Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park Part 12

Individuals of _P. maniculatus_ obtained on the northern end of Wetherill Mesa in May and June of 1962 had numerous ectoparasites. At this time, the population of _P. maniculatus_ was high, but on a downward trend.

My data and observations lead me to conclude that individuals of _P.

maniculatus_ are more heavily parasitized by both botflies and endoparasites than are individuals of _P. truei_. The reasons for this unequal amount of parasitism in two species of mice occurring in the same general area remain obscure.

The kinds of endoparasites and ectoparasites collected from _P.

maniculatus_ and from _P. truei_ are listed below (m = present in _P.

maniculatus_, t = present in _P. truei_).

ACARINA: Ixodidae: _Dermacentor andersoni_ mt, _Ixodes angustus_ mt, _Ixodes spinipalpis_ m. Laelaptidae: _Androlaelaps glasgowi_ m.

Myobiidae: _Blarin.o.bia_ sp. m. Trombiculidae: _Euschoengastia lanei_ mt, _Euschoengastia criceticola_ m, _Euschoengastia dicipiens_ t, _Euschoengastia peromysci_ m, _Leewenhoekia americana_ m, _Trombicula loomisi_ m.

DIPTERA: Cuterebridae: _Cuterebra cyanella_ mt.

SIPHONAPTERA: _Callistopsyllus deuterus_ m, _Catallagia decipiens_ m, _Epetedia stanfordi_ mt, _Malaraeus sinomus_ mt, _Malaraeus telchinum_ mt, _Megarthroglossus procus_ mt, _Monopsyllus wagneri wagneri_ mt, _Orchopeas leucopus_ mt, _Peromyscopsylla hesperomys adelpha_ mt, _Phalacropsylla allos_ t, _Rhadinopsylla sectilis goodi_ t, _Stenistomera macrodactyla_ m, _Stenoponia_ (_ponera_ or _americana_) mt.

CESTODA: _Choanotaenia_ sp. m, _Hymenolepis_ sp. t.

NEMATODA: _Mastophorus numidica_ mt, _Syphacia obvelata_ mt, _Trichuris stansburyi_ t.

ACANTHOCEPHALA: _Moniliformis clarki_ mt.

PREDATION

In order to determine the relative numbers of each species of _Peromyscus_ that were taken on a seasonal basis by predators, scats of coyotes and foxes were collected from trails and roads at least twice each month, from September 1963 through August 1964. Scats were identified, labeled and dried; all bones and samples of hair were later removed from each scat. Scats that were intermediate in size between the droppings of foxes and coyotes, and that could not be identified readily in the field, were not collected. Bones from the scats were identified to species, and hair was identified to genus or species by comparing color patterns or cuticular patterns with samples from known mammals.

More than 200 impression slides and whole mounts of guard hair and underfur were prepared.

Seven individuals of _P. truei_ and three individuals of _P.

maniculatus_ were represented in 114 coyote scats (Table 13). Both species of _Peromyscus_ comprised only 3.9 per cent of the 253 items of food represented in the 114 scats. Rabbits, _Sylvilagus_ sp. and mule deer, _Odocoileus hemionus_ were the major food items of coyotes. Mice of the genus _Peromyscus_ apparently were preyed upon mostly in autumn (September through November), when mouse populations were near their yearly peaks.

Foxes also prey upon _Peromyscus_ in the park. One _P. truei_ was represented in the 16 scats of foxes that were a.n.a.lyzed. This individual was taken in the winter quarter (December through February).

The bobcat may be an important predator upon _Peromyscus_ in this region, but few scats of this animal were found. Since these could not be a.s.signed to a specific month, they were not saved for a.n.a.lysis.

Anderson (1961:58) believed that bobcats and gray foxes were the most abundant predators in the park. My observations over a period of two years led me to conclude that coyotes were more abundant than foxes and that foxes were, in turn, more abundant than bobcats.

TABLE 13--Food Present in 114 Coyote Scats Collected at Mesa Verde National Park each Month from September 1963 through August 1964.

============================+=============+============ Number Percentage Food Item of of total occurrences items ----------------------------+-------------+------------ _Sylvilagus_ sp. 32 12.65 _Spermophilus variegatus_ 5 1.97 _Eutamias_ sp. 12 4.74 _Reithrodontomys megalotis_ 4 1.58 _Peromyscus boylei_ 2 0.79 _Peromyscus maniculatus_ 3 1.18 _Peromyscus truei_ 7 2.76 _Neotoma cinerea_ 2 0.79 _Neotoma mexicana_ 9 3.56 _Neotoma albigula_ 5 1.97 _Neotoma_ sp. 3 1.18 _Microtus longicaudus_ 1 0.39 _Microtus mexica.n.u.s_ 11 4.34 _Microtus monta.n.u.s_ 1 0.39 _Microtus_ sp. 1 0.39 _Odocoileus hemionus_ 59 23.32 Gra.s.s 34 13.44 Juniper berries 23 9.09 Pinyon needles 14 5.53 Pinyon nuts 1 0.39 Arthropods 7 2.76 Juniper needles 3 1.18 Rodent or Lagomorph bones 5 1.97 _Sceloporus_ sp. 1 0.39 Unidentified fruit 2 0.79 Rocks 3 1.18 Paper 4 1.58 Soil 3 1.18 Feathers 5 1.97 +-------------+------------ Total 253 ----------------------------+-------------+------------

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 upon _Peromyscus_ in 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 of _Peromyscus_. Gopher snakes probably are the most abundant snake in the park; they feed mostly on mice and other rodents. Fur of _Peromyscus_ was found in the stomach of a striped whipsnake, _Masticophis taeniatus_ (Douglas, 1966:734).

DISCUSSION

Five species of _Peromyscus_ inhabit Mesa Verde National Park (Anderson, 1961). Two of these species, _P. crinitus_ and _P. difficilis_ are rare, and none was taken in more than 14,000 trap nights. Several individuals of _P. boylei_ were taken in live traps, but this species could not be regarded as common. The two remaining species, _P. truei_ and _P.

maniculatus_, are the most abundant species in the park. Comparison of the habitats and life-cycles of these two forms and a.n.a.lyses of their interrelations.h.i.+ps have been the objectives of this study.

The distribution of _P. truei_ in the park is regulated by the presence of living pinyon-juniper woodland where logs and hollow trees of _Juniperus osteosperma_ provide nesting and hiding places, and where seeds of juniper trees and nuts of pinyon trees provide food. Several other investigators have reported _P. truei_ to be a.s.sociated with trees, but apparently these findings have not a.s.sumed the importance they warrant in understanding the ecology of this species. Bailey (1931:152) observed an individual of _P. truei_ nesting in a tree on Conchas Creek, New Mexico, and thought that this species might be more arboreal than was generally supposed. The type specimen of _P. t. truei_ was 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.... 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 of _P. truei_ usually 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 of _P. truei_, although this species was most abundant where there was stony soil. The coincidence of rock or stones and a high density of _P. truei_ is 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 of _P. truei_ than 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 support _P. truei_. Its absence was noteworthy on Navajo Hill and on the northern end of Wetherill Mesa where only _P. maniculatus_ lived among the mixed shrubs and gra.s.sland. On the Mesa Verde, pinyon and juniper trees must be present in order for _P. truei_ to live in an area; and, these trees must be alive. Dead pinyons and junipers still stand in the burned part of Morfield Ridge, but no _P. truei_ were found there.

Although a few individuals of _P. truei_ were taken in stands of sagebrush adjacent to pinyon-juniper woodlands, this species does not ordinarily venture far from the forest.

_P. maniculatus_ lives almost everywhere in Mesa Verde; the preferred habitats are open and gra.s.sy with an overstory of mixed shrubs.

Individuals of _P. maniculatus_ venture into ecotonal areas lying between gra.s.slands and pinyon-juniper forest, or between sagebrush and pinyon-juniper forest. _P. maniculatus_ is found also in disturbed areas and in stands of sagebrush that occur in clearings of the pinyon-juniper woodland. In such areas, _P. maniculatus_ and _P. truei_ are sympatric; their home ranges overlap and any inter-specific compet.i.tion that might occur would be expected in these places.

The ability of _P. maniculatus_ to live in many different habitats is correlated in part with its ability to build nests in a variety of sites. Whereas _P. truei_ usually builds nests only in dead branches or logs, _P. maniculatus_ builds nests in such varied places as s.p.a.ces 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 of _Peromyscus_ are reproducing. It is reasonable to a.s.sume that young animals that remain dry survive better than those that become wet and chilled. The nestling young of _P. truei_ are in a more favorable position to remain dry and warm than are nestling young of _P.

maniculatus_.

Captives of each species differed in the amounts of water consumed per gram of body weight. Individuals of _P. truei_ consumed more water per gram of body weight than individuals of _P. maniculatus_. Animals may drink more water than they require when allowed to drink _ad 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.

maniculatus_ appears to be better adapted to aridity than _P. 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-juniper forest, where _P. truei_ lives. The temperature extremes were wider in the microenvironments of a thicket of oak brush and of two different stands of sagebrush, where _P. maniculatus_ lives, than in the forest. _P.

maniculatus_ tends 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. maniculatus_ is better adapted to withstand harsh environments than is _P. truei_.

_P. truei_ may 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. a.n.a.lysis of the percentages of moisture contained in the three most common plants in the trapping grid showed that _P. truei_ could obtain the required moisture by eating about ten grams of these plants daily; individuals of _P. maniculatus_ would need to eat less in order to satisfy their water needs.

Individuals of _P. truei_ died more frequently in warm live-traps than did individuals of _P. maniculatus_. This indicates that _P. truei_ can tolerate less desiccation, or a narrower range of temperatures, than can _P. maniculatus_.

Both species of mice eat some of the same plants, but these plants occur widely. _P. truei_ seems to rely more upon the nuts of pinyons and the seeds of junipers than does _P. maniculatus_. Mounds of discarded juniper seeds were a.s.sociated with all nesting sites of _P. 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 of _P. truei_.

However, this is not the only factor, as is shown by the presence of _P.

maniculatus_ but lack of _P. truei_ in a juniper-pinyon a.s.sociation with an understory of bitterbrush. This habitat was seemingly too arid for _P. truei_.

Factors Affecting Population Densities

The production of young, and success in rearing them, is essential to continuity of any population. _P. maniculatus_ is favored in this respect, because the females produce more young and wean them sooner than do females of _P. truei_. In addition, lactating females of _P.

maniculatus_ require significantly less water than do females of _P.

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. maniculatus_ probably is affected less by prolonged drought than is _P. truei_. Since lactating females require the most water of any animal in the population, they are the weakest link in the system. Females of _Peromyscus_ are 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 of _P.

truei_ would be affected more seriously by a shortage of water than would lactating females of _P. 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, _Cuterebra_ sp., revealed that _P. maniculatus_ has a higher incidence of parasitism by these flies than has _P. truei_; possibly the adult flies concentrate in the open, gra.s.sy areas where _P. maniculatus_ is more abundant, rather than in the woodlands where _P. truei_ lives. Perhaps the lower parasitism of _P. truei_ by warbles is related to the physiology of this species of mouse. Near Boulder, Colorado, the incidence of infection by warbles is lower in _P. difficilis_, a species closely related to _P. truei_, than in _P. maniculatus_ (V. Keen, personal communication).

Although predation by carnivores would be expected to be higher on _P.