Table 7—Food and Water Consumption ofPeromyscus maniculatusandP. trueiWhen Fed Diets of Different Protein Content. Food and Water Consumption Are Determined for the Grams, or Milliliters, Consumed per Gram of Body Weight per Day; Daily Totals Are also Given.Peromyscus maniculatus rufinusDiet per cent proteinNo. miceFood /gram /day ± S. D.Total grams /dayWater /gram /day ± S. D.Total water /dayLab Chow 239.201 .0744.455.262 .1835.751Hog Chow 3610.238 .0605.232.496 .18610.749Corn 1111.149 .0443.144.174 .0123.696Peromyscus truei trueiDiet per cent proteinNo. miceFood /gram /day ± S. D.Total grams /dayWater /gram /day ± S. D.Total water /dayLab Chow 2310.216 .0706.353.373 .11910.880Hog Chow 3610.230 .0796.966.653 .18919.571Corn 1110.158 .0104.318.332 .0169.034
Table 7—Food and Water Consumption ofPeromyscus maniculatusandP. trueiWhen Fed Diets of Different Protein Content. Food and Water Consumption Are Determined for the Grams, or Milliliters, Consumed per Gram of Body Weight per Day; Daily Totals Are also Given.
The tendency of both species to eat more of the hog chow than they ate when fed standard laboratory chow may reflect a higher palatability of the hog chow. Both species consumed similar amounts of food per gram of body weight, on each of the diets (Table 7). The largerP. trueirequires moregrams of food per day than the smallerP. maniculatus, but this slight difference in food consumption probably has no effect on the distribution of these species within Mesa Verde.
The results obtained with the low protein diet were strikingly different from those of the first two experiments. In this experiment the same groups of mice were placed on a diet of whole, shelled corn for a period of six weeks. The corn contained less than 11 per cent protein, about three per cent fat, and about 80 per cent carbohydrate.
By the end of the first week, on the low protein diet, all mice had reduced their water intake by about half the amount used per day on the high protein diet (Table 7). There was not a statistically significant difference, for either species, between the average amounts of water drunk in the first and in the sixth weeks of the experiment.
The data inTable 7show that on all three diets, individuals ofP. maniculatusdrank less water per gram of body weight than individuals ofP. truei. Variation in water consumption was high; some individuals ofP. maniculatusthat drank more than the average amount for the species, consumed as much water as some individuals ofP. trueithat drank less than the average amount. In general, individuals ofP. maniculatusdrank about half as much water each day as individuals ofP. truei. Individuals of both species were consistent in their day-to-day consumption.
Table 8—Amounts of Mean Daily Water Consumption as Reported in the Literature for Species ofPeromyscus. Figures in Parentheses are Means; Those Not in Parentheses Are Extremes.Mean daily ml./gm. wt./dayWater consumption total ml. per dayTemperatureHumidityPer cent dietary proteinInvestigator(.262)(5.70)P. m. rufinus.124-.6992.71-15.0720-23low23[A]P. m. rufinus(.101)(2.39)20-2524-47[B]P. m. osgoodi.16-.253.2-4.318-2210-2023[C](.126)(1.74)P. m. bairdii.082-.1771.12-2.722125-68[D]P. m. bairdii.124-.182(2.37-3.17)20-2524-47[B](.372)(10.80)P. t. truei.224-.5617.0-16.9220-23low23[A]P. t. truei(.085)(2.77)20-2524-47[B]P. l. nov..057-.1171.36-2.292125-68[D]P. l. nov.(5.36)1862.5[E][A]Douglas[B]Lindeborg, 1952[C]Williams, 1959[D]Dice, 1922[E]Chew, 1951
Table 8—Amounts of Mean Daily Water Consumption as Reported in the Literature for Species ofPeromyscus. Figures in Parentheses are Means; Those Not in Parentheses Are Extremes.
Table 8shows average water consumption for several species ofPeromyscusas reported in the literature, and as determined in my study. It is difficult to compare my results with most of the data in the literature, because of a lack of information as to protein, fat, carbohydrate, and mineral contents of foods used in other studies. Lindeborg (1952) and Dice (1922) fed mice on amixture of rolled oats, meat scraps, dry skimmed milk, wheat germ, etc. described by Dice (1934). Their data on water consumption inP. maniculatusindicate that this mixture probably is lower in protein content than Purina Laboratory Chow, that was used in my experiments and those of Williams' (tables8and9).
The amount of dietary protein consumed under natural conditions is not known for most wild animals. One index of the minimum amount of protein necessary is the amount required for an animal to maintain its weight. At best, this can be only an approximation of the required amount, for other factors, such as stress, disease, change in tissues during oestrus or gonadal descent, and changes in constituents of the diet other than protein, would all be expected to affect the body weight (Chew, 1965:145-147).
The data inTable 7show that both species vary their food intake with changes in diet.Table 10shows weight changes that took place in individual mice when fed each of the three diets. A change in weight of one gram cannot be considered as important, for the weight of an individual mouse fluctuates depending upon when he last drank, ate, defecated or urinated.
The only significant changes in weight occurred when mice were fed low protein food (Table 10). Individuals ofP. trueilost 15.72 per cent and individuals ofP. maniculatuslost 10.03 per cent of their total body weights on this diet. This indicates that food having a protein content of more than 10 per cent but less than 23 per cent is required for maintenance of weight in these animals.
Although knowledge of the amount of water consumed,ad libitum, by adult mice is valuable information, maintenance of the population depends upon reproduction and dispersal of young individuals. My trapping data indicate that only two to three per cent of the adults live long enough to breed in consecutive breeding seasons. In spring, the breeding population is composed largely of mice that were juveniles or subadults during the latter parts of the breeding season. Therefore, the critical time for the population may well be the time when the season's young are being produced. Any unfavorable circumstances, such as a shortage of food or water, that would affect pregnant or lactating females would be of primary importance to the integrity of the population.
Table 9—A Comparison of Mean Daily Water Consumption of Mice on High Protein Diets. Numbers in Parentheses Are Average Values; All Others Are Ranges of Values.SpeciesMean daily H2O consumptionTemperatureRelative humidityInvestigatorcc./gm. wt.Total cc.P. m. osgoodi(0.27-0.54)(4.6-9.3)18-22 C10-20Williams, 1959(0.496)(10.74)P. m. rufinus0.186-0.7644.54-16.5720-23 ClowDouglas(0.653)(19.57)P. t. truei0.429-1.03113.28-30.2820-23 ClowDouglas
Table 9—A Comparison of Mean Daily Water Consumption of Mice on High Protein Diets. Numbers in Parentheses Are Average Values; All Others Are Ranges of Values.
One would assume that pregnant and lactating females require more waterthan non-pregnant females. One might also assume that juveniles require different amounts of water and food than adults. Juveniles have less dense pelage than adults, and probably are affected more by their immediate environment because of their relatively poor insulation. Juveniles might also be in an unfavorable situation insofar as water conservation is concerned, because they are actively growing, and in most cases, acquiring new pelage; it is well known that these are times of stress for the individual.
Table 10—Weights of Mice at Start and Finish of Experiments, Showing Changes in Weight and Mean Weights, and Means of Changes in Weight (mean delta).Peromyscus truei trueiNo.Lab ChowHog ChowCornStartEndΔStartEndΔStartEndΔ131.031.30.331.332.31.032.329.03.3531.130.50.630.532.82.332.828.74.1627.627.10.527.129.52.429.527.32.2728.026.31.726.327.51.227.522.25.31325.830.64.830.627.03.627.022.24.81426.930.73.830.731.40.731.427.34.11525.429.44.029.429.80.429.824.05.81633.032.90.132.930.52.430.526.04.51937.638.10.538.131.86.331.822.09.82023.525.82.325.826.20.426.222.93.1Ȳ28.930.21.830.229.82.029.825.24.7Peromyscus maniculatus rufinusNo.Lab ChowHog ChowCornStartEndΔStartEndΔStartEndΔ223.020.72.320.721.10.421.118.62.5322.723.10.423.123.80.723.820.73.1422.021.10.921.121.80.721.821.30.5826.328.11.828.115.82.325.823.82.0921.524.02.524.025.11.125.121.83.310………………22.520.02.51121.022.11.122.120.81.320.819.01.81222.323.20.923.221.31.921.320.40.91718.920.01.120.019.20.819.219.40.21817.017.50.517.519.52.019.517.32.22118.918.10.818.120.22.120.217.32.9Ȳ21.421.81.221.821.81.321.919.92.2
Table 10—Weights of Mice at Start and Finish of Experiments, Showing Changes in Weight and Mean Weights, and Means of Changes in Weight (mean delta).
Lindeborg (1950:76) found that 15 days before parturition, pregnant and non-pregnant females ofP. m. bairdiidrank about the same amounts of water, that females consumed more water after the young were born and until they were weaned, and that water consumption increased with an increase in weight in young, growing individuals. He found that in the later stages of pregnancy, females ofP. m. bairdiirequired 36 per cent more water than non-breeding females; at 14 days after parturition, nursing females required 111 per cent more water than non-breeding females, and at weaning time, 158 per cent more water. Dice (1922:35) reported a 217 per cent increase in drinking ofP. m. bairdiibefore parturition, and 171 per cent increase while nursing.
Several females of both species were bred prior to the start of the experiments described herein. As a consequence, it was possible to determine water and food consumption for lactating females of each species, and later, for their litters. Pregnant and lactating females, and newly-weaned litters, were fed laboratory chow throughout this experiment. The litters were separated from their mothers as soon as the young were observed to be eating, or no later than 33 days after birth.
Table 11shows the amounts of water and food consumed by two females of each species while they were either in the later stages of pregnancy, or were nursing. Although the data inTable 11do not cover the full developmental time of the litters involved, it is obvious that both lactating females ofP. trueiand one female ofP. maniculatusconsumed more water than the average for their species (Table 7). Water and food consumption was measured for both females ofP. trueiwhile they were nursing. The female that gave birth to litter A was left in the cage with the male for several days after the litter was born, resulting in another litter being born about 27 days after the first. Therefore, the record of this female represents an extreme case of stress (probably a common occurrence in nature) in which a female is nursing one litter while she is pregnant with a second.
The record of the female ofP. trueithat gave birth to litter B is the most complete, including data from the fifth day after parturition until the young were weaned on the thirty-third day after parturition. The record of the female ofP. maniculatusthat gave birth to litter C covers the last 10 days of nursing before the young were weaned. After being separated from her litter, this female drank more than the average amounts of water, on both high and low protein diets. Although the food and water were lost several times for the female ofP. maniculatuswith litter D, the period of time covered by the 14 days when water and food consumption were measured includes times just prior to parturition and to weaning of the young.
Table 11—Water and Food Consumed by Nursing Females ofP. trueiandP. maniculatus. Consumption Is Calculated on the Basis of Amount (Milliliters or Grams) Consumed per Gram of Body Weight per Day, as well as Total Amounts Used per Day.FemaleWater usedNo. daysAverage weightml. H2O/gm./dayTotal water/dayNo. in litterP. truei(A)4471733.00.79626.293P. truei(B)6762832.70.73824.143P. maniculatus(C)1911019.45.98319.105P. maniculatus(D)1331424.35.2245.466FemaleFood usedNo. daysAverage weightgms. food/gm./dayTotal food/dayNo. in litterP. truei(A)214.72633.00.2508.263P. truei(B)120.52432.70.1535.023P. maniculatus(C)47.81019.45.2464.785P. maniculatus(D)180.12127.42.3128.586
Table 11—Water and Food Consumed by Nursing Females ofP. trueiandP. maniculatus. Consumption Is Calculated on the Basis of Amount (Milliliters or Grams) Consumed per Gram of Body Weight per Day, as well as Total Amounts Used per Day.
It is interesting that the female ofP. maniculatuswith litter C used much more than the average amount of water for the species, and even more per gram of body weight than lactating females ofP. truei. Conversely, water consumption of the female with litter D was within one standard deviation of the mean for all adults ofP. maniculatus. I infer that at least some lactating females ofP. maniculatusare better adapted to aridity than are some lactating females ofP. truei.
Table 11also shows food consumption of the four females discussed above. All females, with the exception of the female with litter D, consumed amounts of food that lie within one standard deviation of the means for their species. The female with litter D had the most young, consumed the most food but drank the least water of the four females. Later, when separated from her litter and placed on the low protein diet, this female drank only .046 milliliters of water per gram of body weight per day. This figure is less than one-third of the average amount (.174) for this species (Table 7).
The records of water and food consumption for litters A, C, and D are given inTable 12; the mice in litter B persisted in placing wood shavings in the opening of the spout on their water bottle, causing loss of the water. The data show that mice in all three litters had an average water and food consumption within one standard deviation of the mean for adults of their respective species (Tables7and12). It is interesting that juveniles of both species require no more food and water per gram of body weight than adults. This indicates that if a young animal survives the rigors of postnatal life until it is weaned, it is then at no disadvantage as far as food and water consumption are concerned. This would be greatly advantageous to the species, as a population, for the young could disperse immediately upon weaning, and go into any areas that would be habitable for adults of the species.
Table 12—Food and Water Consumed by Young Mice in Litters, After Weaning. Consumption Is Calculated on the Basis of the Amount (Milliliters or Grams) Consumed per Gram of Litter Weight per Day; Total Amounts Are Shown and Can Be Divided by Litter Size for Average Individual Consumption. Litter Sizes Are as Follows: A=3; C=5; D=6.LitterTotal water usedTotal correctedNo. daysAverage total weightml. H2O/gm./dayTotal water/dayP. truei(A)120711205758.30.33719.64P. maniculatus(C)142713405776.14.30823.50P. maniculatus(D)7006703158.80.36721.61LitterTotal food usedNo. daysAverage total weightGms./gm. wt./dayTotal food/dayP. truei(A)651.25058.30.22313.02P. maniculatus(C)743.85776.14.17113.04P. maniculatus(D)471.13158.80.25815.19
Table 12—Food and Water Consumed by Young Mice in Litters, After Weaning. Consumption Is Calculated on the Basis of the Amount (Milliliters or Grams) Consumed per Gram of Litter Weight per Day; Total Amounts Are Shown and Can Be Divided by Litter Size for Average Individual Consumption. Litter Sizes Are as Follows: A=3; C=5; D=6.
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 pregnantor 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 ofP. trueiwould be in a more critical position than lactating females ofP. 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 umbellataandPenstemon linarioides, were readily eaten by captive animals. Mice also were observed to eat leaves ofComandraafter 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 ofSolidagoare 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 inFigure 20shows thatComandracontains the highest percentage of water through most of the summer. Water content of bothPenstemonandComandrawas greatly reduced in the dry period that occurred in early July.Solidagomaintained 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. NeitherSolidagonorComandrareached 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 inFigure 20indicate 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 umbellata62.33 per cent;Solidago petradoria53.0 per cent;Penstemon linarioides49.28 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 betweenP. maniculatusandP. trueito 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 ofP. trueimore adversely than individuals ofP. maniculatus.
Fig. 20: Graph showing percentages of moisture contained during the summer of 1964, by three abundant and widely-distributed species of plants in Mesa Verde National Park, Colorado.
Parasitism
Ectoparasites were collected by placing specimens ofPeromyscusin 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. maniculatuswas more heavily infected with warbles than wasP. truei. In 84 individuals ofP. maniculatustaken 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 62P. trueiwere 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 byCuterebra, 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 ofP. maniculatuswas near its peak. The population of this species was reduced considerably in 1962, and remained low through 1964. In 1965, the density ofP. maniculatusappeared to be increasing. Other investigators have reported that increased incidence ofCuterebrainfestation 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 ofP. maniculatuscontained parasites. Endoparasites were less abundant in individuals ofP. truei. This heavier infestation ofP. maniculatusby tapeworms, roundworms, and spiny-headed worms probably reflects the larger proportion of insects eaten byP. maniculatusthan byP. truei.
The most common endoparasite encountered was the nematode,Mastophorus numidicaSeurat, 1914; it was found in the stomachs of many individuals of both species ofPeromyscus. This nematode has been reported fromFelis ocreatain Algeria,Bitis arietansin 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, andEutamias minimus.
Individuals ofP. maniculatusobtained on the northern end of Wetherill Mesa in May and June of 1962 had numerous ectoparasites. At this time, the population ofP. maniculatuswas high, but on a downward trend.
My data and observations lead me to conclude that individuals ofP. maniculatusare more heavily parasitized by both botflies and endoparasites than are individuals ofP. 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 fromP. maniculatusand fromP. trueiare listed below (m = present inP. maniculatus, t = present inP. truei).
Acarina: Ixodidae:Dermacentor andersonimt,Ixodes angustusmt,Ixodes spinipalpism. Laelaptidae:Androlaelaps glasgowim. Myobiidae:Blarinobiasp. m. Trombiculidae:Euschoengastia laneimt,Euschoengastia criceticolam,Euschoengastia dicipienst,Euschoengastia peromyscim,Leewenhoekia americanam,Trombicula loomisim.
Diptera: Cuterebridae:Cuterebra cyanellamt.
Siphonaptera:Callistopsyllus deuterusm,Catallagia decipiensm,Epetedia stanfordimt,Malaraeus sinomusmt,Malaraeus telchinummt,Megarthroglossus procusmt,Monopsyllus wagneri wagnerimt,Orchopeas leucopusmt,Peromyscopsylla hesperomys adelphamt,Phalacropsylla allost,Rhadinopsylla sectilis goodit,Stenistomera macrodactylam,Stenoponia(poneraoramericana) mt.
Cestoda:Choanotaeniasp. m,Hymenolepissp. t.
Nematoda:Mastophorus numidicamt,Syphacia obvelatamt,Trichuris stansburyit.
Acanthocephala:Moniliformis clarkimt.
Predation
In order to determine the relative numbers of each species ofPeromyscusthat 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 ofP. trueiand three individuals ofP. maniculatuswere represented in 114 coyote scats (Table 13). Both species ofPeromyscuscomprised only 3.9 per cent of the 253 items of food represented in the 114 scats. Rabbits,Sylvilagussp. and mule deer,Odocoileus hemionuswere the major food items of coyotes. Mice of the genusPeromyscusapparently were preyed upon mostly in autumn (September through November), when mouse populations were near their yearly peaks.
Foxes also prey uponPeromyscusin the park. OneP. trueiwas represented in the 16 scats of foxes that were analyzed. This individual was taken in the winter quarter (December through February).
The bobcat may be an important predator uponPeromyscusin this region, but few scats of this animal were found. Since these could not be assigned toa specific month, they were not saved for analysis. 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.