Ticks (Ixodoidea)
Two kinds of ticks were found. One adult specimen of Ixodes sculptus Neumann was clinging to the head of a vole, just in front of its eye. This species of tick was taken also from the thirteen-lined ground squirrel (Citellus tridecimlineatus) at Lawrence. One nymph of Dermacentor variabilis (Say) was found attached to the scapular region of a prairie vole. Both of these specimens were taken in June.
Table 7. Host Distribution of Mites on Seven Small Mammals[F]
Column headings:
A: Scalopus aquaticus
B: Cryptotis parva
C: Blarina brevicauda
D: Peromyscus maniculatus
E: Peromyscus leucopus
F: Sigmodon hispidus
G: Microtus ochrogaster
| A | B | C | D | E | F | G | |
| Ascoschöngastia brevipes (Ewing) | .. | .. | .. | .. | X | X | X |
| Liponyssus occidentalis Ewing | .. | X | .. | .. | .. | .. | .. |
| Eulaelaps stabularis (Koch) | X | X | X | .. | X | .. | X |
| Atricholaelaps glasgowi (Ewing) | .. | .. | .. | X | .. | .. | X |
| Atricholaelaps sigmodoni Strandtmann | .. | .. | .. | .. | .. | X | .. |
| Laelaps kochi Oudemans | .. | .. | .. | .. | .. | X | X |
| Myocoptes sp. | .. | .. | .. | .. | .. | .. | X |
| Listrophorus sp. | .. | .. | .. | .. | .. | .. | X |
[F] These data are from material collected at Lawrence, Douglas County, Kansas.
REPRODUCTION
Age Classes
Each prairie vole was assigned to one of three age classes (juvenile, subadult, or adult) principally on the basis of weight, but partly on the quality and color of the pelage. The three age classes are characterized in table 8.
Table 8. Characters of Juvenile, Subadult, and Adult Prairie Voles
| Juvenile | Subadult | Adult |
|---|---|---|
| Less than 21 grams | 21-38 grams | 38 grams or more |
| Weight usually less than 20 grams | Average weight 30-32 grams | Average weight 40-45 grams |
| Entire pelage dull | Pelage of rump dull; rest of pelage glossy | Pelage usually entirely glossy (rump sometimes dull) |
| Dorsal color black | Dorsal color grizzled except on rump | Entire dorsal color grizzled except sometimes on rump |
Fecundity
Hamilton (1941:4) found for Microtus pennsylvanicus that macroscopic tubules of the cauda epididymis were an indication of fecundity. By noting the size of the tubules (whether macroscopic or not) and by making smears from them in approximately every 25th male caught, I found that the presence of sperm was positively correlated with large-sized tubules of the cauda epididymis in Microtus ochrogaster. Inferentially, males with sperm were fecund.
There is a relationship almost positive between the size of the tubules of the cauda epididymis and the length of the testes. Testes longer than 7 mm. have macroscopic tubules in the cauda, and in testes shorter than 7 mm. these tubules cannot be seen with the naked eye, Hamilton (1937b) found that in M. pennsylvanicus testes smaller than 8 × 4 mm. did not contain sperm. The testes of the prairie vole descend into the scrotum in the breeding season. In the two winter months, when the voles did not bring forth young, the testes decreased in size (see [figure 3]) and were withdrawn into the body cavity. The presence of the testes in the body cavity does not mean that a vole is not in breeding condition, for many specimens with abdominal testes were fecund.
The females were considered to be fecund if they were gravid, or if there were placental scars in the horns of the uteri.
Figure 2. Fecundity of Prairie Voles by Months. Adults and Subadults are Considered Together.
Figure 3. Seasonal Changes in the Length of Testes.
Size of Litters
The number of mammae characteristic of a species of vole may be a rough guide to the average size of a litter for that species. The prairie vole has fewer mammae (three pairs) than some other voles in North America, and might, therefore, be expected to have smaller litters. Fifty-eight gravid females of Microtus ochrogaster examined by me had an average of 3.4 embryos each; the number of embryos ranged from one to seven. Hamilton (1936a) gave 5.07 as the average number of young per litter in M. pennsylvanicus. Hatfield (1935) stated that M. californicus has an average of 5.7 young in a litter. Both pennsylvanicus and californicus normally have four pairs of mammae. The expectation as to the size of the litter seems to be realized. In the prairie vole one pair of mammae is pectoral and two pairs are abdominal. Usually a lactating vole showed evidence of only the abdominal mammae having been in use.
The size of litters was found to vary with the season of the year (see [table 9]). Gravid females were collected in every breeding month except September.
Table 9. Average Size of Litters of Microtus ochrogaster by Months[G]
| Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 2.8 | 3.9 | 3.2 | 3.4 | 3.1 | 2.8 | 3.0 | .. | 3.2 | 2.6 | 0 |
| .. | (4) | (10) | (6) | (8) | (9) | (5) | (2) | .. | (5) | (5) | .. |
[G] These months are from October, 1945, until August, 1946. The numbers in parentheses indicate the number of gravid females collected each month.
Table 9 shows that the prairie vole produced the largest litters in March. A comparison of [table 9] with [figure 2] shows that the largest litters were produced at the height of the breeding season. Baker and Ransom (1933), studying Microtus agrestis, also found that larger litters were characteristic of the height of the breeding season; and that at the beginning and at the end of the breeding season the litters averaged smaller.
The size of litters varied also with the age of the female. To place a gravid female in its proper age class, the weight of the embryos was subtracted from the total weight, and the remaining weight was used as the body weight. The average size of the litters of 14 subadults was 2.9, and in 35 adults it was 3.4. Hatfield (op. cit.) found that the younger females of M. californicus gave birth to smaller litters than did the adults.
Not included in either of the above analyses are nine gravid females collected in November in a pasture watered by an artesian spring in Atchison County, Kansas. In this pasture there was a high concentration of prairie voles, and the percentage of fecundity was much higher than in Douglas County at the same time. In November only 29 per cent of the female prairie voles in Douglas County were fecund, as against 59 per cent in Atchison County. The average number of embryos of these nine voles was 4.1. Data from Atchison County are not included in [table 9].
The Breeding Season
In October, 1945, when this study was begun, the prairie vole was bringing forth young. In the winter of 1945-'46 at Lawrence, Kansas, there was a cessation of reproduction. The reproductive activity was measured in terms of the fecundity of the subadults and the adults of both sexes. [Figure 2] suggests that the decline was most marked in December and January; no gravid females were collected in these two months, although two females trapped in the first week of December were lactating. In October, November, and December, 85 per cent of the breeding females were adults. In October, 85 per cent of the adult females were fecund, and in November, this figure was 80 per cent. Reproduction at this season, in the females, it appears, was largely a function of the adults. The proportion of adults to the rest of the population was calculated for each month; and the monthly changes in relative numbers of adults is shown in [figure 4]. In November, December, and January there was a scarcity of adult voles in the population. The autumnal decline in reproduction occurred simultaneously with the disappearance of these adults, and is thought to have been largely a result of it.
Reproductive activity began in February; and in this month one-third of the females contained embryos, and 90 per cent of the males were fecund. Reproduction reached its height in March when fecundity for the females and males was 77 per cent and 100 per cent respectively. In April both sexes showed signs of being less productive, and still later in the spring the percentage of fecundity remained at slightly over 65 for both sexes, this figure being higher for the males than for the females for any one month. From January to February there was a 30 per cent increase in the percentage of adults in the population; and for this period, there was a 33 per cent increase in the fecundity of both males and females. In February, 80 per cent of the fecund females were adults. The breeding in the late winter, as in the fall, is thought to depend upon the percentage of adults in the population. Hamilton (1937b) noted a similar correlation between winter breeding and dominance of adults in Microtus pennsylvanicus in New York. Fisher (1945) found that the prairie vole continued to breed throughout the winter of 1943-'44 in Missouri; in such a case, one would expect to find a large proportion of adults in the population.
Figure 4. Seasonal Changes in the Numbers of Adults in Relation to the Total Population of Prairie Voles.
Throughout the winter of 1945-'46, at Lawrence, the majority of males were fecund; but fecundity in the females was much less, and in January, no females showed signs of reproductive activity. From this it appears that the females, not the males, limit the breeding season of this species.
SUMMARY
In the eleven month period, October, 1945, until August, 1946, in northeastern Kansas, more than five hundred specimens of the prairie vole (Microtus ochrogaster) were examined in the flesh; and forty free-living voles were examined 157 times—an average of slightly less than four times each.
There is a complete molt from juvenal to subadult pelage, and one from subadult to adult pelage. These molts require three weeks each. Subsequent molts are irregular and extend over longer periods of time.
This vole, in summer, inhabits areas of grass, clover, and alfalfa. In winter, habitats with some woody growth may be sought. Twenty-two kinds of plants were found to be used for food. Although most of these were succulent plants, seeds and small woody stems were sometimes eaten. The prairie vole, like some other species of Microtus, lays away stores of food, usually underground; the maximum quantity found in one cache was two gallons.
Nine other species of small mammals occur in the same habitat with the prairie vole, and frequently use its runways. The vole makes a network of paths through the grass, and constructs its own burrows which lead to its nests and food stores. Each of fifteen nests found were underground. Most, if not all, of the underground tunnels are dug when the soil is moist, not when the soil is dry.
The commonest flea on the prairie vole is Ctenophthalmus pseudagyrtes; it averages 1.9 (for subadult voles) to 3.4 (for adult voles) per individual vole. Other fleas on this vole are Orchopeas leucopus, Orchopeas howardii, Nosopsyllus fasciatus, Epitedia wenmanni, and Rectofrontia fraterna. The two species of fleas which were actually common on the vole (C. pseudagyrtes and O. leucopus), parasitized also some other small mammals which lived in the same habitat as the vole. One species of sucking louse (Hoplopleura acanthopus) and two kinds of mites (Laelaps kochi and Atricholaelaps glasgowi) which occur on the prairie vole in Kansas, occur also on Microtus californicus in California and on M. pennsylvanicus in New York. Only three ticks (1 Dermacenter variabilis and 2 Ixodes sculptus) were found on the prairie vole.
Fifty-eight gravid females had an average of 3.4 embryos. Litters at the height of the breeding season are larger than those at the beginning and at the end of the breeding season. Reproduction in Microtus ochrogaster ceased in December, 1945, in northeastern Kansas, and the first evidence of reproduction in 1946 was observed in February.
LITERATURE CITED
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1900. Revision of the American voles of the genus Microtus. N. Amer. Fauna, 17:1-88, June 6, 1900.
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Baker, J. R., and Ransom, R. M.
1933. Factors affecting the breeding of the field mouse (Microtus agrestris). Part 11. Temperature and food. Royal Soc. London Proc., (Ser. B) 112:39-46, November 1, 1932.
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1941. The reproduction of the field mouse, Microtus pennsylvanicus (Ord). Cornell Univ. Agr. Exp. Sta. Memoir 237, pp. 1-23, May, 1941.
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Nelson, E. W.
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Transmitted August 13, 1946.
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