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Small rodents, when confined in traps, are particularly vulnerable to extremes of heat, cold and moisture. The ultimate success of an ecological study involving the capture and release of small mammals often depends upon protection of mammals from these adverse conditions. Plague ecology studies at the San Francisco Field Station involve repeated captures of rodents ranging in size from the comparatively large ground squirrels, Citellus , to the tiny harvest mice, Reithrodontomys . The smaller rodents such as Microtus, Peromyscus, Reithrodontomys and Mus are especially sensitive to the elements and must be protected from them until released. …

Members of the pocket mouse genus, Perognathus, withstand whole-body ir:adiation doses up to 1400 rads without appreciable mortality (1). Several other wild rodents-notably Peromyscus polionotus, P. gossypinus, Sigmodon hispidus, Reithrodontomys humulis, and the Mongolian gerbil, Meriones unguiculatus-are now listed as exhibiting a degree of radiation resistance approaching that of Perognathus (2, 3). The experiments reported here were designed to study the question of radiation resistance in Perognathus. The work was done in six parts as reported herein, and an attempt is made to amalgamate the results of the various experiments in a common discussion.

The specific identificatioin of the fulvous harvest mouse (Reithrodontomyx fglvexcen) in the vicinity of Pittsburg, Kansas has been in doubt. Although this locality is within the probable range set forth by Hall and Kelson (1959) no specimens have been reported taken from Crawford County. Long and Wiley (1961) referred material from owl pellets found near Pittsburg to R. megalotis. Long (1961) reported R. montanvx from McDonald County Missouri and R. montv1Zvx has also been collected in Ottawa County Oklahoma (KU 81948). A specimen (KSCP 620-B) of R. fulvexcenx was collected on 17 March 1966 at the Kansas State College of Pittsburg Natural History Research Reserve. This adult male mouse weighed 13.0 grams and had standard measurements of 158 X92 X 20 X 12 mm. This is the first recorded specimen of R. fglvexcenx collected from Crawford County.

Complete lack of pigment or pied coloration is common in some mammals; in others, such as the genus Reithrodontomys , these conditions are lacking or at least unreported. The western harvest mouse, Reithrodontomys m. megalotis (Baird), now may be added to the long and steadily growing …

Unpublished data by Wooster(12) estimated that during that year there were approximately 7367 Microtus, 1251 Peromyscus and 1529 Reithrodontomys per square mile (640 acres) near Hays, Kansas on the college pasture and adjoining areas. Studies on the effect of drouth on animal population in western Kansas were made by Woostero1-11 and theMicrotus decreased from 2500 per square mile to a point where no signs of their activity could be detected. The identification of rodents and rabbits by their fecal pellets was made by Webb(9) at Fort Hays State College, Hays, Kansas. This information was valuable in organizing data collected for this manuscript in determining food habit studies of the rodents.

Four eastern harvest mice, Reithrodontomys h. humulis , were tagged with gold-198 wires and traced in the field by the use of a Geiger counter. This method is superior to live-trapping because multiple-handling of animals is eliminated, animal movements are unrestricted, and more information about an individual can be obtained in a shorter length of time. When mice were released from live-traps their movements during the following several hours were suggestive of meandering, indicating that they may have been confused, frightened, or hungry from overnight trap confinement. Small globular-shaped grass nests located in or under the herbaceous layer of vegetation were used as den sites. The nests were situated on the periphery of the range and varied from 2 to 4 per animal. Harvest mice movements were mostly limited to night, but during winter occasional short diurnal movements from nests were recorded. Certain areas within the range of each tagged mouse were avoided completely. The behavior and movements of the tagged mice presented a complex picture of spatial relationships. Calculation of range size was not undertaken because presently used methods and concepts were difficult to apply to the data.

Four specimens of the fulvous harvest mouse, Reithrodontomys fulvescens (Allen), were captured by us on three successive nights, January 27–30, 1957, 22 miles south and two miles east of Rodeo, Hidalgo County, New Mexico. The site of capture is located in the Peloncillo Mountains at the point where the Geronimo Trail (the road between Douglas, Arizona and the Animas valley, New Mexico) crosses the crest of the range at an altitude of approximately 5,900 feet. The animals were trapped …

The gestation period of Reithrodontomys h. humulis appears to be approximately 21 days. The sizes of nine litters born in the laboratory ranged from two to four, with a mean of 3.2. A mean of 1.2 grams was recorded from the weights of 27 young at birth. The reproductive efficiencies of two females were 50.4 and 50.8%, close to the highest known reproductive efficiency of any mammal. Harvest mice at birth appear unpigmented, are pink, have a translucent skin, and have tiny, white natal hairs present on most parts of the body. By the end of the first week, the brownish hair on the head and nape almost completely hides the underlying skin. All young are weaned by the end of the third week and are cared for only by the female parent. Parent mice occasionally kill and eat their young and on one occasion two 30-day-old females were killed and eaten by their two litter mates. Males get along well with females, although males rarely tolerate each other in the same cage. Females are exceedingly tolerant of members of their own sex. 7 references, 5 figures, 1 table.

Age structure of populations of Reithrodontomys megalotis longicaudus, Reithrodontomys raviventris raviventris , and Reithrodontomys raviventris halicoetes was investigated through analysis of skulls aged by tooth wear. Age structure is similar among the three taxa. Age classes are not clearly delimited. There is no mass die-off of older adults during a specific period of the year. A long breeding period provides for recruitment into the population over an extended period of time. Sex ratios greatly favor males except in the youngest and oldest of the six age classes recognized. Selection for more males may be advantageous in small mammal populations with organizational types such as are found in these Reithrodontomys .

Two 15-acre areas were selected in a typical cutover, loblolly-shortleaf pine forest near Ruston, Louisiana for a live-trap study. One objective of the study was to determine the effect of controlled burning on small mammals. One hundred live traps were used on each site, and the length of the trapping periods varied from five to six days. The study was begun in January, 1959 and ended in April, 1961. Species trapped included Peromyscus nuttalli, Peromyscus gossypinus, Blarina brevicauda, Reithrodontomys fulvescens , and Microtus pinetorum . All mammals captured were marked for later identification and released. Peromyscus nuttalli and Peromyscus gossypinus were recaptured in sufficient numbers to calculate home range and density per acre. The mean home range of Peromyscus nuttalli was 1.31 acres on the control plot and 1.07 acres on the burned plot. The mean home range of Peromyscus gossypinus was 1.2 acres on the control plot and .74 acres on the burned plot. The highest trap-revealed densities for these two species were 2.79 Peromyscus nuttalli per acre and 1.22 Peromyscus gossypinus per acre. The burn on 23 February 1959 had a negligible effect on the small mammals and was followed by a decrease in trap-revealed rodents for less than a month. Food habit studies by the author have indicated that small mammals are consumers of pine seeds in the loblolly-shortleaf pine association of Louisiana and thus affect pine regeneration. The study was designed to determine the species, abundance, movements and fluctuations of small mammals in a typical cutover, loblolly-shortleaf pine forest, and to evaluate the impact on animal life of the removal of low vegetation and forest litter by fire.

Reithrodontomys humulis in the Oak Ridge, Tennessee, area is found mainly in certain habitats dominated by grasses and other herbaceous plants characteristic of early vegetational succession. Populations seldom are dense, but on a 4 ha, old-field area, trapping success ranged from about 4 to 18 individuals per 100 trap nights for 18 consecutive months. This population on the old-field area disappeared during severely cold weather in February-March of 1960, but a few individuals survived on a radioactive area. More females than males were captured in the periods June-November 1958 and May-November 1959, whereas more males were caught from December 1959 to February 1960. However, the sex ratio was about equal over a period of a year or more. The reproductive season extended from about late spring to late fall, but breeding occurred during winters of some years. Litter sizes ranged from 1-8 (av. 3.4). Cannibalism and abandonment of litters were characteristic of newly caught females. Average weights for females during reproductive periods were consistently greater than average weights of males, but even during nonreproductive periods females weighed as much or more than males. Weights of trapped individuals ranged from 3-17 g. Mortality of nestlings appeared to be severe, but survival after leaving the nests seemed to be relatively long. Huddling was common during cold weather. Territoriality seems poorly developed. Some individuals occupied relatively restricted ranges for periods of several months, but other mice shifted their activities from one part of an area to another. Ranges of the mice were less during the colder months than during the warner months. Only heterosexual pairs of mice were ever trapped together, and these captures usually occurred during breeding periods. Erythrocyte count and mean corpuscular volume in the harvest mouse were not significantly different from the measurements for larger mice, but hemoglobin concentration and oxygen consumption were greater than in the larger mice.

Measurements were made of hemoglobin concentration, hematocrit ratio, mean corpuscular hemoglobin concentration (MCHC) and erythrocyte diameter in 34 species of wild mammals from widely separated areas in the United States and Canada representing 5 orders, 10 families and 25 genera. Included in these determinations were measurements on two races of Tamias striatus , two races of Oryzomys palustris , two races of Peromyscus leucopus and three races of Peromyscus maniculatus . Differences in blood values related to species, age, body size and other factors were evident. Hemoglobin concentrations, hematocrit ratios and MCHC's showed a correlation with body size, smaller species generally having higher blood values. A number of exceptions were noted which were probably related to season, altitude, behavior or special physiological adaptations. Members of the families Soricidae and Vespertilionidae which include many small-bodied forms with high rates of metabolism generally had the highest blood values. Pregnant and lactating females usually had lower mean blood values than non-gravid females and males, but no significant differences were found between adult non-gravid females and males. Hemoglobin and hematocrit values in juveniles were often significantly lower than in adults. There was some indication that erythrocyte size was larger in juveniles than in adults, and this may have compensated in some unexplained way for the lower hemoglobin and hematocrit values. No significant differences were noted between the blood values of immatures (subadults) and adults. Seasonal differences in mean blood values were observed in cotton rats ( Sigmodon hispidus ), harvest mice ( Reithrodontomys fulvescens ) and deer mice ( Peromyscus leucopus , P. maniculatus and P. boylii ). Lower blood values in summer in cotton rats could possibly be correlated with either increase in weight or increase in the ambient temperature of the environment during this season. A tendency for high-altitude forms to have higher mean blood values than forms living at low altitudes was noted, but it was inferred from the data that the differences were probably mainly due to differences in ambient temperature. High blood oxygen capacities associated with high hemoglobin and hematocrit values in species of small body size with high metabolic rates would be a distinct advantage in increasing the delivery of oxygen to the tissues. The advantage of increased blood oxygen capacity of the smaller species coupled with a higher carbonic anhydrase activity in their red cells is discussed.

Populations of fulvous harvest mice, Reithrodontomys fulvescens, were studied on three areas in Nacogdoches Co., Texas. One area in a pine forest and another in a grass-pine plantation provided comparative population data from 1955-1963. Data secured from the third area, located in a grass-pine ecotone, were useful in determining microhabitat distribution and movements of the differcnt sex and age groups. Fulvous harvest mice occurred predominantly in grassland, pinegrass ecotone, and grass-brush habitat. They were significantly absent from pine-type habitat, possibly because this habitat was occupied by cotton mice, Peromyscus gossypinus, and golden mice, Ochrotomys nuttalli. Fulvous harvest mice were most abundant in winter and to a lesser extent in summer. Peak populations occurred in 1957 in the grass-pine plantation and in 1958 in the adjacent pine forest. The exclusive boundary strip method of analysis of trap range data revealed that harvest mice range over approximately one-half acre. Females had a slightly larger trap range (= home range), .59 acre, than did males, .47 acre; young females averaged greatest linear movements. Neither sex displayed marked territorial behavior. Actual and estimated densities revealed approximately three fulvous harvest mice per acre in the pine-grass ecotone area (probably the optimal area of the three for these mice). There were slightly more males than females on all the areas studied. On the pine-grass ecotone, there were significantly greater numbers of adult males than adult females. Two principal breeding peaks, one in March and a second in July, were detected. Fulvous harvest mice seemed extremely tolerant of one another when confined under laboratory conditions. Mice born in captivity became independent of the female at approximately 3 weeks (the time of weaning). Under controlled laboratory conditions, captives displayed an activity rhythm seemingly correlated to the onset of dark periods. In the absence of well-defined light-dark cycles, activity occurred at times of natural darkness.

Eastern harvest mice, Reithrodontomys humulis humulis (Audubon and Bachman), were tagged with subcutaneously inserted 20 gauge gold-198 wires 10 mm long and varying in activity from 0.7 to 4.5 mc. None of the tags inserted by this method were lost or had any apparent effect on the animals. The movements of the tagged mice were successfully traced with a portable Geiger counter.

On March 22 and 23, 1956, small mammal trapping in Whiteside County, 1 mile west of Denrock in northwestern Illinois, resulted in the capture of Reithrodontomys . The selected trapping sites included a railroad right-of-way and an adjacent bluegrass pasture which contained a dry marsh. The pasture contained numerous clumps of bluegrass and an occasional bull thistle. The dry marsh had a band of sedges and cattails around the perimeter with infrequent large clumps of marsh smartweed toward the center. The railroad right-of-way, which is burned each year during late March and early …

Body temperature and metabolism were examined in Scotinomys teguina and S. xerampelinus during 2.5-hour exposures to ambient temperatures between 0 and 35° C Thennal conductance at temperatures below thermoneutrality averaged 0.34 cc O2/g/hr/°C in the smaller species, teguina , and 0.28 cc 02/g/hr/°C in xerampelinus . The difference in mean conductance between species is statistically significant. During exposure to ambient temperatures near 35° C, deaths occurred in xerampelinus but not in teguina . In the range 0 to 5°C, hypothermia was more common in teguina than in xerampelinus . These differences in thermophysiology between the species are correlated with altitudinal distribution; xerampelinus is restricted to cool or cold highlands and teguina is found at lower, warmer altitudes. Although both are montane, tropical species, they adhere to broad patterns of thermoregulatory physiology established in studies of North American peromyscines of the genera Peromyscus, Reithrodontomys , and Baiomys .

In southeastern Arizona the yellow-nosed cotton rat, Sigmodon ochrognathus, inhabits grassy, rocky slopes, in or near the oak belt in the same habitat as Thomomys umbrinus and Reithrodontomys fulvescens. The cotton rats form extensive surface runway systems con- necting surface nests and burrow openings. Surface nests, constructed of grass and placed in dense thickets, are 7 to 16 in. in diameter. Abandoned burrows of Thomomys umbrinus are used by cotton rats and appear to. be a limiting factor in their distribution. Wild animals are much more active during daylight hours than at night. However, one captive male ran an exercise wheel an average of 31,836 ft per night for 100 consecutive nights. In the wild, S. ochrognathus feeds extensively on blue grama; in captivity a diet of salad greens and dry lab chow is adequate. Caged animals do not require free water if fresh plant material is provided every 2 or 3 days. Females breed as early as 45 days of age and 2 individuals had gestation periods of 33 and 36 days. Litter size varies from 2 to 6 and young can be weaned by the 15th day, probably sooner. The furred newborn are precocial with the eyes opening during the first 24 hr. At 2 days of age they can climb the hardware-cloth sides of the cage. Growth is rapid. After 100 days, there is little' increase in total and tail-length; after 40 days, in hind foot-length. By 2 V/2 month of age, the skull has attained over 90% of adult size in greatest length, zygomatic breadth, mastoid breadth, and length of upper toothrow. Estimations of the ages of wild-taken specimens indicate that animals rarely live for more than 4 months in the wild since 96% are less than 4 months old and 83% are less than 2V2 months old. 'Characters for distinguishing S. ochrognathus from S. minimus and S. hispidus in Arizona are listed. Certain distinguishing features of the baculum, interparietal, foramen ovale, and foramen magnum have not been previously reported. The yellow-nosed cotton rat, Sigmodon ochrognathus, has a limited range in the Southwest and, judging from the few specimens in collec- tions, is an uncommon animal. Our own recent field work in Arizona has enabled us to recognize the signs and habitat of this species and once aware of these, we were able to obtain numerous specimens from 15 localities in southeastern Arizona. Discussions of the habitat, be- havior, growth, and general characteristics of this species are based upon field observations in Arizona and upon 7 wild-caught animals and