Читать книгу Life History and Ecology of the Five-Lined Skink, Eumeces fasciatus - Henry S. Fitch - Страница 6
Relationships
ОглавлениеEumeces 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 classification 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 preanal 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 classification 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, oshimensis, and marginatus differ from fasciatus in markedly larger size; elegans, marginatus, oshimensis, and stimsonii differ in lacking a postnasal; all but tamdaoensis tunganus and xanthi differ in having only a single postmental; all but tunganus, E. latiscutatus okadae (and sometimes oshimensis and elegans) differ in reduced number of scale rows; all but tunganus differ in having a lateral postanal scale differentiated, and usually keeled; tunganus, 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 | tunganus | xanthi | elegans | tamadoensis | oshimensis | stimsonii | barbouri | marginatus | latiscutatus | |
| Distribution | E U. S., except Fla. and N New England | Most of E U. S., except N tier of states | SE U. S. | W Szechwan (in N China) | SE China | SW China, Formosa, Pesca- dores I. | Indo- China | Amami- gunto I. | Ishigaki- jima, Riu Kiu I. | Amami- shima | Okinawa | Japan, (main I.) |
| Juvenal Pattern | 5 lined | 5 or 7 lined | 5 or 7 lined | 5 lined | 5 lined | 5 lined | 5 lined | 5 lined | 7 lined | … … | 5 lined | 5 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 postanal scales | undiffer- entiated | undiffer- entiated | undiffer- entiated | undiffer- entiated | differ- entiated | keeled | … … | keeled | keeled | . … . | keeled | keeled |
| Large scales on back of thigh | absent | absent | absent | present | present | present irregular | … … | absent | enlarged; regular | absent | absent | absent |
| Median subcaudals | widened | widened | not widened | widened | widened | widened | … … | widened | not 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 supralabials 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 supralabial 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 latitude 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 competition with fasciatus is greatly reduced where the two species occur together.
Since Eumeces laticeps was recognized by Taylor as a species distinct from fasciatus, numerous authors have accumulated field observations that demonstrate ecological divergence between the two. Conant (1951:33) wrote that in Ohio laticeps prefers a dry habitat of bare rocks, cliffs, dry hillsides, and trees. He summed up the habitat difference as follows: “Fasciatus appears to be essentially terrestrial, to prefer a moist environment and to be at home in ravines in southern Ohio. Laticeps on the other hand, is largely arboreal (particularly adults), prefers dry cliffs, sunny hillsides and hilltops and lives in general above the habitat of fasciatus.” Netting (1939:127) likewise states that in Pennsylvania E. laticeps inhabits drier places than does fasciatus, and is largely arboreal. Other authors with few exceptions agree that laticeps is largely arboreal, but most describe it as at home in forest swamps and bottomlands. My own field experience with it is limited. In the Pigeon Lake area of Miami County, Kansas, the northwesternmost known locality of occurrence for laticeps, the habitat relations described by Conant for Ohio were almost reversed. Eumeces laticeps was relatively scarce, and confined to the vicinity of the swamp chiefly in areas that are flooded in time of high water. All those seen were on or near massive snags of dead trees still standing, but decayed and honeycombed with cavities. Slabs of bark clinging loosely to the tree trunks, with spaces beneath, provided shelter for the skinks and for the abundant arthropod fauna which probably constituted their chief food source. This is one of the few places in Kansas where a remnant of the original bottomland forest remains. In central Louisiana, in 1947 and 1948, persons living on the Kisatchie National Forest told me of large, red-headed skinks living in hollow trees, which must have been E. laticeps. In the literature E. laticeps is frequently referred to as red-headed, although the reddish suffusion on the head of the adult male is ephemeral in this species as it is in E. fasciatus and others. The heightened activity of the adult males in the breeding season seems to have drawn attention to this conspicuous temporary coloration while its absence at other seasons has scarcely been mentioned.
Mansueti (1948:213) describing the habits of laticeps in Maryland, Louisiana and elsewhere in the southern states, emphasizes its arboreal habits, referring to it as “ ‘scorpion’ of the treetops.” He describes it as dashing up and down tree trunks, along fences, and in abandoned buildings. However, he states that it also spends much time on the ground, and may take refuge in holes and cracks near ground level, and gravid females are less arboreal, making their nests in decayed logs of chestnut or oak. He mentions individuals having been found living far above ground in tall trees, in nests of birds of prey. One old male that was frequently seen by him always retreated far up a dead chestnut tree that towered above the surrounding forest of scrub pine. Mansueti also mentions arboreal combats between males and implies that they are territorial. Taylor (1936:59) described laticeps as typically an arboreal form, almost invariably found in trees, and he indicated that it has claws more curved than in other species—an obvious arboreal adaptation. Parker (1948:25), however, stated that “E. laticeps is reputed to be rather arboreal, but field work in western Tennessee has not borne out this belief. A few of the specimens have been found in tall, dead trees, as has E. fasciatus.” This statement evidently was based on a small number of observations.
Cook (1943:15) mentions a female laticeps found in a nest with a clutch of 27 eggs (hence certainly a communal nest of two or more females) in a burrow under a log, on July 8, 1941, in Lee County, Mississippi. This account is under the name Eumeces fasciatus but the large size of the female precludes the possibility of it being either fasciatus or inexpectatus. The remainder of Cook’s account is evidently based on a composite of observations on all three species.
Goin and Goin (1951:29–33) have given an excellent brief account of behavior and seasonal schedule in a small colony of E. laticeps near Gainesville, Florida, based on almost daily observations over a period of years. In view of the greatly different climatic conditions, the seasonal schedule is remarkably similar to that of E. fasciatus in Kansas, and it seems that the minimum threshold temperatures required for activity are much higher in laticeps. Temperatures of 80° F. or above for several consecutive days seemed to be a necessary stimulus for emergence from hibernation; emergence was in the last week in March or the first week in April in Florida. Hatching was found to take place in late June or early July. Adults were last seen before retiring into dormancy in the latter half of September and young of the year remained active into October some two or three weeks later. The skinks observed all lived in hollow water-oaks. When the population was at an especially high level, in the late summer of 1949, each hollow oak was inhabited by one young and one adult. Territoriality and mutual exclusiveness of adults and even of young seems to be implied. The skinks were seen eating spiders, ants, and cockroaches.
Neill (1950:115) mentions one sizable colony of E. laticeps living in a treeless urban area, in Georgia and depending for shelter on piles of metal drums and other industrial equipment. Evidently, however, this was an exceptional situation. In another paper, Neill (1948b:109) described the specialized hibernation site requirements of laticeps in Georgia; the skink retires inside large, rotting pine stumps, especially those that are leaning. He states (1948a:157) that in Georgia, laticeps is most common in the Coastal Plain and is much less numerous above the Fall Line (the line between the Coastal Plain and the Piedmont). Deckert (1918:31) wrote of “Plestiodon fasciatus” in the vicinity of Jacksonville, Florida, where only E. laticeps and E. inexpectatus occur: “Inhabits hollow trees, always near water. Blue-tailed ones often live around human habitations.”
With regard to the ecological traits of E. inexpectatus that distinguish it from fasciatus, authors are much less definite, and evidence is somewhat conflicting as the differences are relatively minor. Engels (1949:269) noted the occurrence of E. inexpectatus on two low islands of submarine origin, off the North Carolina Coast, Harkers Island and Shackelford Banks, and he surmised that the absence from them of E. fasciatus and E. laticeps must have some ecological significance, since all three species occur on the adjacent mainland. Most of the island inexpectatus were taken from beneath loose bark of standing trees, while mainland fasciatus was taken from beneath loose bark of fallen trees.
Barbour and Carr (1940:129) wrote of inexpectatus in the vicinity of Miami, Florida: “… it seems to be the only one [of the five-lined skinks] which has adapted itself to life under the rather specialized environmental conditions existing in its rocky and decidedly tropical habitat. It is one of the very few forms which have established themselves on some of the waterless and poorly vegetated islands on both coasts of the peninsula. E. inexpectatus is much less arboreal than either laticeps or fasciatus. Although it climbs trees when pressed, it is usually found on the ground among leaves or about fallen logs, and particularly about stone walls or old buildings made of cut rock.”
On the other hand, Neill (1948a:157) states that in Georgia, inexpectatus is often observed basking on tree trunks, and though adults often forage on the ground, they dash for the nearest tree when disturbed, usually climbing to a considerable height before halting. The juveniles, however, are said to climb only rarely; they hide beneath objects on the ground when they are pursued. Neill stated that E. inexpectatus occurs in dry pine forests where laticeps and fasciatus are lacking, as well as in moist or even swampy woods. E. inexpectatus often forages on the sides of old buildings.
Hoffman (1953:172), in discussing means of differentiating between inexpectatus and fasciatus in Virginia, states that there are ample differences in color and behavior as well as in scalation. He describes the color difference (blue color of tail of juveniles extending anteriorly beyond pelvis; light stripes reddish-orange on head, sublateral line present, in inexpectatus) but he does not describe the differences in behavior. He states that inexpectatus is the most abundant lizard in southeastern Virginia. Carr (1940:76) also states that inexpectatus is less arboreal than laticeps and is often found under logs and boards in dry sand.
E. inexpectatus thus seems to be adapted to a somewhat drier, more open, habitat than that typical of fasciatus, but it is not clear whether either species is more arboreal in habits. It is to be hoped that the present inconclusive summary will draw attention to the problem and will lead to more critical comparisons of the habitats and behavior of the two species by herpetologists in the southeastern states. The differences, both ecological and morphological, that distinguish inexpectatus from fasciatus are of a degree usually found between subspecies of the same species. The extensive geographic overlap between them is indeed remarkable in view of the slight degree of differentiation, morphologically and ecologically. They are, however, complementary in part in their ranges, while laticeps shares all parts of its range with either one or the other, or both of them (see Figures 4 and 5).
Fig. 4. Geographic distribution of Eumeces inexpectatus, as indicated by published records; only marginal and near-marginal records are shown, excluding those of doubtful validity.
Under present conditions, with these three species so similar in habits and so extensively overlapping in geographic range, it is difficult to visualize a barrier such as would have been required for allopatric speciation of the type, usual in vertebrates, to have occurred. One might be tempted to postulate sympatric speciation, with the parent form, presumably fasciatus, giving rise to the other two by abrupt mutations. However, the demonstrable antiquity of the five-lined skinks would allow ample time for divergence, allopatric speciation, and subsequent disappearance of the barrier and intermingling of populations. The displacement of floras and faunas that occurred in the Pleistocene, with the successive advances and retreats of the continental ice sheets might have had some part in bringing about the present overlapping distribution, after the disappearance of the original barrier. Such a barrier might have been an eastward extension of the central grasslands to the Atlantic Coast at a time when the climate of the continent was warmer and drier.
Fig. 5. Geographic distribution of Eumeces laticeps, as indicated by published records; only marginal and near-marginal records are shown, excluding those of doubtful validity.
