Introduction

The grasscutter or cane rat (Thryonomys swinderianus) is a wild hystricomorph rodent hunted particularly in West Africa for its meat (Vos 1978; Baptist and Mensah 1986; National Research Council 1991; Ntiamoah-Badu 1998). It is desirable for domestication because of its excellent taste, and comparatively higher nutritional value (Asibey and Eyeson 1973) and meat yield (Clottey 1981) than most species of livestock. It contributes in some parts of Africa towards the alleviation of the chronic protein shortage on the continent (Clottey 1968; Asibey 1969; Baptist and Mensah 1986; Anonymous 1993; Ntiamoah-Badu 1998). There is however paucity of information on the biology and breeding habits of the grasscutter, which makes its exploitation difficult (Ajayi 1971; Hartog and Vos 1973; Asibey 1974; Adamu et al 1993; Yeboah and Adamu 1995). Additionally, aggressive hunting of the grasscutter results in the destruction of forest and farmlands as well as reduction in the wild grasscutter populations (Falconer 1992; Addo 1997; Ntiamoah-Badu 1998).

The research reported in this paper sought to provide some of the missing links in the breeding habits of the grasscutter in order to encourage its production in captivity and consequently to help conserve the environment and reduce the pressure on the remaining wild grasscutter populations.

In order to optimise the breeding of animals in captivity, reproductive parameters have to be delineated and breeding strategies established. In the present study, the optimal duration of male-female exposure to achieve pregnancy was investigated. This choice was based on the permanent mating system commonly practised by most farmers, which sometimes results in cannibalism of the neonates by the male (Asibey 1974) and the eventual failure of the breeding programme undertaken.

To determine the optimal duration of male-female exposure to achieve pregnancy, various mating systems could be investigated. These basically entail varying periods of male-female exposure to ensure mating at a time approximating ovulation. Some of the methods are: (i) permanent housing, which provides maximum opportunity for mating, pregnancy and post-partum mating (if postpartum oestrus is manifested by the species under investigation); (ii) temporary mating where the male is separated from the female only during the late stage of pregnancy or on delivery; and (iii) controlled mating, often referred to as ‘hand-mating’ where the male and female are left together until copulation and separated immediately after. In the present study, the hand-mating system was adopted and used in conjunction with individual caging of the female before and after mating. The hand-mating approach provided an enabling environment for comparing various durations of male-female exposures to the outcome of each exposure. Individual caging prior to pairing was necessary, since grasscutters are reflex ovulators like rabbits and could therefore ovulate as the result of an orgasm induced by contact with other females (Hagen 1987), a situation which was most undesirable for the study. Individual caging post-mating also facilitated a closer and better surveillance of the animals, allowing for proper evaluation of the outcome of the male-female exposure. Though grasscutter are induced ovulators, they show variations in reproductive activity or sexual cycle (Adjanohoun 1993; Addo 1997). Therefore in determining the optimal duration of mating for the grasscutter, the various presentations of the sexual cycle namely: period of sexual rest (manifested by an open or secretion-sealed vagina) and sexual repose (manifested by a closed vaginal orifice) were factored into the study.

This paper reports on the results of hand-mating grasscutters, specifically, the effect of the duration of male-female exposure on the female’s acceptance to mate with the male, conceive and deliver.

Materials and methods

Animal management

A total of 30 female and 8 male grasscutters was used in the study. They were initially laboratory and health-conditioned at the conventional animal facility of the Noguchi Memorial Institute for Medical Research (NMIMR) as laid down by the Institutional Animal Care and Use Committee of the Institute. Laboratory conditioning entailed getting the grasscutters acclimatized to laboratory animal housing conditions: caging, ambient temperature (25-30ºC), relative humidity, (60-90%) light-dark cycle (12-12hous), 24-hour natural ventilation, feeding, watering and human handling. Health conditioning involved improving the health status of the grasscutters by routine physical, haematological and microbiological examinations with appropriate veterinary intervention. The grasscutters are hystricomorphs and were therefore maintained under conditions established for guinea pigs, the only hystricomorph rodents maintained at NMIMR. Each animal was individually caged throughout the study, except during the hand mating period. The animals were individually caged to prevent inadvertent ovulation, being induced ovulators.

Selection of breeders and the hand-mating process

Sexually mature, healthy grasscutters were selected for the study. They comprised 8 males and 30 females of an indeterminate age that had been in captivity for at least 6-8 months. The males presented a darkened perineum, a sign of sexual maturity, while the females manifested the vaginal membrane closure and rupture phenomenon, the sign of sexual maturity in females (Addo 1997). No attention was paid to the time of year for the conduct of the study since grasscutters breed all year round. The females were placed in 3 groups of 10 according to the three presentations of the sexual cycle: one group consisted of females in the active phase of the sexual cycle with ruptured vaginal membranes (open vagina), a second group consisted of females in the active phase of the sexual cycle but with secretion-sealed vaginal orifices (sealed vagina) and the third group consisted of females in the resting phase of the sexual cycle with intact vaginal closure membranes (closed vagina). Handmating was conducted by transferring the female to the male’s cage. Prior to the transfer of each female into a male’s cage, its body weight, vaginal status (open, sealed and closed), date and time of the transfer were noted, after which she was left with the male until she had been successfully mated.

Assessment of male-female exposure outcomes: sexual receptivity, pregnancy, parturition

During the female’s stay with the male it was examined daily for signs of sexual receptivity (acceptance to mate with the male). Sexual receptivity was determined by monitoring post-pairing perineal changes as described by Addo (1997) and presence of a copulatory plug in the vagina or cage tray. On observing any of the enumerated signs, the female was immediately and permanently separated from the male, reweighed, transferred to its own cage and the day and time of the appearance of the mating-sign(s) noted. Each of the 8 males was allowed to mate at least 2 females during the course of the study. Pregnancy was determined by palpating the developing foetuses in utero, monitoring changes in body weight and pregnancy associated perineal changes as described by Addo (1997). Animals diagnosed pregnant were observed unobtrusively from the second month until parturition.

Statistical analysis

Statistical analysis was conducted with the statistical package for the social sciences (SPSS). The differences in duration of pairing among the three categories of females (open, sealed, closed) were analysed using analysis of variance (one-way ANOVA) after log transformation of the duration of pairing data. The body weight changes amongst the three categories of females were analysed using one-way ANOVA. Outcome of pairing i.e. female’s acceptance to mate, conceive and deliver were cross-tabulated by vaginal status (open, sealed, closed) to determine if pairing outcome was independent of the vaginal status. Outcome of pairing was also cross-tabulated by duration of pairing to determine if there was any association between duration of pairing and pairing outcome. The significance and strength of the associations were measured using phi coefficient. The predictability of the outcomes based on duration of pairing were computed using PRE (proportional reduction in error: Goodman and Kruskal tau and the uncertainty coefficient). Prior to cross-tabulating duration of pairing and pairing outcome, the duration data was divided into two obvious periods: the first three days which had the highest counts of receptivity, conception and parturition and the rest of the two-week period which had the lowest counts of receptivity, pregnancy and parturition. The differences between the two pairing periods were analysed using the independent samples "t" test. Reproductive performance was computed by the frequencies and case summaries procedures for each of the three categories of females. Reproductive performance was defined as the percentage receptivity, percentage conception and percentage parturition.

Results

Sexual receptivity

Details on the outcome of male-female exposure with respect to the female’s acceptance of the male are shown in Table 1. In summary, 24 (80.0%) of the 30 females were receptive within 18 to 72 hours of pairing, while 5 (16.7%) were receptive between 120 to 192 hours of pairing and 1 (3.3%) was non-receptive after 336 hours of pairing. In the case of the 1 non-receptive female, a new female (with an intact vaginal membrane) was presented to the male. After the introduction of the new female the male abandoned the old partner and successfully mated the new candidate within 18 hours of pairing, in the presence of the non-receptive female. The non-receptive female had extensive lacerations on the trunk and had also lost 357 g in body weight when it was finally separated from the male. It still maintained an intact vaginal membrane at the time of its separation from the male. The rest of the females maintained, lost or gained slightly in body weight at the time of their separation from the males. The changes in body weights among the thirty grasscutters were statistically not significant (p = 0.14). Duration of pairing and receptivity were associated (p= 0.000), and the longer the duration of male-female exposure the lower the receptivity (Phi = - 0.69, p = 0. 000). A 48.3-54.0 reduction in error is achieved predicting receptivity on duration of pairing (PRE: Goodman and Kruskal tau = 0. 48, p = 0.000; Uncertainty coefficient =0.54, p = 0.000). There was no association between sexual receptivity and vaginal status (p = 0.35). There was also no significant difference (p = 0.13) in duration of pairing among the three categories of females.

Conception

Conception in relation to the duration of male-female exposure is shown in Table 2. In summary, 23 (95.8%) of the 24 females that were receptive within the first 18-72 hours of pairing conceived, while all the 5 (100%) females that were receptive between 120-192 hours conceived. Duration of pairing and conception were associated (p= 0.000) and the longer the duration of male-female exposure the lower the conception (Phi = - 0.61 p = 0. 000). A 35.5-37.7% reduction in error is achieved predicting conception on duration of pairing (PRE: Goodman and Kruskal tau = 0. 38, p = 0.000; Uncertainty coefficient =0.36, p = 0.000). Receptivity and conception were associated (p = 0.000) and the relationship was very strong (phi = 0.90, p = 0.000). There was however no association (p= 0.58) between conception and the three sexual states.

Parturition

Parturition in relation to the duration of male-female exposure is shown in Table 3. In summary, 20 (86.9%) of the 23 females that conceived as a result of being receptive between 18-72 hours successfully carried their pregnancies to term, while all the 5 (100%) that were receptive between 120-192 hours successfully carried their pregnancies to term. Duration of pairing and parturition were associated (p= 0.008) and the longer the duration of male-female exposure the lower the parturition (Phi = - 0.43, p = 0. 011). A 14.4-18.2% reduction in error is achieved predicting parturition on duration of pairing (PRE: Goodman and Kruskal tau = 0. 18, p = 0.012; Uncertainty coefficient = 0.14, p = 0.011). There was association between parturition and conception (p = 0.000), which was significantly strong (Phi = 0.85, p = 0.000). No association was found between parturition and vaginal status (p =0.749).

Figure 1. Duration of male-female exposure and exposure outcome

Optimal duration of male-female exposure to achieve pregnancy

Eighty percent (24/30) of sexual receptivity occurred within the first three days of pairing, with 16.7% (5/30) occurring during the rest (11days) of the pairing period (Figure.1). The difference between the two periods was statistically significant (p=0.008). Similarly, 79.3% (23/29) of pregnancies and 71.4% (20/28) of parturitions were the outcome of mating that took place within the first 3 days of pairing, compared to 17.2% (5/29) conception and 17.9% (5/28) parturition for the rest of the pairing period (Figure 1). The differences between the first 3 days and the rest (11 days) of the 2 weeklong mating period were statistically significant for sexual receptivity, conception and parturition (P<0.01) and all the three pairing outcomes as already reported under their respective sections weakened with time. With respect to conception, which is the main variable of interest, a 35.5-37.7% reduction in error is achieved predicting conception on duration of pairing.

Reproductive performance

The reproductive performance of the females according to their vaginal status is presented in tables 1, 2 and 3. The differences observed among the three categories of females with respect to the three reproductive parameters were not statistically significant (p = 0.959).

Discussion

One of the means by which animal production could be improved is by the development of techniques for realizing the full reproductive potential of both male and female breeders. One parameter that has been identified to contribute to the reduction of poor reproductive performance among breeders is optimal duration of pairing to achieve pregnancy (Dukelow 1978). Spontaneous ovulators such as monkeys have been identified to show positive association between duration of male-female exposure times and conception rates (Dukelow 1978). Determination of an optimal duration of pairing to achieve pregnancy is relevant to the successful breeding of spontaneous ovulators, but should be irrelevant to the breeding programme of induced ovulators, since this category of females rather release eggs to meet already waiting spermatozoa. Therefore overnight pairing, which is often used for rabbits should be successfully used for other induced ovulators.

This study on the grasscutter has been necessitated by its status as an unfamiliar species, which has led to the common practice of leaving breeding pairs together for too long, resulting in the cannibalism of neonates by the male and eventual failure of some breeding programmes (Asibey 1974). The findings of the monkey study indicate that the longer the mating pairs stay together the higher the chances of successful mating. The findings of this study suggest otherwise in the case of the grasscutter. Eighty percent of females were successfully mated within the first three days of the 2 week-long mating period, while the female that was kept beyond a week was severely traumatized and the male also lost interest in it. Duration of male-female exposure and resultant sexual receptivity, pregnancy and parturition were significantly negatively associated and the difference between the first three days and the rest of the pairing period was also significant.

These findings show that most matings occur within the first few days of pairing and not many matings occur thereafter. More importantly, 79.3% of pregnancies and 71.4% of parturitions occurred as the result of matings that took place within those first 3 days of pairing, compared to 17.2% of pregnancies and 17.9% of parturition for the rest of the pairing period. From the study it appears that long exposure periods may not be beneficial to the breeding of grasscutters. Having to keep a breeding pair together beyond one week is an indication of poor breeding prospects and the pair should be separated after a week of unsuccessful mating. Separation may be the best option, since persistence could lead to severe physical injury of one of the partners, most probably the female, as was observed in the case of the non-receptive grasscutter. Probably for yet unknown reasons some reproductive processes become refractory with long male-female contact. The fact that the differences in reproductive performance for all three categories of females were not statistically significant suggests that the findings on optimal duration is applicable to all grasscutters (Thryonomys swinderianus) irrespective of their sexual cycle, provided they are sexually mature.

Conclusion

The 80% sexual receptivity and 79.3% resultant pregnancies that occurred within the first three days of male-female exposure suggest that mating in the grasscutter takes place shortly after the female is presented to the male. The female that was not receptive within the first week but was maintained an extra week with resultant injuries could suggest that overly long exposures (when the female remains non-receptive throughout the first week) could be detrimental to the grasscutter-breeding programme. It appears that if a female grasscutter will accept to mate, it would do so within the first week of pairing.

The fact that the differences between the first three days and the rest of the pairing period for the three reproductive parameters were negatively associated and statistically significant, show that the most favourable duration of male-female exposure to achieve pregnancy in the grasscutter is three days or at most one week.

Therefore, to ease the breeding of grasscutters and consequently enhance the grasscutter industry, duration of male-female exposure to achieve pregnancy should be appreciated as an important consideration in grasscutter production.

Acknowledgements

The authors thank the Noguchi Memorial Institute for Medical Research for funding the study. The authors also acknowledge the technical assistance of Messrs Emmanuel Atta Tioh, Samuel Mensah, Daniel Osei-Boakye, and David Appiah for the care and management of the grasscutters.

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Received 29 April 2002; Accepted 24 December 2002

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