Life History and Ecology of the Five-lined Skink, Eumeces fasciatus Part 1

Life History and Ecology of the Five-lined Skink, Eumeces fasciatus.

by Henry S. Fitch.

Introduction

The common five-lined skink (or common blue-tailed skink) is a small woodland lizard, abundantly and widely distributed over the eastern United States. Many authors have casually discussed this lizard or have treated in detail some phase of its biology. Excellent brief summaries of the known facts concerning its life history have been published by Smith (1946:349-350 and 1950:187-188) and Pope (1947:153-157).

Nevertheless, no thoroughgoing study of its life history and ecology has heretofore been made.

In 1932, taxonomic studies by Dr. Edward H. Taylor revealed that the lizards previously referred to in the literature as _Eumeces fasciatus_, actually were three closely related and similar, partly sympatric species. Although Taylor's work was careful and detailed, and indicated numerous minor differences by which the three species could be distinguished, many herpetologists were reluctant to accept his findings for nearly a decade thereafter. Consequently a large amount of literature concerning five-lined skinks is either obviously composite in the sense that it is based upon two or three species, or is not definitely a.s.signable to any one species. In the study here reported upon, all pertinent literature available to me has been examined, and evaluated, and important findings of other authors have been incorporated in the discussion. However, mine was primarily a field study, and in one small part of the geographic range of the one species.

The University of Kansas Natural History Reservation is a tract of 590 acres preserved as a natural area, available for the pursuit of ecological studies. The studies undertaken include intensive investigations of selected species of vertebrate animals. The main criteria used in selecting these species have been whether or not they were sufficiently abundant and generally enough distributed to play an important role in the over-all ecology of the area, and whether a species was sufficiently accessible for study with available techniques.

Among the 300 species of vertebrate animals recorded from the Reservation, the five-lined skink is one of those most frequently noticed in the field. In actual numbers it is probably exceeded only by the cricket frog (_Acris gryllus_), the leopard frog (_Rana pipiens_), the ring-necked snake (_Diadophis punctatus_), the prairie vole (_Microtus ochrogaster_) and perhaps the white-footed mouse (_Peromyscus leucopus_). Although numerous, the skink is not easy to study because it is secretive in its behavior, and is inactive in inaccessible shelters during the greater part of the year.

The five-lined skink generally occurs along with a characteristic set of community a.s.sociates in a particular type of situation. It is a predator on various small animals, mostly invertebrates. For some of the many prey species the effect is certainly negligible, but for others its predation may be a major ecological factor. In areas where optimum habitat conditions exist its bioma.s.s may exceed that of any other insectivorous animal, and in such situations it a.s.sumes a major role as a predator and as a compet.i.tor with other insectivorous types. In turn it provides part of the food source of various larger predators, including reptiles, birds and mammals. It is a host and carrier of various parasites, including at least one species that regularly attacks humans--the common chigger. It is not evident on the basis of the present findings that the skink is either harmful or beneficial to any perceptible degree, in its over-all effect on human affairs and economy.

Nevertheless, there probably are various unsuspected relations.h.i.+ps.

In the course of my field study many workers on the University of Kansas Natural History Reservation helped by capturing skinks; especially Sydney Anderson, Richard Freiburg, John Hawken, Dennis G. Rainey and Lewis L. Sandidge. Mr. Robert Gordon very kindly furnished information on specimens in the Tulane University collection, which served as a basis for comparing the breeding schedule of the southern population with that of _E. fasciatus_ in northeastern Kansas. Dr. W. J.

Breckenridge kindly permitted examination of material in the University of Minnesota Museum of Natural History. Dr. Edward H. Taylor has made helpful suggestions from time to time. Mr. Richard B. Loomis helped me in various ways with the field work, and made available his personal field notes with records of predation on _Eumeces_ by various snakes.

Dr. E. Raymond Hall, Director of the Museum of Natural History, has critically examined the ma.n.u.script, and has been helpful in various ways. The line drawings and graphs, with the exception of Figures 8 and 9, were made or completed by Mrs. Louise Brunk, artist for the Museum.

The study here reported on was initiated in May 1949, and was continued through 1950, 1951 and 1952. A few observations made in 1948 have been included. Various separate items of information obtained in 1953 have likewise been incorporated especially where histories of individual skinks are presented, but the ma.n.u.script was completed in essentially its present form in the fall of 1952.

Methods

Skinks were obtained by active search; rocks and boulders were lifted up and the skinks thus exposed were seized by hand before they had time to escape. This method was effective when the skinks were using rocks for shelter and when temperatures were low enough so that they were slow and sluggish, but in hot weather the skinks were so quick and active that those exposed usually escaped. Usually skinks could be obtained much more easily by trapping. At the pond rock pile (Fig. 26), for instance, shelter was so readily available that the skinks could seldom be caught by hand. Gallon cans buried with the tops open flush with the surface of the ground served as pitfalls and were effective when they were carefully placed, at the bases of rock ledges or logs or stumps, where the skinks were most likely to fall into them. Most of the skinks recorded at the rock pile were caught by this method, and sometimes several were caught together in the same pitfall. Ordinarily each pitfall was covered with a large flat rock, propped against a nearby object to leave ample s.p.a.ce for the skink to enter beneath it. The rocks provided protection from direct sunlight, from rain, and from predators.

Still another method of catching skinks was with wire screen funnel traps (Fitch 1951:77). These funnel traps were of different sizes, and were made of different kinds of wire mesh. They were set for reptiles that were mostly larger than five-lined skinks, and those having quarter-inch wire mesh permitted many of the immature skinks to escape.

Most of these funnel traps were from about one foot long and five inches in diameter, to about twice these dimensions, with funnel openings about 1.5 inches in diameter. Some made of 1/8 inch wire mesh, six or seven inches long, and three or four inches in diameter, with funnel openings only a little larger than the body diameter of an adult skink, were found to be suitable for skinks of all sizes, and were used successfully at the pond rock pile. Most of the skinks trapped were adult males, and they were taken chiefly in May. The funnel traps were generally placed at the edges of rock outcrops, boulders or logs, where skinks were likely to be intercepted in their usual travel routes. Each method of collecting skinks resulted in occasional mortality to them but most losses were in those caught in funnel traps. In these traps they sustained rapid loss of moisture, and were usually somewhat desiccated.

Two or more adult males were often caught together, and in most of these instances the first one caught probably served as bait attracting another and arousing his pugnacious interest. Injuries were frequent, and some deaths occurred because in the close confines of a trap the loser in a fight was unable to escape further attacks.

Most of the skinks caught were examined, and released within a few minutes. Snout-vent length was measured by holding the skink against a rigid transparent plastic millimeter ruler and exerting a slight pull on each end of the lizard until it tired and relaxed its muscles, eliminating bends and kinks. Even with such precaution, precise measurements could not be obtained and the readings often varied a millimeter or more for the same skink measured two or more times on the same day. Tail length was similarly recorded with separate readings for the original and regenerated portions. Also recorded were s.e.x (when discernible), color and pattern, breeding data, injuries, general condition, and sometimes temperature. Many of the skinks were brought to the laboratory, and were weighed to the nearest tenth of a gram.

Occasional trips were made to localities away from the Reservation to collect skinks. Some of those obtained were kept under observation in terraria where their behavior was studied. Most were preserved and were used for data on habitat preferences, seasonal changes in the gonads, size group, stomach contents, and various other items of information.

Description

The scutellation and osteology have been described in detail by Taylor (1936:39-48 and 199-206) and others, and need not be repeated. The five-lined skink is slender and elongate, somewhat snake-like (though much less so than many other skinks) as the head, neck, body, and tail are not well set off from each other, and the sleek, streamlined contours are broken only by the small limbs protruding from the sides of the body. The body is slightly flattened laterally, tending toward quadrangular shape in cross section. The head is wedge-shaped, with a short, rounded snout. The nostrils are laterally placed, well back from the tip of the snout. The eyes are small and deep set; the iris is dark.

The neck is thick and strong, nearly as long as the head. The torso is 3-1/2 to 4 times as long as it is wide. The tail is almost square in cross section at its base, but is circular in cross section for most of its length. The limbs are moderately developed; when adpressed along the sides of the body, the forelimb and hind limb overlap by a length about equal to the longest toes of the forelimb. The limbs are pentadactyl and all the toes are well developed and have claws (Figures 1 and 2). The claws are short, and are curved in such a manner that their tips are directed downward, each approximately at right angles to the axis of the toe (Figure 2b). The limbs are moderately thick and muscular. The upper arm and forearm segments are of approximately equal length, as are the femoral and tibio-fibular segments of the hind limb.

[Ill.u.s.tration: FIG. 1. Antipalmar view of right front foot, 9.]

[Ill.u.s.tration: FIG. 2. A. Antiplantar view of right hind foot, 9.

B. Terminal part of second toe of left hind foot, and its claw, in lateral view, 9.]

The five-lined pattern is characteristic of the hatchling, but gradual ontogenetic change results in its dulling, suppression, and eventual loss. In the hatchling the ground color of the head and body is black or dark brown, with five milky white longitudinal stripes extending the length of the head and body, and on the basal one-fourth of the tail.

The five light lines are of approximately equal width, and are separated by dark inters.p.a.ces 1-1/2 to 2 times as wide. The mid-dorsal stripe includes most of the two mid-dorsal scale rows. Posteriorly it extends onto the base of the tail, where it becomes increasingly suffused with the blue color of the tail, widens, and loses its ident.i.ty. In the nuchal region, this dorsal stripe narrows and splits into left and right branches, which diverge anteriorly to form a lyrate pattern on the head.

On either side of the dorsal stripe are the dark inters.p.a.ces, nearly twice as wide as the stripe itself and tapering to a point posteriorly on the tail, likewise tapering anteriorly to a point immediately above and in front of the eye. Lateral to these dark areas are the dorsolateral stripes; they extend from the basal one-fourth of the tail anteriorly onto the head along the superciliary region, tapering to a point on the anterior superciliary. Below these stripes are the dark lateral areas which extend from the basal part of the tail anteriorly along the sides of body and neck region (including the upper half of the aperture of the ear), eye region, and loreal region. Below this dark area on each side is the lateral stripe. It extends along the sides just above the level of the limb insertions (broken or pinched to a fraction of its average width above the hind limb insertion), broken by the ear opening, and extending anteriorly to include all the supral.a.b.i.al scales (with the exception of their upper edges) and the rostral. Here the left and right lateral stripes may be said to join; however in the facial region these stripes are not well defined, partly because the dark areas that border their lower edges do not extend so far forward. This lowermost dark area is about equal in width to the lateral stripe. It extends from the posterior infral.a.b.i.als posteriorly, to include the fore- and hind-limbs, and onto the basal part of the tail. The ventral surface of the head and body is dull white or pearly gray.

Thus, there are 12 longitudinal bands of color on the body: the five narrow, subequal, pale lines separated by the six dark areas, of which the dorsal and dorsolateral are broad and of approximately equal width, while the ventrolateral is narrower; and lastly the broad, pale ventral area.

[Ill.u.s.tration: FIG. 3. A. Osteoderm of an old adult male, from near the midline of the back, 25.

B. Another osteoderm from same male, from belly near midline, 25.

C. Another osteoderm from side of same male, at a point approximately halfway between foreleg and hind leg, 25.

D. Osteoderm of a juvenile obtained in April, from near midline of back, 25.

E. Tongue from dorsal view, shown in its normal position in the lower jaw, 2-1/2.]

The tail in young individuals is bright blue. In _Eumeces_ the tail characteristically has a color different from that of the body, and is usually more conspicuous; in many species it is blue, but in others it may be purple, greenish-blue, red, pink, or orange. Hatchlings have the most brightly colored tails, and as growth proceeds the colors gradually become duller. In _E. fasciatus_ the bright colors of the tail are mostly or entirely lost in old adults, especially in males, and in individuals of either s.e.x that have lost their original tails and regenerated new tails. Young which lose their tails and regenerate them at an early age have the regenerated portions colored almost as brightly as the originals at first.

The skin is tight fitting and relatively thick, stiffened by a bony armor. A small bony plate or osteoderm underlies each scale. Oliver (1951:127) has called attention to the pattern of ornamentation on the osteoderms, which becomes more complex with advancing age. He has suggested the possibility that age might be accurately determined on the basis of extent of osteodermal ornamentation. I have compared osteodermal ornamentation in marked individuals of known age, but have found it to be of limited applicability as a method of age determination; size and pattern are probably more satisfactory bases for estimating age, even though they do not permit definite aging of old adults and are not infallible for skinks short of adult size. In adult _E. fasciatus_ the pattern of ornamentation is closely similar to that figured for _E. laticeps_ by Oliver (op. cit.) and also resembles the pattern shown for an Old World skink, _Mabuya multifasciata_, as figured by Smith (1935: 2). The pattern differs somewhat in osteoderms on different parts of the body, and is most nearly symmetrical in those near the midline on either dorsal or ventral surface (Figure 3).

Relations.h.i.+ps

_Eumeces_ is a widespread genus occurring in the New World in southern Canada and southward into Costa Rica. The greatest number of forms is in Mexico. In the Old World numerous species occur in southeastern Asia and on adjacent islands, and other species occur westward across southern Asia, and across North Africa to Morocco, with a major break in the continuity of distribution in the Himalayan region. Taylor in his revision recognized 57 forms with fifty full species, belonging to 15 major groups within the genus. Since then only relatively minor changes in cla.s.sification have been proposed. Several new species and subspecies have been named, and several species have been relegated to the status of subspecies.

Within the genus there are several groups that have representatives in both the New World and the Old World. Smith and Etheridge (1953:159) point out that the most primitive line of _Eumeces_ is best represented in the Old World, where there are two groups and nine species, while in the New World this line has only three tropical relict forms. For this reason, Smith and Etheridge concur with Taylor (1936:67) in considering the genus to be of Old World origin; but the two main lines of the genus (the four-lined and five-lined stocks) are both regarded as being of New World origin. According to this idea, the Asiatic members of these two groups migrated from the New World. In the early Tertiary, warm temperate climates extended north to the Arctic Circle, and _Eumeces_, or at least some of its species, may have had a distribution straddling migration routes to both North America and Asia.

Of the 15 groups within the genus, the _fasciatus_ group, with a dozen species, has more representatives than any other. The _fasciatus_ group is characterized by having the tail bright blue with dorsal body pattern of five light lines on a darker ground color; mid-dorsal line bifurcating on head to form lyrate markings (this striped pattern and bright color of the tail becoming dull or obsolete in the adults); medial prea.n.a.l scales overlapped by those lateral to them; two pairs of nuchals; no postfemoral pocket; four supraoculars; scales on sides of body in parallel rows. The characters that separate members of the _fasciatus_ group from each other are minor. The width and position of the light lines differ somewhat among them. The mid-dorsal light line bifurcates either on the nuchals or on the parietals. The complex of scales in the temporal region differ in shape and relative size.

The following table, compiled mostly from information set forth by Taylor (1936:186-283), indicates some of the main differences and similarities between species in the chief characters upon which the cla.s.sification is based.

The close resemblance between _E. fasciatus_ and its Asiatic relatives is remarkable considering the great distance separating them and the long time that must have elapsed since their isolation began. Some of the Asiatic forms differ from each other almost as much as they differ from _fasciatus_. Of the Asiatic species, _elegans_, _tamdaoensis_, _os.h.i.+mensis_, and _marginatus_ differ from _fasciatus_ in markedly larger size; _elegans_, _marginatus_, _os.h.i.+mensis_, and _stimsonii_ differ in lacking a postnasal; all but _tamdaoensis tunga.n.u.s_ and _xanthi_ differ in having only a single postmental; all but _tunga.n.u.s_, _E. latiscutatus okadae_ (and sometimes _os.h.i.+mensis_ and _elegans_) differ in reduced number of scale rows; all but _tunga.n.u.s_ differ in having a lateral posta.n.a.l scale differentiated, and usually keeled; _tunga.n.u.s_, _xanthi_ and _elegans_ differ in having a patch of enlarged scales on the posterior side of the thigh; and in all, the primary temporals and upper and lower secondary temporals differ in size and proportions. Although some of the Asiatic forms seem to be directly derived from others, _fasciatus_ is somewhat intermediate between the more divergent forms, and fulfills most of the conditions to be looked for in an ancestral type.

Table 1. Distribution, Pattern, Size, and Lepidosis of the "Five-lined" Skinks (Fasciatus Group of the Genus Eumeces)

================================================================================================================================== _fasciatus_ _laticeps_ _inexpectatus_ _tunga.n.u.s_ _xanthi_ _elegans_ _tamadoensis_ _os.h.i.+mensis_ _stimsonii_ _barbouri_ _marginatus_ _latiscutatus_ ----------------+---------+---------+---------+----------+--------+---------+-------+--------+---------+-------+-------+-------------- Distribution E U. S., Most of SE U. S. W Szechwan SE SW Indo- Amami- Is.h.i.+gaki- Amami- Okinawa j.a.pan, except E U. S., (in China China, China gunto I. jima, Riu s.h.i.+ma (main I.) Fla. and except N China) Formosa, Kiu I. N New N tier Pesca- England of states dores I. Juvenal Pattern 5 5 or 7 5 or 7 5 5 5 5 5 7 ..... 5 5 lined lined lined lined lined lined lined lined lined lined lined Max. snout-vent length in mm 80 130 89 81 76 96 ..... 99 63 66 93 80 Postnasal present present present present present absent present absent absent present absent present Postmental 2 2 2 2 2 1 2 1 1 1 1 1 No. scale rows 28-30 30-32 30-32 28 22-24 26-28 ..... 26-28 26 22 26 26 (or 24) Lateral posta.n.a.l undiffer- undiffer- undiffer- undiffer- differ- scales entiated entiated entiated entiated entiated keeled ..... keeled keeled ..... keeled keeled Large scales on present enlarged; back of thigh absent absent absent present present irregular ..... absent regular absent absent absent Median not not subcaudals widened widened widened widened widened widened ..... widened widened widened widened widened ----------------+---------+---------+---------+----------+--------+---------+-------+--------+---------+-------+-------+--------------

The American _Eumeces laticeps_ and _E. inexpectatus_ seem to be more specialized than _E. fasciatus_ and might have been derived from it or from a common ancestor differing but little from the modern _fasciatus_.

Both differ from _fasciatus_ in having more scale rows. _E. laticeps_ also differs in having eight instead of seven supral.a.b.i.als and in having the median subcaudal scales greatly widened, in having intercalated plates on the outer side of the fourth toe nearly to the ultimate phalanx, posterior supral.a.b.i.al low and elongate, young sometimes seven-lined instead of five-lined, and especially in much larger size, stocky build, and in early loss of striped pattern. _E. inexpectatus_ differs in having the median subcaudals not at all enlarged, and in having the dorsolateral stripes a little more widely separated from the midline.

_Eumeces fasciatus_ and its relatives present a curious exception to Jordan's Rule, which states that the nearest relatives of any given species are to be found neither in the same area nor in a remote one, but in an adjacent region separated by a barrier. _E. fasciatus_ is absent from almost all of Florida; otherwise its range overlaps most of the ranges of both _laticeps_ and _inexpectatus_, the former including the southeastern United States south of about lat.i.tude 40, and the latter being mainly in the Atlantic and Gulf states from Chesapeake Bay into eastern Louisiana. Presumably both of these species began their differentiation as southern populations of an ancestral _fasciatus_ and later became isolated from it and continued their differentiation until they overlapped it again as distinct species. The differentiation of _laticeps_, being much greater, presumably took place at an earlier time than did that of _inexpectatus_, and at present it overlaps _fasciatus_ more extensively. _E. laticeps_ probably diverged to such an extent that compet.i.tion with _fasciatus_ is greatly reduced where the two species occur together.