Stinkpot (Sternotherus odoratus) COSEWIC assessment and status report: chapter 6
Biology
Reproduction
Peak mating times for stinkpots are in the spring and fall when the turtles are congregated at hibernation sites (Risley 1933; McPherson and Marion 1981b; Mendonça 1987). Although courtship and mating behaviours have been described in detail (Mahmoud 1967), the mating system of this species (e.g. polygynous, random, etc.) is unknown. It is likely that multiple paternities are possible for individual clutches of eggs. Females can store viable sperm from a fall mating through the winter (Gist and Jones 1989), and there is an account of a female copulating with two different males during same breeding season (Ernst 1986).
Age and size at maturity vary across latitude, with northern individuals maturing at a later age and a larger size than southern ones (Tinkle 1961; Edmonds and Brooks 1996). Males mature younger and at a smaller size than females (Tinkle 1961; Mahmoud 1967; McPherson and Marion 1981a,b; Mitchell 1988, Edmonds 1998). In the Twelve Mile Bay population, males matured at an average carapace length of 63.6 mm (between 5 and 6 years old) and females matured at an average carapace length of 80.7 mm (between 8 and 9 years old) (Edmonds 1998).
Nesting can occur from February through July, depending on the latitude of the population. In Canada, nesting usually occurs between June and early July (Logier 1939; Lindsay 1965; Johnson 1989, Edmonds 1998). Clutch frequency varies with latitude; southern females lay from 2 to 4 clutches per year and northern females lay one or fewer clutches per year (Risley 1933; Tinkle 1961; Gibbons 1970; McPherson and Marion 1983; Edmonds 1998). Individual clutch size increases with body size (Gibbons 1970) and can range from 1 to 9 eggs (Ernst et al. 1994). The incubation time ranges from 65 to 86 days (Ernst et al. 1994). Often, more than one female will nest in the same place (Cagle 1937; Edgren 1942). Female stinkpots may exhibit year-to-year nesting site fidelity. During the radio-telemetry study of the Twelve Mile Bay population, 7 of 10 radio-tracked females with a mean home range of 28 ha nested along the shoreline of a single 2.5 ha bay (Edmonds 1998). Three females were tracked in 2 consecutive years, and two of them nested at the same bay in both years (The third was not gravid in the second year) (Edmonds and Brooks unpubl. data).
In Ontario, stinkpot nesting behaviour has been described only twice. At Big Clear Lake, individual females laid single clutches of 2 to 6 eggs (average 4) between 27 June and 23 July (Lindsay 1965). At Twelve Mile Bay, individual females laid single clutches of 3 to 7 eggs (N = 10 clutches, mean = 4.7 eggs, SE = 0.12) between 6 June and 20 July (Edmonds 1998; Edmonds and Brooks unpubl. data). In both of these Ontario populations, nests were in shallow, gravel or soil-filled rock crevices close to the shoreline. The crevices were located on rock faces exposed to direct sunlight.
Lindsay (1965) noted that several stinkpot nests laid close to the shoreline were exposed by the wave action of the water. There are insufficient data to estimate nesting success, hatchling survival or recruitment rates in stinkpot populations. Typical freshwater turtle life-history patterns are such that recruitment and early juvenile survivorship are low (Iverson 1990; Congdon et al. 1993).
Physiology
Stinkpots are ectotherms and regulate their body temperatures through environmental temperatures. They rarely emerge from the water to bask, instead basking close to the surface of the water. The thermal activity range is 10° to 34°C, with a preferred field body temperature of 24°C (Mahmoud 1967). In laboratory experiments, stinkpots would only accept food if their body temperatures were between 13°C and 35°C (Mahmoud 1967). Out of water, stinkpots are highly susceptible to desiccation and are relatively quick to show signs of distress (Ernst 1968).
Food Habits
Stinkpots are omnivorous and will eat clams, snails, crayfish, aquatic insects, earthworms, fish eggs, minnows, tadpoles, carrion, algae, and parts of vascular plants (Lagler 1943; Mahmoud 1968; Bancroft et al. 1983). Their primary food items are mollusks and aquatic insects. They actively forage while walking on the substrate and search for food by probing their heads into soft mud, sand, and rotting vegetation.
Growth and Survivorship
The growth pattern of stinkpots is logarithmic. Growth rates of young turtles are greater than in older turtles, and as the turtle ages, growth approaches an asymptote (Edmonds 1998). Males and females reach maturity at 58% and 74% of their maximum body size, respectively (Edmonds 1998). A captive stinkpot lived for more than 54 years (Snider and Bowler 1992). Two wild stinkpots in Pennsylvania were estimated to be at least 27 and 28 years old at the time of their last capture (Ernst 1986). Little is known of age specific mortality for this species. In a Virginia population, annual survival rates for all age and sex groups were estimated at 84% to 86% (Mitchell 1988). However, this estimate was biased because the time interval of the study was short (three years), relative to the life span of these turtles (27+ years). In such a short time period, temporary emigration or cryptic behaviour may have been mistaken for mortality. Adults make up the majority of most stinkpot populations (Ernst 1986, Dodd 1989, Edmonds and Brooks 1996). Typical freshwater turtle species tend to have very high hatchling and juvenile mortality rates and low adult mortality rates (Brooks et al. 1990; Iverson 1990; Congdon et al. 1993). Populations with this pattern of survivorship can withstand periods of zero or low recruitment into the population. However, unusually high adult mortality rates can have serious negative impacts on the population, and even slight increases (as low as 1-2% annually) in these rates can lead to the decline of and eventual extirpation of a population (Brooks et al. 1990; Congdon et al. 1993, Compton 1999).
Hibernation
Stinkpots hibernate underwater, buried in approximately 30 cm of mud. They begin hibernation when water temperatures drop below 10°C (Ernst et al. 1994). They sometimes congregate in high densities in suitable hibernacula (Ernst et al. 1994).
Movement and Migration
There is no evidence of stinkpots defending territories. Estimates of size of home ranges vary considerably among populations: from 0.05 ha in a Florida population (Mahmoud 1969) to 1.50 ha in a Pennsylvania population (Ernst 1986) to between mean values (minimum convex polygon) of 50 and 155 ha for females and males respectively in a population in Ontario (Edmonds 1998). Because they rarely leave the water, stinkpot home ranges are likely confined to single bodies of water (Ernst et al. 1994). During their active season, stinkpots disperse from their hibernation site. Males tend to move more than females (Ernst 1986; Edmonds 1998). Females in a Parry Sound district population may have restricted their movements because they were forced to use a specific nesting area because nest sites were a limited resource (Edmonds 1998).
Behaviour
Provided their habitat remains intact, stinkpot populations can usually coexist with human populations. However, there are still two potential threats to the stinkpots. First, stinkpots are often captured when they attempt to eat bait from fishing hooks, and consequently, are often killed by the fishermen or from the injuries from hooks (Mahmoud 1969). The second threat occurs from motorboat traffic. Stinkpots bask at the surface of the water and can be wounded or killed by propellers when boats pass over the turtles (Bancroft et al. 1983; Edmonds 1998). Mortality from motorboat traffic was a major source of stinkpot mortality in a Florida population (Bancroft et al. 1983).
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