Graphs of the condylobasilar lengths...Fig. 13.Graphs of the condylobasilar lengths, body lengths and weights of a series of voles of known age. Within each age group, the youngest vole is on the left in the graphs.
Fig. 13.Graphs of the condylobasilar lengths, body lengths and weights of a series of voles of known age. Within each age group, the youngest vole is on the left in the graphs.
When other cranial measurements, and ratios of pairs of measurements, were plotted in the same order, individual variation obscured some of the variation due to age and the curves resembled those of weight and length of body rather than that of condylobasilar length. When the cranial measurements were averaged for the age groups the curves showed a relationship to age but the relationship of mean measurements is of little use in determining the age of individual specimens. The data described above indicated that a study of the relationship of the condylobasilar length and age in a large sample might provide useful information.
Anyone who has examined mammalian skulls knows of many other characters which vary with age but which are difficult to measure and describe with precision.Figs 14and15are drawings of skulls of voles of known age. The most obvious change, related to aging, evident in the dorsal view of the skulls (Fig. 14) is the increasing prominence and closer approximation of the temporal ridges in older specimens. The lambdoidal ridge is also more prominent in older voles, and their skulls have a generally rougher and more angular appearance. The individual variation evident in these ridges is probably due to variations in the development of the muscles operating the jaws (Howell, 1924:1003). There is an increased flattening of the roof of the skull of older voles.
Dorsal views of skulls of voles of known ageFig. 14.Dorsal views of skulls of voles of known age. (Ages 11⁄2, 21⁄2, 3, 31⁄2, 4, 41⁄2, 6 and 12 months). All × 3.View larger image
Fig. 14.Dorsal views of skulls of voles of known age. (Ages 11⁄2, 21⁄2, 3, 31⁄2, 4, 41⁄2, 6 and 12 months). All × 3.
View larger image
Palatal views of skulls of voles of known ageFig. 15.Palatal views of skulls of voles of known age. (Ages 11⁄2, 21⁄2, 3, 31⁄2, 4, 41⁄2, 6 and 12 months). All × 3.View larger image
Fig. 15.Palatal views of skulls of voles of known age. (Ages 11⁄2, 21⁄2, 3, 31⁄2, 4, 41⁄2, 6 and 12 months). All × 3.
View larger image
From a palatal view (Fig. 15) the skulls of voles also showed agevariation which was apparent but not easily correlated with precise age. The median ridge on the basioccipital bone increases in prominence in older voles. The shape of the posterior margin of the palatine bones changes from a V-shape to a U-shape. On the skull of the oldest (12 months) vole the pterygoid processes are firmly fused to the bullae, a condition not found in any of the other specimens. The anterior spine of the palatine approaches the posterior projection of the premaxillae more closely as age increases and, in the oldest vole is firmly attached and forms a complete partition separating the incisive foramina.
Tooth wear during the life of a vole causes a considerable variation in the enamel patterns, especially of the third upper molar. Howell (1924:1012) considered such variation to be independent of age, but Hinton (1926:103) related the changes to age and interpreted them as a recapitulation of the evolution of microtine molars. In my series, an indentation on the medial margin of the posterior loop of the third upper molar seemed to be related to age. This indentation was absent in the youngest vole (one and one-half months), absent or indefinite in those voles less than 31⁄2months of age, and progressively more marked in the older voles.
The prairie vole, like other members of the genusMicrotus, feeds mostly on growing grass in spring and summer. Piles of cuttings in the runways are characteristic sign of the presence of voles. The voles cut successive sections from the bases of grasses until the young and tender growing tips are within reach. The quantity of grass destroyed is greater than that actually eaten, a fact which will have to be considered in any attempt to evaluate the effects of voles upon a range.
In all piles of cut plants that were examined,Bromus inermiswas the most common grass, andPoa pratensiswas the grass second in abundance. These were, by far, the most common grasses present on the areas studied; in most places,B. inermiswas dominant. Other grasses present on the areas were occasionally found in the piles of cuttings. Jameson (1947:133-136) found no utilization ofB. inermisby voles but that grass was present in a relative abundance of only one per cent in the areas studied by him. The voles that he studied ate alfalfa in large amounts and alfalfa was, perhaps, the most common plant on the particular areas where his voles were caught. Seemingly, the diet of voles is determined mostly by the species composition of the habitat.
Other summer foods included pokeberries, blackberries and a few forbs and insects. Forbs most commonly found in the piles of cuttings were the leaves of the giant ragweed (younger plants only) and dandelion. Insect remains were found in the stomachs of voles killed in summer and occurred most frequently in those killed in August and September. At no time did insects seem to be a major part of the diet but they were present in most vole stomachs examined in late summer. Laboratory experiments with summer foods gave inconclusive results but suggested that the voles chose grasses on the basis of their growth stage rather than according to their species. Young and tender grasses were chosen, regardless of species, when various combinations ofTriodia flava,Bromus inermisandPoa pratensiswere offered to the voles. Attimes the voles showed a marked preference for dandelion greens, perhaps because of their high moisture content; the voles' water needs were satisfied mostly by eating such succulent vegetation.
Winter foods consisted of stored hay and fruits and of underground plant parts.Bromus inermismade up nearly all of the hay and was stored in lengths of up to ten inches in underground chambers specially constructed for storage. Underground parts of plants were reached by tunnelling and were an especially important part of the voles' diet in January and February. The fruit ofSolanum carolinensewas eaten throughout the winter and one underground chamber, opened in February, 1952, was packed full of these seemingly unsavory fruits. Fisher (1945:436), in Missouri, found this fruit to be an important part of the winter diet of voles. An occasional pod of the honey locust tree was found partly eaten in a runway. Fitch (1953,in litt.) often observed girdling of honey locust and crab apple (Pyrus ioensis) root crowns on the Reservation but I saw no evidence of bark eating, perhaps because my study plots were mostly grassland. On two occasions when two voles were in the same trap one of them was eaten. In both traps, all of the bait had been eaten and the captured voles probably were approaching starvation. Because the trapping procedure offered abundant opportunity for cannibalism, the low frequency of its occurrence suggested that it was not an important factor in satisfying food requirements under normal conditions.
Perhaps the most characteristic sign of the presence ofMicrotus ochrogasterwere their surface runways and underground tunnels. Only rarely was a vole observed to expose itself to full view. When a trapped vole was released it immediately dove out of sight into a runway. Once in a runway, the vole showed no further evidence of alarm and was usually in no hurry to get away. The runways seemed to provide a sense of security and the voles were familiar with their range only through runway travel. The urge to seek a runway immediately when exposed has obvious survival value.
Surface runways were usually under a mat of debris. In areas where debris was scanty or lacking, runways were usually absent. Jameson (1947:136) reported that in alfalfa and clover fields the voles did not make runways as they did in grassland, even in fields where trapping records showed voles to be abundant. Typical surface runways are approximately 50 mm. wide, only slightly cut into the ground and bare of vegetation while in use. Usually they could be distinguished from the runways of the pine vole, which were cut more deeply into the ground, and those of the cotton rat which were wider and not so well cleared of vegetation. Some runways ended in surface chambers and some of these were lined with grass. Their size varied from a diameter of 90 mm. to 250 mm. and they seemed to be used primarily for resting places.
A runway system usually consisted of a long, crooked runway and several branches. Two typical systems are illustrated inFig. 16. The runway systems often were not clearly limited; they merged with other systems more or less completely. One map showed a runway system extending across 140 square meters and including 12 underground burrows. All of these runways seemed to be part of a single runway system but the system probably was used by more than one vole or family group of voles. Sixteen of the 22 maps that were made extendedacross areas between 50 and 90 square meters. One map, mentioned above, was larger and the remaining five smaller. The smallest extended across only 20 square meters. Of course, the area encompassed by a set of runways changed almost daily, as the voles extended some runways, added some and abandoned others in the course of their daily travels.
Maps of runway systemsFig. 16.Maps of runway systems of the prairie vole. The runways follow an irregular course and are frequently changed. The solid lines represent surface runways and the dotted lines underground passages.
Fig. 16.Maps of runway systems of the prairie vole. The runways follow an irregular course and are frequently changed. The solid lines represent surface runways and the dotted lines underground passages.
Each runway system contained underground nests. These were in chambers from 70 mm. to 200 mm. below the surface and were up to 200 mm. in diameter. Most systems that were mapped had from two to six of these burrows. Most of these were lined with dried grass and seemed to be used for delivering and nursing litters. Each burrow was connected to a surface runway by a tunnel. Often the tunnel was short and the hole opened almost directly into the burrow from the surface runway. Others had tunnels several meters long. Jameson (1947:137) reported every burrow to have two connections with the surface. In the present study, however, I found three arrangements in approximately equal frequency of occurrence: (1) one hole to one tunnel leading to a burrow; (2) two holes to two short tunnels which joined a long tunnel leading to a burrow; and (3) two separate tunnels from the surface to a burrow. The size, depth and number of underground burrows in the systems that I studied varied and so did those reported in the literature. Jameson (loc. cit.) found burrows in eastern Kansas as deep as 18 inches, far deeper than any found in my study. Fisher (1945:435) reported none deeper than five inches in central Missouri. The soil data in my study, as well as in the two reports cited immediately above, were not adequate to permit conclusions, but the type and condition of the soil probably determine the extent of burrowing by the voles of any given locality.
The number of voles using a runway system at one time was difficult to ascertain. In one system, however, four adult individuals were trapped in a ten day period. In August, 1952, at the conclusion of the live-trapping program, a runway system was mapped which had included two trapping stations. In the preceding ten days, four adult voles (three males and one female) had been taken in both traps. During that time, therefore, the runway system was shared by at least four voles. The voles used an area that was considerably larger than that encompassed by any one runway system, a fact obvious when the sizes of home ranges as computed from trapping data were compared with the sizes of the runway systems mapped. A runway system seemed not to be a complete unit, but was only a part of the network of runways used by a single individual.
Although no special investigation of activity was made, some conclusions concerning it were apparent in the data gathered. There have been a few laboratory studies of the activity pattern ofMicrotusby various methods. Calhoun (1945:256) reportedM. ochrogasterto be mainly nocturnal with activity reaching a peak between dark and midnight and again just before dawn. Davis (1933:235), working withM. agrestis, and Hatfield (1935:263), working withM. californicus, both found voles to be more nocturnal than diurnal. In a field study ofM. pennsylvanicus, Hatt (1930:534) found the species to be chiefly nocturnal, although some activity was reported throughout the day. Hamilton (1937c:256-259), however, reported the same species to be more active in the daytime. Agreement on the activity patterns of these species ofMicrotushas not yet been attained.
From occasional changes in the time of tending a trap line, and from running lines of traps at night a few times in the summer of 1951, I gained the impression that these voles were primarily diurnal. Relatively few of them were caught in the hours of darkness. In summer, however, their activitywas mostly limited to the periods between dawn and approximately eight o'clock and between sunset and dark. In colder weather, there was increased activity on sunny days.
Although voles were a common item of prey for many species of predators on the Reservation, no marked effect on the density of the population of this vole could be attributed to predation pressure. Only when densities reached a point that caused many voles to expose themselves abnormally could they be heavily preyed upon. Their normally secretive habits, keeping them more or less out of sight, suggest that they are an especially obvious illustration of the concept that predation is an expression of population vulnerability, rising to high levels only when a population is ecologically insecure, rather than a major factor regulating population levels (Errington, 1935; 1936; 1943; Erringtonet al, 1940).
Scats from predatory mammals and reptiles and pellets from raptorial birds were examined. Most of these materials were collected by Dr. Henry S. Fitch, who kindly granted permission to use them. The results of the study of the scats and pellets are summarized inTable 5. Remains of voles were identified in 28 per cent of the scats of the copperhead snake (Ancistrodon contortix) examined. Copperheads were moderately common on the Reservation (Fitch, 1952:24) and were probably important as predators on voles in some habitats. Uhleret al(1939:611), in Virginia, reported voles to be the most important prey item for copperheads. A vole was taken from the stomach of a rattlesnake (Crotalus horridus) found dead on a county road adjoining the Reservation. Rattlesnakes were present in small numbers on the Reservation but were usually found along rocky ledges rather than in areas where voles were common (Fitch,loc. cit.). The rattlesnakes probably were less important as predators on voles than on other small mammals more common in the usual habitat of these snakes. The blue racer (Coluber constrictor) was common in grassland situations on the Reservation (Fitch, 1952:24) and twice was observed in the role of a predator on voles; one small blue racer entered a live-trap in pursuit of a vole and another blue racer was observed holding a captured vole in its mouth. The blue racer seems well adapted to hunt voles and probably preys on them extensively. The pilot black snake (Elaphe obsoleta) has been reported as a predator onM. ochrogasterin the neighboring state of Missouri (Korschgen, 1952:60) and was moderately common on the Reservation (Fitch,loc. cit.).M. pennsylvanicus, with habits similar to those ofM. ochrogaster, has been reported as a prey for all of the above snakes (Uhler,et al, 1939).
Table 5. Frequency of Remains of Voles in Scats and Pellets
PredatorNo. of scats or pellets examinedNo. containing remains of volesPercentageCopperhead25728Red-tailed hawk25312Long-eared owl251872Great horned owl32619Crow25416Coyote25312
The red-tailed hawk (Buteo jamaicensis), the long-eared owl (Asio otus),the great horned owl (Bubo virginianus) and the crow (Corvus brachyrhynchos) fed onMicrotus. All four birds were fairly common permanent residents on the Reservation (Fitch, 1952:25). The low density and the strict territoriality of the red-tailed hawk (Fitch,et al, 1946:207) prevented it from exerting any important influence on the population of voles, even though individual red-tailed hawks ate many voles. Predation by the long-eared owl was especially heavy; remains of voles were identified in 72 per cent of its pellets examined. Korschgen (1952:39) found remains of voles in 70 per cent of 704 pellets of the long-eared owl. The reason for the heavy diet ofMicrotusseems to be that both the owl and the vole are especially active at dusk. A group of long-eared owls, living near the edge of Quarry Field, probably exerted an influence on the density of the local population of voles because of the high ratio of predator to prey animals. The crows ate some, and perhaps most, of their voles after the animals had died from other causes. Other birds, mostly raptors, occurring in northeastern Kansas and reported to prey on voles include the sharp-shinned hawk (Accipiter striatus), Cooper's hawk (A. cooperi), red-shouldered hawk (Buteo lineatus), broad-winged hawk (B. platypterus), American rough-legged hawk (B. lagopus), ferruginous rough-legged hawk (B. regalis), marsh hawk (Circus cyaneus), barn owl (Tyto alba), screech owl (Otus asio), barred owl (Strix varia) and shrike (Lanius excubitor) (Korschgen, 1952:26; 28; 34; 35; 37; McAtee, 1935:9-27; Wooster, 1936:396).
Coyotes, house cats and raccoons were identified as predators on voles in the study areas. Remains of voles were present in 12 per cent of the scats of the coyote (Canis latrans) examined. In Missouri, Korschgen (1952:40-43) reported remains of voles in slightly more than 20 per cent of the coyote stomachs that he examined. Fitch (1948:74), Hatt (1930:559) and others have reported other species ofMicrotusas eaten by the coyote. Although coyotes were rarely seen on the Reservation, coyote sign was abundant (Fitch, 1952:29) and coyotes probably ate large numbers of voles. House cats (Felis domesticus), seemingly feral, were observed to tour the trap lines on several occasions and were noted by Fitch (loc. cit.) as important predators on small vertebrates. Four cats were killed in the course of the study and remains of voles were found in the stomachs of all of them. On several occasions, raccoon tracks were noted following the trap line when the traps had been overturned and broken open, suggesting that raccoons are not averse to eating voles although no further evidence of predation on voles by raccoons was obtained. Fitch (loc. cit.) reported raccoons (Procyon lotor) to be moderately common on the Reservation. Reports of predation by raccoons on voles are numerous (Hatt, 1930:554; Lantz, 1907:41). The opossum (Didelphis marsupialis), common on the Reservation, occasionally eats voles (Sandidge, 1953:99-101). Other mammals which are probably important predators on voles on the Reservation, though no specific information is available, are the striped skunk (Mephitis mephitis), spotted skunk (Spilogale putorius), weasel (Mustela frenata) and the red fox (Vulpes fulva). Eadie (1944; 1948; 1952), Shapiro (1950:360) and others have reported that the short-tailed shrew (Blarina brevicauda) was an important predator onMicrotus. Shrews were present on the Reservation but were not trapped often enough to permit study.
The variety of vertebrates preying on voles suggests that they occupy a position of importance in many food chains. Errington (1935:199) and McAtee (1935:4) refer to voles as staple items of prey for all classesof predatory vertebrates. An attempt to evaluate prey species was made by Wooster (1939). He proposed a formula which involved multiplying the density of a species, its mean individual weight, the fraction of the day it was active and the fraction of the year it was active to give a numerical index of prey value. Although his methods of determining population densities would now be considered questionable, the purpose of his investigation merits further consideration. He reportedM. ochrogasterto be second only to the jack-rabbit (Lepus californicus) as a prey species in west-central Kansas.
In the course of live-trapping operations several species of small mammals other thanMicrotus ochrogasterwere taken in the traps. Also, from time to time, direct observations of certain mammals were made and various types of sign of larger mammals were noted. These records gave a picture of the mammalian community of which the voles were a part. The three associated species which were most commonly trapped wereSigmodon hispidus,Reithrodontomys megalotisandPeromyscus leucopus. These three species have been commonly found associated withMicrotusin this part of the country (Fisher, 1945:435; Jameson, 1947:137).
The Texas cotton rat,Sigmodon hispidus, was the most commonly trapped associate of the voles between November, 1950, and February, 1952. Although a greater number of individuals of the harvest mouse were taken in a few months, the cotton rat had a greater ecological importance because of its larger size (Figs 17,18,19). The cotton rat was an especially noteworthy member of the community for two reasons. It has arrived in northern Kansas only recently and its progressive range extension northward and westward has attracted the attention of many mammalogists (Bailey, 1902:107; Cockrum, 1948; 1952:183-187; Rinker, 1942b). Secondly,Sigmodonhas long been considered to be almost the ecological equivalent ofMicrotusand to replace the vole in the southern United States (Calhoun, 1945:251; Svihla, 1929:353). Since the two species are now found together over large parts of Kansas their relationships in the state need careful study.
Variations in density and mass of three common rodents on House FieldFig. 17.Variations in density and mass of three common rodents on House Field. The upper graph shows the sum of the biomass of the three rodents. In the two lower graphs the solid line representsMicrotus, the broken lineSigmodon, and the dotted lineReithrodontomys.
Fig. 17.Variations in density and mass of three common rodents on House Field. The upper graph shows the sum of the biomass of the three rodents. In the two lower graphs the solid line representsMicrotus, the broken lineSigmodon, and the dotted lineReithrodontomys.
Variations in density and biomass of three common rodents on House FieldFig. 18.Variations in density and biomass of three common rodents on Quarry Field. For key, see legend ofFig. 17.
Fig. 18.Variations in density and biomass of three common rodents on Quarry Field. For key, see legend ofFig. 17.
Variations in density and biomass of three common rodents on House FieldFig. 19.Changing biomass ratios of three common rodents on House Field and Quarry Field. In late 1951 and early 1952 the cotton rats attained relatively high levels and seemingly caused compensatory decreases in the numbers of voles. The solid line representsMicrotus, the broken lineSigmodon, and the dotted lineReithrodontomys.
Fig. 19.Changing biomass ratios of three common rodents on House Field and Quarry Field. In late 1951 and early 1952 the cotton rats attained relatively high levels and seemingly caused compensatory decreases in the numbers of voles. The solid line representsMicrotus, the broken lineSigmodon, and the dotted lineReithrodontomys.
Both this study and the literature (Black, 1937:197; Calhoun,loc. cit.; Meyer and Meyer, 1944:108; Phillips, 1936:678; Rinker, 1942a:377; Strecker, 1929:216-218; Svihla, 1929:352-353) showed that, in general, the habitat needs ofMicrotusandSigmodonwere similar. Studies on the Natural History Reservation, both in connection with my problem and otherwise, suggested, however, thatSigmodonoccurred in only the more productive habitat types used by voles, where the vegetation was relatively high and rank. On the Reservation the cotton rat was found mostly in the lower meadows; they were more moist and had a more luxuriant vegetation than the higher fields. Although a few cotton rats were taken in Quarry Field and still fewer in Reithro Field, the population of those hilltop areas did not approach, at any time, the levels reached on House Field, which produced a more luxuriant cover. Only when the levels of population were exceptionally high did the cotton rats spread into less productive habitats. At all times, there were areas on the Reservation used byMicrotuswhich could not support a population ofSigmodon.
The cotton rats reacted differently to the floods of July, 1951, than did the voles. Although the population of the cotton rat decreased slightly immediately after the wet period, this decrease wasinsignificant when compared with the drop in population level of other species of small mammals on the same area. During the autumn of 1951 and until March, 1952, the cotton rat became the most important mammal on the House Field study area in terms of grams per acre (Fig. 17), although the number of cotton rats per acre never matched the density of the voles. A similar, though less pronounced, trend was observed on the Quarry Field study area (Fig. 18). One factor in the success of the cotton rat at this time seemed to be the greater resistance to wetting shown by very young individuals. Few adults (of any species) marked before the heavy rains of July, 1951, were trapped in September, 1951, when trapping was resumed after a lapse of one month. Several subadults and some juvenal cotton rats did survive, however,and provided a breeding population from which the area was repopulated. Cotton rats are born fully furred and able to move well, and are often weaned at ten days (Meyer and Meyer, 1944:123-124). Voles, on the other hand, are born naked and helpless and are often not weaned for three weeks. It seems, therefore, that extremely wet soil would harm the voles more than it would the cotton rats.
Several instances of cotton rats eating voles, caught in the same live-trap, were noted. There is reason to believe that young voles, unable to leave the nest, are subject to predation by cotton rats. This would accentuate any competitive advantage gained otherwise by the cotton rats.
The population ofSigmodonretained its high level, relative toMicrotus, until February, 1952. In March only one individual was captured and after that none was trapped until August, 1952, when a single subadult male was captured. Early in March, 1952, before thetrapping period for the month had begun, the area suffered three successive days of unusually low temperature, with snow, which lay more than six inches deep in places. As suggested by Cockrum (1952:185), such conditions proved detrimental to the cotton rats and, at least to the end of the study period in August, 1952, the population of cotton rats had failed to recover. Perhaps the extremely dry weather which followed the heavy winter mortality delayed the recovery of the population.
These limited data seem to indicate competition betweenSigmodonandMicrotusin Kansas. Extremely wet conditions seem to giveSigmodona competitive advantage whereasMicrotusis better able to survive dry summers and severe winters. However, these relationships need further clarification by an intensive study of the life history ofSigmodonin Kansas (especially the more arid western part), including its coactions with the communities it has invaded successfully recently.
The harvest mouse (Reithrodontomys megalotis) also was a common inhabitant of the study plots, but this small rodent seemed not to be a serious competitor of the voles, as its food consists almost entirely of seeds (Cockrum,op. cit.:165) not usually used by voles. In this study, at least, no conflict over space was apparent. Harvest mice frequently were taken in the runways of voles and even in the same trap with voles. Reithro Field, the part of the Reservation having the heaviest population of the harvest mouse, differed from the habitats that were better for voles in being higher, drier and less densely covered with vegetation. However, during the summer of 1951 when the voles were most abundant, Reithro Field supported a large population of voles. Estimates of population of the harvest mouse were of doubtful validity in summer because it was readily trapped only in winter and early spring. Many individuals marked in late spring were not trapped again until late autumn although presumably they remained on the area. This seasonal variation in trapping success seemed to be a matter of acceptance and refusal of bait (Fitch, 1954:45).
The presence of the wood mouse (Peromyscus leucopus) on the study plots indicated an overlapping of habitats. Both House and Quarry Fields were on the ecotone between forest and meadow and a mixture of mammals from both types of habitat occurred. No sign of the homes of the wood mouse was found on the study plots, and on the larger trap line, operated by Fitch, wood mice were captured only near the edge of the woods.
Only six deer mice (Peromyscus maniculatus) were taken on the study plots. This small number probably provided an inaccurate index of the association of the deer mouse and the prairie vole, because samples from snap-traps and the data of other workers on the Reservation showed a more common occurrence of the two species together. The deer mice seemed to prefer a sparser vegetation and did not approach so closely to the forest edge as did the voles. It may have been, in part, the presence ofP. leucopusin the ecotonal region which made it unsuitable forP. maniculatus.
Other mammals noted on the study areas were the following:Didelphis marsupialis,Blarina brevicauda,Scalopus aquaticus,Canis familiaris,Canis latrans,Procyon lotor,Felis domesticus,Sylvilagus floridanus,Microtus pinetorum,Mus musculusandZapus hudsonius.
In the 23-month period from October, 1950, to August, 1952, the ecology of the prairie vole,Microtus ochrogaster, was investigated on the Natural History Reservation of the University of Kansas. In all, 817 voles were captured 2941 times in 13,880 "live-trap days." For some aspects of this study, Dr. Henry S. Fitch, resident investigator on the Reservation, permitted the use of his trapping records. He had captured 1416 voles 5098 times. The total number of live voles used in the study was thus 2233, and they were captured 8039 times. In addition to the voles, I caught 96 cotton rats, 108 harvest mice, 29 wood mice, 2 pine voles and 6 deer mice in live traps. When Fitch's records were used, the live-trapping data covered a thirty-month period and general field data were available from July, 1949, to August, 1952.
Hall and Cockrum (1953:406) stated that probably all microtine rodents fluctuate markedly in numbers. Certainly the populations I studied did so, but the fluctuations were not regularly recurring forM. ochrogasteras they seem to be for some species of the genus in more northern life zones. The changes in the density of populations described in this paper can be explained without recourse to cycles of long time-span and literature dealing specifically withM. ochrogastermakes no references to such cycles. There is, however, an annual cycle of abundance: greatest density of population occurs in autumn, and the least density in January.
This annual pattern is often, perhaps usually, obscured because of the extreme sensitivity of voles to a variety of changes in their environment. These changes are reflected as variations in reproductive success. In this study, some of these changes were accentuated by the great range in annual precipitation. Annual rainfall was approximately average in 1950 (36.32 inches, 0.92 inches above normal), notably high in 1951 (50.68 inches, 15.28 inches above normal) and notably low in 1952 (23.80 inches, 11.60 inches below normal).
Among the types of environmental modification to which the populations of voles reacted were plant succession, an increase in competition withSigmodon, abnormal rainfall and concentration of predators. In the overgrazed disclimax existing in 1948 when the study areas were reserved, no voles were found because cover was insufficient. After the area was protected a succession of good growing years hastened the recovery of the grasses and the populations of voles reached high levels. In areas where the vegetation approachedthe climax community, the densities of voles decreased from the levels supported by the immediately preceding seral stages. The higher carrying capacity of these earlier seral stages was probably due to the greater variety of herbaceous vegetation which tended to maintain a more constant supply of young and growing parts of plants which were the preferred food of voles. Later in the period of study the succession from grasses to woody plants on parts of the study areas also affected the population of voles. Not only did the voles withdraw from the advancing edge of the forest, but their density decreased in the meadows as the number of shrubs and other woody plants increased. These influences of the succession of plants on the population density of voles were exerted through changes in cover and in the quality, as well as the quantity, of the food supply.
Whenever voles were in competition with cotton rats, there was a depression in the population levels of voles. Primarily, the competition between the two species is the result of an extensive coincidence of food habits, but competition for space, cover and nesting material is also present. There was one direct coaction between these two species observed. Cotton rats, at least occasionally, ate voles, especially young individuals. In extremely wet weather, as in the summer of 1951, the high survival rate of newborn cotton rats resulted in an increase in their detrimental effect on the population of voles. However, cotton rats proved to be less well adapted to severe cold or drought than were voles.
Heavy rainfall reduced the densities of populations of voles by killing a large percentage of juveniles. During the summer of 1951 the competition of cotton rats further depressed the population level of the voles, but the relative importance of competition with cotton rats and superabundant moisture in effecting the observed reduction in population density is difficult to judge. Perhaps most of the decrease in population which followed the heavy rains was due to competition rather than to weather. Subnormal rainfall, as in 1952, reduced the population of voles by inhibiting reproduction. Presumably because of an altered food supply, reproduction almost ceased during the drought. Utilization of the habitat was further reduced in the summer of 1952 because the voles did not grow so large as they otherwise did.
Predation, as a general rule, does not significantly affect densities of populations, but large numbers of predators concentrating on small areas may rapidly reduce the numbers of prey animals. In the course ofmy study, such a situation occurred but once, when a group of long-eared owls roosted in the woods adjacent to Quarry Field. The population of voles in that area was probably reduced somewhat as a result of predation by owls.
Population trends in either direction may be reversed suddenly by changes in the factors discussed above. In the fall of 1951, a downward trend in the density of the voles was evident. At this time, populations of cotton rats were increasing rapidly and competition between cotton rats and voles was intensified. In February, 1952, the population of cotton rats was decimated suddenly by a short period of unusually cold weather. The voles were suddenly freed from the stress of competition and the population immediately began to rise. The upward trend began prior to the annual spring increase and was subsequently reinforced by it. In the last part of May, 1952, the upward trend of the population was reversed, as the drought became severe, and the density of the population decreased rapidly. This drop was too sudden and too extreme to be only the normal summer slump. The relatively rapid response of voles to a heavy rain after a dry period, first by increased breeding and later by increases in density, is one more example of abrupt changes in population trends caused by altered environmental conditions.
In the population changes that I observed, no evident "die-off" of adults accompanied even the most drastic reductions in population density. The causative factor directly influences the population either by inhibiting reproduction or by increasing infant and prenatal mortality. The net reduction is due to an inadequate replacement of those voles lost by normal attrition.
Most voles, under natural conditions, live less than one year. Those individuals born in the autumn live longer, as a group, than those born at any other time. Since the heaviest mortality is in young voles, adults which become established in an area may live more than 18 months and, if they are females, may produce more than a dozen litters. No decrease in vigor and fertility was found to accompany aging. A relationship between the condylobasilar length of the skull and the age of a vole was discovered and, with further study, may yield a method of aging voles more accurately than has been possible heretofore. Other characteristics, varying with age, were described. The most reliable indicator of age seemed to be the prominence of the temporal ridges.
Runway systems and burrows are used by groups of voles rather than by individuals. Most of the activity of voles is confined to theserunways and an exposed individual is seldom seen. A home range may include several runway systems, and the ranges of individuals overlap extensively. Both home ranges and patterns of runway systems change constantly. Runways seem to be primarily feeding trails, and are extended or abandoned as the voles change their feeding habits. Groups of adult voles using a system of runways seem to have no special relationship. Juveniles tend to stay near their mothers, but as they mature, they shift their ranges and are replaced by other individuals. Males wander more than females, and shift their ranges more often. No intolerance of other voles exists and, in laboratory cages, groups of voles lived together peaceably from the time they are placed together. Crowding does not seem to be harmful directly, therefore, and high densities will develop if food and cover resources permit.
As a prey item, the prairie vole proved to be an important part of the biota of the Reservation. It was eaten frequently by almost all of the larger vertebrate predators on the Reservation and was, seemingly, the most important food item of the long-eared owl. The ability of the prairie vole to maintain high levels of population over relatively broad areas enhances its value as a prey species.
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1937. Ecology of a mixed prairie in west-central Kansas. Ecol. Monog., 7:481-547.
Bailey, V.
1902. Synopsis of the North American species ofSigmodon. Proc. Biol. Soc. Washington, 15:101-116.
1924. Breeding, feeding and other life habits of meadow mice. Jour. Agric. Res., 27:523-536.
Baker, J. R., andR. M. Ransom.
1932a. Factors affecting the breeding of the field mouse (Microtus agrestis). Part I. Light. Proc. Roy. Soc. London, Series B, 110:313-322.
1932b. Factors affecting the breeding of the field mouse (Microtus agrestis). Part II. Temperature. Proc. Roy. Soc. London, Series B, 112:39-46.
Black, T. D.
1937. Mammals of Kansas. Kansas State Board Agric., 13th Biennial Rep., 1935-36:116-217.
Blair, W. F.
1939. Some observed effects of stream valley flooding on small mammal populations in eastern Oklahoma. Jour. Mamm., 20:304-306.
1940. Home ranges and populations of the meadow vole in southern Michigan. Jour. Wildlife Mgmt., 4:149-161.
1941. Techniques for the study of small mammal populations. Jour. Mamm., 22:148-157.
1948. Population density, life span and mortality rates of small mammals in the bluegrass meadow and bluegrass field associations of southern Michigan. Amer. Midland Nat., 40:395-419.
Bodenheimer, F. S., andF. Sulman.
1946. The estrous cycle ofMicrotus guentheriD. and A. and its ecological implications. Ecol., 27:255-256.
Bole, B. P., Jr.
1939. The quadrat method of studying small mammal populations. Cleveland Mus. Nat. Hist. Sci. Publ., 5:1-77.
Brown, H. L.
1946. Rodent activity in a mixed prairie near Hays, Kansas. Trans. Kansas Acad. Sci., 48:448-458.
Brumwell, M.
1951. An ecological survey of the Fort Leavenworth Military Reservation. Amer. Midland Nat., 45:187-231.
Burt, W. H.
1943. Territoriality and home range concepts as applied to mammals. Jour. Mamm., 24:346-352.
Calhoun, J. B.
1945. Diel activity rhythms of the rodentsMicrotus ochrogasterandSigmodon hispidus hispidus. Ecol., 26:251-273.
Chitty, D., andD. A. Kempson.
1949. Prebaiting of small mammals and a new design of a live trap. Ecol., 30:536-542.
Cockrum, E. L.
1947. Effectiveness of live traps vs. snap traps. Jour. Mamm., 28:186.
1948. Distribution of the hispid cotton rat in Kansas. Trans. Kansas Acad. Sci., 51:306-312.
1952. Mammals of Kansas. Univ. Kansas Mus. Nat. Hist. Publ., 7:1-303.
Davis, D. H. S.
1933. Rhythmic activity in the short-tailed vole,Microtus. Jour. Animal Ecol., 2:232-238.
Dice, L. R.
1922. Some factors affecting the distribution of the prairie vole, forest deer mouse and prairie deer mouse. Ecol., 3:29-47.
Eadie, W. R.
1944. The short-tailed shrew and field mouse predation. Jour. Mamm., 25:359-362.
1948. Shrew-mouse predation during low mouse abundance. Jour. Mamm., 29:35-37.
1952. Shrew predation and vole populations on a limited area. Jour. Mamm., 33:185-189.
1953. Response ofMicrotusto vegetative cover. Jour. Mamm., 34:262-264.
Elton, C.
1949. Population interspersion. An essay in animal community patterns. Jour. Ecol., 37:1-23.
Errington, P. R.
1935. Food habits of midwestern foxes. Jour. Mamm., 16:192-200.
1936. What is the meaning of predation? Smithsonian Inst. Rep., 1936:243-252.
1943. An analysis of mink predation upon muskrat in north central United States. Agric. Exp. Sta., Iowa State Coll. Agric. Mech. Arts, Res. Bull., 320:799-924.
1946. Predation and vertebrate populations. Quart. Rev. Biol., 21:144-177.
Errington, P. L.,F. Hamerstrom, andF. N. Hamerstrom, Jr.
1940. The great horned owl and its prey in north central United States. Agric. Exp. Sta., Iowa State Coll. Agric. Mech. Arts, Res. Bull., 277:759-831.
Fisher, H. J.
1945. Notes on the voles in central Missouri. Jour. Mamm., 26:435-437.
Fitch, H. S.
1948. A study of coyote relationships on cattle range. Jour. Wildlife Mgmt., 12:73-78.
1950. A new style live trap for small mammals. Jour. Mamm., 31:364-365.
1952. The University of Kansas Natural History Reservation. Univ. Kansas, Mus. Nat. Hist. Misc. Publ., 4:1-38.
1954. Seasonal acceptance of bait by small mammals. Jour. Mamm., 35:39-47.
Fitch, H. S.,F. Swenson, andD. F. Tillotson.
1946. Behavior and food habits of the red-tailed hawk. Condor, 48:205-237.
Goodpastor, W. W., andD. F. Hoffmeister.
1952. Notes on the mammals of eastern Tennessee. Jour. Mamm., 33:362-371.
Gunderson, H. L.
1950. A study of some small mammal populations at Cedar Creek Forest, Asoka County, Minnesota. Univ. Minnesota Mus. Nat. Hist., 4:1-49.
Hall, E. R.
1926. Changes during growth in the skull of the rodentOtospermophilus grammarus beecheyi. Univ. California Publ. Zool., 21:355-404.
Hall, E. R., andE. L. Cockrum.
1953. A synopsis of North American microtine rodents. Univ. Kansas, Mus. Nat. Hist. Publ., 5:373-498.
Hamilton, W. J., Jr.
1937a. Growth and life span of the field mouse. Amer. Nat., 71:500-507.
1937b. The biology of microtine cycles. Jour. Agric. Res., 54:779-790.
1937c. Activity and home range of the field mouse. Ecol., 18:255-263.
1940. Life and habits of the field mouse. Sci. Monthly, 50:425-434.
1941. The reproduction of the field mouse,Microtus pennsylvanicus. Cornell Univ. Agric. Exp. Sta. Mem., 237:1-23.
1949. The reproductive rates of some small mammals. Jour. Mamm., 30:257-260.
Hatfield, D. M.
1935. A natural history ofMicrotus californicus. Jour. Mamm., 16:261-271.
Hatt, R. T.
1930. The biology of the voles of New York. Roosevelt Wildlife Bull., 5:513-623.
Hayne, D. M.
1949a. Two methods of estimating populations from trapping records. Jour. Mamm., 30:399-411.
1949b. Calculation of the size of home range. Jour. Mamm., 30:1-18.
1950. Apparent home range ofMicrotusin relation to distance between traps. Jour. Mamm., 31:26-39.
Hinton, M. A. C.
1926. Monograph of the voles and lemmings (Microtinae) living and extinct. British Museum of Nat. Hist., London, xvi + 488 pp. 15 pls.
Hopkins, H. H.,F. W. Albertson, andD. A. Riegel.
1952. Ecology of grassland utilization in a mixed prairie. Trans. Kansas Acad. Sci., 55:395-418.
Howard, W. E.
1951. Relation between low temperature and available food to survival of small rodents. Jour. Mamm., 32:300-312.
Howell, A. B.
1924. Individual and age variation inMicrotus montanus yosemite. Jour. Agric. Res., 28:977-1015.
Jameson, E. W.
1947. Natural history of the prairie vole. Univ. Kansas, Mus. Nat. Hist. Publ., 1:125-151.
1950. Determining fecundity in male small mammals. Jour. Mamm., 31:433-436.
Johnson, M. S.
1926. Activity and distribution of certain wild mice in relation to the biotic community. Jour. Mamm., 7:245-277.
Korschgen, L. J.
1952. A general summary of the food of Missouri predatory and game animals. Conserv. Comm., Div. Fish and Game, State of Missouri. July, 1952. 61 pp.
Lantz, D. E.
1907. An economic survey of the field mice (genusMicrotus). USDA Biol. Surv. Bull, 31:1-64.
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1940. The mortality, fertility and rate of natural increase of the vole (Microtus agrestis) as observed in the laboratory. Jour. Animal Ecol., 9:27-52.
Llewellyn, L. M.
1950. Reduction of mortality in live-trapping mice. Jour. Wildlife Mgmt., 14:84-85.
McAtee, W. L.
1935. Food habits of common hawks. USDA Circ., 370:1-36.
Meyer, B. J., andR. K. Meyer.
1944. Growth and reproduction of the cotton rat,Sigmodon hispidus hispidus, under laboratory conditions. Jour. Mamm., 25:107-129.
Mohr, C. O.
1943. A comparison of North American small mammal censuses. Amer. Midland Nat., 29:545-587.
1947. Table of equivalent populations of North American small mammals. Amer. Midland Nat., 37:223-249.
Phillips, P.
1936. The distribution of rodents in overgrazed and normal grassland in central Oklahoma. Ecol., 17:673-679.
Poiley, S. M.
1949. Raising captive meadow voles (Microtus p. pennsylvanicus). Jour. Mamm., 30:317.
Rinker, G. C.
1942a. Litter records of some mammals of Meade County, Kansas. Trans. Kansas Acad. Sci., 45:376-378.
1942b. An extension of the range of the Texas cotton rat in Kansas. Jour. Mamm., 23:439.
Sandidge, L. L.
1953.Food and densof the opossum (Didelphis virginiana) in northeastern Kansas. Trans. Kansas Acad. Sci., 56:97-106.
Selle, R. M.
1928.Microtus californicusin captivity. Jour. Mamm., 9:93-98.
Shapiro, J.
1950. Notes on the population dynamics ofMicrotusandBlarinawith a record of albinism inBlarina. Jour. Wildlife Mgmt, 14:359-360.
Stickel, L. F.
1946. Experimental analysis of methods of measuring small mammal populations. Jour. Wildlife Mgmt., 10:150-159.
1948. The trap line as a measure of small mammal populations. Jour. Wildlife Mgmt., 12:153-161.
Strecker, J. K.
1929. Notes on the Texas cotton and Atwater wood rats. Jour. Mamm., 10:216-220.
Summerhayes, V. S.
1941. The effects of voles (Microtus agrestis) on vegetation. Jour. Ecol., 29:14-48.
Svihla, A.
1929. Life history notes onSigmodon hispidus hispidus. Jour. Mamm., 10:352-353.
Townsend, M. T.
1935. Studies on some small mammals of central New York. Roosevelt Wildlife Annals, 4:1-120.
Uhler, F. M.,C. Cottam, andT. E. Clarke.
1939. Food of the snakes of George Washington National Forest, Virginia. Trans. 4th N. A. Wildlife Conf., 605-622.
Wooster, L. D.
1935. Notes on the effects of drought on animal populations in western Kansas. Trans. Kansas Acad. Sci., 38:351-352.
1936. The contents of owl pellets as indicators of habitat preferences of small mammals. Trans. Kansas Acad. Sci., 39:395-397.
1939. An ecological evaluation of predatees on a mixed prairie area in western Kansas. Trans. Kansas Acad. Sci., 42:515-517.
Transmitted May 19, 1955.
Institutional libraries interested in publications exchange may obtain this series by addressing the Exchange Librarian, University of Kansas Library, Lawrence, Kansas. Copies for individuals, persons working in a particular field of study, may be obtained by addressing instead the Museum of Natural History, University of Kansas, Lawrence, Kansas. There is no provision for sale of this series by the University Library which meets institutional requests, or by the Museum of Natural History which meets the requests of individuals. However, when individuals request copies from the Museum, 25 cents should be included, for each separate number that is 100 pages or more in length, for the purpose of defraying the costs of wrapping and mailing.
* An asterisk designates those numbers of which the Museum's supply (not the Library's supply) is exhausted. Numbers published to date, in this series, are as follows:
Vol. 1.
Nos. 1-26 and index. Pp. 1-638, 1946-1950.
Index. Pp. 605-638.
*Vol. 2.
(Complete) Mammals of Washington. By Walter W. Dalquest. Pp. 1-444, 140 figures in text. April 9, 1948.
Vol. 3.
*1. The avifauna of Micronesia, its origin, evolution, and distribution. By Rollin H. Baker. Pp. 1-359, 16 figures in text. June 12, 1951.
*2. A quantitative study of the nocturnal migration of birds. By George H. Lowery, Jr. Pp. 361-472, 47 figures in text. June 29, 1951.
3. Phylogeny of the waxwings and allied birds. By M. Dale Arvey. Pp. 473-530, 49 figures in text, 13 tables. October 10, 1951.
4. Birds from the state of Veracruz, Mexico. By George H. Lowery, Jr., and Walter W. Dalquest. Pp. 531-649, 7 figures in text, 2 tables. October 10, 1951.
Index. Pp. 651-681.
*Vol. 4.
(Complete) American weasels. By E. Raymond Hall. Pp. 1-466, 41 plates, 31 figures in text. December 27, 1951.
Vol. 5.
1. Preliminary survey of a Paleocene faunule from the Angels Peak area, New Mexico. By Robert W. Wilson. Pp. 1-11, 1 figure in text. February 24, 1951.
2. Two new moles (Genus Scalopus) from Mexico and Texas. By Rollin H. Baker. Pp. 17-24. February 28, 1951.
3. Two new pocket gophers from Wyoming and Colorado. By E. Raymond Hall and H. Gordon Montague. Pp. 25-32. February 28, 1951.
4. Mammals obtained by Dr. Curt von Wedel from the barrier beach of Tamaulipas, Mexico. By E. Raymond Hall. Pp. 33-47, 1 figure in text. October 1, 1951.
5. Comments on the taxonomy and geographic distribution of some North American rabbits. By E. Raymond Hall and Keith R. Kelson. Pp. 49-58. October 1, 1951.
6. Two new subspecies of Thomomys bottae from New Mexico and Colorado. By Keith R. Kelson. Pp. 59-71, 1 figure in text. October 1, 1951.
7. A new subspecies of Microtus montanus from Montana and comments on Microtus canicaudus Miller. By E. Raymond Hall and Keith R. Kelson. Pp. 73-79. October 1, 1951.
8. A new pocket gopher (Genus Thomomys) from eastern Colorado. By E. Raymond Hall. Pp. 81-85. October 1, 1951.
9. Mammals taken along the Alaskan Highway. By Rollin H. Baker. Pp. 87-117, 1 figure in text. November 28, 1951.
*10. A synopsis of the North American Lagomorpha. By E. Raymond Hall. Pp. 119-202, 68 figures in text. December 15, 1951.
11. A new pocket mouse (Genus Perognathus) from Kansas. By E. Lendell Cockrum. Pp. 203-206. December 15, 1951.
12. Mammals from Tamaulipas, Mexico. By Rollin H. Baker. Pp. 207-218. December 15, 1951.
13. A new pocket gopher (Genus Thomomys) from Wyoming and Colorado. By E. Raymond Hall. Pp. 219-222. December 15, 1951.
14. A new name for the Mexican red bat. By E. Raymond Hall. Pp. 223-226. December 15, 1951.
15. Taxonomic notes on Mexican bats of the Genus Rhogeëssa. By E. Raymond Hall. Pp. 227-232. April 10, 1952.
16. Comments on the taxonomy and geographic distribution of some North American woodrats (Genus Neotoma). By Keith R. Kelson. Pp. 233-242. April 10, 1952.
17. The subspecies of the Mexican red-bellied squirrel, Sciurus aureogaster. By Keith R. Kelson. Pp. 243-250, 1 figure in text. April 10, 1952.
18. Geographic range of Peromyscus melanophrys, with description of new subspecies. By Rollin H. Baker. Pp. 251-258, 1 figure in text. May 10, 1952.
19. A new chipmunk (Genus Eutamias) from the Black Hills. By John A. White. Pp. 259-262. April 10, 1952.
20. A new piñon mouse (Peromyscus truei) from Durango, Mexico. By Robert B. Finley, Jr. Pp. 263-267. May 23, 1952.
21. An annotated checklist of Nebraskan bats. By Olin L. Webb and J. Knox Jones, Jr. Pp. 269-279. May 31, 1952.
22. Geographic variation in red-backed mice (Genus Clethrionomys) of the southern Rocky Mountain region. By E. Lendell Cockrum and Kenneth L. Fitch. Pp. 281-292, 1 figure in text. November 15, 1952.
23. Comments on the taxonomy and geographic distribution of North American microtines. By E. Raymond Hall and E. Lendell Cockrum. Pp. 293-312. November 17, 1952.
24. The subspecific status of two Central American sloths. By E. Raymond Hall and Keith R. Kelson. Pp. 313-317. November 21, 1952.
25. Comments on the taxonomy and geographic distribution of some North American marsupials, insectivores, and carnivores. By E. Raymond Hall and Keith R. Kelson. Pp. 319-341. December 5, 1952.
26. Comments on the taxonomy and geographic distribution of some North American rodents. By E. Raymond Hall and Keith R. Kelson. Pp. 343-371. December 15, 1952.
27. A synopsis of the North American microtine rodents. By E. Raymond Hall and E. Lendell Cockrum. Pp. 373-498, 149 figures in text. January 15, 1953.
28. The pocket gophers (Genus Thomomys) of Coahuila, Mexico. By Rollin H. Baker. Pp. 499-514, 1 figure in text. June 1, 1953.
29. Geographic distribution of the pocket mouse, Perognathus fasciatus. By J. Knox Jones, Jr. Pp. 515-526, 7 figures in text. August 1, 1953.
30. A new subspecies of wood rat (Neotoma mexicana) from Colorado. By Robert B. Finley, Jr. Pp. 527-534, 2 figures in text. August 15, 1953.
31. Four new pocket gophers of the genus Cratogeomys from Jalisco, Mexico. By Robert J. Russell. Pp. 535-542. October 15, 1953.
32. Genera and subgenera of chipmunks. By John A. White. Pp. 543-561, 12 figures in text. December 1, 1953.
33. Taxonomy of the chipmunks, Eutamias quadrivittatus and Eutamias umbrinus. By John A. White. Pp. 563-582, 6 figures in text. December 1, 1953.
34. Geographic distribution and taxonomy of the chipmunks of Wyoming. By John A. White. Pp. 584-610, 3 figures in text. December 1, 1953.
35. The baculum of the chipmunks of western North America. By John A. White. Pp. 611-631, 19 figures in text. December 1, 1953.
36. Pleistocene Soricidae from San Josecito Cave, Nuevo Leon, Mexico. By James S. Findley. Pp. 633-639. December 1, 1953.
37. Seventeen species of bats recorded from Barro Colorado Island, Panama Canal Zone. By E. Raymond Hall and William B. Jackson. Pp. 641-646. December 1, 1953.
Index. Pp. 647-676.
*Vol. 6.
(Complete) Mammals of Utah,taxonomy and distribution. By Stephen D. Durrant. Pp. 1-549, 91 figures in text, 30 tables. August 10, 1952.
Vol. 7.
*1. Mammals of Kansas. By E. Lendell Cockrum. Pp. 1-303, 73 figures in text, 37 tables. August 25, 1952.
2. Ecology of the opossum on a natural area in northeastern Kansas. By Henry S. Fitch and Lewis L. Sandidge. Pp. 305-338, 5 figures in text. August 24, 1953.
3. The silky pocket mice (Perognathus flavus) of Mexico. By Rollin H. Baker. Pp. 339-347, 1 figure in text. February 15, 1954.
4. North American jumping mice (Genus Zapus). By Philip H. Krutzsch. Pp. 349-472, 47 figures in text, 4 tables. April 21, 1954.
5. Mammals from Southeastern Alaska. By Rollin H. Baker and James S. Findley. Pp. 473-477. April 21, 1954.
6. Distribution of some Nebraskan Mammals. By J. Knox Jones, Jr. Pp. 479-487. April 21, 1954.
7. Subspeciation in the montane meadow mouse, Microtus montanus, in Wyoming and Colorado. By Sydney Anderson. Pp. 489-506, 2 figures in text. July 23, 1954.
8. A new subspecies of bat (Myotis velifer) from southeastern California and Arizona. By Terry A. Vaughn. Pp. 507-512. July 23, 1954.
9. Mammals of the San Gabriel mountains of California. By Terry A. Vaughn. Pp. 513-582, 1 figure in text, 12 tables. November 15, 1954.
10. A new bat (Genus Pipistrellus) from northeastern Mexico. By Rollin H. Baker. Pp. 583-586. November 15, 1954.
11. A new subspecies of pocket mouse from Kansas. By E. Raymond Hall. Pp. 587-590. November 15, 1954.
12. Geographic variation in the pocket gopher, Cratogeomys castanops, in Coahuila, Mexico. By Robert J. Russell and Rollin H. Baker. Pp. 591-608. March 15, 1955.
13. A new cottontail (Sylvilagus floridanus) from northeastern Mexico. By Rollin H. Baker. Pp. 609-612. April 8, 1955.
14. Taxonomy and distribution of some American shrews. By James S. Findley. Pp. 613-618. June 10, 1955.
15. Distribution and systematic position of the pigmy woodrat, Neotoma goldmani. By Dennis G. Rainey and Rollin H. Baker. Pp. 619-624, 2 figs. in text. June 10, 1955.
Index. Pp. 625-651.
Vol. 8.
1. Life history and ecology of the five-lined skink, Eumeces fasciatus. By Henry S. Fitch. Pp. 1-156, 2 pls., 26 figs. in text, 17 tables. September 1, 1954.
2. Myology and serology of the Avian Family Fringillidae, a taxonomic study. By William B. Stallcup. Pp. 157-211, 23 figures in text, 4 tables. November 15, 1954.
3. An ecological study of the collared lizard (Crotaphytus collaris). By Henry S. Fitch. Pp. 213-274, 10 figures in text. February 10, 1956.
4. A field study of the Kansas ant-eating frog, Gastrophryne olivacea. By Henry S. Fitch. Pp. 275-306, 9 figures in text. February 10, 1956.