Life History and Ecology of the Five-lined Skink, Eumeces fasciatus Part 8

These records show that after minimum adult size of 65 mm. is attained, growth slows abruptly, and that by the time a length of approximately 75 mm. is attained in most instances growth has become extremely slow in males and has virtually stopped in females. Males attain a maximum size several millimeters larger than that of females. Individuals differ greatly in their growth, however; some adults continue to grow rapidly till they near the maximum size, whereas others apparently stop growing when they are still below average adult size. Unusually large specimens are not necessarily old, but may have attained their size only a year or two after reaching maturity through the accelerated growth resulting from abundant food and predisposing genetic factors. Likewise, unusually old individuals are not necessarily the largest, but may be only a little above average adult size. It may be a.s.sumed that no growth occurs during the period of winter dormancy, which occupies approximately half the year in the population studied. To compute growth rates, in those recaptured after an intervening hibernation, periods of hibernation, arbitrarily estimated as six months, were subtracted from the time elapsed between captures.

Table 11. Average Growth Rate in a Selected Sample of Skinks of Adult Size.

============+====================+==================== Males Females ----------+---------+----------+--------- SIZE GROUP Average Number Average Number growth mm. of skinks growth mm. of skinks per month in sample per month in sample ------------+----------+---------+----------+--------- 65-68 mm. 1.4 11 .8 12 69-72 mm. .7 12 .4 21 73-76 mm. .7 13 .3 21 77-80 mm. .4 7 .... ....

Opportunity to compare the rapid growth of young during their first year of life with the relatively slow continued growth after attainment of s.e.xual maturity is afforded by the records of skinks caught and marked while yet immature and recaptured in two or more successive years after their attainment of s.e.xual maturity. The records of selected individuals of this group are presented below. With the exception of number three, all in this series are of the 1949 brood, and probably all hatched within a two-week period.

Table 12.--Records of Individual Skinks Marked as Young and Recaptured Repeatedly After Attainment of Adult Size, Showing Trend of Progressively Slowing Growth.

=======+==================+==========+=====================+======+============================ Snout-vent Weight Date length Tail length in in mm. in mm. grams Remarks -------+------------------+----------+---------------------+------+---------------------------- No. 1. Male At an age of 33 months April 12, 1950 43 71 1.5 this male was far short August 30, 1950 56 56 + 21 5.4 of maximum size, and May 23, 1951 68 59 + 32 .... smaller than some April 28, 1952 73 62 + 38 6.6 males a year younger; all four captures within a 65-foot diameter.

No. 2. Male At an age of approximately July 5, 1950 61 92-1/2 (regenerated) 5.2 one year this July 28, 1950 64 97 (regenerated) 5.4 male was approaching May 3, 1951 68 96 (broken stub) 5.8 small adult size; when June 21, 1951 72 101-1/2 (regenerated) .... last captured at an age May 1, 1952 78 101 (regenerated) .... of 34 months, he was a large adult. All five records within a 190-foot diameter.

No. 3. Male This skink was nearly a June 22, 1949 65 111 .... year old and nearing May 4, 1950 72-1/2 115 7.3 adult size when first June 17, 1950 73 116 7.8 captured; recaptured May 15, 1951 80 125 .... in each of the four May 13, 1952 82 125 .... succeeding years, he April 6, 1953 82 104 (regenerated) .... showed slowing growth.

He was near the maximum size at the time of his last capture when he was about 57 months old, and evidently had stopped growing (for movement see No. 2, p. 110).

No. 4. Female This individual, marked July 13, 1949 27 34-1/2 .... when less than two June 1, 1950 54-1/2 93-1/2 3.1 weeks old, had grown August 21, 1951 74 119 .... to nearly the maximum May 1, 1952 76 123 10.0 female size at an age of 34 months; all four captures within a 175-foot diameter.

(See Figure 24).

No. 5. Female April 15, 1950 43 70 1.4 All six records within a June 5, 1950 52-1/2 87 2.8 65-foot diameter (See May 25, 1951 71 82 + 29 .... Figure 21).

September 28, 1951 73 111 (regenerated) .... April 26, 1952 74 113 (regenerated) 7.4 April 24, 1953 76 114 (regenerated) .... No. 6. Female Hatched in July 1949, April 21, 1950 46 75 2.1 this skink had attained May 7, 1950 48 15 (broken stub) 2.0 the maximum female May 3, 1951 74 29 + 57 8.5 size at an age of a little May 2, 1952 78 25 + 64 .... more than three years; August 27, 1952 79-1/2 95 (regenerated) 8.3 (for movement see Figure 25).

No. 7. Female Hatched in July 1949, June 5, 1950 51 82 2.5 this skink was 11 July 13, 1950 59 93 3.9 months old and about July 29, 1950 64 98 4.4 half-grown when it was August 21, 1951 69 80 (broken stub) 5.0 marked. When last May 28, 1952 73 83 + 9-1/2 .... caught at an age of 35 months it was of average adult female size, having grown less than numbers 4 and 6 at the same age. All five captures were within a 60-foot diameter (Fig. 24).

No. 8. Female Hatched in July 1949, April 26, 1950 50-1/2 78-1/2 2.7 this skink was of average May 24, 1951 74 107 (regenerated) .... adult female size April 28, 1952 78 93 (regenerated) 8.5 and was breeding in April 23, 1953 80 93 (regenerated) .... May 1951; it grew nearly to maximum female size in the next 11 months. All captures within a 70-foot diameter.

No. 9. Female July 5, 1950 60 95 4.5 All three captures at the August 6, 1951 71 106-1/2 5.6 same site.

May 28, 1952 72 110 8.5 No. 10. Male Hatched in July 1949, April 23, 1950 46-1/2 66 (regenerated) 1.8 this male grew less June 13, 1950 52-1/2 26 + 3 2.7 rapidly than most, and September 2, 1950 66 32 + 51 6.2 in the spring of 1953 May 29, 1951 67 33 + 58 .... was smaller than some August 3, 1951 70 94 (regenerated) .... others that were a year March 27, 1953 74 78 (regenerated) 7.1 younger, or even two years younger.

No. 11. Female April 26, 1950 50-1/2 78-1/2 2.7 This skink had attained May 24, 1951 74 87 .... maximum female size April 28, 1952 78 72 + 21 8.5 when she was a little April 23, 1953 80 73 + 20 .... less than four years old.

Differences in their growth rates therefore reflect differences in s.e.x, individual vigor, and local situation, in individuals living at the same time and within the same general environment.

Changing weather, and other factors that vary from year to year cause marked differences in the dates of important events in the annual cycle, and in the stage of development at any given date. Data are available for five successive annual broods of young, those of 1948, 1949, 1950, 1951, and 1952, and each brood differs from the others to some extent, as shown in Figures 11 to 13. In 1949, for instance, young hatched relatively early, and probably most of them were active by the middle of July. They made rapid growth in August, averaging larger than young hatched in other years on any given date in late summer. However, they retired into dormancy early in the fall. Cool and dry weather in early September ended their activity for the season. In 1950, young hatched, on the average, at least three weeks later, about the first of August, but they remained active until late in September, and by hibernation time had partly caught up to the stage of development attained by the young of 1949. Most young of 1951 hatched late in the first half of August, and at first were smaller than those of 1950 and much smaller than those of 1949 on corresponding dates, but favorable weather in the early fall hastened their development. By early September they had caught up and pa.s.sed the stage of development of young of 1950 and by the time they retired to dormancy in late September, they had reduced by half the size-advantage of the young of 1949 at the time these latter retired into hibernation. The young of 1951 appeared to be few in numbers, and a lack of compet.i.tion may have been a factor in their rapid early development.

[Ill.u.s.tration: FIG. 15. Records of growth in another group of recaptured young that grew less rapidly than those of Fig. 14.]

The young of 1948, first sampled after their emergence from their first hibernation in mid-April of 1949, were then somewhat intermediate in size as compared with those of 1949 and 1950 at the same times of year.

Their subsequent development was rapid; by late May they had caught up and pa.s.sed the stage reached by the 1949 young at the same time of year.

The young of 1950 after having a late start, were further set back by cold weather in April 1951 delaying their emergence from hibernation. As a result they were still unusually small in late April and May. Even though they grew rapidly subsequently, they were consistently smaller than those of other broods on corresponding dates. Favorable fall weather prolonging the 1951 growing season into late September beyond the time of retirement in other years may have permitted many of them to attain adult size.

[Ill.u.s.tration: FIG. 16. Records of immature individual skinks marked and recaptured within the same growing season, showing the trend of rapid growth, and differences in growth rate between individuals.]

The varying fortunes of the several annual broods studied were closely correlated with weather trends, and suggest possible effects of slight changes in climate. An unfavorable sequence of weather might bring about drastic reduction of the population without causing any direct mortality. A late spring in two successive years would have c.u.mulative effect in delaying emergence and breeding of adults the first year, and delaying in the second year emergence of the young, already r.e.t.a.r.ded by the lateness of their hatching. If this sequence were followed by onset of unusually cool and dry weather in early September, or even in late August, the young might be "caught short," and forced to hibernate while still in the 50-60 mm. size cla.s.s. Emerging the following spring, they might have failed to mature s.e.xually, reducing by perhaps half the number of productive adults. At the northern extreme of the species'

range, length of growing season may be more critical than extremes of temperature in limiting the numbers and distribution. Growing seasons that average long enough and warm enough to permit attainment of maturity by onset of the second hibernation period may be essential to the species. While no two annual broods of young in the same locality come under exactly the same weather influences, extremes of r.e.t.a.r.dation or acceleration continuing throughout development are relatively rare.

r.e.t.a.r.ding effects of unfavorable weather causing delayed breeding and hatching, may be offset by prolongation of warm weather in the fall thus delaying hibernation, or by warm spring weather hastening emergence from hibernation.

Under favorable conditions an adult female produces about ten offspring annually of which about half are females. It is calculated that if all survived, after ten breeding seasons, the progeny of an original female might have increased to a population of more than 97,000, under the climatic conditions of eastern Kansas, permitting attainment of breeding maturity late in the second year of life. In the same ten year period under climatic conditions delaying maturity until late in the third year of life (as seems normally to occur in _E. septentrionalis_ and _E.

skiltonia.n.u.s_, and probably in _E. fasciatus_ at the northern edge of its range) the original female would have produced a population of somewhat less than 7,800 a.s.suming that all survived. With a long growing season such as occurs in the southern part of the range, it seems theoretically possible (though not probable) that individuals might mature before the end of their first year, in time to partic.i.p.ate in the next breeding season. If this should occur the original female might produce a population of more than 120 million by the end of the tenth breeding season.

CHANGES IN PATTERN

Progressive alteration of the color pattern is more rapid in males than in females and is synchronized with growth. During the first year of life changes in the pattern are gradual, and consist chiefly of loss in vividness. The blue of the tail is slightly dulled. The light lines become suffused with brown and the dorsolateral dark areas become paler, with light brown areas appearing on the corners of the scales and gradually spreading to replace the original black. In skinks that are in the second year of life the striped pattern although still conspicuous is made up of two shades of brown instead of the earlier black and white markings.

[Ill.u.s.tration: FIG. 17. Records of another group of immature skinks marked and recaptured within the same growing season.]

Even in hatchlings, the dorsal part of the rostrum and the inter-nasals are of a somewhat neutral brownish color, matching neither the light lines nor the dark inters.p.a.ces of the striped body pattern. With advancing age this neutral brown color gradually spreads posteriorly on the head so that the striking lyrate marking of the bifurcated dorsal stripe on the head in the juvenile become obscured by the time the skink has grown to small adult size, at 21 months. The top of the head is then dull brown, with a slightly mottled appearance caused by the different intensity of pigmentation in different areas. The stripes though still discernible, are faint and inconspicuous. Dorsally, on the body, the stripes are still conspicuous, but are dull and lacking in contrast. At this stage, the dark lateral area is retained with intensity of pigmentation scarcely diminished.

Table 13. Normal Range of Variation in Dorsal Striping of Head and Body, and in Color of Tail According to Age and s.e.x.

===================+================================+============================ Condition of stripes Color of tail Age, Size and s.e.x -----+--------+----+-----+------+--------------+------------- Sharp Distinct Dull Faint Absent Original Regenerated -------------------+-----+--------+----+-----+------+--------------+------------- Small young bright blue bright blue body X head X Larger young bright blue duller blue body X head X X Young adult female dull blue gray-blue body X X to brown head X X X Young adult male mostly dull gray-blue body X X blue to brown head X X X 3 year adult female mostly mostly body X X brown and brown and head X X gray with gray scattered blue scales 3 year adult male mostly brown body X brown; head X occasional bluish scales Old adult female brown; brown body X X X occasional head X bluish scales Old adult male brown brown body X head X -------------------+-----+--------+----+-----+------+--------------+-------------

In tracing the gradual ontogenetic changes in the striped pattern, from the vividly contrasting colors of hatchlings to the dull, patternless coloration of old adult males, five descriptive terms have been applied to the successive stages: "sharp," "distinct," "dull," "faint," and "absent." To most individuals below minimum adult size, the term "sharp"

is applicable, although there is some loss in vividness in the larger young, as compared with hatchlings. Fading of the original striped pattern proceeds more rapidly on the head than on the body. Upon emergence from their second hibernation at an age of about 21 months, the skinks, mostly grown to adult size, and ready to mature s.e.xually, still show but little s.e.xual difference. They retain the hatchling pattern essentially unchanged, but with colors dulled and contrasts reduced. Within a few weeks the newly matured males undergo relatively rapid color change as the breeding season progresses. The stripes tend to fade and blend into the dark areas adjacent to them. In the two-year-old males stripes are distinct to dull on the body and faint or absent on the head, while in females of the same age group, body stripes are sharp or distinct.

Table 13 refers to adult pattern and coloration as they appear in the breeding season. After the breeding season, in late spring and early summer, when the red suffusion of the head and neck has faded in adult males, the original striped pattern, after having been almost completely suppressed may again become discernible. Individuals of the same size differ in extent of pattern change, and the color descriptions made of individuals were not sufficiently detailed to show fully the changes occurring between successive dates of capture. However, most large adult males taken later than mid-June had at least some trace of the striped body pattern and many of them had become so much like females in appearance that close scrutiny was necessary to determine their s.e.x.

They were especially like females in having the dark lateral area extending forward onto the cheek and setting it off sharply from the paler temporal region above it. In breeding males the head has no such dark markings and is suffused with red.

GROWTH AND REGENERATION OF THE TAIL

Even among those skinks which have never broken their tails there is a wide range of variation in relative length of tail. This is partly a matter of relative growth since the proportions change during the course of development. Also there may be slight s.e.xual difference and there is much individual variation. In fetuses still well below hatching size, the tail length is less than the snout-vent length. For instance, an egg in a natural nest 12 days short of hatching contained a fetus that had a snout-vent length of 14 mm. and tail length of 12 mm. (Figure 18). In the late stages of fetal development the tail growth is relatively rapid. At hatching, the tail is considerably more than half the total length. In a large series of young with snout-vent lengths from 30 mm.

down to hatching size of 25 mm. or less, the tail length averaged 130.8 per cent of snout-vent length. In larger young, up to a snout-vent length of 40 mm. or more, the tail continues to lengthen more rapidly than the body. In skinks that are about two thirds grown, the tails average relatively longer than in either larger or smaller individuals.

In the sample representing the size cla.s.s 50-54 mm. snout-vent length, the tails average 163.3 per cent of the snout-vent lengths, whereas in groups of adults of various sizes and both s.e.xes, the tail length is near 155 or 156 per cent of the snout-vent length. s.e.xual dimorphism in tail length is slight if it exists at all; in adult males, tails averaged a little longer than in adult females.

[Ill.u.s.tration: FIG. 18. Diagram showing relative tail-length (as a percentage of snout-vent length) in skinks of different size groups that retain their original tails unbroken; in the early stages of growth the tail becomes relatively longer as size increases, but the trend is reversed before adult size is attained.

For each series the mean, standard error, standard deviation, and extremes are shown.]

When a skink's tail is broken, there is almost no loss of blood. The fractured surface is rough and irregular, with exposed muscle ma.s.ses protruding on the detached end and corresponding concavities on the end of the stump tail retained by the lizard. The concavities are soon filled with oozing blood, and a thick scab forms. As healing begins, the broken end presents a flat, slightly irregular surface. When the scab is sloughed off, a slightly convex surface of delicate, pale-colored new skin of the regenerating tail, is exposed. At first, no scale structure is discernible. As growth proceeds, the new tail takes on a bluntly conical shape. During the early stages of growth, it is well set off from the original portion by the abrupt taper at the point of contact and by its paler coloration and different texture, with no scales discernible at first, and later with fine and granular scalation. The new tail elongates until the more abrupt taper beyond the point of the break is no longer noticeable, and the coloration, surface texture and scalation match that of the original portion so closely that it is difficult to determine where the break occurred or even to ascertain that there has been one. On the regenerated tail, however, the scales are less uniform in size and less regular in shape. The regenerated tail, being different from the original in internal structure, with a cartilaginous rod replacing the vertebral column, is less fragile and subsequent fractures are most likely to be on the part proximal to the regeneration. Nevertheless, fractures of regenerated tails occur occasionally. In old skinks especially, the tail eventually may consist of three or more distinct segments including the basal remnant of the original tail and the successive regenerations. When a break in the regenerated tail occurs, the detached portion is relatively inert, and is capable of only feeble twitching movements in contrast with the lively wriggling normally displayed in a newly detached tail that includes part of the central nervous system.

[Ill.u.s.tration: FIG. 19. Relative lengths of original and regenerated portions of tails in skinks which have had their tails broken and regenerated; for each individual, length of each part of the tail is expressed as a percentage of the snout-vent length.]

Rate of growth in the regenerating tail is controlled by a variety of factors, such as age, condition, and activity of the individual, and site of the fracture. A break occurring early in the skink's lifetime results in regeneration more complete than occurs in an adult sustaining the same type of injury. The regenerated tail eventually may be longer and thicker than the lost part if the lizard is young and still growing.

But the regenerated tail is never so long as the original one would have been. Regeneration is most extensive in those tails broken near the base. The farther from the base the break occurs the shorter is the part regenerated. As a result, tails that have had time to regenerate do not differ greatly in total length regardless of where the break occurred.

However, the nearer the break is to the base, the shorter is the total tail-length after regeneration (Figures 19 and 20). If only the tip of the tail is lost, regeneration may not occur. In the skinks examined that had regenerated tails the proportions varied over a wide range.

Presumably, in many, growth of the regenerated portion was still incomplete.

[Ill.u.s.tration: FIG. 20. Diagrams showing typical extent of tail regeneration in skinks having tails broken at different points, approximately 2/3. Original parts of the tails are at the right.]

Table 14. Records of Regeneration of the Tail in Individual Skinks Marked and Recaptured.

=================+==================+==========+======================== Snout-vent Age and s.e.x Date length Tail length in mm. in mm.

-----------------+------------------+----------+------------------------ No. 1. June 1, 1951 73 38 (newly broken) Adult male June 26, 1951 73 38 + 14 August 17, 1951 76 40 + 45 April 29, 1952 76 40 + 45 No. 2. May 28, 1949 51 46 (newly broken) Immature October 15, 1949 69 51 + 21 No. 3. June 11, 1950 49 49 (newly broken) Immature September 2, 1950 63 63 + 31 No. 4. August 10, 1950 62-1/2 60 (newly broken) Immature female June 14, 1951 68 62 + 17 No. 5. August 14, 1950 67-1/2 77 + 4-1/2 Adult male September 3, 1950 68-1/2 77 + 6-1/2 April 27, 1951 69 78 + 9 No. 6. April 7, 1950 67 38 (newly broken; Adult male separated end 66) July 27, 1950 71 39 + 47-1/2 No. 7. May 28, 1951 70 18 (newly broken; Adult male separated end 100) June 14, 1951 71 18 + 1 September 22, 1951 76 19 + 29 + 7-1/2 No. 8. June 12, 1951 72 33 (recently broken) Adult male July 28, 1951 76 36 + 31 No. 9. May 2, 1951 45 47 + 1 Juvenile May 14, 1951 45 47 + 3 May 19, 1951 45 47 + 5 No. 10. June 7, 1952 64 51 (recently broken) Subadult female June 24, 1952 64 51 + 11 No. 11. August 19, 1949 42 40 (newly broken) Juvenile June 13, 1950 58-1/2 58 + 28 August 5, 1950 63 62 + 31 No. 12. May 17, 1951 78 20 (newly broken) Adult male June 12, 1951 78 20 + 13 -----------------+------------------+----------+------------------------

Under favorable conditions regeneration occurs at a relatively rapid rate. After a period of healing the new tail grows with a sudden spurt, making most of its gain in length within a few weeks. Then growth abruptly slows or ceases altogether. In young similarly rapid growth of the regenerating tail occurs, but subsequently the increase is more gradual corresponding to the over-all growth of the lizard. In numerous adult skinks marked, and recorded as having well-regenerated tails, the proportions recorded at subsequent captures months or years later were still just the same, demonstrating that extent of regeneration is not proportional to elapsed time. Those adult skinks having unusually long regenerated tails presumably are individuals in which the original tail was lost early in life, and the potentiality for regeneration is probably somewhat less in older individuals, especially those that have stopped growing.

Successive records of selected individuals are listed in Table 14 to ill.u.s.trate trends in regeneration of the tail. In those instances in which the tail is referred to as "newly broken" the separation usually occurred as an accident at the time the lizard was captured, while in those designated as "recently broken" separation had already occurred in some earlier accident but regeneration was not yet perceptibly underway.

In the "Tail length" column, plus signs separate the original portion of tail, on the left, from the regenerated portion, on the right.

As in many other kinds of lizards, the tail in the five-lined skink serves as a reservoir for fat, which may be drawn upon for nutrition in time of food scarcity. An individual that is in good condition has a plump and rounded tail. Fat comprises much of its bulk. Upon emergence from hibernation this fat supply is not noticeably depleted. Brooding females in the latter part of the incubation period have the supply of caudal fat most noticeably depleted, and their tails may appear emaciated, with kinks on the terminal portion. It is my impression that in adults the capacity for storage of fat is most developed in the females, and that their tails vary in proportions more than do those of males. The capacity to shed the tail easily seems somewhat inconsistent with this function of fat storage. Loss of the tail sometimes involves loss of a large amount of reserve fat. Many detached tails that were broken accidentally at the time of the skinks' captures were weighed. In those that were broken off near the base and were not previously regenerated, weights were usually 16 to 20 percent of the lizards' total weights.