Part 17

NATURAL ENEMIES

Little is known concerning the kinds of predators that destroy five-lined skinks, or their importance in its ecology. In studies of the food habits of various predatory birds and mammals, workers often have been interested chiefly in items of direct economic bearing, and have tended to lump as "lizard" or "reptile" material that might have included Eumeces fasciatus. I have been able to find only a few specific references to predation on it. Nevertheless many kinds of predators probably utilize it as food, at least occasionally. Owls probably seldom have opportunity to prey on these skinks, which are not known to be active after dark. Nestling broad-winged hawks observed in 1954 were found to have eaten an adult and a subadult five-lined skink on June 13 and June 23. The Cooper's hawk and red-shouldered hawk also are probable predators as both are known to feed upon small reptiles. Mammalian predators which might be expected to take skinks occasionally include the red fox, gray fox, bobcat, mink, weasels, skunks, opossum, armadillo, moles, and shrews. Snakes, especially those of the genera Elaphe, Lampropeltis, Cemophora, Micrurus and Ancistrodon, may include some of the chief predators on the skink. Certain larger lizards also may prey upon it.

Of these several potential predators, only the opossum, armadillo, and snakes (Elaphe obsoleta, E. guttata, Lampropeltis triangulum, L. calligaster, L. getulus, and Ancistrodon contortrix), Sonoran skink and the greater five-lined skink (in confinement) have actually been recorded as preying on Eumeces fasciatus but circumstantial evidence has been obtained for the mole (Scalopus aquaticus) and short-tailed shrew (Blarina brevicauda). The short-tailed shrew may be one of the major predators on the skink. This shrew prefers the same habitats and occurs throughout the skink's extensive range. Like the skink, it is a characteristic inhabitant of the hardwood forests of the eastern United States, but its range extends farther north and west. A high proportion of the skinks examined had scars, usually on the sides or dorsal surface of the body, or of the tail near its base--wounds which must have been made by a small, sharp-toothed animal. For example, in May 1951, eighteen per cent of 155 skinks captured on the study areas had such scars. The incidence seemed to vary according to age and possibly sex; the scars were present in 22.9 per cent of the adult males, 25.5 per cent of the adult females, and only 9.8 per cent of the yearlings (these three groups being represented by approximately equal numbers in the sample). As the scars are more or less permanent, adults could be expected to show a much higher incidence than young. Females, being inclined to stay in their nest burrows and defend them against small predators, may receive more wounds than the males, which are quicker to escape. None of the invertebrates present on the study area is sufficiently large or powerful to inflict such wounds, and none of the birds, reptiles, or amphibians has a dentition capable of producing them. The wood mouse (Peromyscus leucopus) is the most abundant small mammal in the skink's habitat; other rodents present in relatively small numbers include the prairie vole (Microtus ochrogaster), harvest mouse (Reithrodontomys megalotis) and pine vole (Microtus pinetorum). Both voles and harvest mice have been known to kill skinks caught in the same traps with them, but individuals experimentally placed with skinks in captivity have failed to molest them and it seems likely that the attacks in traps were motivated by extreme hunger or self defense. The irregular scars from lacerated wounds characteristic of the skinks bear little similarity to rodent bites, in which the long, sharp-edge incisors make slit-like punctures. Other small mammals abundant in the places where skinks were studied were the insectivores: the common mole, short-tailed shrew, and least shrew (Cryptotis parva).

On one occasion when a large five-lined skink was put in a terrarium with a recently captured short-tailed shrew, each displayed strong aversion for the other. The skink crouched, attempting to conceal itself in the end of the terrarium farthest from the shrew, and resisted efforts to drive it toward the shrew. In exploring the terrarium the shrew several times sensed the skink's presence, and then scampered away in frantic haste. The skink also rushed away several times when the shrew came close enough to disturb it. Three days later, when the shrew had become accustomed to the terrarium, the test was repeated, with different results. The shrew, having finished the food left for it, was noticed moving about the terrarium, sniffing and testing objects with its tactile snout, obviously hungry and searching for more food. The skink was then dropped near it. In a few seconds the shrew sensed the skink's presence and pounced upon it, and bit hard on its back. The skink reacted with a violent flexure of its body which caused the shrew to release it instantly, and both rushed away in opposite directions. After a few seconds the shrew located the skink again, and moved up to it with little hesitation but with nervous alert sniffing, and delivered another quick bite after which the two separated as before, the skink showing signs of injury. Soon the shrew attacked a third time, and bit the skink's tail severing it near the base. As the skink rushed away, the detached tail performed lively squirming movements, but the shrew seized it, held it down, and began to eat the exposed flesh on the broken end as the tail writhed. After rapid nibbling it would drop the tail, and leaving it temporarily would explore the terrarium. Several times on these trips it encountered the skink and renewed its attack. As death of the skink seemed imminent, it was then removed, and it survived with no apparent ill effects. The wounds inflicted by the shrew bore close resemblance to those noticed on skinks in the wild. It seemed almost certain that Blarina had inflicted most of these wounds or all of them. On subsequent occasions several other captive shrews that were tested, quickly killed and ate skinks that were introduced into their containers. The least shrew, Cryptotis, likewise occurred in all situations where skinks were taken, and in some localities was more abundant than the larger Blarina. Bites inflicted by these two kinds of shrews might be indistinguishable, but because of its larger size, Blarina would seem by far the more formidable enemy.

Reynolds (1945:367) found E. fasciatus to be the most frequent reptile in a collection of opossum scats from Missouri, with two occurrences in 100 fall scats and ten occurrences in 100 spring scats. Sandidge (1953:98 and 101) recorded one of these skinks among numerous other items identified from stomach contents of sixty-six opossums. Probably the opossum is a frequent predator on this skink. Although no specific instances were obtained on the area of the study, flat rocks a few inches in diameter frequently have been found flipped over, larger ones and those solidly anchored in the ground have been found partly undermined by opossums scratching away the loose dirt at their edges. The rocks found disturbed by opossums were typical of those used as shelter by the skink. On many occasions wire funnel traps set for skinks and other reptiles along hilltop rock ledges were found to have been disturbed, either shifted in position or with their rock shelters removed, or rolled downhill or broken open. Similarly, heavy flat rocks used to cover pitfalls, to protect the small animals falling into them from predators, often were found to have been shifted somewhat, or completely removed. When such raids became frequent and troublesome, steel traps were set beside the reptile traps to discourage the raiders or catch them and determine their identity. On several occasions opossums were caught and somewhat less frequently, spotted skunks (Spilogale interrupta). These skunks probably prey regularly on lizards including the five-lined skink. However no definite records were obtained. Crabb (1941:356-358) in his food habits study of the spotted skunk in southeastern Iowa, did not record this or any other species of reptile among the items identified in 834 scats. On the Reservation both opossums and skunks were, in many instances, attracted to the reptile traps by the insects and other arthropods in them, rather than by lizards. The striped skunk (Mephitis mephitis) is another of the predators which probably feeds upon the five-lined skink occasionally on this area.

In the contents of 103 armadillo stomachs collected in west-central Louisiana, in 1947 and 1948 I found the broken tail of one Eumeces fasciatus. The lizard itself evidently had escaped (Fitch, 1949a:88). Many clutches of lizard eggs were found in the contents of the armadillo stomachs and some of these probably were eggs of Eumeces, which are similar to those of other small lizards in the same region (Anolis carolinensis, Sceloporus undulatus) in size, shape, and color.

Among 217 identified prey items from stomachs and scats of Sonoran skinks (Eumeces obsoletus) from northeastern Kansas were remains of three hatchling five-lined skinks. Taylor (1953b:212) recorded that a Eumeces laticeps shipped from Arkansas to Kansas ate an E. fasciatus that was with it in the container. Several authors have recorded predation on Eumeces fasciatus by snakes of various kinds in captivity. Conant (1951:211) recorded that one was eaten by a blacksnake (Coluber constrictor) placed in the collecting sack with it. Anderson (1942:211 and 216) recorded that a king snake (Lampropeltis getulus holbrooki) and a young copperhead (Ancistrodon contortrix) each fed upon them. Hurter (1911:184) recorded that a milk snake, Lampropeltis triangulum syspila, placed in a bucket with a Eumeces fasciatus was found swallowing it a short time later and its tail had been broken off.

Ruthven (1911:268) mentioned that stomachs of milk snakes, L. t. triangulum, collected in Michigan contained remains of five-lined skinks. Ditmars (1907:352) wrote that stomachs of several L. t. elapsoides contained Eumeces, and Wright and Bishop (1915:167) wrote of the same kind of king snake in the Okefinokee Swamp region: "It feeds on ground lizards, skinks, swifts, and other snakes and lizards."

Mr. Richard B. Loomis is of the opinion that the five-lined skink is one of the chief food sources for the milk snake (L. t. syspila). Having kept many of these snakes in captivity and experimentally offered them different types of prey, he found that individuals inclined to feed would avidly seize and eat skinks and young mice, but other proffered prey, small adult rodents, snakes, or lizards other than Eumeces were either rejected or were taken with some hesitation. These milk snakes have habitat preferences similar to the skink, which would seem to be one of the most available food sources. Loomis recorded in his field notes that a juvenal blotched king snake (L. calligaster) 310 mm. in total length, taken on April 8, 1950, seven miles southwest of Tulsa, Oklahoma, had eaten a large adult E. fasciatus. Another juvenal blotched king snake that he found under a flat rock near Sunflower, Johnson County, Kansas, regurgitated an adult five-lined skink. Loomis also recorded a juvenal rat snake (Elaphe guttata emoryi) and a juvenal pilot black snake (E. obsoleta) each feeding on individuals of Eumeces fasciatus in captivity. Uhler, Cottam and Clarke (1939:622) in a study of the contents of the alimentary tracts of 893 snakes of 18 species, from the George Washington National Forest, Virginia, found among the prey items only one skink (species undetermined but most probably E. fasciatus). It had been eaten by one of the two corn snakes (Elaphe guttata) that were examined in the study.

On June 11, 1950, in Skink Woods, a young copperhead 335 mm. in snout-vent length and weighing 27.6 grams, had a gravid female skink in its stomach. Another young copperhead (335 mm., 36.1 grams) trapped near Rat Woods on August 28, 1953, had in its stomach a bob-tailed adult five-lined skink. Many copperheads collected on the Reservation were kept in captivity for short periods, and from them a total of 44 scats were obtained, each scat containing the remains of one or more prey animals eaten in the wild. Of this total, five scats contained remains of Eumeces fasciatus, which was one of the more frequent items, although small mammals collectively made up the bulk of the scat contents.

PARASITES

Skinks, like many other lizards, are likely to be infested with parasites. Little attention was devoted to the endoparasites in the present study, but they were noted from time to time. On several occasions small nematodes and flukes were seen in feces voided by lizards which were handled. Small white cysts were seen in the body cavities of several that were dissected.

Harwood (1932:65) examined for endoparasites nine E. fasciatus along with many other reptiles and amphibians collected near Houston, Texas. Most of them were infested and five kinds of helminths were identified. Two of the skinks were infested with Oswaldocruzia pipiens, a spirurid nematode that was also present in various other lizards, snakes, toads and frogs from the same region; four had Comocercoides dukae, an oxyurid nematode also present in various lizards, snakes, turtles, and frogs; one had in its intestine Oochoristica eumecis, named as a new species by Harwood, and found only in Eumeces; one contained Cysticercus sp. in its body cavity, present in great abundance as white globular structures .6 mm. in diameter (Harwood states that possibly these were larvae of Oochoristica). One skink contained Mesocoelium americanum, a dicrocoelid trematode which was found also in the brown skink (Scincella laterale) and DeKay's snake (Storeria dekayi).

The ectoparasites of these skinks consist mainly of chiggers. Wharton (1952:135) lists three species; Trombicula alfreddugesi, T. splendens, and T. gurneyi. The first species is the common pest chigger of humans and domestic animals in the United States, and south through tropical America. Wharton lists 136 known hosts which are fairly evenly divided among mammals, birds and reptiles; he lists four kinds of frogs and toads. Trombicula splendens is a similar and closely related species which has been recorded from thirty-eight vertebrate hosts including mammals, birds, reptiles, and a tree-toad. Trombicula gurneyi belongs to a separate subgenus and it was originally recorded from Eumeces fasciatus which seems to be one of the principal hosts.

Two of these mites, T. alfreddugesi and T. gurneyi, were on skinks collected on the Reservation, and nearby areas. A four year study of the chiggers in this general region by Loomis (MS), Wolfenbarger (1953) and Kardos (MS) has clarified the ecological relationships of the several kinds of chiggers present, including their local distribution with respect to vegetation, soil type, moisture and temperature, host preferences, and seasonal occurrence. At the quarry, Rat Woods and the pond rock pile, the chigger population consisted chiefly of T. alfreddugesi, while at Skink Woods T. gurneyi was also abundant. In some local situations where they are among the most abundant of vertebrates the skinks probably are important as hosts of T. gurneyi. An individual skink may have dozens of chiggers on it at one time but usually there are fewer.

There are several favorable sites of attachment. The most favored site is in the axilla. There the scales are minute and granular with exposed areas of thin and tender skin, and the chiggers are well protected from dessication and are not likely to be rubbed off as the skink moves about. Other favorite sites of attachment are: about the insertion of the hind limb, about the cloacal opening, on the eyelids and on the toes. Only occasionally are chiggers found attached on the dorsal surface. When attached in protected spots in the tender skin of the axilla or groin, they are often in dense clusters of a dozen or more. Damage to the skin resulting from the attachment of the first chiggers renders conditions more favorable for the attachment of others. At Rat Ledge and at the quarry, many of the larger Sonoran skinks (Eumeces obsoletus) were captured, and individuals were far more heavily infested than were E. fasciatus from the same places. A single Sonoran skink might be found to have hundreds of chiggers, widely distributed over its body with concentrations at the axillae, groins, lateral neck region, and any injured spots where the protective armor of scales was broken. The reasons for the greater susceptibility of E. obsoletus are not entirely clear. It is a larger, less active species with coarser scalation, and is more subterranean in its habits.

The chiggers that attach to skinks seem to occasion but little discomfort. There is no local swelling and inflammation such as occurs in humans. The infestations observed in five-lined skinks were not sufficiently severe to cause debilitation or any noticeable symptoms. There is, however, a possibility that chiggers are vectors of microorganisms causing diseases in reptiles, just as they are for certain mammals (including humans) in some parts of the world.

Bishopp and Trembley (1935:42) record a single kind of tick, Ixodes ricinus scapularis Say, the black-legged tick, as parasitic in its immature stages on Eumeces fasciatus. This tick, however, has been recorded principally from mammals, of which many kinds serve as hosts for its larval, nymphal, and adult stages.

Population

COMPOSITION

Population structure obviously differs from place to place and from time to time. Because of the differences in secretiveness and elusiveness between young and adults and between males and females, true sex ratios and age ratios are obscured. In the period of weeks between the emergence from hibernation and the onset of the breeding season, these skinks tend to be less secretive than at other times, and secondary sexual and age differences in behavior are minimized. A sample at this season should be more representative of the true population composition than samples taken at other times of year. In a sample of 308 skinks available for the month of April, including the collections made on the Reservation and on nearby areas, in 1949, 1950, 1951, and 1952, 36.7 per cent were adult males, 28.3 per cent were adult females, and 35.0 per cent were young. That these figures cannot, however, be accepted as an accurate indication of the population composition is shown by the data from the areas where intensive population studies were made. Data are most complete from Skink Woods. For 292 adults taken there over a four year period, the sex ratio was 100:122.6. On this area after the first year of study a substantial proportion of the individuals recorded were repeaters from one year to the next, and in some cases for three or even four successive years. Many could be definitely assigned to a known age group. By analogy the majority of others could be tentatively assigned with some assurance on the basis of measurements, and relatively few were of indeterminate status. By assigning each of these indeterminate individuals to one or another age group, on the basis of greatest probability, the approximate composition of the population could be determined. Of 611 adults, 55 per cent were "two-year olds" (in the season between their second and third hibernations, which was their first breeding season). The percentage was not significantly different in the two sexes.

On the average, a pair of adults produces somewhat more than nine eggs per year. From the time individuals of a brood start their development in the egg until they are breeding adults two years later, they undergo such drastic reduction in numbers that, on the average, approximately only one per brood survives. Most of the mortality probably occurs early, especially before hatching, also in the inexperienced hatchlings, and in the first hibernation. In spring, after emergence from hibernation, young are generally taken in smaller numbers than are adults. Their relative scarcity is only apparent, owing to greater secretiveness, and greater elusiveness when found. In spring, newly matured adults (age class about 21 months) may be taken in somewhat larger numbers than young (age class about 9 months). The latter obviously must be more numerous, in a stable population however, as the 21 month age class necessarily has sustained some loss since it was 9 months old.

Success of the annual brood varies greatly from year to year, depending on the weather and various other factors. In 1949 evidently conditions were near optimum; young hatched early and were especially numerous in late summer. In 1950 these young hatched in 1949 made up 40 per cent of the total catch (excluding hatchlings) in Skink Woods and were relatively more numerous than young of the corresponding age group in other years. In 1951, these young of the 1949 brood, grown to adults, made up 70 per cent of the breeding populations, as against 36 per cent for the corresponding class in 1950 and 58 per cent for the corresponding class in 1952.

Even after attainment of adulthood, any given age group evidently is subject to annual reduction amounting to at least half its numbers. Within six or seven years, at the most, the original numbers would be reduced to an insignificant percentage. At an age of four or five years individuals probably have attained their maximum size, with obscured pattern and changed proportions suggestive of advanced age. Occasional individuals possibly attain much greater age, but certainly few live more than five years. Like most small animals, the five-lined skink has a short life expectancy and a rapid population turnover. As compared with mammals of comparable sizes, the small rodents and insectivores that are this lizard's community associates and are subject to many of the same hazards, the skink is notably successful, with a much longer life expectancy. For these small mammals the life span is seldom as long as a year. Most kinds of small birds likewise have a life expectancy less than that of the five-lined skink, although somewhat greater than that of small mammals.

DENSITY

The population density changes constantly, following an annual cycle with gradual reduction to its lowest ebb in late June or early July, then rapid increase to a high point a few weeks later when hatching of the single annual brood has been completed. In a normally successful breeding season the population is at least doubled, but reproductive success varies from year to year, as the population responds to weather conditions that are favorable or unfavorable, even where the environment remains fairly stable. In most places, however, local populations continue upward or downward trends for periods of years in response to successional changes which cause progressive improvement or deterioration of local habitats. Local populations are likely to be more or less isolated from others by areas where the habitat does not exist. Even in an area of favorable habitat such as a wooded hillside of several acres, the population is not at all evenly distributed, but concentrations occur along rock outcrops, and about decaying logs, or stone piles. In intervening areas lacking such abundant shelter, and less productive of food, the population is sparse, or there may be no permanent residents.

In view of these traits, and the difficulty of obtaining a representative sample, no precise measurements of population density can be made. During the time required to secure a sample, the population undergoes change. At the pond rock pile, an area of approximately .05 acre, the skinks were found in remarkably high concentrations, 57 in 1949, 85 in 1950, 37 in 1951, and 51 in 1952. These numbers represent population densities of, respectively, 1120 per acre, 1960 per acre, 746 per acre, and 1000 per acre. No such concentrations were found elsewhere, and probably do not occur in natural habitat. The Skink Woods study area of 2-1/4 acres is typical of favorable habitat in the region of the study, and the numbers taken there are more significant. For 1949 the 74 skinks recorded comprise an incomplete sample, and the population density of 33 per acre represented is certainly somewhat too low. For other years the following population densities (exclusive of hatchlings) are indicated: 1950, 92 per acre; 1951, 61 per acre; 1952, 49 per acre. These figures are only approximate, of course, and it is difficult to judge how accurately they reflect the true numbers. Even the most intensive collecting may be insufficient to obtain every individual on a small area. Within each season there are shifts of range by some individuals, off the study area and corresponding shifts onto it by others, so that the numbers caught in the course of an entire season are somewhat too high. The individuals taken on the study area may regularly range beyond its boundaries to some extent, so that the seeming population density is somewhat too high. Actually this was probably a minor source of error for the Skink Woods study area, as nearly half its perimeter was bordered by an open field uninhabitable for the skinks, and the remaining perimeter adjoined areas much less favorable than the central portion.

Census of the population of the study area by a ratio such as the "Lincoln Index" used in game management studies was scarcely practicable because of the changing seasonal habits distorting the recorded ratios of the sexes and of age groups somewhat differently at different stages of the season. These changing ratios tend to produce an erroneously high population figure, unless separate computations are made from the data for adult males, adult females, and young. Census figures obtained by this method were erratic but seemed to bear out in a general way, the population figures based on total numbers of individuals taken.