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.
