Changes in Pattern

Fig. 12.Sizes of immature skinks of successive annual broods, grouped in biweekly or monthly intervals, with mean, standard error, standard deviation, and extremes shown for each group.

A certain small percentage fail to attain minimum adult size or breeding maturity by the time of emergence from their second hibernation. Among 77 individuals marked as young either soon after hatching or in spring and early summer, and recaptured the following spring, only one had failed to grow to adult size. It was 46.5 mm. in length when marked on June 13. When recaptured on April 25 of the following year, it had grown to a length of 59 mm., still short of minimum adult length. During the interval between captures it had maintained about the average growth rate. Its failure to attain maturity was obviously the result of its early retardation, and probably late hatching was primarily responsible. Although this is the only individual with known history, which failed to attain breeding maturity after its second hibernation, occasional specimens are taken in spring which are somewhat below adult size but seem too large to be young hatched the preceding summer. Obviously, the incidence of such failure from year to year would be influenced by weather conditions, and an unusually cool summer may result in such delayed laying and hatching that an unusually large proportion of young might fail to attain sexual maturity at the usual time. At more northern localities, the percentage of such failures might be expected to increase. At the northern edge of the range attainment of breeding maturity may normally require more than two years. Such delayed development would result in a drastic reduction of the reproductive potential which might be critically limiting to the species, even in an otherwise favorable environment, as the population would be unable to replace rapidly enough the individuals eliminated by normal mortality factors.

In contrast to the delayed development of those that have failed to attain maturity at an age of two years, is the accelerated development of those that have already more than doubled in length before the first hibernation, and continue to grow rapidly after emergence. By late spring they are already approaching adult size, perhaps even before laying has occurred, and while breeding is still in progress. It is certain that in northeastern Kansas there is no breeding by such accelerated individuals approaching adult size at an age of nine or ten months. Farther south in the species’ range with a much longer growing season, there is perhaps some possibility of such early breeding by first-year individuals. This would reduce by more than half the length of time required for a generation, and would tremendously increase the reproductive potential. With such added impetus to its reproduction the species might be able to withstand greatly increased predation pressure, or other mortality factors.

Fig. 13.Growth curves of successive annual broods (designated by the year of hatching), superimposed to bring out differences in trends resulting from changes in weather from year to year.

Extremes of acceleration or retardation are relatively rare in the population studied. Nevertheless, in April there are some individuals between 50 and 60 mm. in snout-vent length which cannot be classified with certainty as to their age group, and might be either accelerated individuals about nine months old or retarded individuals about 21 months old.

The spread in size for any given age group is especially large, if data from different years are combined. A typical individual, having a snout-vent length of 25 mm. at hatching in mid-July may have attained 30 mm. by early August, 35 mm. by late August, and 45 mm. by the time it hibernates late in September. Emerging shortly before the middle of April it may grow to 50 mm. by the end of May, 58 mm. by the end of June, and more than 60 mm. by the end of July when it is a little more than a year old. By the time of its second hibernation it may have attained a length of from 65 mm. to 70 mm., and emerges from this hibernation as a breeding adult.

Fig. 14.Records of growth of immature individual skinks, both hatchlings and yearlings, that were marked in one year and recaptured the next.

In reptiles in general there is a wide range in adult size, and the extent and rapidity of continued growth after attainment of sexual maturity and minimum adult size is still insufficiently understood. Information bearing on this problem was obtained in the present study from the recapture of marked skinks already measured as adults. It is evident that the growth rate of the young, amounts to as much as 15 mm. per month in snout-vent length in the late summer period from hatching until hibernation, averages perhaps three or four mm. per month in the summer after emergence from the first hibernation, and tapers off rapidly as adult size is approached.

One hundred of the skinks marked as adults or subadults and recaptured after intervals of months, including, in most instances, one or more hibernation periods, represent in the aggregate, 87 years ofgrowth. 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 assumed 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.

Opportunity to compare the rapid growth of young during their first year of life with the relatively slow continued growth after attainment of sexual 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 sexual 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.

Differences in their growth rates therefore reflect differences in sex, 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 Figures11to13. 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 ofAugust, 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 passed 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 competition may have been a factor in their rapid early development.

Fig. 15.Records of growth in another group of recaptured young that grew less rapidly than those ofFig. 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 passed 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.

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 cumulative effect in delaying emergence and breeding of adults the first year, and delaying in the second year emergence of the young, already retarded 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 mightbe “caught short,” and forced to hibernate while still in the 50-60 mm. size class. Emerging the following spring, they might have failed to mature sexually, 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 retardation or acceleration continuing throughout development are relatively rare. Retarding 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 inE. septentrionalisandE. skiltonianus, and probably inE. fasciatusat the northern edge of its range) the original female would have produced a population of somewhat less than 7,800 assuming 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 participate 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.

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 althoughstill conspicuous is made up of two shades of brown instead of the earlier black and white markings.

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 interspaces 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 andinconspicuous. 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 Sex.

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 sexually, still show but little sexual 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 breedingseason 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 13refers 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 sex. 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.

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 sexual 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 class 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 sexes, the tail lengthis near 155 or 156 per cent of the snout-vent length. Sexual dimorphism in tail length is slight if it exists at all; in adult males, tails averaged a little longer than in adult females.

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 masses 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 abrupttaper 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.

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, andsite 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 (Figures19and20). 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.

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