Detection of mating, pregnancy and imminent parturition in the grasscutter (Thryonomys swinderianus)

Phyllis Addo

Noguchi Memorial Institute for Medical Research, Box LG581,
University of Ghana, Legon, Ghana
paddo@noguchi.mimcom.net

Abstract

This study was undertaken to establish methods for detecting mating, pregnancy and imminent parturition in the grasscutter to aid in the captive breeding of the species. Nineteen female grasscutters were hand-mated and subsequently individually caged to determine the outcome of male-female contact (i.e. mating, conception and parturition) and the cardinal signs that accompany them. The signs of mating were detected by monitoring changes in the perineum of the female before and after mating; the signs of pregnancy by monitoring changes in the perineum of the female, changes in weight gained post-mating and presence of foetuses in utero by abdominal palpation; while the signs of imminent parturition were detected by monitoring expectant mothers with distended abdomens for change in feeding/drinking habits, behaviour and posture.

The findings of the study suggest that mating in grasscutters is always manifested by vulval congestion, which is sometimes accompanied by vulval oedema and protrusion of the vaginal wall. Pregnancy is manifested by a definite change in body weight four weeks after mating and by intermittent vaginal bleeding and palpation of foetuses in utero five weeks after mating. Parturition is imminent within three days when the expectant mother stands on only its hind legs, and it is a day away when the change in posture is combined with frequent looks at the lower abdomen. The grasscutters give birth to precocious young after 148-158days of gestation. Mating, pregnancy and imminent parturition are therefore detectable in the grasscutter by visible cardinal signs. 

Keywords: Grasscutter (Thryonomys swinderianus), mating, pregnancy, imminent parturition

Introduction

The grasscutter is a hystricomorph rodent widely distributed in the African sub-region and exploited in most areas as a source of animal protein (Vos 1978; Asibey 1974; National Research Council 1991). Being the preferred and most expensive meat in West Africa (Baptist and Mensah 1986; Asibey and Addo 2000), it contributes to both local and export earnings of many West African countries (Asibey 1969; National Research Council 1991; Baptist and Mensah 1986; GEPC 1995, Ntiamoa-Baidu 1998) and is therefore hunted aggressively. At a local market for example, approximately 73 tonnes of grasscutter meat were sold in one year, which represented more than 15,000 animals (National Research Council 1991). Unfortunately its collection from the wild is attended by destruction of the environment, through the setting of bush fires by hunters (National Research Council 1991; Yeboah and Adamu 1995; Ntiamoa-Baidu 1998). To address this problem, attempts are being made in the sub-region to domesticate the grasscutter (National Research Council 1991; Anonymous 1993) and make it more readily available, gain economic benefit and also reduce the environmental destruction that accompanies its collection from the wild (National Research Council 1991; Ntiamoa-Baidu 1998). The attempts have not been as successful as desired (Baptist and Mensah 1986) due to paucity of information on the biology of the species (Adu et al 1999).

The main determinant of successful domestication is the maintenance of reproductive competence (Zeuner 1963; Fox 1987; Dukelow 1978; Adams 1989), therefore in investigating the biology of the grasscutter, information that would have bearing on its successful breeding in captivity is of immediate importance. To facilitate the study, parameters known to enhance the captive breeding of already-domesticated animals should be investigated for their applicability in the grasscutter. For example, the detection of mating, diagnosis of pregnancy and detection of imminent parturition in laboratory (Arrington 1972) and farm (Arthur et al 1989) animals have been shown to enhance animal production (Arrington 1972; UFAW 1989). Some of the methods used were perineal examination post-mating, abdominal palpation, monitoring of changes in body weight during gestation, biological pregnancy assays and unobtrusive observation of pregnant animals for changes in behaviour and posture as parturition approaches (Hafez 1970; Arrington 1972; Arthur et al 1989; UFAW 1989). This study sought to provide information on the reproductive biology of the grasscutter, specifically, the detection of mating, diagnosis of pregnancy and detection of imminent parturition, using all the aforementioned methods except biological pregnancy assays. The methods were used in conjunction with hand-mating and individual caging of the female before and after mating, in order to make it easier to evaluate the outcome of each mating session, as well as facilitate better assessment of the progression of pregnancy and manifestation of any signs of pending or approaching parturition.

This paper reports on the methods used in detecting mating, pregnancy and imminent parturition in the grasscutter, and the periods during which each method provides an unequivocal positive result.

Materials and Methods

Experimental animals

Twenty-eight sexually mature grasscutters made up of 19 females and 9 males were used for the study. They formed part of a colony of wild and laboratory-bred grasscutters maintained in the conventional animal facility of the Noguchi Memorial Institute for Medical Research (NMIMR).

Animal management

The animals were initially laboratory and health-conditioned at the NMIMR conventional animal facility for at least six months. Laboratory conditioning entailed getting the grasscutters acclimatized to laboratory animal housing conditions namely: caging (standard laboratory rabbit cages measuring 50cm (H) by 40cm (W), by 40cm (L), ambient temperature (25-30ºC), relative humidity (60-90%), light-dark cycle (12-12 hours), 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. Each animal was individually caged throughout the study, except during the hand-mating period. They were also fed, watered, maintained and experimentally manipulated as approved by the Institutional Animal Care and Use Committee of NMIMR.

Hand-mating and detection of mating

Grasscutters are induced ovulators (Stier et al 1991; Adjanohoun 1993; Addo et al 2001) and breed all year round (Asibey 1974) therefore no consideration was given to the time of mating. However, though grasscutter are induced ovulators, they sometimes show variations in their reproductive activity or sexual cycle (Adjanohoun 1993), which is manifested by the status of their vaginal membrane. Sometimes when sexually ready or in oestrus, the female grasscutter may present a perforated vaginal membrane with (sealed) or without (open) a hardened vaginal secretion. When not sexually ready or in anoestrus it may present an intact (closed) vaginal membrane (Adjanohoun 1993). The hand-mating process was therefore conducted taking cognisance of these variations. Hand-mating was conducted by transferring the female to the male’s cage. Prior to the transfer, the female’s body weight, vaginal status (open, sealed, closed), date and time of the transfer were noted, after which it was left with the male until it had been mated. During the female’s stay with the male it was examined daily for post-pairing perineal changes namely: perforation of the vaginal membrane (in females that presented closed or sealed vaginal membranes at the time of pairing), nature of vaginal secretion if any and presence of a copulatory plug in the vagina. Other mating signs looked for were copulatory plug on the cage floor and scratches on the female’s trunk inflicted by the male in its attempt to mount the female (Asibey 1974). On observing any of the signs, the female was immediately and permanently separated from the male, weighed, transferred to its own cage and the date and time of the appearance of the mating-sign(s) noted. The female was thereafter monitored either daily or weekly for signs of fertile mating.

Pregnancy diagnosis

Pregnancy diagnosis was conducted either daily or weekly, depending on the parameter under investigation. The daily-monitored parameters were: status of the vaginal membrane (closed, sealed or open) and presence/characteristics of vaginal secretion. The weekly-monitored parameters were: change in body weight and presence of developing foetuses in utero, which was determined by abdominal palpation (Figure 1). To conduct the palpation, the animal was partially restrained in a restraining net designed by the author, while an attendant held the animal firmly but gently at the shoulders. The examiner held the animal by the tail and placed one hand between its hind legs, moved the hand gently until the animal’s lower abdomen was cupped in the hand. The uterine horns were gently passed in between the fingers and the foetuses were felt as slippery marbles during the early stages of gestation. All the methods, except change in body weight were used for eight weeks after mating. Animals that were diagnosed pregnant by the end of the eighth week were transferred to maternity cages (H: 50cm, W: 40cm, L: 80cm). Weight change was monitored until the 21^st week of pregnancy because it was considered to be the least stressful of the pregnancy diagnostic methods.

Figure 1. Diagnosing pregnancy in the grasscutter by abdominal palpation

Detection of imminent parturition and determination of gestation length

Pregnant animals were monitored for peculiar signs that could be indicators of imminent delivery, therefore those with distended abdomens were observed unobtrusively daily, from 6am to 6pm at three-hourly intervals, for change in eating and drinking habits as well as change in behaviour and posture. Gestation length was estimated to be from the day of the appearance of the mating sign (day 1) to the day of parturition.

Data analyses

Statistical analysis was conducted with the Statistical Package for the Social Sciences (SPSS, Standard version, Release 10.01) (SPSS Inc. 1989-1999). The mating data were analysed by the cross-tabulation procedure as follows: The association between mating and mating signs was determined using the Pearson’s chi square statistic, while the degree of the association for each sign (scratch, vulval oedema and vulval congestion) was determined by the Phi coefficient. The predictability of mating by the principal mating sign (vulval congestion) was computed by the Phi, Lambda, Goodman and Kruskal tau and uncertainty coefficients. The two principal signs observed post-mating (vulval oedema and vulval congestion) were compared by the Pearson’s chi square statistic to determine if there was a difference in their use as indicators of successful mating. The data on pregnancy diagnosis was analysed by the cross-tabulation procedure as follows: The Pearson’s chi square statistic was used to determine if there was any association between intermittent vaginal bleeding and pregnancy. The use of intermittent vaginal bleeding as a predictor of pregnancy was analysed by the Phi, Lambda, Goodman and Kruskal tau and uncertainty coefficients. The Pearson’s chi square statistic was used to determine if there was any association between gestational age and the results of abdominal palpation on one hand and change in body weight on another hand within the first eight weeks of pregnancy. The Pearson’s chi square statistic was also used to compare the performance of the two tools (abdominal palpation and change in body weight) as pregnancy diagnostic methods. The changes in body weight data were further analysed by the case summaries procedure to compute averages of the weekly weights gained by the colony of pregnant grasscutters during the 21week gestation period. The weight data were thereafter log transformed after screening for normality and homogeneity of variance. The weights gained by individual females during pregnancy were compared using one-way analysis of variance (ANOVA). ANOVA and post hoc multiple comparisons were used to compare the differences in the weekly weights gained by the entire colony of pregnant grasscutters during the 21-week period. Pearson correlation coefficient was used to determine if there is any relationship between weight gained and age of pregnancy, while linear regression was used to determine if age of pregnancy is a predictor of weight gained during pregnancy.

Results

Mating signs

Eighteen (94.7%) of the 19 females presented obvious changes in the perineal region namely; vulval congestion (figure 2) or vulval congestion with vulval oedema, and occasional protrusion of the anterior walls of the vagina. The mating signs disappeared within 3 hours of the female’s separation from the male.

Figure 2. Vulval congestion observed in a grasscutter after mating

Two (10.52%) of the 19 animals presented scratches on the trunk and one of them maintained an intact vaginal membrane throughout its stay with the male. The scratches on the female that failed to mate were deep wounds that needed veterinary attention. None of the animals given for mating presented a copulatory plug in the vagina or cage tray. Vulval congestion (100%) was found to be positively associated with mating (?² = 20.000, P<0.001; Phi = 1.000, P<0.001) and a perfect predictor of successful mating (Phi = 1.000, P<0.001; Lambda = 1.000, P<0.001; Goodman and Kruskal tau = 1.000, P<0.001; Uncertainty coefficient = 1.000, P<0.001). Surprisingly, no association (?² = 1.955; P>0.05) was found between vulval oedema (72.2%) and mating. Scratches (10.5%) were found to be negatively associated with mating (?² = 9.474, P<0.01; Phi = -0.688, P<0.01). The details of the changes that occurred after pairing the male and female are presented in Table 1.

Pregnancy diagnosis by perineal changes

Perforation and closure of the vaginal membrane

The vaginal membranes of the 18 successfully mated females reformed between 8-27days (14.4 ± 5.32 days) after appearance of the mating sign(s). The closure of the vagina occurred irrespective of whether the female conceived or not. The vaginal membranes of the 18 females later perforated, between 30-42 days (35.22 ± 4.56 days) after mating and was irrespective of whether the female conceived or not.

Perforation of the vaginal membrane with intermittent vaginal bleeding

Sixteen (88.89%) of the 18 successfully mated females bled intermittently between the 35^th –39^th day (36.27 ± 1.44 days) after appearance of the mating sign(s). An association was found between intermittent vaginal bleeding and conception (?² = 18.000, P<0.001). Intermittent vaginal bleeding was also noted to be a perfect predictor of pregnancy ((Phi = 1.000, P<0.001; Lambda = 1.000, P<0.001; Goodman and Kruskal tau = 1.000, P<0.001; Uncertainty coefficient = 1.000, P<0.001).

Pregnancy diagnosis by change in body weight

The body weights of the 18-mated females fluctuated until the third week after mating. During the three-week period, 16 (88.9%) of the 18 females had an average weight change of 41.2 ± 33.2 g (0-150 g), while two (11.1%) had an average weight change of 1.67 ± 5.16 (-5.00 – 10 g). The body weights of 14 of the 16 females increased by 122 ± 39.8 g (80-182 g) in the fourth week to 1098 ± 184 g (860 -1310 g) in the 21^st week of pregnancy (Table 2).

The 14 females did not differ significantly (P>0.05) from each other in weight gained during the 21week gestation period; though two experienced partial foetal resorption. One of the two stagnated at 200 g for three weeks (8^th – 10^th week), after which it steadily gained a total of 1232 g at the 21^st week, and the other lost 160 g from the 9^th to the 14^th week, after which it also steadily gained a total of 902 g in the 21^st week. The remaining two of the 16 females experienced a total cessation of growth (total foetal resorption) at the 11^th (260 g) and 14^th week (410 g) of pregnancy. The two females that showed no appreciable change in weight the first three weeks after mating continued to maintain a low weight change (18.4 ± 10.0) to the end of pregnancy. The progression of gestation in the 14 females showed that statistically significant weight gains were made in the 3^rd, 4^th, 7^th, 9^th, 13^th, 15^th, 16^th, 17^th, 18^th, 19^th, and 20^th weeks of pregnancy. The significant change in the third week was the beginning of the turning point between pregnant and non-pregnant status; while the significant change in the fourth week was the point of unmistakable distinction between pregnant and non-pregnant status. From the 13^th week the foetuses showed consistent growth up till the 20^th week of gestation. A strong positive correlation was found between age of pregnancy and weight gained (r = 0.93, P<0.01). Age of pregnancy was computed to be a good predictor of weight gained during pregnancy. The dependence of weight gained, on age of pregnancy was described by:

Weight gained = -102 + 51.9*(gestational age)

The categorization of females into pregnant and non-pregnant states (pregnancy diagnosis by change in body weights) differed significantly amongst the weeks (?² = 13.3, P>0.005), with the highest (maximum) categorization rate being recorded in the fourth week of gestation. Subsequently, the fifth to the eighth week turned out to be weeks of confirmatory diagnosis.

Pregnancy diagnosis by abdominal palpation

The foetuses were felt as slippery marbles of varying sizes two weeks after appearance of the mating sign(s). The diagnosis was categorized as tentative at this stage, due to the semblance of the foetuses to faecal pellets, which could also be felt during the palpation. In the third week, the paired uteri were as one swollen mass with an elongated content and in the fourth week as a voluminous bag of soft consistency. In the fifth week the uteri still maintained their voluminous nature but contained long rods. In the sixth week the uteri became comparatively less voluminous and the foetuses were felt as distinct short rods, a pattern that continued till the eight week. The detection rates of pregnancy by abdominal palpation differed significantly amongst the weeks (?² = 19.429 P>0.001), and the highest (maximum) detection rate was recorded in the third week of gestation. Consequently, the fourth to the eighth week turned out to be confirmatory diagnosis. Comparison of pregnancy detection rates by abdominal palpation and change in body weight showed that there was no difference between the two methods for the eight-week monitoring period (?² = 0.000, P>0.05). Analysis of individual week rates for the two methods also showed no significant differences: week one: ?² = 2.33, P>0.05; week two: ?² = 0.57, P>0.05; week three through week eight: ?² = 2.15, P>0.05. A comparison of the pregnancy-related features observed using the four pregnancy diagnostic methods during the eight-week monitoring period is provided in Table 3.  

Signs of imminent parturition, parturition and gestation length

The abdomen of the pregnant animals distended and took on the shape of a ‘rugby ball’ in the 4th month. Nine (64.3%) of the 14 expectant mothers walked on only the hind limbs (personally termed the "penguin posture") three days before delivery and combined the ‘penguin posture’ with frequent downward looks at the lower abdomen a day before delivery. Parturition occurred between 148-158 days (152 ± 2.98 days) after appearance of the mating sign. The parturition process could not be observed in all the animals except two. The two littered whilst standing on only the hind limbs. The mothers ate the placenta after the delivery of each baby, before proceeding on to deliver the next baby. The neonates were born fully haired with the eyes open. They stood by their mothers during the delivery of their littermates and followed the mother about within minutes of their delivery, which took approximately 32-40 minutes.

Discussion

The findings of this study show that mating, pregnancy and imminent parturition are visibly manifested in the grasscutter and therefore can be detected to aid in the practical management of grasscutter breeding programmes. Asibey’s (1974) assertion that scratches on the trunk of female grasscutters are signs of mating was not confirmed by this study. Mating scratches were observed in only two of the nineteen females given for mating, one of which also presented perineal congestion and oedema. The female that had only scratches (deep wounds) maintained an intact vaginal membrane after weeks of stay with the male. Statistically it was shown that scratches and mating were strongly inversely related (Phi = - 69%) suggesting that the presence of scratches may rather be an indication of non-successful mating. It could be hypothesized that probably females that persistently resist the males are those that receive scratches, and not necessarily those that have been mated. In the bush or in wider enclosures, the chances of prolonged courting and frequent running away from the male are increased, which lead to a more frequent struggle between the couple, and hence the greater number of females with scratched trunks observed in Asibey's collections from the wild. Probably it is for the same reason that fewer females with scratches were recorded in this study since the cages were of a smaller dimension (H40cm x L50cm x W40cm). This goes to support Adjanohoun's (1993) recommendation that it is preferable to use cages of reduced dimensions (W: 50cm L: 65cm H: 37cm) to protect the female from being wounded during the violent phase of courtship and also provide the male a more congenial environment i.e., a reduced area to enable it court more efficiently by preventing the female from slipping away. The smaller enclosure would invariably increase the male-female contacts, which are likely to favour the appearance of oestrus (Adjanohoun 1993) since the grasscutter is an induced ovulator (Addo et al 2001).

The most consistent and reliable sign of mating in the grasscutter according to the findings of this study is perineal congestion, since it was the only sign that was consistently observed in all mated females among the post-mating signs observed. This finding goes to confirm Adjanohoun’s report (1993) that a strong and brief congestion of the ano-genital zone of the grasscutter is observed immediately after coitus. The briefness of the congestion was confirmed by this study, and since the perineal sign is of a short duration, females given for mating should be examined daily for its detection. Though one of the females that showed perineal signs did not conceive, it does not mean that the sign is not reliable, because about 20-25% of female rabbits fail to conceive following coitus due to failure to ovulate which in turn is due to a deficiency of LH (Fox and Krinsky 1968), a situation that might have arisen in the case of the grasscutter. Above all, post-mating vulval congestion also turned out to be a perfect predictor of successful mating by all the measures examined (Predictability = 100%, P <0.001). Surprisingly, vulval oedema could not be confirmed statistically as a mating sign. This outcome may be because vulval oedema was indeed an incidental finding or that the sample size was not large enough to allow its statistical confirmation.

Pregnancy was successfully diagnosed employing three methods, each yielding an unequivocal result at a particular time. First and foremost, the present findings suggest that the reformation of the vaginal closure membrane after mating should not be considered a sign of conception, as assumed by some (Asibey 1974), since the vaginal closure membrane was observed to reform in all mated females between the 8th and 27th day after mating, irrespective of the outcome of the mating. Furthermore, irrespective of the outcome of mating, the vaginal membrane re-ruptured between 30-42 days post-mating, a phenomenon that sometimes occurs mid-pregnancy in guinea pigs. The phenomenon in the guinea pig is attributed to the surge of progesterone and increase in relaxin production (Zarrow 1947; Jagiello 1965). The same explanation may hold for the occurrence of the phenomenon in the grasscutter. Since the grasscutter is an induced ovulator, mating results in ovulation and the formation of corpora lutea with consequential production of progesterone in all mated females, leading to the rupture of the vaginal membrane, irrespective of the mating outcome. It was also demonstrated in the study that the vaginal membrane of all the pregnant animals perforated with intermittent bleeding five weeks after mating. Fourteen (87.5%) of the sixteen animals that bled, delivered, while those that did not bleed, were not diagnosed pregnant by the other two methods (abdominal palpation and weight gain) and above all, never delivered, suggesting that the bleeding only occurs in those that are pregnant. Hoar et al (1957) considered vaginal bleeding in the guinea pig to be a sign of embryo resorption or abortion of the litter, whilst in monkeys it is considered a sign of implantation (Valerio et al 1969). The findings of this study suggest that the observation made in the grasscutter in the fifth week of gestation may be implantation bleeding, rather than embryo resorption or abortion. The reason being that all the grasscutters that bled continued to gain weight normally, and even though two stopped gaining weight, the set back occurred in the eleventh and fourteenth week of gestation, times that were quite far removed from the fifth week. The cessation in the eleventh and fourteenth week must have been total embryo resorption, while that which occurred from the eighth to the tenth week must have been partial embryo resorption. To buttress this stand, intermittent vaginal bleeding was statistically shown to be a perfect predictor of conception by all the measures examined (Predictability = 100%, P <0.001).

Pregnancy diagnosis by abdominal palpation was performed from the first to the eight week after mating and yielded unequivocal results the third week after mating. Use of the technique before the third week of pregnancy should be done appreciating the difference between embryos and faecal pellets. The latter are hard and more fixed with respect to mobility. The embryos on the other hand, a few days post-coitus are felt between the fingers as small, slippery marbles, which get progressively bigger and elongated as the pregnancy advanced. The technique should be done gently so as not to disturb the pregnancy, since grasscutters are prone to embryo resorption, which is an unelucidated phenomenon.

Pregnancy diagnosis by monitoring changes in body weight was found to be very reliable. It brought out an evident difference between the pregnant and non-pregnant in the fourth week of a five and a half month long pregnancy. It is also a method worth employing weekly until delivery, since it efficiently reveals both partial and total resorption, a reproduction problem of grasscutters (Asibey 1974; Adu and Yeboah 2000). Above all, it is an easier and safer method of diagnosing pregnancy in the grasscutter. Special training is not required to enable one to use this method and could therefore be conveniently practiced by the semi-literate farmer.

The findings of this study suggest that the grasscutter manifests cardinal signs of imminent parturition. The signs are both unique and conspicuous and will therefore be easily recognised by all categories of grasscutter breeders, the young, the old, the literate and illiterate. Such conspicuous changes which appear days before parturition, will provide the breeder adequate time for proper preparation and, in good time, to institute timely managerial preparations that would cut down on losses, which could usually result from inappropriate caging, feeding, mishandling and obtrusive observation. In all species studied, the onset of parturition is manifested by the occurrence of myometrial activity (Arthur et al 1989). Most probably the frequent glances at the lower abdomen by the grasscutter during the last 24 hours of pregnancy were due to increased myometrial activity of the labour process. Parturition in the grasscutter as is also usual in most species (Arrington 1972), is very infrequent during the day and that is why only two females were observed in this study. However, when stumbled upon it is found to be normally over within 32 to 40 minutes, a finding which is in agreement with Asibey’s report (1974) that grasscutters delivered within 57 minutes.

The findings on gestation length in this study did not confirm the observations by Ewer (1960) and Asibey (1974), who reported gestation lengths of 70 days and 108 ± 5.3 days, respectively. The finding of this study (152 ± 2.98 days [148 - 158 days]) are in agreement with reports by Mensah and Baptist (1986) [152 ± 2 days], Adjanohoun (1988) [156 ± 3 days] and Stier et al (1991) [152 days]. The long gestation length observed in the grasscutter is typical of hystricomorph rodents (Weir 1974) and therefore cannot be considered unusual.

Conclusion

The findings of this study have shown that mating, pregnancy and imminent parturition are manifested visibly in the grasscutter and therefore can be detected as an aid in the practical management of grasscutter breeding programmes.

·        Successful mating in grasscutters is manifested by vulval congestion sometimes accompanied by vulval oedema and protrusion of the vaginal wall.

·        Pregnancy can be diagnosed by abdominal palpation three weeks after mating, by a definite change in body weight four weeks after mating and by intermittent vaginal bleeding five weeks after mating.

·        Imminent parturition is manifested by the expectant mother adopting the ‘penguin posture’ three days before delivery and a combination of the ‘penguin posture’ with frequent looks at the lower abdomen a day before delivery.

·        The grasscutter delivers precocious young after 148-158days of gestation.

Acknowledgements

The author thanks the Noguchi Memorial Institute for Medical Research for funding the study. The author also acknowledges the technical assistance of Messrs Emmanuel Atta Tioh, Samuel Mensah, Daniel Osei-Boakye, David Appiah for the care and management of the grasscutters, Atu Jones for constructing the grasscutter maternity cages, Messrs Alfred Dodoo and Isaac Hudson Odoi for photography.

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Received 29 April 2002

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