Population structure obviously differs from place to place and from time to time. Because of the differences in secretiveness and elusiveness between young and adults and between males and females, true sex ratios and age ratios are obscured. In the period of weeks between the emergence from hibernation and the onset of the breeding season, these skinks tend to be less secretive than at other times, and secondary sexual and age differences in behavior are minimized. A sample at this season should be more representative of the true population composition than samples taken at other times of year. In a sample of 308 skinks available for the month of April, including the collections made on the Reservation and on nearby areas, in 1949, 1950, 1951, and 1952, 36.7 per cent were adult males, 28.3 per cent were adult females, and 35.0 per cent were young. That these figures cannot, however, be accepted as an accurate indication of the population composition is shown by the data from the areas where intensive population studies were made. Data are most complete from Skink Woods. For 292 adults taken there over a four year period, the sex ratio was 100:122.6. On this area after the first year of study a substantial proportion of the individuals recorded were repeaters from one year to the next, and in some cases for three or even four successive years. Many could be definitely assigned to a known age group. By analogy the majority of others could be tentatively assigned with some assurance on the basis of measurements, and relatively few were of indeterminate status. By assigning each of these indeterminate individuals to one or another age group, on the basis of greatest probability, the approximate composition of the population could be determined. Of 611 adults, 55 per cent were “two-year olds” (in the season between their second and third hibernations, which was their first breeding season). The percentage was not significantly different in the two sexes.

On the average, a pair of adults produces somewhat more than nine eggs per year. From the time individuals of a brood start their development in the egg until they are breeding adults two years later, they undergo such drastic reduction in numbers that, on the average, approximately only one per brood survives. Most of the mortality probably occurs early, especially before hatching, also in the inexperienced hatchlings, and in the first hibernation. In spring, after emergence from hibernation, young are generally taken in smaller numbers than are adults. Their relative scarcity is only apparent, owing to greater secretiveness, and greater elusiveness when found. In spring, newly matured adults (age class about 21 months) may be taken in somewhat larger numbers than young (age class about 9 months). The latter obviously must be more numerous, in a stable population however, as the 21 month age class necessarily has sustained some loss since it was 9 months old.

Success of the annual brood varies greatly from year to year, depending on the weather and various other factors. In 1949 evidently conditions were near optimum; young hatched early and were especially numerous in late summer. In 1950 these young hatched in 1949 made up 40 per cent of the total catch (excluding hatchlings) in Skink Woods and were relatively more numerous than young of the corresponding age group in other years. In 1951, these young of the 1949 brood, grown to adults, made up 70 per cent of the breeding populations, as against 36 per cent for the corresponding class in 1950 and 58 per cent for the corresponding class in 1952.

Even after attainment of adulthood, any given age group evidently is subject to annual reduction amounting to at least half its numbers. Within six or seven years, at the most, the original numbers would be reduced to an insignificant percentage. At an age of four or five years individuals probably have attained their maximum size, with obscured pattern and changed proportions suggestive of advanced age. Occasional individuals possibly attain much greater age, but certainly few live more than five years. Like most small animals, the five-lined skink has a short life expectancy and a rapid population turnover. As compared with mammals of comparable sizes, the small rodents and insectivores that are this lizard’s community associates and are subject to many of the same hazards, the skink is notably successful, with a much longer life expectancy. For these small mammals the life span is seldom as long as a year. Most kinds of small birds likewise have a life expectancy less than that of the five-lined skink, although somewhat greater than that of small mammals.

Density

The population density changes constantly, following an annual cycle with gradual reduction to its lowest ebb in late June or early July, then rapid increase to a high point a few weeks later when hatching of the single annual brood has been completed. In a normally successful breeding season the population is at least doubled, but reproductive success varies from year to year, as the population responds to weather conditions that are favorable or unfavorable, even where the environment remains fairly stable. In most places, however, local populations continue upward or downward trends for periods of years in response to successional changes which cause progressive improvement or deterioration of local habitats. Local populations are likely to be more or less isolated from others by areas where the habitat does not exist. Even in an area of favorable habitat such as a wooded hillside of several acres, the population is not at all evenly distributed, but concentrations occur along rock outcrops, and about decaying logs, or stone piles. In intervening areas lacking such abundant shelter, and less productive of food, the population is sparse, or there may be no permanent residents.

In view of these traits, and the difficulty of obtaining a representative sample, no precise measurements of population density can be made. During the time required to secure a sample, the population undergoes change. At the pond rock pile, an area of approximately .05 acre, the skinks were found in remarkably high concentrations, 57 in 1949, 85 in 1950, 37 in 1951, and 51 in 1952. These numbers represent population densities of, respectively, 1120 per acre, 1960 per acre, 746 per acre, and 1000 per acre. No such concentrations were found elsewhere, and probably do not occur in natural habitat. The Skink Woods study area of 2¹/₄ acres is typical of favorable habitat in the region of the study, and the numbers taken there are more significant. For 1949 the 74 skinks recorded comprise an incomplete sample, and the population density of 33 per acre represented is certainly somewhat too low. For other years the following population densities (exclusive of hatchlings) are indicated: 1950, 92 per acre; 1951, 61 per acre; 1952, 49 per acre. These figures are only approximate, of course, and it is difficult to judge how accurately they reflect the true numbers. Even the most intensive collecting may be insufficient to obtain every individual on a small area. Within each season there are shifts of range by some individuals, off the study area and corresponding shifts onto it by others, so that the numbers caught in the course of an entire season are somewhat too high. The individuals taken on the study area may regularly range beyond its boundaries to some extent, so that the seeming population density is somewhat too high. Actually this was probably a minor source of error for the Skink Woods study area, as nearly half its perimeter was bordered by an open field uninhabitable for the skinks, and the remaining perimeter adjoined areas much less favorable than the central portion.

Census of the population of the study area by a ratio such as the “Lincoln Index” used in game management studies was scarcely practicable because of the changing seasonal habits distorting the recorded ratios of the sexes and of age groups somewhat differently at different stages of the season. These changing ratios tend to produce an erroneously high population figure, unless separate computations are made from the data for adult males, adult females, and young. Census figures obtained by this method were erratic but seemed to bear out in a general way, the population figures based on total numbers of individuals taken.

In favorable habitat where they occur in high populations of 50 to 100 per acre in spring, these lizards must attain a biomass of a pound or more per acre. Biomass in a population probably fluctuates but little during the course of the annual cycle, even though the number of individuals changes greatly. The steady elimination of individuals through various mortality factors, is compensated for by rapid growth of the young.