Western skink (Eumeces skiltonianus) COSEWIC assessment and status report: chapter 6

Biology

General

In California, body temperatures of field-caught E. skiltonianus are highly variable and range from 14°C to 34°C (Brattstrom 1965, Cunningham 1966); the lizards appear to become lethargic at temperatures below 14°C (Brattstrom 1965). Brattstrom (1965) determined that after a two-week acclimation period at 13-14°C the critical thermal maximum for one animal was 41.3°C. In the field, the lizards appear to retreat underground during unsuitable thermal conditions.

Vitt and Pianka (1994) compared the stamina of several lizards, including seven species of skinks, in treadmill-endurance tests. The stamina of E. skiltonianus was relatively low (2.0-3.8 min at running speeds of 1 km/h), reflecting its small body size (1.4-7.8 g for the animals tested) and possibly also its habits of remaining near cover in the natural habitat; this behaviour reduces the need for sustained running to avoid predators. In the Creston Valley, the skinks seldom run for more than 2 m from a disturbance before seeking shelter (Pam Rutherford, pers. comm.).

Farley (1997) examined running speeds of E. skiltonianus on level and sloping surfaces and concluded that the power-producing capacity of the muscular system did not limit maximum speeds. The efficiency of locomotion on vertical surfaces was relatively low when compared to that of the arboreal gecko, Coleonyx variegatus (Farley and Emshwiller 1996). The mechanics of locomotion of the skinks was similar to that of other vertebrates with legs, although the skinks exhibited pronounced lateral bending of the trunk while running (Farley and Ko 1997).

Reproduction

The reproductive biology of E. skiltonianus has been studied in the southern portion of the species' range, mainly in Utah (Tanner 1943, 1957) and California (Rogers and Memmler 1943, Punzo 1982); very little information exists on British Columbia populations. The species is oviparous with one clutch laid per year. The clutch size is 2-6 eggs (Tanner 1957, Stebbins 1985). The number of oviductal eggs is positively correlated with the body size of the female (Punzo 1982).

Mating takes place in May-June, females lay their eggs in June-July, and the eggs hatch in July-August (Rogers and Memmler 1943, Smith 1946, Tanner 1957, Stebbins 1985). The exact timing of the events varies among geographic locations. During an ongoing study in the Creston Wildlife Management Area, British Columbia, two egg masses, each consisting of 4 eggs, were found in July, and hatchling-sized skinks were found in August and early September (Pam Rutherford, pers. comm.).

Tanner (1943, 1957) described nests of E. skiltonianus from Utah in detail. The clutches were typically located in enlarged, 5-9 cm (2-3.5") wide and 2.5-3.8 cm (1-1.5") high chambers at the end of burrows terminating under rocks. The chambers were up to 25 cm (10") underground. A female was in attendance of each nest. Female parental care is widespread among Eumeces species and probably functions in protection from predators (Noble and Mason 1933). Females of some species rotate the eggs, possibly facilitating normal development. They may also increase the temperature of their clutches with their body heat by periodically leaving the nest to bask in the sun (Noble and Mason 1933). Female E. skiltonianus have been reported to repair the nest, move eggs, and show aggression in response to disturbance (Tanner 1943, 1957).

Growth and Survivorship

Based on data from California and Utah, the mean SVL at hatching is ca 25 mm (Rogers and Memmler 1943, Tanner 1957). Rogers and Memmler (1943) estimated mean growth rates of 25 mm, 15 mm, and 3-4 mm for the first three years of life, respectively; after the third year, growth was minimal, approximately 1-2 mm/year.

Tanner (1957) and Rogers and Memmler (1943) both recognized three distinguishable size classes in the populations studied: hatchlings or same-year young, yearlings or previous year's young, and larger animals, including both breeding adults and maturing individuals. In the California population, the smallest breeding individuals were 61-62 mm in SVL but most were between 68-75 mm (Rogers and Memmler 1943). In Utah, the SVL at first breeding was somewhat smaller (males: 53 mm, females: 56 mm); maximum adult body size was also smaller (70 mm; Tanner 1957).

Based on growth rates and the distribution of body sizes, Rogers and Memmler (1943) estimated that in California, both females and males typically reach sexual maturity at the age of three years, although some individuals might mature at the end of their second year of life. Similarly, Tanner (1957) concluded that in his study population in Utah, maturation occurred at 2.5 years of age but noted that many younger males (1.5 years old in spring) might have bred. Longevity of E. skiltonianus is poorly known. A reproductive life span of 5-6 years has been suggested, resulting in a maximum age of 9 years (Rogers and Memmler 1943). Survival rates also have not been studied.

Adult males and females are either similar in mean body size (Rogers and Memmler 1943) or females are slightly larger (ca 5%) than males (Tanner 1957, Stamps 1983). Demographic parameters -- growth rates, body size at sexual maturity, and mean body size -- might vary along altitudinal and latitudinal gradients within the wide geographical range of the species (Tanner 1957), but little information of such variation is available.

Movements and Home Range

Home ranges and movement patterns of E. skiltonianus have not been documented. Both sexes excavate burrows (Tanner 1957), suggesting that individuals might maintain defined home ranges in the vicinity of these shelters. In spring and fall, movements of the skinks appear to be restricted to areas under and near rocks and other cover objects, whereas they appear to be more active on the surface in summer when vegetation growth provides additional escape cover (Rogers and Memmler 1943). Whether there is a corresponding seasonal change in movement distances and home range sizes is presently unknown.

Food Habits

E. skiltonianus will consume a variety of insects at different life history stages, including eggs of various species -- caterpillars, moths, beetles, grasshoppers, and crickets (Van Denburgh 1922, Tanner 1943, 1957, Smith 1946, Stebbins 1954, 1966). Ants have not been found in stomach contents (Tanner 1957). The skinks also consume a variety of other invertebrates, such as spiders and isopods, but less frequently. The skinks may occasionally be cannibalistic (Stebbins 1954, Zweifel 1954), but this behaviour appears not to occur in the Great Basin populations (Tanner 1957).

Predation, Parasites and PredatorDefense

A variety of birds, mammals, and snakes probably prey on E. skiltonianus, but known predators are few. The species has been found in stomach contents of rattlesnakes (Crotalus viridis) and garter snakes (Thamnophis elegans; Tanner 1943, 1957); ring-necked snakes (Diadophis spp.) will prey on western skinks in captivity (Vitt et al. 1977). In British Columbia, a yellow-bellied racer (Coluber constrictor) was observed attempting to catch E. skiltonianus (M. Sarell, pers. comm.), and adult skinks were found in the stomachs of two juvenile rubber boas (Charinae bottae; Pam Rutherford, pers. comm.). Ectoparasites include ticks (Ixodes pacificus) and chigger mites (Tromicula belkini; Tanner 1957). Endoparasites include flagellates (Proteromonaslacertaeviridis, Monocercomonas colubrorum, Hexamastix spp., Retortamonas saurarum), amoebas (Endoclimax and undetermined spp.), nematodes (Psylaloptera retusa), and cestodes (Mesocestoides spp. larvae; Telford 1970).

The initial response of the skinks to predators is usually a rapid retreat (Vitt et al. 1977); females attending eggs may either retreat or attempt to defend the nest by biting (Tanner 1943, 1957). The skinks readily autotomize their tails if seized by a predator or a collector (Carl 1944, Stebbins 1966, 1972, Vitt et al. 1977). Tail autotomy is widespread among lizards and is thought to distract the attention of the predator away from the body while the lizard makes its escape. The trashing of the detached tail and its blue colour, which is particularly vivid in juveniles, might enhance this distractive effect in E. skiltonianus (Carl 1944, Tanner 1957, Stebbins 1966, 1972); the bright colour of the tail of juveniles may also reduce attacks by adult males (Stebbins 1972). Vitt et al. (1977) found a high frequency of tail breaks (62%) in natural populations of E. skiltonianus, suggesting that tail autotomy is highly effective against predators. In captivity, the tails regenerated relatively slowly (mean of 0.35 mm/day) when compared to other lizard species tested (Coleonyx variegatus, E. gilbertii, Gerrhonotus (Elgaria) multicarinatus). The regenerated tails were similar in volume to, or larger than the original tails (Vitt et al. 1977). Regenerated tails are seldom, if ever, blue in colour (Pam Rutherford, pers. comm.).

Daily and Seasonal Activity

The skinks are diurnally active on the surface, but the period of activity appears to be variable. Stebbins (1954) found them most active in late afternoon in California, whereas Tanner (1957) reported an apparent preference for both morning and afternoon hours in Utah. Applegarth (1994) noted that the skinks were active during the warm parts of the day in Oregon. Similarly, in the Creston Valley, British Columbia, Pam Rutherford (pers. comm.) has seen skinks active on the surface during hottest parts of the day when the sympatric alligator lizards were inactive.

The seasonal activity period of E. skiltonianus ranges from spring to fall, but skinks are observed most easily during various periods between March and July, depending on the locality (Rogers and Fitch 1947, Stebbins 1954, Tanner 1957). The skinks suspend activities during cold periods in winter (Rodgers and Fitch 1947) and probably hibernate in underground refugia. Western skinks have been found together with northern alligator lizards (Elgaria coerulea) and rubber boas (Charina bottae) in an apparent hibernaculum under and among rocks in Klickitat County, Washington (Storm and Leonard 1995). During a study of rattlesnakes (Crotalus viridis oreganus) in southern Washington, 17 individual skinks were caught entering or leaving an enclosure around a snake hibernaculum from April to October (Columbia NWR files 1989). Whether the skinks used the location for communal or individual hibernation is unknown. In the Creston Wildlife Management Area in British Columbia, E. skiltonianus was trapped or found under surface cover objects from April to September; whether activity occurred before or after this period was not studied (Pam Rutherford, pers. comm.).

Behaviour

The skinks are secretive and most often found when turning over rocks or other cover (Smith 1946, Tanner 1943, 1957, Stebbins 1954, 1985). Both sexes excavate burrows in loose soil, often under or by rocks or other cover objects (Tanner 1943, 1957, Smith 1946). The burrows of males are longer (up to 48 cm (18") long) and narrower than those of females; nesting females construct enlarged terminal chambers to their burrows (Tanner 1957). Smith (1946, p. 383) noted that the skinks "burrow with some ease, utilizing the snout while adpressing the limbs to the sides of the body and tail."

There is no evidence of territoriality with the possible exception of nesting females (Tanner 1957, Stamps 1983). Tanner (1957) noted that the burrows of adult males, juveniles, and non-breeding females were irregularly spaced in the habitat and concluded that the protection of territories was improbable. Females attending egg clutches, however, were more regularly spaced and resisted intrusions from predators; they might similarly attempt to repel intruding conspecifics from their burrows.

Vulnerability

With their blue tails and striped patterns, the skinks are potentially attractive to the pet trade. Such collecting poses a major threat to populations of the similarly attractive E. fasciatus in Ontario (COSEWIC report by C. and D. Seburn cited in Green 1998). No information, however, is available on whether E. skiltonianus is collected for the pet trade or what the extent of such activities might be. The secretive habits of the skinks might protect them to some extent from collecting and other direct human disturbance. Illegal collecting appears not to take place in the Creston Wildlife Management Area (Pam Rutherford, pers. comm.).

The greatest immediate threats to E. skiltonianus are habitat loss and alteration (see section on Habitat Trends). In particular, residential development, road construction, and talus/gravel extraction associated with increasing human populations are a major threat. The small geographic range of the species in Canada, together with the local distribution of populations, increases its vulnerability to habitat-related disturbances.

Page details

Date modified: