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.