Prairie skink (Eumeces septentrionalis) COSEWIC assessment and status report: chapter 6

Biology

General

The Prairie Skink is a small, insectivorous lizard growing to approximately 85 mm SVL (Bredin 1988, 1989). Females may grow larger than males (Nelson 1963, Bredin 1989). Juveniles have bright blue tails. The bright blue coloration fades by the time most juveniles reach approximately 45 mm SVL after their first winter (Nelson 1963). Although Prairie Skinks are largely terrestrial, they have been observed to dive into water to avoid capture. They have not been observed to climb trees or shrubs (Nelson 1963), unlike the closely related Five-lined Skink. Skinks are able to thermoregulate while under cover and hence are rarely seen. They require loose soil to allow them to bury deep enough to avoid freezing during winter.

Reproduction

Breeding occurs in the spring. Males develop orange breeding colour on the chin at this time and are aggressive to other males. Females lay 4-18 eggs, with larger females laying more eggs than smaller females (Somma 1987b); however, the number of eggs laid by a particular female can vary markedly from one year to the next (Nelson 1963). During drought years, females laid an average of only 1.8 eggs (n=5) compared with an average of 8.4 eggs (n=8) in wetter years (Bredin 1988). Eggs are laid from late June to early July and they hatch approximately 30 days later (Nelson 1963, Bredin 1988).

Females will nest communally and up to 3 nests have been found under a single cover object in Manitoba (Bredin, personal observation) and in Iowa (Freese, pers. com., 2003). The eggs are laid under cover and females remain with the eggs until hatching. In the only population that has been tracked using radioactive tags, females constructed subterranean nests 3-9 cm below the surface of the sand despite the presence and the use of cover objects during other parts of the year (Nelson 1963).  The nests were small oval cavities dug in open sandy areas with no cover objects. Hatching success in the lab is less than 40% (Somma and Fawcett 1989) although there are data on natural nests. Females generally remain with the hatchlings for a few days after they hatch (Somma 1987a). Hatchlings average approximately 25 mm SVL (Breckenridge 1943, Nelson 1963, Bredin 1989). Sexual maturity is reached at approximately 65 mm SVL and females breed for the first time after their second or third winter (Breckenridge 1943, Bredin 1989). Females typically breed annually and frequently return to the same general area to nest (Nelson 1963). Longevity of Prairie Skinks is unknown. Few marked skinks have been captured in more than two years although one skink first caught as an adult was re-captured 5 years later, suggesting a longevity of at least 7 years (Bredin 1999).

Survival

Little is known about the mortality of any size or age class of Prairie Skinks. Like most reptiles, Prairie Skinks are apt to be most vulnerable at the egg and hatchling/juvenile stages. Skinks, particularly juveniles, are fed upon by a variety of birds, mammals, snakes and even other Prairie Skinks (Breckenridge 1943). American Kestrels may be a significant predator as observations on a nesting pair revealed numerous skinks were fed to the young. Bredin (1989) observed an American Kestrel dive and carry off a skink. House cats have also been reported to kill skinks (Bredin 1989). Adult predation levels are probably low given the amount of time Prairie Skinks spend under cover or underground; however 77% of adults either were missing their tails or had a regrown tail in Minnesota (Nelson 1963). This suggests that attempted predation may be common.

The fact that females brood their eggs may increase nesting success compared with other reptiles that abandon their eggs. Egg mortality is significantly reduced when the female remains with her eggs (Somma and Fawcett 1989). Egg mortality increases with very low or very high soil moisture contents. Hatchling size is positively correlated with soil moisture (Somma 1989). Larger hatchlings likely have a higher survival rate but there are no studies on juvenile survival or recruitment in this species. Overwinter mortality has not been assessed in this species, but it could be a significant source of mortality especially for hatchling skinks.

Physiology

Prairie Skinks emerge from hibernation in mid- to late April in Manitoba (Bredin 1988). Males generally emerge first and juveniles tend to emerge 3-4 weeks after adults (Nelson 1963). Prairie Skinks enter hibernation by mid-September and hibernate for over 7 months of the year. Hibernation sites are generally not within the summer home range, but up to 25 m from the summer home range.  Skinks do not hibernate communally but sometimes return to the same site year after year. Prairie Skinks hibernate at depths up to 66 cm below the surface (Nelson 1963).

Through basking, Prairie Skinks can maintain a body temperature greater than the ambient air or substrate temperature (Nelson 1963). Preferred body temperatures appear to range from 22-35° C and the lethal thermal limit is 41-44° C (Nelson 1963). On a diurnal basis, skinks are often not active until late morning, with activity declining by mid-afternoon. Prairie Skinks spend the night under cover or buried 5-8 cm under the sand (Nelson 1963).

Movements/dispersal

Prairie Skinks make limited movements over the course of the active season and are frequently found under the same piece of cover within a year and even from one year to the next (Bredin, personal observation). Home ranges are typically small. Some Prairie Skinks remain in an area of only 30 m in diameter for up to 4 years while the maximum known home range length is approximately 100 m (Nelson 1963). Prairie Skinks may occasionally make larger movements. A single skink was observed along a railway line running through a Tamarack bog, over 1 km from the edge of the bog (Bredin 1989). There are no data on the dispersal of hatchlings; however, Five-lined Skink hatchlings have been observed to move over 100 m (Seburn 1993).

Nutrition and interspecific interactions

Prairie Skinks feed on a wide variety of invertebrates and it is unlikely that food is a limiting resource to this ectotherm. Adults eat mainly crickets, grasshoppers and spiders in Minnesota (Breckenridge 1943). By frequency of occurrence, the diet consists of orthoptera (27%), arachnida (29.5%), and homopterans (mainly leaf hoppers), coleopterans and lepidopterans (mainly larvae; 28%). The diet of hatchlings is similar, consisting of spiders (46%), orthopterans (15%), dipterans (15%) and homopterans (13%) mainly in the 4-9 mm length (Nelson 1963). Ants, a common invertebrate, were only found in one adult’s stomach (Breckenridge 1943) and in none of the hatchlings examined (Nelson 1963). Cannibalism can also occur and Breckenridge (1943) found one adult female had consumed a yearling skink. Captive feeding experiments suggest Manitoba skinks prefer spiders and crickets (Bredin 1989). Grasshoppers and meal worms were only consumed if captive skinks had gone without eating for several days. Beetles, caterpillars and a chopped meat mixture were all refused.

Behaviour/adaptability

Prairie Skinks appear to tolerate and possibly even benefit from many human modifications to the landscape. Skinks are often very numerous in areas with lots of debris such as old dumps (Bredin 1988). They also adapt to low density human modification and use woodpiles, debris, porches, etc. as cover and nest sites. However, they are vulnerable to predation by pets, especially cats. They are also common in some areas that are grazed by cattle. The largest threat to their habitat is through succession to Aspen Parkland as a result of humans suppressing fire or through the invasion of Leafy Spurge.

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