Part 11

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 22 deg.C. 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 assigned by Taylor, seemed plausible. However, in the light of present knowledge, it is certain that the seeming intervals between his assumed 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 retardation 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 skiltonianus 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 skiltonianus belong to groups separate from each other and from that including fasciatus. While septentrionalis and skiltonianus resemble each other in their growth pattern and in the time required to reach sexual 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, skiltonianus 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 cumulative 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 retarded 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 sexually and constitute 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 retardation 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 retarded |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. | -------+---------------+------+---------------------+------+--------------------------

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.

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.

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 of growth. 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.

============+====================+==================== | 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 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.