to the Washington population states that age of maturity is reached by females at 10-15 years and at 8-12 years by males (WPTP website). Copulation has been observed in the field in April, May, June and late August (Ernst et al. 1994; IEP interim report). Nesting occurs from late May to early June in the Mojave River population (Lovich and Meyer In Press), from mid-June to mid-July in the Trinity River population (Reese and Welsh 1997), and from May to August in the Washington state populations (IEP interim report; WPTP website).
Prior to nesting, females in the Trinity River population made multiple trips (ranging from 2-11) onto land during which they burrowed into leaf litter for up to three days at a time (Reese and Welsh 1997). This may allow the females to increase their body temperature to optimize preovipositional development of the embryo as land temperatures are greater than water temperatures in spring (ibid.), or to assess the environment for favourable oviposition sites. Females dig flask-shaped nests in open, upland sites with a southern exposure (Ernst et al. 1994; Reese and Welsh 1997). Clutch size ranges from 2-11 and there is some evidence that multiple clutches may be laid in the same year (Ernst et al. 1994).
Hatchlings in the Trinity River population did not emerge from nests until the following spring (Reese and Welsh 1997). It is unknown whether the hatchlings overwintered in the nest cavity or if the embryonic development was suspended until just before emergence in March. The incubation time for turtles in Washington state ranged from 90-130 days (WPTP website). Hatchling sex is determined by incubation temperature (M. Ewert, pers. comm.).
Growth and survivorship
Hatchling Pacific pond turtles have a carapace length between 25 and 29 mm and reach 27.8-33.9 mm by the beginning of their second season (Ernst et al. 1994). The maximum carapace length for this species is recorded as 18 cm in Carr (1952) and 21 cm in the IEP database (IEP interim report), with an average carapace length of 15 cm (Carr 1952). The longest-lived turtle in captivity lived over 12 years (Ernst et al. 1994); however, maximum age attained in the wild is unknown and estimates vary from 20+ years (ibid.) to 30-40 years (IEP interim report) to 50 years (WPTP website).
A demographic study of the Trinity River population indicated that the age-structure of this population is skewed towards adults. Overall, only 25% of the total captures were juveniles, and significantly fewer juveniles than adults were recaptured throughout the study (Reese and Welsh 1998b). It is possible that juveniles have a lower capture probability; however, the low rate of recapture may also be due to lower survival probability. Despite high adult survivorship, naturally low nest survivorship in Pacific pond turtles makes high juvenile survivorship crucial for a population to maintain long-term stability (Reese and Welsh 1998b; Congdon et al. 1993).
Feeding habits
The Pacific pond turtle is an opportunistic forager and scavenger. In the wild it has been observed feeding on various plants, algae, crustaceans, adult and larval insects, fish, frogs, snakes, and duck and mouse carrion, and in captivity it will eat anything from dog food to earthworms to romaine lettuce (Ernst et al. 1994). Males consume larger prey and more animal matter than females, who eat more algae. Juveniles eat smaller food items, and more of them, than adults (Bury 1986a).
Behaviour
Pacific pond turtles generally forage at sunrise (0530-0800h), but may forage throughout the day in the summer months (Ernst et al. 1994). They will move upstream and downstream to different pools in early morning and evening in search of suitable feeding and basking opportunities (ibid.). The majority of basking takes place between 0900 and 1000h (Bury 1972). Basking Pacific pond turtles aggressively defend their place in the basking site if it becomes crowded. They will ram and push, threaten with an open-mouth gesture and occasionally bite one another (Bury 1986b). Adult turtles will sometimes use the open-mouth gesture to threaten juveniles already occupying good basking sites, and may even push the smaller turtle off its perch (Ernst et al. 1994).
Hibernation
Hibernation takes place both on land and in the water (Ernst et al. 1994; Reese and Welsh 1997). Of the 12 radio-equipped turtles monitored by Reese and Welsh (1997), 10 chose hibernation sites in woodlands (hardwood, coniferous and mixed) and two overwintered in lentic bodies of water. All turtles moved onto land in September, but changed locations as many as four times before early December when they selected their final sites. Emergence began in February and was completed in June (Reese and Welsh 1997). In the Willamette Valley, the earliest occurrence of the Pacific pond turtle was 28 February and the latest was 19 November (Evenden 1948 in Ernst et al. 1994). This species will also estivate during summer drought periods by burrowing into the muddy bottoms of streams or pools (Bury 1986b; Ernst et al. 1994).
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