Table 13—Food Present in 114 Coyote Scats Collected at Mesa Verde National Park each Month from September 1963 through August 1964.Food ItemNumber of occurrencesPercentage of total itemsSylvilagussp.3212.65Spermophilus variegatus51.97Eutamiassp.124.74Reithrodontomys megalotis41.58Peromyscus boylei20.79Peromyscus maniculatus31.18Peromyscus truei72.76Neotoma cinerea20.79Neotoma mexicana93.56Neotoma albigula51.97Neotomasp.31.18Microtus longicaudus10.39Microtus mexicanus114.34Microtus montanus10.39Microtussp.10.39Odocoileus hemionus5923.32Grass3413.44Juniper berries239.09Pinyon needles145.53Pinyon nuts10.39Arthropods72.76Juniper needles31.18Rodent or Lagomorph bones51.97Sceloporussp.10.39Unidentified fruit20.79Rocks31.18Paper41.58Soil31.18Feathers51.97Total253
Table 13—Food Present in 114 Coyote Scats Collected at Mesa Verde National Park each Month from September 1963 through August 1964.
Total
Hawks, owls and eagles live in the park. Red-tailed hawks were seen frequently in the burned area on the northern end of Wetherill Mesa. Both hawks and owls probably prey uponPeromyscusin Mesa Verde, for they are well-known predators upon mice and small rodents in other areas. I tried to find owl and hawk nests that were occupied, but located only nests that were abandoned or impossible to reach.
Captive gopher snakes,Pituophis melanoleucus, ate adults of both species ofPeromyscus. Gopher snakes probably are the most abundant snake in the park; they feed mostly on mice and other rodents. Fur ofPeromyscuswas found in the stomach of a striped whipsnake,Masticophis taeniatus(Douglas, 1966:734).
Discussion
Five species ofPeromyscusinhabit Mesa Verde National Park (Anderson, 1961). Two of these species,P. crinitusandP. difficilisare rare, and none was taken in more than 14,000 trap nights. Several individuals ofP. boyleiwere taken in live traps, but this species could not be regarded as common. The two remaining species,P. trueiandP. maniculatus, are the most abundant species in the park. Comparison of the habitats and life-cycles of these two forms and analyses of their interrelationships have been the objectives of this study.
The distribution ofP. trueiin the park is regulated by the presence of living pinyon-juniper woodland where logs and hollow trees ofJuniperus osteospermaprovide nesting and hiding places, and where seeds of juniper trees and nuts of pinyon trees provide food. Several other investigators have reportedP. trueito be associated with trees, but apparently these findings have not assumed the importance they warrant in understanding the ecology of this species. Bailey (1931:152) observed an individual ofP. trueinesting in a tree on Conchas Creek, New Mexico, and thought that this species might be more arboreal than was generally supposed. The type specimen ofP. t. trueiwas taken by Shufeldt from a "nest protruding from an opening in the dead and hollow trunk of a small pinon, at least 2 feet above the ground#8230. The nest, composed of the fine fibers of the inner bark of the pinon, was soon pulled out, and its owner dislodged...." (Shufeldt, 1885:403). Individuals ofP. trueiusually build nests in trees, or in hollow logs, and are therefore more abundant in pinyon-juniper woodland where there are many such nesting sites.
Rocks and stones are not necessary in the habitat ofP. truei, although this species was most abundant where there was stony soil. The coincidence of rock or stones and a high density ofP. trueiis thought to be explainable in terms of vegetation. Stony soils support mixed shrubs as well as pinyon and juniper trees; the additional cover and source of food probably allow a greater abundance ofP. trueithan would be possible without the shrubs. Secondarily, the rock provides nesting sites for more mice.
Stands of mixed shrubs, lacking a pinyon-juniper canopy, do not supportP. truei. Its absence was noteworthy on Navajo Hill and on the northern end of Wetherill Mesa where onlyP. maniculatuslived among the mixed shrubs and grassland. On the Mesa Verde, pinyon and juniper trees must be present in order forP. trueitolive in an area; and, these trees must be alive. Dead pinyons and junipers still stand in the burned part of Morfield Ridge, but noP. trueiwere found there.
Although a few individuals ofP. trueiwere taken in stands of sagebrush adjacent to pinyon-juniper woodlands, this species does not ordinarily venture far from the forest.
P. maniculatuslives almost everywhere in Mesa Verde; the preferred habitats are open and grassy with an overstory of mixed shrubs. Individuals ofP. maniculatusventure into ecotonal areas lying between grasslands and pinyon-juniper forest, or between sagebrush and pinyon-juniper forest.P. maniculatusis found also in disturbed areas and in stands of sagebrush that occur in clearings of the pinyon-juniper woodland. In such areas,P. maniculatusandP. trueiare sympatric; their home ranges overlap and any inter-specific competition that might occur would be expected in these places.
The ability ofP. maniculatusto live in many different habitats is correlated in part with its ability to build nests in a variety of sites. WhereasP. trueiusually builds nests only in dead branches or logs,P. maniculatusbuilds nests in such varied places as spaces under rocks, at the bases of rotten trees, and in abandoned tunnels of pocket gophers. This adaptability is advantageous for the dispersal of young individuals and the movement of adults into new areas.
Nesting sites have important bearing on survival of the young. In Mesa Verde the rainy season occurs in July and August, while both species ofPeromyscusare reproducing. It is reasonable to assume that young animals that remain dry survive better than those that become wet and chilled. The nestling young ofP. trueiare in a more favorable position to remain dry and warm than are nestling young ofP. maniculatus.
Captives of each species differed in the amounts of water consumed per gram of body weight. Individuals ofP. trueiconsumed more water per gram of body weight than individuals ofP. maniculatus. Animals may drink more water than they require when allowed to drinkad libitum, but Lindeborg (1952) has shown that species which consume less water when it is not restricted also fare better on a reduced ration.P. maniculatusappears to be better adapted to aridity thanP. truei. The preferred habitats of each species are in accord with these findings.
Within the trapping grid, the most moderate microenvironment, in terms of temperature and humidity, was in the pinyon-juniperforest, whereP. trueilives. The temperature extremes were wider in the microenvironments of a thicket of oak brush and of two different stands of sagebrush, whereP. maniculatuslives, than in the forest.P. maniculatustends to live in the harsher, more arid parts of Mesa Verde. Because of its propensity to build nests under things, or in the ground, and because of its ability to use less water per gram of body weight,P. maniculatusis better adapted to withstand harsh environments than isP. truei.
P. trueimay be restricted to the pinyon-juniper woodland because of its need for more mesic conditions. Still, Mesa Verde is semi-arid and there are few permanent sources of water available for animals. The primary source of moisture for rodents must be their food. Analysis of the percentages of moisture contained in the three most common plants in the trapping grid showed thatP. trueicould obtain the required moisture by eating about ten grams of these plants daily; individuals ofP. maniculatuswould need to eat less in order to satisfy their water needs.
Individuals ofP. trueidied more frequently in warm live-traps than did individuals ofP. maniculatus. This indicates thatP. trueican tolerate less desiccation, or a narrower range of temperatures, than canP. maniculatus.
Both species of mice eat some of the same plants, but these plants occur widely.P. trueiseems to rely more upon the nuts of pinyons and the seeds of junipers than doesP. maniculatus. Mounds of discarded juniper seeds were associated with all nesting sites ofP. truei. Bailey (1931:153) also noticed the fondness of this species for pine nuts and juniper seeds. Apparently, the availability of these foods is one of the major factors affecting the distribution ofP. truei. However, this is not the only factor, as is shown by the presence ofP. maniculatusbut lack ofP. trueiin a juniper-pinyon association with an understory of bitterbrush. This habitat was seemingly too arid forP. truei.
Factors Affecting Population Densities
The production of young, and success in rearing them, is essential to continuity of any population.P. maniculatusis favored in this respect, because the females produce more young and wean them sooner than do females ofP. truei. In addition, lactating females ofP. maniculatusrequire significantly less water than do females ofP. truei. Since young mice of both species require no more water per gram of body weight than do adults, the young can disperse into any area that is habitable by their species.P. maniculatusprobably is affected less by prolonged drought than isP. truei.Since lactating females require the most water of any animal in the population, they are the weakest link in the system. Females ofPeromyscusare known to reabsorb embryos when conditions are unfavorable for continued pregnancy. If prolonged drought occurred in the reproductive season, and desiccated the vegetation upon which the mice depend for moisture, the populations should diminish the following year. Lactating females ofP. trueiwould be affected more seriously by a shortage of water than would lactating females ofP. maniculatus.
Of two species, the one producing the more young probably would be subjected to more parasitism and predation than the species producing fewer young. A favorable season for botflies,Cuterebrasp., revealed thatP. maniculatushas a higher incidence of parasitism by these flies than hasP. truei; possibly the adult flies concentrate in the open, grassy areas whereP. maniculatusis more abundant, rather than in the woodlands whereP. trueilives. Perhaps the lower parasitism ofP. trueiby warbles is related to the physiology of this species of mouse. Near Boulder, Colorado, the incidence of infection by warbles is lower inP. difficilis, a species closely related toP. truei, than inP. maniculatus(V. Keen, personal communication).
Although predation by carnivores would be expected to be higher onP. maniculatus, because this species does not climb, my data show that more individuals ofP. trueiwere taken by coyotes. I lack confidence in these findings, suspecting that another sample might indicate the reverse. Birds of prey probably catch more individuals ofP. maniculatus, because this species lives in more open habitats. My data do not warrant firm conclusions regarding predation.
The length of time females must care for their young influences the rate at which individuals can be added to the population. Females ofP. trueinurse their young longer and keep them in the nest longer than do females ofP. maniculatus. Although this may enhance the chances of survival of young ofP. truei, it also reduces the number of litters that each female can have in each breeding season. Females ofP. maniculatuscan produce more young per litter, and each female probably can produce more litters per year than females ofP. truei.
Captives ofP. trueiwere tolerant of other individuals of the same species, even when kept in close confinement. However, when there was slight shortage of food or water they killed their litter mates, or females killed their young. Only a short period oftime was necessary for one mouse to dispatch all others in the litter. The attacked mice were bitten through the head before being eaten; the brains and viscera were the first parts consumed. The population might be decimated rapidly if drought forced this species to cannibalism. When the supply of food or water was restored, the captive mice resumed their tolerant nature.
In captivity,P. maniculatusis amazingly tolerant of close confinement with members of the same species; individuals did not tend to kill their litter mates, or their young, even during shortage of food and water. This tolerance, especially under stressful conditions, probably enablesP. maniculatusto persist in relatively unfavorable areas.
Adaptations to Environment
Each of the two species ofPeromyscusillustrates one or more adaptations to its environment.P. trueiis adapted to climbing by possession of long toes, a long tail, and large hind feet. The tail is used as a counterbalance when climbing (Horner, 1954). When frightened, individuals ofP. trueioften ran across the ground in a semi-saltatorial fashion, bounding over clumps of grass that were as much as 18 inches high. Such individuals usually ran to the nearest tree and climbed to branches 10 to 20 feet above the ground.
Large eyes are characteristic of thetrueigroup of mice, and may be an adaptation to a semi-arboreal mode of life. A similar adaptation is shared by some other arboreal mammals, and of arboreal snakes. The large eyes ofP. trueiin comparison to those ofP. maniculatus, probably increase the field of vision, and permit the animal to look downward as well as in other directions.
The above-mentioned adaptations ofP. trueipermit these graceful mice to use their environment effectively. By climbing, this species can nest above-ground in the hollow branches of trees, and can rear its young in a comparatively safe setting. The ability to climb also permits vertical as well as horizontal use of a limited habitat. Because of the three-dimensional nature of the home range oftruei, its range is actually larger than that ofmaniculatusalthough the standard trapping procedures makes the home range of the two appear to be about the same size. Finally, trees may offer safety from predators, and a source of food that probably is the winter staple of this species.
Peromyscus maniculatushas adapted differently to its environment. Small size of body and appendages permit this species to use a variety of nesting sites and hiding places even though it isrestricted, by its anatomy, to life on the ground. The tail and hind feet are shorter than inP. truei, andP. maniculatusis an inefficient climber. I have placed individuals in bushes, and found that many walk off into space from a height of several feet. Perhaps the relative smallness of their eyes accounts for their seeming lack of awareness of how high they are above the ground.
When frightened, individuals ofP. maniculatusran rapidly in a zig-zag path and dove into the nearest cover. Mice, released from live traps, often stuck their heads under leaves, leaving their bodies exposed. This species tends to hide as rapidly as possible, and remain motionless. This tactic would not be of much value as an escape from carnivores, but it could be effective against birds of prey.
In Mesa Verde,P. maniculatusinhabits the more arid, open areas. When the population is dense, individuals of this species are found also in pinyon-juniper woodland. ApparentlyP. maniculatusprefers the grassy areas and the thickets of oak brush. Although such habitats have harsh climatic conditions, they offer innumerable hiding places, and thus have great advantage for a species confined to the ground.
The low requirements of water per gram of body weight, the ability to eat diversified foods, the use of varied habitats, the high fecundity, and the ability to use any nook for retreat or nesting makeP. maniculatusa successful inhabitant of most parts of Mesa Verde, and indeed, of most of North America.
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Transcriber's NotesAll obvious typographic errors corrected. For consistency, the species listings forboyliihas been standardised toboylei.Typographical CorrectionsPageCorrection429nuaseosus ⇒ nauseosus430Orthocarpos ⇒ Orthocarpus447unbellata ⇒ umbellata450ludovociana ⇒ ludoviciana456phrheliometer ⇒ pyrheliometer480rudale ⇒ ruderale481rates ⇒ rats482bases ⇒ basis499clumbs ⇒ clumps
Transcriber's Notes
All obvious typographic errors corrected. For consistency, the species listings forboyliihas been standardised toboylei.
Typographical Corrections