Reproduction in a Laboratory Colony of the Smith's Red-Backed Vole, Eothenomys Smithii*

J. Mamm. Soc. Japan 13( 1) : 11-20 March 1988

Reproduction in a Laboratory Colony of the Smith's Red-backed Vole, Eothenomys smithii*

Akiro ANDO, Satoshi SHIRAISHIt and Teru Aki UCHIDA

Zoological Laboratory, Faculty of Agriculture, Kyushu University 46-06, Fukuoka 812, Japan (Accepted January 6, 1988)

Abstract. Reproduction in the Smith's red-backed vole, Eothenomys-smithii, was investigated under laboratory conditions. A total of 188 litters produced by 39

females from three generations were used in this study. Litter size was 4.45 (a range of 1-9) en bloc, showing no significant difference between these generations.

The sex ratio of 540 young in 124 litters was 1 male : 0.97 female (274 : 266).

The shortest and most frequent litter intervals were day 19 and day 22, respective ly. Post-partum mating was common. The maximum number of litters per a female

was 19. The oldest female at parturition was 980 days old ; the oldest male sired a

litter at 1,180 days of age. The maximum longevity was 1,280 days. Compared

with other well-studied microtine species in the reproductive parameters, E. smithii

possesses a relatively great reproductive capacity. As to litter size of this species, mean litter size in a laboratory colony exceeded mean embryo counts (4.1 in Kyushu

and •`2.5 in Honshu and Shikoku) in wild populations. On the basis of this fact, it is concluded that reproduction of E. smithii is apt to be influenced by its life en

vironment and field conditions seem to suppress its reproduction.

Key words : Reproductive parameters ; Litter size ; Number of litters ; Interbirth in-

terval ; Smith's red-backed vole.

Introduction

Reproduction occupies a main and important position in the life cycle of a given animal, but it is difficult to obtain data on reproduction by field studies •\ especially the litter size, the number of litters in lifetime, interbirth intervals, etc. Accordingly, such reproductive traits in a laboratory colony are in- dispensable to understanding the life cycle and population dynamics of the species concerned. In addition, the success in breeding in captivity furnishes a signifi cant clue for clarification of the relationship between two related forms which have been taxonomically disputed, by crossbreeding experiments. As to reproduction in the members of the subfamily Microtinae in captivity, many studies have been carried out in the genera Clethrionomys (Langford &

Clulow, 1979; Gustafsson et al., 1980, 1983; Fujimaki & Kuwahata, 1984, etc. ),

Microtus (Negus & Pinter, 1966; Kudo & Oki, 1982; Clarke & Egan, 1984,

*The scientific species name of smithii was adopted in accordance with the revision of the Interna- tional Code of Zoological Nomenclature (1985), instead of smithi used in the previous paper. Reprint request should be addressed to the second author. 12 A. Ando et al. etc.), Pitymys (Jemiolo, 1983) and so forth. On the other hand, the genus Eothenomys consisting of 11 species occurs mainly in China, Taiwan and Japan (Corbet, 1978). Few studies on this genus are available in any aspect. In Eothenomys smithii, which is an endemic species of Japan, the reproductive activ ity and population dynamics in the field have been only fragmentarily reported (Tanaka, 1964; Miyao, 1967; Yoshida, 1973; Yukawa, 1976; Igarashi, 1980) ; however, no information has been published on the reproduction under laboratory conditions. The purpose of this study is to clarify the reproductive capacity as a part of the fundamental study on the laboratory-reared E. smithii and to discuss the reproductive characteristics and particularly some factors influencing the litter size.

Materials and Methods

The breeding colony was derived from 24 voles (12 males and 12 females) live-trapped in Mt. Wakasugi (an elevation of 681 m) near Fukuoka in

1981-1984. The above 12 females of the first generation (wild-caught), 18 females of the second and 9 females of the third each produced 41, 93 and 54 litters. Data used in this study were based on a total of 188 litters from 39

females (see Table 1). The colony was maintained at temperatures of 15 - 20•Ž

on photoperiods of 12 -13 hr light : 12 -11 hr dark throughout the rearing

period, except for that from mid-December 1983 to mid-April 1984 (room temperatures of 1-14•Ž, natural daylengths of 10- 13 hr) during which the con

trolled room was under reconstruction. Humidity was not controlled throughout the experimental period. Animals used in breeding were randomly paired re-

gardless of their age and generation, but inbreeding was avoided. Suitable pairs were kept until they ceased to reproduce consecutively ; when the male died, a new fertile male was supplied. All animals were housed in stainless steel cages

(43 •~ 25 •~ 23 cm), and given ad libitum a commercial diet (CMF, Oriental Yeast Co., Ltd., Tokyo) and water, and sometimes fresh cabbages. The majority of the pregnant females were inspected daily for deliveries so

that their litters were discovered within 24 hr post partum. The day when neo-

nates were found by checking was designated the day of parturition and day 0 of

both the newborn and the consecutive litter interval. In some instances (29/ 188), the presumed day of parturition was determined by the body weight of the

young and changes in their external characters (Ando et al., 1987), because they were discovered within a few days after birth. The number of young on the

day when they were found was regarded as the litter size. The significant dif ference between two means was examined by Student's t-test after equal

variances were confirmed by. F test (p<0.05). Mean values were given with

SE. Reproduction in Smith's Red-backed Voles 13

Results

1. Litter size, interbirth intervals and the number of litters

Mean litter size in each generation was 4.12 •} 0.24 (N = 41, a range of 1- 8) in the first, 4.66 •} 0.17 (N = 93, a range of 1-9) in the second and 4.33 •} 0.22 (N =54 , a range of 1-7) in the third (Table 1). In all generations a distribution of litter size showed a high frequency in 4 and 5, and especially in the second and third generations the distribution had a tendency to concentrate in litter size 4 -

6. The differences in variances and mean litter sizes between these three gen-

Table 1. Frequency distributions of the litter size and mean litter size in 39 females from three generations of Eothenomys smithii.

erations were not significant (p >0.05): the overall mean litter size was 4.45 •} 0.12. However, litter size was influenced by parity ; i. e. mean litter size in successive pregnancies obtained from 27 females (the second and third generations) with a known litter sequence increased gradually from the first towards the fourth litter (Table 2). Afterwards, although mean litter size tended to de- crease slightly, it remained more than 4. Mean litter size was 3.85 •} 0.29 (N =

Table 2. Consecutive litter sizes in 27 laboratory-reared female Eothenomys smithii. 14 A. Ando et al.

27, a range of 1-7) in primiparous females, whereas 4.82 •} 0.11 (litters 2 - 6 ; N =78 , a range of 1-9) in multiparous ones : the difference in mean litter size between these two groups was significant (p<0.01). Furthermore, mean litter

size varied widely with the individual level. In 13 females (the second and third

generations) having over five litters, the lowest and highest prolificacy was 2.8 (N =12, a range of 1-4) and 6.8 (N = 5, a range of 5- 9) young per litter, respectively. As for the interbirth interval, a frequency distribution of 113 intervals of

below 50 days was shown in Fig. 1. Most of these intervals (93 instances)

distributed between days 19 and 28, and in particular about a half of the whole

(55 instances) centered on days 21-23. The mode and minimum of litter intervals were day 22 and day 19, respectively.

One female having the maximum number of litters produced 19 litters with a

total of 79 offspring from 238 to 767 days of age (Table 3). The number of

young per partum was 4.16 ranging from 2 to 7 ; however, litter size was less in the first two, 16th and last two litters. Interbirth intervals of days 20 - 25 occupied 78% (14118) of all.

Table 3. A maximum record of parturition obtained from one individual of Eothenomys smithii .

* 238 days old ; •õ 767 days old Reproduction in Smith's Red-backed Voles 15

2. Sex ratio, puberty and longevity

The sex ratio of 540 young in 124 litters was 1 male to 0.97 female (274 males : 266 females), which was not significantly different from 1 : 1 ratio (ƒÔ2 test, p>0.05). No attempt to investigate the age at puberty and the end of the reproductive life was carried out. However, the six youngest females whose vaginae perforated were 23 (3), 25 (2) and 28 (1) days old. The youngest males, in which presence of spermatozoa in their cauda epididymidis was recognized by the smear technique, were 31-43 days old. On the other hand, the oldest female at parturition was at the age of 980 days and a certain male sired a litter at 1,180 days of age. The maximum longevity was 1,280 days in males and 1,140 days in females.

Discussion

This is the first report on reproduction in the genus Eothenomys under laboratory conditions. Several important parameters for reproduction in seven species belonging to four genera of Microtinae are given in Table 4. Our dis cussion deals with some factors affecting litter size, in addition to the reproduc tive variables of the laboratory-reared E. smithii. Values based on a long-term experiment with a large sample size are sufficiently reliable as indices of a reproductive life span and a physiological longevity which have been unknown. Interbirth intervals ranged from 19 to 28 days (Fig. 1) and postpartum mat ing was usual in E. smithii. Judging from the shortest litter interval of this species, the gestation period is considered to be - 19 days. In order to know exactly the duration of pregnancy, however, the period between mating and par turition should be decided : for example, the gestation periods determined by the latter method are 18.3 days (Gustafsson et al., 1980) and 19.5 days (Clarke & Hellwing, 1983) in Clethrionomys glareolus, 18.5 days in Clethrionomys gapperi (Langford & Clulow, 1979) and 19.7 days in Microtus agrestis (Breed, 1969). The youngest age at parturition is oldest in E. smithii (120 days of age) among seven species listed in Table 4. However, the vagina opened at days 23-28 in the some youngest female E. smithii. This fact suggests that females have already attained puberty at this age. The females did not conceive at less than 100 days of age which was calculated backward from the date of the first parturition. In this connexion, in C. glareolus the female fertility at the first mating below 100 days of age is low because of the insufficient sensitivity compared with a normal mating (Westlin & Gustafsson, 1984). In E. smithii, on the other hand, the oldest age at parturition was day 980; this age did not much differ from those in other species (Table 4). Furthermore, it is evident that the males also maintain normal breeding ability for a long period, because they reached puberty at days 31-34 and one of them sired a litter at day 1,180. Judging from the survival record of a male (1,280 days), the physiological longev ity of this species is over three years. 16 A. Ando et al.

Table 4. A comparison of the reproductive traits and longevity under laboratory conditions among seven microtine species.

* Converted age in months into in days. MLS, mean litter size ; SIT, shortest interbirth interval ; MIT, mode of interbirth interval ; MIN, maximum number of litters ; YAP, youngest age at parturition ; OAP, oldest age at parturition ; ML, maximum longev ity.

Mean embryo counts of E. smithii from wild populations have been reported to be 2.6 (N =19, a range of 1- 4) (Yukawa, 1976) in Honshu, 2.6 (N =120, a range of 1-5) (Tanaka, 1964), 2.4 and 2.5 (N = 96, a range of 1- 4 and N = 41, a range of 1- 4, respectively) (Igarashi, 1980) in Shikoku, and 4.1 (N =13, a range of 2 - 7) (Yoshida, 1973) in Kyushu. The mean litter size (4.5) of this species with a range of 1- 9 under rearing conditions was close to the mean embryo counts in Kyushu (4.1), but much larger than those in Honshu (2.6) and Shikoku (2.4 - 2.6), although litter size does not exceed the number of embryos theoreti cally. Reproduction in Smith's Red-backed Voles 17

Fig 1. Frequency distributions of interbirth intervals in a laboratory colony of Eothenomys smithii.

Regarding the relation between embryo counts and litter size, in Microtus montebelli average embryo counts from field studies are 3.5 - 5.2 (Imaizumi & Yoshiyuki, 1957; Watanabe, 1962; Miyao et al., 1966; Shiraishi, 1967; Abe, 1974; Kaneko, 1978; Saito et al., 1980), while average litter sizes are 3.4 - 4.7 (Table 4) ; i. e. the latter is slightly smaller than the former. A similar relationship is found in C. rufocanus bedfordiae (Fujimaki & Kuwahata, 1984). Addi- tionally, in most species of the New World Microtus, with exception of M. orego ni and M. pinetorum, values for laboratory colonies (litter size) are lower than the highest values observed under field conditions (embryo counts) (Kelley, 1985). Thus, it is characteristic of E. smithii that the litter size in a breeding colony exceeds the number of embryos in field populations, although these two values are obtained under different conditions. Furthermore, E. smithii has a relatively large mean litter size among seven species listed in Table 4 and the members of the Microtinae (the minimum 1.8-the maximum 6.0) (Hasler, 1975). However, it is necessary to note that litter size is affected by rearing conditions and diets (Hasler, 1975; Kuwahata, 1976; Alibhai, 1985) and difference in litter size is also recognized between breeding colonies of C. glareolus derived from different regions in spite of the same rearing conditions (Gustafsson et al., 1983). On the other hand, prenatal loss of ova and embryos is known to be one of important factors which bear upon litter size in several murine mammals under field or laboratory conditions (Beer et al., 1957; Hamajima, 1961; Shiraishi, 18 A. Ando et al.

1967; Langford & Clulow, 1979; Fujimaki, 1981; Clarke & Egan, 1984; Nadeau, 1985). In this connexion, although prenatal mortality of E. smithii has not yet been investigated, it seems possible that alteration in ovulation rate and/ or prenatal loss may bring about the difference in embryo counts between dif ferent regions (i. e. Kyushu and the other regions) and the discrepancy between litter sizes from a laboratory colony and embryo counts from field populations. Much remains to be learned as to the reproduction in E. smithii. On the basis of the fact that the mean number of young in a laboratory colony excelled that of embryos under field conditions, it is concluded that reproduction in E. smithii is apt to be influenced by its life environment and field conditions seem to suppress its reproduction.

Acknowledgements

We wish to thank the staff and graduate students of the Zoological Labora- tory, Faculty of Agriculture, Kyushu University for their help and encourage- ment. We are also indebted to Professor E. W. Jameson, Jr. of the University of California for comments on the manuscript. This work was supported in part by Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.

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