beside station P4b. Chewed cones of pinyon pine and chewed juniper seeds
were on the ground. A small nest of shredded juniper bark, and a few
leaves of serviceberry, were found inside the log. Chewed pinyon nuts
and juniper seeds also were present in the nest.
[Illustration: FIG. 16: (above) Photograph of juniper log at station
G3d, which contained the nest of _P. truei_ # 241.]
[Illustration: FIG. 17: (below) Photograph of dissected juniper log
at station G3d, showing the nest of _P. truei_ # 241, at end of
mattock handle. The nest of shredded juniper bark contained
chewed seeds of juniper trees.]
No. 267, _P. truei_, juvenile. This male ran into a fallen juniper log
40 feet southwest of station P7a and then disappeared into a hole
leading under an adjacent rock. Dissection of the log revealed many
chewed juniper seeds inside and beneath the log, but no nest. I did not
overturn the large rock or excavate under it.
No. 268, _P. truei_, adult. This pregnant and lactating female ran into
a hollow branch of a partly-dead juniper tree 10 feet south of station
O7d. The limb and base of the tree were hollow, and there were large
numbers of chewed juniper seeds nearby. Because of time limitations, the
branch was not dissected.
No. 74, _P. maniculatus_, juvenile. This female ran into a small
circular hole in the ground 13 feet north of station J3a. Excavation
revealed that this hole led into the abandoned tunnel of a pocket gopher
(_Thomomys bottae_). The tunnel was followed for about four feet, but no
nest was found and the tunnel led under a thicket of oak brush which
made further excavation impractical (Fig. 15).
No. 247, _P. maniculatus_, adult. This male was followed to a large nest
situated at the base of a stump and under a juniper log lying beside the
stump, five feet from station I2c. This large nest was built on the
ground and was constructed of grasses (_Poa fendleriana_, _Stipa
comata_, and _Koeleria cristata_), and contained a few leaves of Gambel
oak. It was the largest nest found. Chewed pinyon nuts were in the nest.
(Fig. 15).
No. 276, _P. maniculatus_, juvenile. This male ran into a small hole at
the base of a dead juniper tree 40 feet north of station O2c. It would
have been necessary to cut the tree down to uncover the nest, and this
was not deemed to be worthwhile.
The preceding accounts indicate that, in Mesa Verde, nests of _P. truei_
usually are associated with hollow juniper logs or branches. In one
instance a nest of _P. truei_ was found on the ground, under a rock.
Shredded juniper bark, and, in one case, grasses were the materials most
commonly used for nest building.
Individuals of _P. maniculatus_ did not build nests in trees. One nest
was found under a stump and adjacent log. Another site was in the
abandoned tunnel of a pocket gopher, and a third was under a large rock.
The only nest that was unquestionably built by a _P. maniculatus_ was
constructed of grasses and a few leaves.
It seems unlikely that competition for nesting sites between the two
species of _Peromyscus_ affects the local distribution of each species.
The analysis of nesting sites suggests that _P. truei_ is restricted, in
Mesa Verde, by the availability of fallen logs, hollow branches, or
hollow trunks of juniper trees. My observations lead me to think that
within the pinyon-juniper woodland there is a surplus of nesting sites
for individuals of _P. truei_. Many juniper trees have dead branches,
and hollow juniper logs are abundant throughout the forest. It is
inconceivable to me that the population of _P. truei_ could reach
densities sufficient to saturate every nesting site available to them in
the trapping grid.
Sagebrush areas, or brushy zones adjacent to the pinyon-juniper woodland
usually do not contain juniper logs; when hollow juniper trees or logs
are not available, _P. truei_ is not found as resident of such areas. As
mentioned earlier, individuals of _P. truei_ may venture into such areas
to feed if they are adjacent to pinyon-juniper woodland.
An individual of _P. truei_ may have more than one nest within its home
range (for example Nos. 105 and 241 cited above). Each mouse probably
has refuges, each containing a nest, strategically located in its home
range. Thus, if a mouse is chased by a predator, or by another mouse, it
need not return to its main nest, but can seek refuge in one of its
secondary nests. These secondary nests were small and were invariably
constructed from shredded juniper bark. Some of these nests were little
more than a scant handful of shredded bark that formed a platform to sit
upon. Other nests were larger and ball-shaped, with one opening on the
side. All of the secondary nests that were found were inside hollow
juniper logs. The bark used in construction of the nests had, in each
case, been transported from nearby living trees. The logs had previously
lost their bark through decay.
The evidence indicates that these secondary refuges are prepared with
considerable care. Not only is the bark transported for some distance,
but it is shredded into a soft mass of fibers. When a mouse first
establishes itself in a new area, perhaps it begins several such nests
before settling upon the most favorable site. The less desirable sites,
if still within the animal's range, are then available (barring
competition by a new inhabitant) for outlying refuges.
My data do not indicate whether individuals of _P. maniculatus_ use a
similar arrangement of nests within their home ranges. The population of
_P. maniculatus_ was sparse in the trapping grid, and the habitat these
mice occupied was such as to make following them extremely difficult.
In captivity, both species constructed nests that were indistinguishable
to me, when the mice were given cotton, kapok, or pieces of burlap as
building material. The cotton or kapok was used directly, but the burlap
was shredded into a fine mass of fluffy fibers. The burlap seemed to me
to be the best building material, for it maintained its shape best.
Both species constructed nests that resembled inverted bowls. Solitary
mice naturally built smaller nests than those built by females with
young.
The entrance to the closed nests varied; often the female would bolt
through the side of the nest where there was no opening. Sometimes the
mice would exit and enter through the top of the nest. In some cases it
appeared that the entire nest was closed; probably the occupant had
closed the entrance. Such a closed nest would have the advantage of
greatly moderating the microenvironment within the nest, and would allow
the animal within to remain comfortable with a minimum expenditure of
energy. The larger nests found in the trapping grid resembled those
built by captives. Nests built of grasses were always larger than those
built of juniper bark. Juniper bark is as easily worked into nests as
are grasses, in my judgment. Therefore, difficulty of construction of
nests from this material probably does not account for the smaller size
of the nests composed of bark. I think the difference in insulating
characteristics between the two materials probably accounts for the
difference in size of the nests.
REPRODUCTION
In Mesa Verde, _Peromyscus_ reproduces from April through September.
Reproduction is greatly reduced in the autumn, and most females complete
reproduction before October.
Ten of the 20 females of _P. maniculatus_, taken in May, contained
embryos; five others were lactating. Lactating and pregnant females were
collected on May 5, 1962, indicating that reproduction in some females
began in early April. In September, 15 of 34 females were pregnant or
lactating, whereas in October only two out of 15 females of _P.
maniculatus_ were reproducing. Only one female of _P. maniculatus_ was
found to contain embryos in October. This large adult was taken on
October 3, 1963, and had six embryos, each five millimeters long. She
probably would have produced a litter later in October, and would have
been nursing into November. A report of October breeding in
north-central Colorado described nine of 23 females of _P. maniculatus_
as being in a reproductive state; seven were lactating and one was
pregnant between October 26 and 31, 1952 (Beidleman, 1954:118).
In the Museum of Natural History, the University of Kansas, there are 35
females of _P. maniculatus_ more than 144 millimeters in total length
taken from Mesa Verde in November, 1957 (Anderson, 1961:53). None of
these contained embryos, and no pregnant females have been taken from
the park in November.
_P. truei_ and _P. maniculatus_ reproduce at about the same time. A
female of _P. truei_ prepared as a specimen on May 10, 1964, contained
four embryos, each 20 millimeters long, indicating a breeding time in
mid-April. Svihla (1932:19) reported the gestation period for
non-lactating _P. truei_ to be 25 to 27 days and for lactating
individuals, 40 days. Lactation tends to increase the gestation period
of other _Peromyscus_ by about five days (Asdell, 1964:266). The
gestation period of nine non-lactating females of _P. m. rufinus_ was
reported by Svihla to be 23 to 24 days. Lactation increased the length
of the period of gestation in this subspecies to between 23 and 32 days
(mean for seven females 26.57 +- 0.73, Svihla, 1932:19).
Females of _P. truei_ were observed in various stages of reproduction
from June through September. Ten of the 20 females of _P. truei_ taken
in September were reproducing; four contained embryos and the other six
were lactating. In October, only one of 17 females caught in snap traps
was lactating. Lactating females were caught in live-traps as late as
October 23, although most females had ceased reproduction by then. No
pregnant or lactating females were observed in November.
In _P. maniculatus_, puberty has been placed at 32 to 35 days for
females weighing 13 grams, and in males at from 40 to 45 days, at
weights of 15 to 16 grams (Jameson, 1953:45). In _P. truei_, the weight
of the testes is reported to rise in March and diminish through
September, with accessory organs following the same cycle (Asdell,
1964:267). Young of _P. truei_ nurse for about one month, although some
litters may not be weaned until 40 days of age. Young of _P.
maniculatus_ are weaned between 22 and 37 days of age (Svihla, 1932:30).
Twenty-six pregnant females of _P. maniculatus_, taken in the breeding
seasons of 1961-1964, contained from one to eight embryos each; the mean
was 4.65 +- 1.67. Other investigators have found similar mean values in
this species (Asdell, 1964:266).
Thirteen females of _P. truei_ taken in the breeding seasons of
1961-1964, contained from three to six embryos each; the mean was 4.0 +-
.912. Svihla (1932:25) reported litter sizes, at birth, of two to five
and a mean of 2.84, in 19 litters. Other investigators have reported
litter sizes of one to five with a mean of 3.4, and one to six with a
mean of 3.6 (Asdell, 1964:268). Apparently _P. truei_ does not have more
than six young per litter.
In captivity, females of both species began reproduction in early
February. These captives had been kept for several months at a
temperature of 21 degrees Centigrade, and on a daily photoperiod of 15
hours. Some captive males had enlarged, scrotal testes in January; the
extended photoperiod and warm temperature probably influenced the
breeding condition. In both species testes of wild males caught in
autumn after late September and on through the winter were abdominal,
except for one male of _P. maniculatus_ which had enlarged, scrotal
testes on October 15.
Dates at which different animals arrived at breeding condition varied,
in part owing to subadults (young of the year) appearing in the catch
from early summer to late autumn. Some adult females appeared to be
pregnant or lactating throughout much of the summer and early autumn,
whereas other females, that were caught a number of times, apparently
reproduced only once in the summer.
Some females may fail to breed even though they are mature enough to do
so. One female of _P. truei_ captured eight times (August 30 to
September 20) was a juvenile when first caught, and was classed as young
(in postjuvenal molt) on September 10. She did not reproduce in her
first breeding season, unless she did so after September 20, which is
unlikely. Another female of _P. truei_ was an adult when first caught,
and was caught 12 times (August 21 to October 25). At no time were her
mammae enlarged and she was not lactating or pregnant. It is improbable
that she reproduced earlier in the season, for teats of mice that have
reproduced earlier usually are enlarged to such a degree that previous
parturition is clearly indicated. It was surprising to catch a female,
of any age, 12 times in two months without sign of reproductive
activity.
Only one female of _P. maniculatus_ did not show reproductive activity.
She was a juvenile on July 19 when first caught; a subadult on August 28
when caught the third time, and an adult on October 23 when caught the
fifth time.
Burt reported a rest period of a month or more in the summer, in
Michigan, during which many females of _P. leucopus_ did not reproduce.
They began to breed again in late summer at about the time when young of
the year began reproducing (Burt, 1940:17, 19). Abundant mast was
correlated with reproductivity in autumn, according to Jameson
(1953:54), who thought that "food is a basic determinant of the autumn
reproduction" of _P. leucopus_.
Little has been written about the length of time males remain in
breeding condition. Difficulties in determining breeding condition are
many. Fertility customarily is determined by sectioning testes and
noting the presence or absence, and relative abundance, of sperm. This
procedure necessarily sacrifices the individual and indicates the
breeding condition at only one moment and for only the individuals
sacrificed. My observations of males caught a number of times in live
traps shed some light on the breeding condition of males, but the
investigator is likely to err in extrapolating physiological data from
morphology when he notes whether the testes are abdominal or scrotal and
whether they are enlarged or small. It was assumed that testes that have
not descended, and that lie within the abdominal cavity, are not capable
of producing viable sperm. This is the condition in most juveniles, and
in all males during winter. As the breeding condition is attained,
testes descend into the scrotum. Soon the testes and their accessory
organs enlarge and are readily apparent.
Howard (1950:320) reported that numerous males of _P. leucopus_ sired
litters when their testes appeared to be abdominal, and therefore
questioned whether the criterion of descended testes is valid as an
indicator of breeding condition. My captive males of _P. maniculatus_
and _P. truei_ did not sire litters when their testes were abdominal,
even though such males were left with adult females for as long as four
to five months (August through December). Captive pairs of both species
yielded no evidence of reproductive activity until January when, as
mentioned earlier, some of the males had scrotal testes. Young were born
first in early February, although their parents had been confined
together since the preceding August. Jameson reported the testes of
fecund males of _P. maniculatus_ as almost always 8.0 millimeters or
larger (Jameson, 1953:50). Testes that are at least partly scrotal must
be considered as being capable of producing motile sperm, even though
this may not be the case for all individuals.
Toward the beginning and end of the breeding season the testes and
accessory organs of wild mice were small and probably produced few if
any sperm. At these times some males apparently were so frightened by
being handled that the testes were retracted into the inguinal canals.
It would have been easy to consider such males as having abdominal
testes when in fact they did not. In such cases the scrotum usually was
noticeably enlarged; it was found also that in many cases the testes
returned to the scrotal position if the mouse was held gently for a few
minutes. Careful handling of animals was found to prevent, or at least
retard, retraction of the testes. Retraction of the testes from the
scrotum was not a problem at the height of the breeding season when the
testes were engorged.
I had originally assumed that all adult males would be fertile
throughout the breeding season, and that any males with abdominal testes
would be subadults or young of the year. This assumption was an
oversimplification; all adult males did not reach breeding condition at
the same time of year. My data do not support a firm conclusion, for it
is difficult to follow non-captive individuals throughout a breeding
season, owing to sporadic appearance of animals in traps. Nevertheless,
observations of mice that were trapped a number of times indicated the
following:
1) Some adult males that had abdominal testes in the middle of July
reached breeding condition as late as late August and even late
September.
2) Some juvenal males had scrotal testes at the time their postjuvenal
molt was just beginning to be apparent on their sides. Most juvenal
males did not have scrotal testes, and many juveniles that appeared
repeatedly in traps from mid-July through late October did not attain
breeding condition. A mouse that was a juvenile in mid-July must have
been born in mid-June.
3) Apparently animals born early in the breeding season may reproduce
later in that season, whereas those born later in the breeding season
tend not to breed until the following year.
Possibly cooler evening temperatures in July and August, due to the
relatively larger amounts of precipitation in those months, inhibit
reproductive development of late-born young. Most plants have ceased
vegetative growth and have produced seeds by this time; but the
interrelationships between growing seasons, climatic conditions, and
reproductive physiology are unknown.
Only one adult of each species had scrotal testes after late September;
the _P. truei_ had scrotal testes on October 24, 1963, and the _P.
maniculatus_ had scrotal testes on October 15 of that year.
GROWTH
Growth of captive _P. maniculatus_ and _P. truei_ is discussed in
several reports. One of the most complete is that of McCabe and
Blanchard (1950) on _P. m. gambelii_ and _P. t. gilberti_ in California.
A detailed discussion of the dentition in _P. truei_ and wear of the
teeth in different age groups is given by Hoffmeister (1951). Molt in
these species has been considered by a number of authors (Collins, 1918;
McCabe and Blanchard, 1950; Hoffmeister, 1951; Anderson, 1961). The
report by McCabe and Blanchard is valuable because molt is compared
between the two species from the first to the twenty-first week of
postnatal development.
[Illustration: FIG. 18: Scatter diagram of postnatal growth of
captive mice, showing increase in length of bodies from birth to
70 days of age. The records for _P. truei_ represent 11
individuals of five litters; those for _P. maniculatus_ represent
17 individuals of four litters.]
The thoroughness of the above-mentioned studies is readily apparent to
those who have worked with mice of the genus _Peromyscus_. Nevertheless,
the ecology of local populations of _P. maniculatus_ and _P. truei_ as
reported for the San Francisco Bay area (McCabe and Blanchard, 1950) has
little relationship to the ecology of mice of other subspecies of these
species, in southwestern Colorado. Indeed, the preferred habitats, and
to some extent the behavior, differ strikingly in Colorado and
California.
[Illustration: FIG. 19: Graphs showing postnatal growth of solitary
captive individuals of _P. truei_ and _P. maniculatus_,
representing the only young in each of two litters.]
Figures 18 and 19 show that some litters grow appreciably faster than
others, but the end results are about the same. Since the young were
measured at irregular intervals, statistical procedures for calculating
confidence limits of the curves were not applicable.
Solitary young reared by one female of each species, attained maximum
size more rapidly than animals having litter mates (Fig. 19).
Nevertheless, solitary individuals and individuals from litters all
reach essentially the same size 50 days after birth.
The gestation time of _P. truei_ is several days longer than that of _P.
maniculatus_, and the young of _truei_ are fewer and heavier than those
of _maniculatus_. As would be expected, _truei_ remains in the nest
longer and nurses longer than _maniculatus_.
Young of each species grow rapidly for the first month, and attain, in
that time, the largest percentage of their adult size; they grow rapidly
up to sometime between the thirtieth and fiftieth days. Thereafter the
rate of growth diminishes and the animals begin to gain weight rather
than continuing to extend the lengths of the body and appendages.
Figure 19 reveals that the appendages of young _maniculatus_ attain most
of their length about a week earlier than those of _truei_. Young
_truei_ acquire mobility and coordination somewhat later than young
_maniculatus_, but both species are seemingly equal in these respects by
about the end of the second week.
Length of gestation period, number and size of embryos, amount of time
spent in the nest, and time required for bodily growth are all of major
importance in determining the relative success of _truei_ and
_maniculatus_. These parameters will be considered further in the
discussion.
PARENTAL BEHAVIOR
In the laboratory, pregnant females were supplied with either kapok,
cotton, or a piece of burlap with which to make a nest. The kapok or
cotton was used directly by the mice in constructing a hollow, compact,
moundlike nest. When burlap was used for nest building, the female first
completely frayed the cloth by chewing it into a fluffy mass of fibers.
When the top of a nest was opened to inspect young, the female would
attempt to pull the nesting material back into shape by means of
forefeet and teeth. The mother's defensive posture was to cover the
young with her body, often lying over them and facing upward, toward the
investigator. In this semi-recumbent position, the female would attack
the investigator's fingers with her forefeet and teeth. Often the female
would stand bipedally and use the forefeet and teeth to mount the
attack. If at this time a young chanced to wander away from the mother,
she would quickly pick it up and place it in the nest at her feet.
When disturbed, females of both species, but especially _P.
maniculatus_, often dove headlong under their nest or into the wood
shavings on the floor of the cage. This type of retreat was most often
used when young were nursing. Time is required even by the mother to
disengage nursing young, and this mode of escape is the most expedient.
The mother disengaged nursing young by licking around their faces and
pushing with her paws.
Nursing females of both species tolerated the male parent in the nest. A
male and female often sat side by side in the nest and by means of their
bodies participated in covering the young. Males were not observed to
attempt any defense of the nest, or of the young. Females were tolerant
of older young in the nest when another litter was born and was being
nursed. In one nest, a female of _P. truei_ gave birth to a litter of
three when her older litter was 29 days old. The three older young
continued to nurse until they were 37 days old, at which time they were
removed from the cage. The female appeared tolerant of this nursing by
members of the older litter, but appeared to give preference to the
wants of the younger offspring.