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 2-1/4 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.
Summary
Five-lined skinks were studied for four consecutive years in four small
areas, totalling approximately ten acres, on the University of Kansas
Natural History Reservation, Douglas County, Kansas. The information
gained from intensive study on these areas has been supplemented by data
from skinks collected elsewhere in northeastern Kansas, and from an
extensive literature pertaining to this species.
The genus _Eumeces_, to which the common five-lined skink belongs, has
more than 50 species, occurring throughout Central America, North
America to the latitude of southern Canada, and, in the Old World,
across southern Asia and North Africa. Within the genus, the five-lined
skinks, comprising a dozen species, form a natural group of closely
related forms. In this "_fasciatus_ group" nine of the species occur in
the Orient, Japan and neighboring islands and the adjacent mainland. The
remaining three, including _E. fasciatus_, occur in the eastern United
States. Specific differences are to be found in details of pattern,
scalation, and size, and, in some instances, they were long
unrecognized. _E. fasciatus_ coincides closely in its distribution with
the Deciduous Forest Biome of southeastern North America. An early
Tertiary deciduous forest in Alaska and probably in the Bering Strait
area, evidently growing in a humid, mild-temperate climate, included
genera of plants that are now most characteristic of southeastern North
America along with other kinds now characteristic of forest remnants in
southeastern Asia, and still others characteristic of the western United
States. The _fasciatus_ group seemingly dispersed from a northern center
that may have coincided with the early Tertiary deciduous forest of
Alaska.
_Eumeces laticeps_ almost coincides in distribution with _E. fasciatus_,
but does not occur quite so far north, and unlike _fasciatus_ it occurs
throughout Florida. Young are similar in appearance but _laticeps_ is a
larger, more powerful species, notably arboreal in its habits. _E.
inexpectatus_ much more closely resembles _fasciatus_, and ecological
divergence is slight. It is characteristic of hot and dry rocky areas in
open woods, and is more southern in distribution, although there is
extensive overlap with _fasciatus_ and _inexpectatus_ shares nearly all
of its range with _laticeps_.
_Eumeces fasciatus_ is most abundant in well-drained, open, rocky
situations within its forest habitat. It is scarce or absent in
bottomland forest that is subject to flooding and requires a forest with
openings in the leaf canopy so that sunshine patches for basking are
available. In northeastern Kansas, at least, woodlands that are browsed
by livestock, and have scanty undergrowth, provide better habitat than
those that are protected. _E. fasciatus_ is likely to be most abundant
in cutover woodland, and may reach greatest numbers in artificial
situations, such as old rock piles, or the vicinity of deserted
sawmills. In the north, the species is increasingly confined to open
situations, while in the south it may inhabit heavily wooded areas. An
abundant supply of moisture is a necessity and the species is limited to
a climate of high humidity. Dew normally supplies the source of drinking
water, without which the skinks rapidly become emaciated and die.
Optimum body temperature was determined to be near 34 deg.C., from a series
of temperature readings taken both under natural conditions and in
confinement under conditions permitting behavioral thermoregulation. By
thermoregulatory behavior, active skinks in the wild tend to maintain
their body temperatures near this level over a wide range of
environmental temperatures. They can tolerate body temperatures only a
few degrees higher, but, within a range of several degrees below 34 deg.C.,
efficiency is little impaired and incentive to make readjustment is
slight. At progressively lower temperatures skinks become slower and
less efficient. They are, however, capable of copulation at temperatures
down to 21 deg.C, and of feeding at 16 deg.C. At 10 deg.C. they are slow and clumsy,
barely capable of normal locomotion. At temperatures near freezing they
are torpid; they can survive temperatures a little below freezing, but
cannot survive being frozen solid. More than half the year is spent in
hibernation in northeastern Kansas. Weight loss is slight during
hibernation.
Normally the skinks emerge from hibernation in early April in
northeastern Kansas, several weeks earlier in the southern states and
correspondingly later in the northern part of the range. Maximum
activity occurs in the period of weeks following emergence; interrupted
from time to time by cold weather which necessitates return to
torpidity. After approximately three weeks of activity the adults attain
breeding condition. Breeding males acquire a salmon red suffusion of the
head region. They become pugnacious and fight on sight. Fighting does
not involve territorial defense. In confinement males may mutilate or
kill each other. In their search for females, and fighting, the breeding
males are so much more active and conspicuous than they are at other
times of year that published descriptions usually refer to males as
red-headed, with no cognizance of the fact that this condition exists
for only a few weeks in the annual cycle. Old adult males lose the
striped pattern and blue color of the tail of the young, and are golden
brown, usually a little darker on the sides. Males find females by a
combination of sight and scent. Sexual relations are promiscuous, and
there is but little courtship behavior. The male pursues the female and
grasps in his jaws loose skin at or behind her shoulder region, and
maintains this hold during copulation which lasts about five minutes.
Within a few days after insemination, usually in early May, females
become actively hostile to males. In late May or early June the gravid
females become unusually secretive and excavate nest burrows in damp
soil under flat rocks, or in rotten wood of decaying logs and stumps.
The single annual clutch of eggs is laid in June. The average clutch is
somewhat more than nine eggs, with larger and older females slightly
exceeding younger and smaller females in average productivity. The
female remains in the nest burrow with her clutch most of the time, from
laying until after hatching. She alters the nest burrow, dampens it in
time of drought, keeps the cavity from being filled with loose soil,
prevents the eggs from adhering to the sides or floor of the cavity, and
she may repulse certain small predators capable of destroying the eggs
if they were left undefended. When they are laid, the eggs are
approximately 11 x 7-1/2 mm. and weigh .4 grams or a little less. By
hatching time they have enlarged to 15 x 11 mm. and each weighs about a
gram. Recorded incubation periods vary from 27 days to 47 days;
development of the embryo is slowed at low temperatures, and eggs
experimentally kept in a refrigerator at 11 deg. to 12 deg.C. for periods of
days hatched later than others of the same clutch that developed under
normal conditions, indicating that development was almost halted in the
eggs kept at such low temperatures. Eggs are, however, tolerant of a
wide range of temperature, and can develop in nearly dry soil, or can
survive partial submersion in water for at least two days. Under weather
conditions prevailing in 1951, incubation periods of about six weeks
were recorded. Incubation may be shortened by retention of ova in the
oviducts in early stages of embryonic development. Hatching may occur
from the first week of July to mid-August, but in any one year most
clutches hatch within two weeks of each other. Hatching of eggs in a
clutch extends over a day or two. The hatchling gradually becoming
active inside the egg, slits the leathery shell with its egg tooth, and
spends several hours in the early stages of emergence. After resting
with head and shoulders protruding, becoming adjusted to the outside
environment and gaining strength, it lunges from the egg. For a day or
two after hatching, the young remains in the nest, being slow and
feeble, and handicapped in its movements by the protruding belly
distended by the yolk mass. The female usually remains in the nest
cavity for a day or two after the eggs hatch, showing affinity for the
young by curling around them protectively. Family ties are broken as
soon as the young leave the nest, and they do not return. Hatchlings
average a little less than an inch in snout-vent length, and have a
sharply defined five-lined pattern on a black ground color, and vivid
blue tail. Hatchlings make rapid growth in late summer, and by the time
of their retirement into hibernation, the more successful may have
doubled in length, and may have increased their original weight, of
approximately .3 or .4 grams, more than eight-fold. After emergence from
their first hibernation the young continue their rapid growth. When they
are a year old, some of them are as large as small adults. However, they
can usually be distinguished from adults by the more sharply defined
pattern. These grown young retain the hatchling pattern but the contrast
between stripes and ground color, and between body and tail is not quite
so sharp. Especially in those with regenerated tails, the vivid blue of
the hatchling's tail has become much dulled. By the time they retire to
their second hibernation, the young have mostly grown to small adult
size. A small percentage are retarded in their growth and fail to
mature. Upon emergence from their second hibernation, the grown young
mature sexually and participate in the annual breeding season, in early
May, and they may comprise the majority of breeding adults. The ratio of
new adults to old adults however varies from year to year depending on
the varying fortunes of successive annual broods. The new adults overlap
older ones in size, but are usually distinguishable on the basis of
their coloration, as they retain the striped body pattern (dulled,
especially on the head) and with distinctly blue color on the tail. In
skinks that are three years old or more, the dorsal stripes have become
obscured and partly blended with the ground color, which becomes
progressively paler with advancing age. Metamorphosis is most complete
in old males, which retain no trace of the stripes or of the blue color
on the tail. Old females usually retain the dark lateral area, and the
tail is usually bluish gray, with a blue scale remaining here and there
if the tail has not been regenerated.
Most individuals lose their original tails, however. By the time the
young are approximately two months old, about half have had their tails
broken, and by the time they are a year old and have grown to small
adult size, three-fourths have regenerated tails. Some individuals may
have had their tails broken and regenerated many times. Tails regenerate
rapidly and most of the growth is made within the first few weeks. The
regenerated tail is not so long as the original lost portion.
Individuals tend to stay within small areas which are their regular home
ranges. These ranges are only a fraction of an acre in extent, but vary
considerably in size and shape according to the individual and the
situation. Home ranges of approximately 90-foot diameter for adult males
and young, and a little more than 30-foot diameter for adult females are
indicated. A home range generally centers about some environmental
feature providing shelter and food, such as a log, hollow tree, or rock
outcrop. Activity tends to be concentrated in the central part of the
home range. An individual may continue to occupy the same home range
throughout its lifetime, or it may gradually alter its range, shifting
by slow stages into a new area. Some individuals seem to "get lost" or
voluntarily shift, and settle in a new area which may be hundreds of
feet removed from the original range. Shifts are most likely to occur
after emergence from hibernation, when the lizard finds its habitat
somewhat altered. Individuals released in areas strange to them settled
down and established new home ranges, either immediately or after brief
wandering.
The five-lined skink is a predator, occasionally taking small
vertebrates (lizards and possibly newborn mice) but depending for most
of its food on invertebrates. Of these it takes a wide variety. Spiders
are the mainstay of the diet, and various salticids and lycosids are the
kinds most frequently preyed upon. Phalangids are also eaten. Of
insects, orthopterans (including roaches, ceuthophilid and gryllid
crickets, grouse locusts, and small grasshoppers) are most important in
the food. Larvae of moths and both larvae and adults of beetles are also
taken in quantity. Small snails make up an important part of the diet,
and the skinks often eat their own sloughed skins. Less frequently
taken food items include certain large ants, centipedes, moths, and
miscellaneous insects. Rarely the adult skinks may even eat eggs or
young of their own species. Prey is found by sight and scent, and
consists of almost any small animals within a certain size range (small
enough to be swallowed entire, but large enough to make up a substantial
part of a meal) of types which are not too heavily armored, lack noxious
defensive secretions, and live on or in the ground or decaying wood.
They are ordinarily crushed in the jaws or battered against the ground,
and then swallowed entire.
Natural enemies of the five-lined skink certainly include the
broad-winged hawk and probably other kinds of hawks and also include
various predatory mammals such as the opossum, armadillo, skunks, moles
and shrews; snakes (the copperhead, milk snake, king snake, corn snake,
and probably others), the Sonoran skink and even the closely related but
larger _Eumeces laticeps_. On the Reservation, the short-tailed shrew
was certainly the commonest, and probably by far the most important
natural enemy. A high proportion of the skinks examined had scars
resembling those inflicted by shrews experimentally confined with skinks
in captivity. In time of danger escape reactions vary according to the
type of enemy and the attending circumstances. Frequently an alarmed
skink may escape into a hole or crevice, running directly to it from a
distance of several yards. Under other circumstances a skink may burrow
into ground litter of dry leaves and other debris, or may even dive and
hide underwater. One of the commonest escape reactions is climbing tree
trunks. It occurs even in gravid females that are slow and clumsy, being
weighed down with eggs. Generally the skink stays on the main trunk of
the tree, attempting to conceal itself by utilizing the screening
vegetation that is available. In the young, especially, the bright blue
tail seems to be used as a decoy, for it is carried, arched high and
waved conspicuously as the lizard moves about. Rapid lashing movements
of the conspicuous tail as the animal darts erratically for shelter may
serve to confuse a pursuer, at least as to the direction that the skink
has taken.
This skink is parasitized by various helminths, both cestodes and
nematodes, which inhabit the digestive tract and body cavity. Some of
these infest many kinds of amphibian and reptilian hosts, but others may
be confined to the five-lined skink. Their life cycles, and effect upon
the host are not well known. Ectoparasites consist principally of
chiggers. Three kinds have been recorded on the skinks; _Trombicula
alfreddugesi_, _T. splendens_, and _T. gurneyi_. The first two are
common pest chiggers which attack humans as well as a wide variety of
other mammalian, avian and reptilian hosts. _T. gurneyi_ is a less
common species found mainly on _Eumeces fasciatus_ and confined to its
woodland habitat.
LITERATURE CITED
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1909. Notes on some salamanders and lizards of North Georgia. Science,
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1945. Principal game birds and mammals of Texas. Texas Game Fish
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1950. Studies in late Tertiary paleobotany. Carnegie Inst. Wash., Publ.
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1950. The reptiles of Big Black Mountains, Harlan County, Kentucky.
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1940. _Eumeces_ in the Miami area. Copeia, 1940:129.
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1949. Thermoregulation in reptiles, a factor in evolution. Evolution,
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1927. Notes on the amphibians and reptiles of the Dismal Swamp.
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