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Taxonomy:

The western pond turtle has been known under a variety of taxonomic classifications, most notably Clemmys marmorata and Emys marmorata. The former is characterized by a plastron (hard under shell) fused to the carapace, whereas the latter includes a plastral hinge (Bettelheim, 2004). Although the western pond turtle has a fixed plastron, phylogenetic analysis reveals that the species is likely most closely related to the Emys genus (Bettelheim, 2004). In light of this data, the western pond turtle is commonly identified as Emys marmorata. The species is divided into two subspecies: the northwestern pond turtle (Emys marmorata marmorata) and the southwestern pond turtle (Emys marmorata pallida).

Physical Characteristics:

The species name of the western pond turtle, marmorata, comes from the Latin root marmor, meaning marbled (Bettelheim, 2004). Western pond turtles indeed showcase a dark green or brown marbled carapace (top shell)and similarly mottled skin consisting of yellow, grey, and brown hues. The southwestern subspecies tends to be the paler of the two. The plastron ranges from pale yellow to dark green, usually consisting of pale yellow covered to varying degrees with green blotches. Adult carapace length typically ranges from 160 mm to 190 mm, the northwestern pond turtle being slightly larger than the southwestern pond turtle (Bettelheim, 2004). The largest recorded carapace length is 210 mm, although specimens approaching that size are rare (Buskirk, 2002).

Behavior and Sexuality:

Western pond turtles are generally peaceful, but occasionally exhibit aggressive behaviors such as ramming, pushing, or biting one another. Such behaviors are most common in areas with limited basking structures, or during mating season (Bettelheim, 2004). Mating generally occurs between March and November and females lay eggs in early spring to mid summer, depending on geographical location (Bettelheim, 2004). Some bear eggs every year or even twice per year, but it is more common for females to lay eggs every other year (Bettelheim, 2004).

Figure 1: Neck Mottling (Buskirk, 2002)

Figure 2: Carapace Marbling (Buskirk, 2002)

Age can be determined by counting scute rings occurring on the carapace and plastron (Bettelheim, 2004). The average lifespan of a western pond turtle is 40 years of age. A western pond turtle becomes sexually mature at a carapace length of 110-120 mm but continues to grow with age (Buskirk, 2002). The age of sexual maturity varies with geographical location; in Oregon, western pond turtles reach sexual maturity between ages 8 and 10 (Bettelheim, 2004). A healthy population should be comprised of 70 percent adults and 30 percent juveniles (Adamus, “status”, 9). There are several pronounced sexually dimorphic features of the western pond turtle. Females have a shorter more blunt nose, a taller and deeper carapace, and a longer, thinner tail than males (Bettelheim, 2004, 8).

Figure 3: Sexual Dimorphism in Carapace shape (Buskirk, 2002)

Figure 4: Sexual Dimorphism in Nose shape, female left, male right (Buskirk, 2002)

Species Range:

The western pond turtle historically ranges from Southern British Columbia, to Northern Baja California, in between the Cascade-Sierra Nevada crest and the Pacific Ocean (Bettelheim, 2004). Although typically found below 500 m, western pond turtles exist up to elevations of 1500 m (Buskirk, 2002). The ranges of the subspecies overlap from the San Joaquin Valley to San Francisco Bay.

Figure 5: Historic Range (Bettelheim, 2004)

Aquatic Habitat:

Adult western pond turtles are aquatic habitat generalists, preferring slow-moving, shallow water, with adequate refugia such as undercut banks, large rocks, and aquatic plants (Bettelheim, 2004, 11). Western pond turtles are ectothermic; they regulate body temperature by taking advantage of environmental sources of heat and cold. Thermoregulation requires that basking structures with adequate sun exposure be present, especially in the Northern portion of their range (see figure 6 for an example of aquatic habitat). They are also dietary generalists. The typical diet of a western pond turtle includes aquatic invertebrates, terrestrial invertebrates, tadpoles, egg masses, algae, and aquatic plants (Bettelheim, 2004). It is assumed that western pond turtles only feed while submersed, however, recent field sightings have demonstrated that they are capable of feeding on land.

Upland Habitat:

Western pond turtles have specific upland habitat requirements. When temperatures fall in autumn, they travel up to 500 m to locate a suitable overwintering site (Bettelheim, 2004, 10). Individuals located at the South end of the species range may overwinter for only one month out of the year, whereas those inhabiting the Northernmost reaches of the species range overwinter for eight months. In Oregon, the species typically begins to overwinter between late September and late November (Bettelheim, 2004, 10). Lacustrine populations often overwinter on muddy lake bottoms, whereas inhabitants of flowing waterways tend to move into the upland to overwinter, seeking pre-existing holes or humus (Bettelheim, 2004).

Nesting is another primary use of upland habitat. Areas of sparse vegetation, clay or silt soils, and gentle South or West facing slopes are desirable, but nests often exist in locations failing to meet those specifications (Bettelheim, 2004). Nesting sites are generally found within 2500 m of aquatic habitat (Buskirk, 2002). Throughout their range, western pond turtles lay eggs in June or July and Hatchlings emerge from the nest 80 to more than 100 days after nesting (Bettelheim, 2004). See figure 6, below, for an example of upland habitat.

Figure 6: Aquatic Habitat with Basking Structures and Nearby Upland Habitat (Buskirk, 2002)

biology.txt · Last modified: 2011/12/19 09:36 (external edit)
 
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