Life History and Ecology of the Five-lined Skink, Eumeces fasciatus Part 7

The young were much more active than the female. These and other young observed in the open were almost constantly in motion. Pauses to bask at any one spot were of only a few seconds duration. A certain log in Skink Woods evidently was the site of one or more successful skink nests each year that observations were made, although a nest was actually found in it only in 1948. On July 26, 1950, recently hatched young were active on this log. Temperature was about 22C. and the young were alternating frequently between shade and sunshine to maintain their body temperature. Collectively they seemed to cover every square inch of the log surface, poking and probing into niches, crevices and insect borings. They had a tendency to seek out the highest points on the log as resting places.

In moving about, foraging or sunning, the young often carry the tail arched high, and keep it in motion with slow squirming undulations.

These undulations may be continued even when the lizard itself has come to rest momentarily. The movements of the tail together with its vivid blue color serve to attract attention to it. Such behavior has not been observed in adults or partly grown young. Jopson (1938:90) observed an instance in which two dogs cornered a young five-lined skink (either the present species or _E. laticeps_) but were distracted by the wriggling of its bright blue tail "either dropped by autotomy or knocked off" so that the skink itself was allowed to escape. On another occasion these same two dogs attacking an adult male skink, were not distracted by the wriggling but dull colored broken tail, and they killed the lizard.

GROWTH

The subject of growth in _Eumeces_ was briefly discussed by Taylor (1936:66) in his revision of the genus. Sorting fairly large series of museum specimens into seeming age-size groups, Taylor concluded that skinks require as much as 9 or 10 years to attain adult size. For _fasciatus_, for instance, the snout-vent length of 65.7 mm. (small adult size) was considered typical of individuals in their ninth year of life, with yearly gain of only 6 or 7 mm. in length in the young. I have seen the original data on which this conclusion was based, and the age groupings, as a.s.signed by Taylor, seemed plausible. However, in the light of present knowledge, it is certain that the seeming intervals between his a.s.sumed age groups would have disappeared with a still larger series of specimens. The eight or nine size groups that Taylor recognized as distinct annual age groups actually comprise only two age groups, each having such wide dispersion of individuals (by r.e.t.a.r.dation of some and acceleration of others) that there is overlapping in size between them.

Growth in reptiles is now much better understood. Many species have been studied by a variety of methods, including observation of growth in captives, recording of growth in marked individuals living under natural conditions, and sorting of large series into age-size groups. Two species of _Eumeces_ have been studied in some detail. Breckenridge (1943:601-602) marked all the individuals of _septentrionalis_ that could be found in a small colony in Minnesota and he concluded from the growth recorded in several that were recaptured, that these skinks grow to mature size (65 mm. and larger) at the end of their second year of life and are ready to breed the following spring. Rodgers and Memmler (1943:61) plotted the size distribution of a large year-round collection of _skiltonia.n.u.s_ from near Berkeley, California. They found that in this species hatching occurs in July and August, hatchlings are about 25 mm. in snout-vent length, and grow to about 50 mm. by the time they are one year old, and to about 65 mm. at two years of age, but most of them breed at the end of their third year. Within the genus the species _septentrionalis_ and _skiltonia.n.u.s_ belong to groups separate from each other and from that including _fasciatus_. While _septentrionalis_ and _skiltonia.n.u.s_ resemble each other in their growth pattern and in the time required to reach s.e.xual maturity, _fasciatus_ is notably different in its more rapid growth and the shorter time it requires to reach breeding maturity. This would scarcely be expected, as all three are of similar size. Furthermore, _skiltonia.n.u.s_ in the region of Rodgers' and Memmler's study has a longer growing season than _fasciatus_ in northeastern Kansas, while _septentrionalis_ in Minnesota has a growing season markedly shorter than either. It is noteworthy that each of these three skinks is the northernmost lizard in the section of the country where it occurs.

In the present study growth was investigated by measuring and marking large numbers of young, many of which were recaptured for subsequent records, and by sorting into age-size groups all available measurements.

An understanding of the latter set of data was facilitated by correlating it with the growth records of marked individuals. Changes in the phenology of growth from year to year according to weather conditions were noted.

As already indicated, hatching occurs from early July to mid-August in northeastern Kansas. Unseasonably cool weather with frequent rains may cause c.u.mulative delay in breeding and incubation so that hatching may average several weeks later than it does in years with relatively warm and dry weather during the breeding season. Within any one year hatching time is concentrated, so that the majority of the young hatch within a period of two weeks, but microclimates in the situations where the nests are made may differ enough to cause this much spread. Individuals living on north slopes in thick woods, and receiving the minimum amount of sunlight may have their emergence from hibernation and attainment of breeding condition delayed. Later, nesting in the same situations, they may have incubation of their clutches similarly delayed.

Newly hatched young average just under an inch in snout-vent length (23-27 mm.) and weigh .2 to .45 grams. Most rapid growth occurs in the period of weeks following hatching. The growth rate during this late summer period cannot be well shown by comparing average size of series taken on successive dates, because each series is likely to include some newly hatched young.

In 1949, a series of recently hatched young averaged 26.7 mm. on July 10. By August 26, average length in a series collected was 42.9 mm., indicating an average gain of at least .35 mm. per day. One that may be considered typical was marked on July 23, 1950, soon after hatching, and it had a snout-vent length of 26.5 mm. and weighed .25 grams. It was recaptured just a month later when it had grown to 36 mm. snout-vent length, and weighed .8 grams. Potential growth rate under favorable conditions is shown by the fact that some individuals have attained a snout-vent length of 50 mm. by the third week of August, thus approximately doubling their hatching length. A maximum growth rate of about .5 mm. per day is indicated for these accelerated individuals, but on the average, young are considerably less than 50 mm. in length even when they enter hibernation. At the other extreme, representing r.e.t.a.r.ded growth, is an individual having a snout-vent length of only 34 mm. on May 1. It must have been approximately nine months old on that date, but of course had spent at least six months in hibernation. Even if it made rapid growth subsequently, this yearling could scarcely have attained by midsummer the pre-hibernation length of the most accelerated individuals.

During the growing season following their first hibernation period, the young grow to small adult size in most instances. After emerging from a second hibernation they mature s.e.xually and const.i.tute an important part of the breeding population.

Many of the skinks marked before their first hibernation, as hatchlings, when they were a few days or a few weeks old, were subsequently recaptured as well-grown yearlings or small adults, affording ample information as to the usual growth rate and the extremes of acceleration or r.e.t.a.r.dation that occasionally occur. Records of selected individuals in this group of skinks, marked early in life and recaptured after a hibernation, are recorded below.

Table 8. Records of Individual Skinks Marked as Hatchlings (Before the First Hibernation) and Recaptured the Following Year. Rapid Rate of Early Growth Is Shown.

========+=================+==========+=====================+======+============================= Snout-vent Weight Date length Tail length in in mm. in mm. grams Remarks --------+-----------------+----------+---------------------+------+----------------------------- No. 1. August 8, 1951 23-1/2 30-1/2 .25 Had just hatched when April 28, 1952 39 55 + 1/2 1.3 first recorded; second June 7, 1952 48 69 + 1 .... capture was made soon after emergence from hibernation. All three captures within a 50-foot diameter.

No. 2. July 8, 1952 25 25 (broken stub) .3 April 23, 1953 42 17 + 26 .... June 23, 1953 56 22 + 36 .... No. 3. July 16, 1948 26-1/2 37 .... Caught at the same place July 5, 1949 68 101-1/2 .... on both occasions; in a little less than a year this female grew to small adult size.

No. 4. August 23, 1950 36 55 .9 The interval between May 19, 1951 46 69-1/2 1.7 captures included about two months of active life, plus the hibernation period; caught at the same place on both occasions.

No. 5. September 2, 1950 34-1/2 33 (broken stub) .... Tail broken at first capture; June 12, 1951 45 48 + 3 2.0 recaptured 40 feet from original location.

No. 6. July 28, 1949 36 56 .... Recaptured 75 feet from April 21, 1950 49 83 2.5 original location.

No. 7. August 31, 1951 38 58 .... All three captures within May 25, 1952 48 82 .... a 70-foot diameter.

June 30, 1952 63-1/2 57 + 26 .... No. 8. August 23, 1950 36 44 (broken stub) .7 Tail broken at first capture.

July 23, 1951 69 37 + 49 .... Capture sites 150 feet apart.

No. 9. August 23, 1949 39 53-1/2 (regenerated) .... This male was r.e.t.a.r.ded June 7, 1950 46 70-1/2 (regenerated) 2.1 in growth, being still July 23, 1950 58 88 (regenerated) 3.7 well short of small September 3, 1950 62 91 (regenerated) 4.9 adult size as its second hibernation period approached; all four captures recorded within a few yards.

No. 10. July 31, 1949 38 23 (broken stub) .... Capture sites June 17, 1950 58 43 + 36 3.6 20 feet apart.

No. 11. August 13, 1949 40 66 .... Approximately a year August 8, 1950 63 90 (regenerated) 5.0 after its original record this skink was recaptured 80 feet away, still short of small adult size.

No. 12. August 19, 1949 42 40 (broken stub) .... All three captures within June 13, 1950 58-1/2 58 + 28 4.1 a 50-foot diameter.

July 5, 1950 63 62 + 31 5.9 --------+-----------------+----------+---------------------+------+-----------------------------

Many other young were not caught and marked until the growing season following their first hibernation, and were recaptured within this second growing season weeks or months after they were originally marked, and after they had made substantial growth. Those recaptured near the end of this second growing season, when they were a year old, or a little more, usually had attained small adult size or were nearing it.

Selected records of these yearlings are presented below.

Table 9. Selected Records of Individual Skinks Marked as Yearlings (After Emergence From the First Hibernation) and Recaptured One or More Times the Same Year. Rapid Growth Is Shown.

=======+==================+==========+=================+======+================= Snout-vent Weight Date length Tail length in in mm. in mm. grams Remarks -------+------------------+----------+-----------------+------+----------------- No. 1. May 2, 1951 38 53-1/2 .... Capture sites September 25, 1951 62 25 + 31 .... 30 feet apart.

No. 2. May 8, 1951 39 57 .... Capture sites August 2, 1951 60 67 + 25 .... 150 feet apart.

No. 3. April 17, 1952 39 55 1.1 Capture sites June 23, 1952 57 73 (regenerated) .... 30 feet apart.

No. 4. May 20, 1952 45 67 .... Capture sites May 28, 1952 47 71 .... 15 feet apart.

June 9, 1952 53 82 .... No. 5. May 22, 1952 48-1/2 77-1/2 2.0 Capture sites July 20, 1952 63 106 5.3 10 feet apart.

No. 6. June 11, 1950 49 49 (broken stub) 2.4 Capture sites September 2, 1950 63 63 + 31 4.9 20 feet apart.

No. 7. April 14, 1950 47 72 1.9 Capture sites May 29, 1950 50 82-1/2 2.5 50 feet apart.

No. 8. May 12, 1952 49 77 .... Capture sites June 18, 1952 61-1/2 98 .... 60 feet apart.

No. 9. June 4, 1950 54 89 2.8 Both captures at August 1, 1950 64-1/2 101 (broken stub) 5.7 same site.

No. 10. June 11, 1950 49 49 (broken stub) 2.4 Capture sites September 2, 1950 63 63 + 31 4.9 20 feet apart.

No. 11. June 13, 1949 57 68 (regenerated) .... August 8, 1949 70 37 + 11 .... -------+------------------+----------+-----------------+------+-----------------

Adult skinks can be found in greatest numbers in the breeding season and many of the young that were marked were recaptured as newly matured breeding adults soon after their second hibernation, often still short of average adult size. Selected records of such individuals are presented below.

Table 10. Records of Individual Skinks Marked as Young and Recaptured as Adults.

=======+===============+======+=====================+======+========================= Snout- vent Tail length Weight Remarks Date length in mm. in in mm. grams -------+---------------+------+---------------------+------+------------------------- No. 1. Male Probably less than a August 21, 1950 34 48 .7 month old at first May 30, 1952 69 37 + 49 .... capture; at second capture 21 months later and 185 feet away, he had red facial suffusion already somewhat faded as the breeding season waned.

No. 2. Male July 31, 1949 39 64 .... All three captures August 22, 1949 47 75 .... within a 70-foot May 19, 1951 73 69 (regenerated) .... diameter.

No. 3. Male August 5, 1949 36 57 .... Capture sites May 3, 1951 67 103 5.1 10 feet apart.

No. 4. Male June 16, 1951 44 41 (broken stub) .... Capture sites May 28, 1952 63 77 (regenerated) .... 535 feet apart.

No. 5. Male April 12, 1950 45 73 1.9 Capture sites May 1, 1951 67 17 + 48 .... 100 feet apart.

No. 6. Male This individual had April 12, 1950 46 4 + 15 1.3 attained approximately August 10, 1950 67 75 (regenerated) 5.3 average adult size by May 12, 1951 71 77 (regenerated) .... the 1951 breeding season; all three captures were within a distance of 90 feet.

No. 7. Male April 30, 1950 48-1/2 78-1/2 2.4 June 15, 1950 56 94 2.9 May 19, 1951 67 90 (broken stub) .... No. 8. Male May 3, 1950 47 51 + 4 1.7 Capture sites May 29, 1951 75 115 (regenerated) .... 450 feet apart.

No. 9. Male June 2, 1949 51 46 (broken stub) .... Capture sites May 2, 1950 66-1/2 31-1/2 + 51 7.0 90 feet apart.

No. 10. Male May 20, 1950 58 92-1/2 4.0 Capture sites June 21, 1950 61 95 4.7 within 40 feet.

August 21, 1950 70 108 (broken stub) 7.2 No. 11. Male June 25, 1950 62 100 5.1 May 1, 1951 71 113 7.1 No. 12. Female April 15, 1950 46-1/2 73-1/2 1.5 Capture sites May 20, 1951 72 113 .... 160 feet apart.

No. 13. Female June 11, 1950 51 69 2.5 Capture sites May 25, 1951 66 40 .... 20 feet apart.

No. 14. Female June 6, 1949 52 47 (regenerated) .... Capture sites May 20, 1950 68-1/2 69 (regenerated) 7.5 20 feet apart.

June 9, 1950 71 71 (regenerated) .... No. 15. Female July 2, 1950 60 100 4.2 Capture sites May 21, 1951 74 33 + 35 .... 20 feet apart.

No. 16. Female June 12, 1950 57 83 3.1 Capture sites May 1, 1951 71-1/2 53 (broken stub) 6.4 35 feet apart.

No. 17. Female This female probably June 22, 1949 62 24 (broken stub) .... hatched in July 1948 May 22, 1950 72 27 + 7 9.0 and was nearing adult size when first caught at an age of a little less than a year. By the next breeding season it was an average sized adult; both captures at same site.

No. 18. Female This female probably was July 4, 1950 64 30 + 55 4.3 Approximately a year May 23, 1951 73 31 + 62 .... old when first caught, and she grew to average adult size by the next spring; both captures at same site. No. 19. Female This female was about a July 5, 1950 61-1/2 92-1/2 (regenerated) 4.7 year old when first June 14, 1951 73 111 (regenerated) 8.2 captured; loss of weight June 29, 1951 74 106 (regenerated) 5.0 in July 1951 was caused by its laying a clutch of eggs. All three captures were within a 15-foot diameter. -------+---------------+------+---------------------+------+--------------------------

[Ill.u.s.tration: 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 r.e.t.a.r.dation, 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 s.e.xual 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.

[Ill.u.s.tration: 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 r.e.t.a.r.dation 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 cla.s.sified with certainty as to their age group, and might be either accelerated individuals about nine months old or r.e.t.a.r.ded 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.

[Ill.u.s.tration: 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 s.e.xual 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 of growth.