Part 18

=====================+=======+=========+=====+=========+========+====== | Total | | | Average | ml. | Total Litter | water | Total | No. | total |H{2}O/ | water | used |corrected|days | weight |gm./day | /day ---------------------+-------+---------+-----+---------+--------+------ P. truei (A) | 1207 | 1120 | 57 | 58.30 | .337 | 19.64 P. maniculatus (C) | 1427 | 1340 | 57 | 76.14 | .308 | 23.50 P. maniculatus (D) | 700 | 670 | 31 | 58.80 | .367 | 21.61 ---------------------+-------+---------+-----+---------+--------+------ | Total | | Average | Gms./ | Total Litter | food | No. | total |gms. wt.| food | used |days | weight | /day | /day -----------------------------+---------+-----+---------+--------+------ P. truei (A) | 651.2 | 50 | 58.30 | .223 | 13.02 P. maniculatus (C) | 743.8 | 57 | 76.14 | .171 | 13.04 P. maniculatus_ (D) | 471.1 | 31 | 58.80 | .258 | 15.19 -----------------------------+---------+-----+---------+--------+------

The young of pregnant and lactating females are the animals in the population most likely to be affected by a deficient supply of water. Drought could reduce the water content of the vegetation to such a level that pregnant or lactating females might find it difficult, if not impossible, to raise litters successfully. If such a drought persisted throughout an entire breeding season, the next year's population would be reduced in numbers, for even under normal climatic conditions it is almost exclusively the juveniles that survive from one breeding season to the next. If such a hypothetical drought occurred, lactating females of P. truei would be in a more critical position than lactating females of P. maniculatus.

In order to determine how much water was available to mice in the peak of the breeding season, samples of the three most common plants in the study area were collected each week for analysis of their moisture content. Plants were placed in separate plastic bags that were sealed in the field. About a dozen plants of each species were used in each determination. Only the new tender shoots of the plants were collected, for it was assumed that mice would eat these in preference to the tougher basal portions of the plants. The plants were taken immediately to the laboratory and were weighed in the bag. Then the bag was opened and it and the contents placed in an incubator at 85 degrees Fahrenheit for a period of at least 72 hours. About 48 hours were required to dry the plants to a constant weight. The dried plants were weighed and their percentages of moisture were determined. Plants lose some water upon being placed in a closed bag; small drops of water appear immediately on the inner surface of the bag. Therefore, the bag must be weighed at the same time as the plants and the weight of the dried bag must be subtracted later.

The three kinds of plants chosen were among the most widely distributed species in the study area, and all three grow close to the ground, within reach of mice. Stems and leaves of two of the plants, Comandra umbellata and Penstemon linarioides, were readily eaten by captive animals. Mice also were observed to eat leaves of Comandra after being released from metal live traps. The third species, Solidago petradoria, differs from the other two in having a short woody stem that branches at ground level. The more succulent shoots arise from this woody stem. The leaves of Solidago are coarse and were not eaten by captive mice. Nevertheless, this species was chosen because it is widely distributed and has the growth form of several other species of plants in the area.

The graph in Figure 20 shows that Comandra contains the highest percentage of water through most of the summer. Water content of both Penstemon and Comandra was greatly reduced in the dry period that occurred in early July. Solidago maintained a relatively constant percentage of moisture; perhaps its woody stem serves for water storage. The rains of July and August increased the percentage of moisture in the plants, but not to the extent expected. Neither Solidago nor Comandra reached the levels of hydration of early June. All plants were collected at or about 11 A. M. At night, when mice are active, these plants would be expected to contain a higher percentage of water than in the daytime.

The data in Figure 20 indicate that mice probably are not endangered by water shortages in most years. The average percentage of moisture in the plants studied was as follows: Comandra umbellata 62.33 per cent; Solidago petradoria 53.0 per cent; Penstemon linarioides 49.28 per cent. If a mouse were to eat ten grams of plant material containing 50 per cent moisture, it would provide him with five grams of food and five grams of water, both of which exceed the minimum daily needs for non-pregnant adults of either species.

The data indicate that there are sufficient differences in water consumption between P. maniculatus and P. truei to account for their habitat preferences in Mesa Verde National Park. In years having average precipitation, water present in the vegetation has the potential for providing enough moisture for the needs of both species. Extended drought would affect individuals of P. truei more adversely than individuals of P. maniculatus.

PARASITISM

Ectoparasites were collected by placing specimens of Peromyscus in separate plastic bags soon after death, adding cotton saturated with carbon tetrachloride, closing the bag for about five minutes, then brushing the fur of the specimen above a sheet of white paper. The ectoparasites were sorted and sent to specialists for identification. Endoparasites were saved when stomach and intestinal contents were examined. Larvae of botflies were collected from mice in the autumn of 1962, placed in sand in containers, and kept over winter until they hatched. Eyelids of alcoholic specimens were inspected for mites by an authority on these organisms.

In 1961, the incidence of parasitism by botflies was the highest for the period 1960-1966. P. maniculatus was more heavily infected with warbles than was P. truei. In 84 individuals of P. maniculatus taken in September 1961, from Morfield Ridge, 32.1 per cent had warbles. The average number of warbles per animal was 1.24, and it was not uncommon to find two or three warbles per mouse. Sixty-nine per cent of the warbles were in the third instar stage, and the rest were in the second instar stage. Warble infestation was higher in the first half of September (40 per cent of mice infected) than in the second half of the month (30 per cent infected), but a larger percentage of the warbles were found (69 per cent) in the second half of the month.

In October 1961, 12.9 per cent of 62 P. truei were infected with warbles. The average number of warbles per infected mouse was 1.37. Seventy-three per cent of the warbles were in the third instar stage; the rest were in the second instar stage. Warble infestation was higher in the first half of October (16 per cent of the mice infected) than in the second half of the month (5.5 per cent infected). These mice were collected from several localities on Chapin Mesa, in pinyon-juniper woodland.

In Mesa Verde the greatest incidence of infestations is in late September and early October. This agrees with the finding of other investigators (Sealander, 1961:58).

Sealander (1961) investigated hematological values in deer mice infected with botflies, and found that infected mice had significantly lower concentrations of hemoglobin than non-infected mice. Myiasis, associated with infection by Cuterebra, is likely to lead to a lowering of the physiological resistance of a segment of the population, and perhaps to a subsequent decline in the population (Sealander, 1961:60).

Mice infected by warbles were less agile than non-infected mice. Other investigators also have reported awkwardness in locomotion in infected mice (Scott and Snead, 1942:95; Sealander, 1961:58). Test and Test (1943:507) noted that parasitized mice did not appear to be emaciated, and this was also true of parasitized mice at Mesa Verde. Healed wounds, where warbles had emerged, were apparent on a number of mice. The warbles, and wounds, usually were found on the flanks and backs of the mice. The large, third instar larvae weighed about one gram apiece; there is little doubt that such large larvae induce trauma in their hosts.

The highest rate of infestation by botflies occurred in 1961, the year in which the population density of P. maniculatus was near its peak. The population of this species was reduced considerably in 1962, and remained low through 1964. In 1965, the density of P. maniculatus appeared to be increasing. Other investigators have reported that increased incidence of Cuterebra infestation in deer mice coincides with lower population densities and with a downward trend in the population (Scott and Snead, 1942:95; Wilson, 1945). My data indicate that this may not be the situation in Mesa Verde.

The intestines or stomachs of almost all individuals of P. maniculatus contained parasites. Endoparasites were less abundant in individuals of P. truei. This heavier infestation of P. maniculatus by tapeworms, roundworms, and spiny-headed worms probably reflects the larger proportion of insects eaten by P. maniculatus than by P. truei.

The most common endoparasite encountered was the nematode, Mastophorus numidica Seurat, 1914; it was found in the stomachs of many individuals of both species of Peromyscus. This nematode has been reported from Felis ocreata in Algeria, Bitis arietans in the Congo, and from the following mammals in the United States: Canis latrans, Peromyscus crinitus, P. gossypinus, P. maniculatus, P. truei, Onychomys leucogaster, Dipodomys ordii, Reithrodontomys megalotis, and Eutamias minimus.

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: Blarinobia 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.