Comparative characterization of the oestrous cycles of the grasscutter (Thryonomys swinderianus) and the guinea pig (Cavia porcellus) by the hystricomorph vaginal membrane perforation phenomenon

P G Addo, B Awumbila*, E Awotwi* and N-A Ankrah

Noguchi Memorial Institute for Medical Research, Box LG581, University of Ghana, Legon, Ghana

* Department of Animal Science, Box LG 25, University of Ghana, Legon, Ghana

paddo@noguchi.mimcom.net   ;   adokaley@yahoo.com
 

Abstract

The grasscutter (Thryonomys swinderianus) is a hystricomorph rodent undergoing domestication; therefore to facilitate and enhance its breeding in captivity its oestrous cycle was characterized. For better elucidation, its oestrous cycle was studied alongside that of the guinea pig (Cavia porcellus), a domesticated spontaneous ovulating hystricomorph rodent. Sexually mature grasscutters and guinea pigs were monitored for the manifestation of the periodic vaginal membrane perforation phenomenon, which is a sign of oestrous cycling in hystricomorph rodents. The oestrous cycle was characterized with respect to its time of occurrence, constituent phases and duration.

The grasscutters manifested erratic oestrous cycles with significantly different lengths, while the guinea pigs manifested uniform oestrous cycles with no significant differences. The oestrous cycles in both species consisted of two phases: one of sexual activity and the other of sexual inactivity. In the grasscutter, the sexual activity phase was represented by an open or secretion-sealed vagina, while in the guinea pig it was solely represented by an open vagina. The phase of sexual inactivity in both species was represented by an inaccessible vagina closed off the exterior by an epithelial membrane. The mean length of the oestrous cycle was 17.5 ± 21.3 days and 15.9 ± 2.1 days in the grasscutter and guinea pig respectively. The grasscutter presented a mean open/sealed period of 11.1 ± 25.9 days and a mean closed period of 12.5 ± 22.4 days. The guinea pigs presented a mean open period of 3.1 ± 1.1 days and a mean closed period of 12.9 ± 1.71 days.

Since the grasscutters manifested an erratic oestrous cycle while the spontaneous ovulating guinea pigs manifested a uniform cycle it could be inferred that the grasscutter is an induced ovulator like the cuis (Galea musteloides), an induced ovulating hystricomorph that also manifests the phenomenon in a sporadic manner.

Key words: Grasscutter (Thryonomys swinderianus), Guinea pig (Cavia porcellus), Oestrous cycle, Vaginal membrane

Introduction

Grasscutters (Thryonomys swinderianus) are wild hystricomorph rodents found currently only in Africa (Adoun 1993). Attempts are underway in various West African countries to breed them in captivity for use as microlivestock (National Research Council 1991) and laboratory animals (Addo 1997; Asibey and Addo 2000). Information on the biology of the grasscutter, which could facilitate and enhance its maintenance in captivity, is however lacking (Den Hartog and De Vos 1973; Adu et al 1999), and therefore research into its husbandry requirements and reproductive biology is being encouraged across the sub-region. In the captive breeding of animals, a bewildering range of reproductive responses and problems are encountered (Dukelow 1978; Follet 1989). However, one reproductive parameter that could be readily exploited to advantage if known, is the female reproductive or oestrous cycle (D'Souza 1978; Betteridge 1986; Short 1990).

In view of the importance of the reproductive cycle in the captive breeding of animals, this study aimed at characterizing the oestrous cycle of the grasscutter. The study was conducted on the premise that the grasscutter being a hystricomorph rodent may manifest its oestrous cycle by exhibiting the vaginal membrane perforation phenomenon, which is known to be a sign of oestrous cycling in most hystricomorphs (Weir 1974). For better and easier elucidation of the grasscutter's oestrous cycle it was studied alongside the guinea pig (Cavia porcellus), a well-investigated and documented spontaneous ovulating hystricomorph rodent.

This paper therefore reports on the characteristics of the oestrous cycle of the grasscutter as deduced from comparison with the oestrous cycle of the guinea pig.
 

Materials and methods

Experimental animals

Twenty-six adult female hystricomorph rodents, made up of twenty grasscutters and six guinea pigs were used in the study. The grasscutters were 6-7 months old, while the guinea pigs were 3 months old. Both species had been born and maintained at the conventional animal facility of the Noguchi Memorial Institute for Medical Research (NMIMR).

Animal management

The grasscutters and guinea pigs were maintained in separate rooms, but under similar laboratory conditions, in accordance with the guidelines of the Institutional Animal Care and Use Committee of NMIMR. The housing conditions were: caging in standard laboratory cages measuring 50cm (H) by 40cm (W), by 40cm (L), which were placed in a room with an ambient temperature and relative humidity ranging between 25-30ºC, and 60-80% respectively. The animals were also provided with a 12-12 hour light-dark cycle by fluorescent lighting and 24 hour natural ventilation. The grasscutters and guinea pigs were provided feed and water ad libitum. The grasscutters' laboratory diet consisted of commercial rodent feed pellets (produced by Ghana Agro Food Complex), guinea grass (Panicum maximum) and sugar cane (Saccharum spp.). The guinea pigs' laboratory diet consisted of smaller-sized commercial rodent feed pellets (produced by Ghana Agro Food Complex), guinea grass (Panicum maximum) or 200mg of ascorbic acid/litre of water whenever guinea grass was not provided. Both species were provided with pre-boiled water (from an industrial boiler) in sipper bottles. The grasscutters were individually caged, while the guinea pigs were kept in groups of two throughout the study. The grasscutters and guinea pigs were given fresh provision of feed and water daily and the animal rooms were swept and disinfected daily with 1% Virkon S (A broadspectrum bactericidal, fungicidal and virucidal disinfectant produced by Antec International Ltd, England;. it consisted of potassium peroxymonosulfate, sulfamic acid and sodium alkyl benzene sulfonate). The cage trays, feeding troughs and water bottles were washed and autoclaved twice weekly, while cages were soaked and scrubbed in hot water monthly. Both species were experimentally manipulated in accordance with the guidelines of the Institutional Animal Care and Use Committee of NMIMR.

Characterisation of the oestrous cycle

The grasscutter was monitored for the manifestation of the periodic vaginal membrane perforation phenomenon (a sign of oestrous cycling in hystricomorph rodents (Weir 1974)), in order to characterize its oestrous cycle. For better elucidation of the grasscutter's oestrous cycle it was studied alongside the spontaneous ovulating guinea pig, a well investigated and documented hystricomorph rodent.

The grasscutters and guinea pigs were monitored for a minimum of one hundred days and a maximum of 110 days. Both species were monitored for visual changes in the perineum specifically: (i) manifestation of the vaginal membrane perforation and closure phenomenon (oestrous cycle), (ii) time of vaginal opening and duration of the open vaginal periods, (iii) time when vagina was sealed by vaginal secretion and duration of the secretion-sealed vaginal periods, (iv) time of vaginal closure and duration of the closed vaginal periods, (v) time of perineal colour change and duration of perineal colour change and (vi) time of perineal swelling and duration of perineal swelling. The oestrous cycle length was defined as the interval between the first day of spontaneous vaginal membrane perforation, through the period of vaginal closure, to the day before the next spontaneous perforation (Weir 1974). The grasscutters and guinea pigs were monitored once daily about the same time each morning (8-10 GMT) to ensure consistency. Each animal was restrained in the dorsal decubitus position on a non-slippery surface while its perineum was observed for the aforementioned changes.

Data analyses

Statistical analysis was conducted with the Statistical Package for the Social Sciences (SPSS), Standard version, Release 12.0.1 (SPSS Inc. 1989-2003). The frequencies procedure was used to determine the mean, standard deviation and mode of the durations of (i) the oestrous cycle, (ii) open vaginal periods, (iii) secretion-sealed vaginal periods, (iv) closed vaginal periods and (v) phases of the oestrous cycles, namely: sexual activity and sexual inactivity. After conducting the above-mentioned determinations on the grasscutter and guinea pig data, they were screened for normality and homogeneity of variance. None of the grasscutter data was normally distributed or homogenous. The oestrous cycle lengths of the grasscutters were log transformed, but the rest of the grasscutter data could not be successfully transformed and were therefore analysed using nonparametric tests. The one-way analysis of variance (ANOVA) procedure was used to compare the oestrous cycle lengths manifested by 15 grasscutters that presented complete oestrous cycles, while the Kruskal-Wallis test was used to compare the duration of the open vaginal periods, sealed vaginal periods, closed vaginal periods and the sexual activity and sexual inactivity phases of the oestrous cycle exhibited by the 15 grasscutters. The Mann-Whitney test was used to compare the sexual activity and inactivity phases of the oestrous cycle on one hand and the open and sealed periods of the oestrous cycle on another hand. In the case of the six guinea pigs, the oestrous cycle lengths and durations of the closed periods were power transformed to attain normality, while the durations of the open periods were log transformed. The oestrous cycle lengths and the open and closed periods exhibited by the6 guinea pigs were compared by the ANOVA procedure, while the independent samples t test was used to compare the differences observed between the durations of the open (sexual activity) and closed (sexual inactivity) periods of the oestrous cycle.
 

Results

Manifestation of the vaginal membrane perforation phenomenon in the grasscutter

The female grasscutters also manifested the periodic vaginal membrane perforation and closure phenomenon usually observed in hystricomorph rodents. However, the phenomenon was manifested erratically; therefore no definite pattern could be associated with individual grasscutters or the colony of grasscutters (Table 1).

A significant difference (P<0.01) was noted among the grasscutters with respect to the length of the oestrous cycle. The oestrous cycle had a mean duration of 17.5 ± 21.3 days, with a mode of 6 days. The shortest duration of the oestrous cycle was 3 days, while the longest was 98 days. Five of the grasscutters did not manifest a complete cycle within the 110-day monitoring period; they remained for long periods in one phase (Table 1) without making a full cycle within the period of observation. The mean duration of the open periods was 2.05 ± 1.82 days with minimum and maximum durations of 1 and 12 days respectively and a mode of 1 day. The differences in the duration of the open periods amongst the grasscutters were statistically significant (P<0.05). Apart from the classic open and closed periods known to occur in the guinea pig, the grasscutter presented sealed vaginal periods. Specifically, the vagina was sealed off the exterior by a hardened vaginal secretion, (usually light or dark yellow in colour and occasionally white or grey), which normally dried up and detached from one edge of the vaginal orifice, and left the vagina open for 1-12 days. An accidental removal of the seal in one grasscutter revealed an open vagina. The seal reformed within 24 hours, followed by an alternation of an open and sealed vagina for 19 days, after which the vagina was patently closed by an epithelial membrane. The mean duration of the sealed vaginal periods was 3.50 ± 3.77 days with minimum and maximum durations of 1 and 26 days respectively and a mode of 1 day. The differences in the durations of the sealed periods amongst the females were not statistically significant (P>0.05). The mean duration of the closed vaginal periods was 19.1 ± 26.9 days with minimum and maximum durations of 1 and 100 days respectively and a mode of 1 day. The differences in the durations of the closed periods amongst the females were not statistically significant (P>0.05). When the epithelial membrane closed the vagina, the female perineum appeared like that of the male grasscutter and distinguishing between the two sexes was usually tricky for the unacquainted, principally because the female grasscutter has a large clitoris which could be mistaken for the penis. The vaginal membrane was noted to be fragile in some of the animals resulting in accidental opening of the vagina during the period of restraint or other forms of experimental manipulation. The accidentally opened vagina usually closed within 24 hours of the accident by an epithelial membrane. The various durations of the grasscutter's oestrous cycle and that of the open, sealed and closed periods are presented in table 2.

Phases of the grasscutter oestrous cycle

Since there could be no secretion without the vagina being open, the sealed periods were considered to be another manifestation of the open periods and therefore the two periods were considered to be one and the same phase of the oestrous cycle, while the closed period was considered a contrasting phase. Consequently, the open and sealed periods were together categorised as the sexual activity phase of the oestrous cycle, since the vagina was open at the time and therefore accessible for mating; while the closed period was categorized as the sexual inactivity phase (anoestrus) of the oestrous cycle, since a membrane had closed off the vagina, making it inaccessible for mating. The sexual activity phase had a mean duration of 11.1 ± 25.9 days, with minimum and maximum durations of 1 and 100 days respectively; while the sexual inactivity phase had a mean duration of 12.5 ± 22.4 days with minimum and maximum durations of 1 and 100 days respectively. As a colony, there was no significant difference between the durations of the sexual activity and sexual inactivity phases (P>0.05). However, there were significant differences (P<0.05) amongst the 15 grasscutters with respect to the sexual activity phase, but not with the sexual inactivity phase (P>0.05).

Manifestation of the vaginal membrane perforation phenomenon in the guinea pig

As expected, the female guinea pigs also manifested the periodic vaginal membrane perforation and closure phenomenon usually observed in hystricomorph rodents. The phenomenon was manifested in a regular and predictable manner (Table 1). The mean duration of the oestrous cycle was 15.9 ± 2.1 days with a mode of 16 days and a minimum and maximum duration of 10 and 19 days respectively. There was no significant difference amongst the guinea pigs with respect to the duration of the oestrous cycle (P>0.05). The mean duration of the open vaginal periods was 3.1 ± 1.1 days with a mode of 3 days and minimum and maximum duration of 1 and 6 days. There were no significant differences amongst the guinea pigs with respect to the duration of the open period (P>0.05). The mean length of the closed vaginal period was 12.9 ± 1.71 days with a mode of 14 days and a minimum and maximum duration of 8 and 15 days. There were no significant differences amongst the guinea pigs with respect to the duration of the closed period (P>0.05). There was however a significant difference between the duration of the open periods and closed periods (P<0.05). Though all the guinea pigs manifested a uniform oestrous cycle, on one occasion one female manifested a split oestrous cycle. Specifically, the vaginal membrane perforated on the 14^th day of the cycle, re-formed the following day and remained closed until the 17^th day, before going through the normal opening period, which lasted three days. A summary of the durations of the oestrous cycle, and the open and closed periods of the oestrous cycle that were manifested by the guinea pigs are presented in table 2.

Manifestation of perineal colour change and perineal swelling

None of the grasscutters manifested perineal colour change or perineal swelling as accompanying signs of the vaginal opening and closure phenomenon (oestrous cycle). However, the perineum of the young grasscutters took on a more pigmented shade as they advanced in age; the change was irrespective of the vaginal status (open, sealed, closed). None of the guinea pigs manifested perineal colour change or perineal swelling as accompanying signs of the vaginal membrane perforation and closure phenomenon.
 

Discussion

The findings of this study have shown that the grasscutter, like the guinea pig, does manifests the hystricomorph vaginal membrane perforation phenomenon and goes to endorse its classification as a hystricomorph rodent (Temminck 1827). According to Weir (1974), characteristics such as the presence of a vaginal closure membrane among others, should be considered diagnostic of the hystricomorph group. The erratic or sporadic manifestation of the vaginal membrane perforation phenomenon by the grasscutter, as against its predictable manifestation by the spontaneous ovulating guinea pig suggests that it is an induced ovulator like the cuis (Galea musteloides). The cuis is an induced ovulating hystricomorph rodent that also manifests the hystricomorph vaginal membrane perforation phenomenon in a sporadic manner (Weir 1974).

The findings of this study showed that the grasscutter has an array of oestrous cycle lengths, which are significantly different amongst the females investigated. This finding could be explained by Weir's (1974) statement that cycle length is difficult to define for species in which ovulation is induced by copulation, because to a large extent, corpora lutea determine the length of the oestrous cycle (Weir 1974), and therefore in their absence the oestrous cycle has no definite length. This is in agreement with histological findings by Addo et al (2001) that corpora lutea are formed only in grasscutters that have been mated. Apart from the large array of oestrous cycle lengths observed in the grasscutter, a large array of open, sealed and closed periods were also noted, a finding that confirms Adjanohoun's (1993) report on the sexual behaviour of the adult female grasscutter, which he described as being without a fixed duration or frequency of appearance.

The observations made in this study about the existence of two phases in the oestrous cycle of the grasscutter confirmed the findings of Adjanohoun (1993), who reported that the sexual cycle of the grasscutter consists of two distinct periods: one of sexual activity and the other of sexual rest. According to Adjanohoun (1993), the two sexual periods occurred with a certain frequency in individual females, though not as a group, ranging from between a few days to over twelve months. The findings of this study confirmed that the two phases had variable durations which ranged from a few days to the entire duration of the study (110 days). Adjanohoun's (1993) observation that individual females manifested the active and resting phases with a certain frequency was however not confirmed in our study. The findings of this study are rather in agreement with findings on wild rabbits, which are reported to show variations in their reproductive activity, which are interspaced with periods of anoestrus (Hafez 1970). Our grasscutters may be exhibiting these variations because they are only the second filial generation in captivity and therefore are far from being domesticated. Adjanohoun's grasscutters manifested the two sexual phases with a certain frequency probably because they have been bred in captivity for over a decade, and as such are becoming more domesticated (like the domestic rabbit, an induced ovulator, which though without a regular oestrous cycle exhibits a certain rhythm or frequency in its sexual cycle (Hafez 1970)). Adjanohoun's finding is plausible since seasonal breeders have also been known to convert to continuous breeding patterns under favourable environmental conditions after some years of captivity (Valerio and Dalgard 1975). The non-existence of regularity in the manifestation of the two sexual phases in our study would not impact negatively on the reproductive manipulation of grasscutters in captivity, because by virtue of their induced ovulatory mechanism their breeding would for the most part be determined by the farmer and not necessarily their oestrous phase. The induced ovulatory mechanism is beneficial to the grasscutter domestication programme because one does not need to be conversant with the oestrous cycle to mate induced ovulators; as such, both children and adult grasscutter farmers (most grasscutter keepers in Ghana are between 13-76 years of age (Adu et al 1999)) would be able to breed their grasscutters without acquiring skills for the detection of oestrus. For it is well known that failure of farmers to detect oestrus in spontaneous ovulating animals is a major cause of infertility in herds (Short 1990); hopefully this will not be an issue to contend with in the production of grasscutters in captivity.
 

Conclusion

• The study has shown that the grasscutter does manifest the spontaneous vaginal membrane perforation phenomenon, which is a diagnostic of hystricomorph rodents. However, unlike the spontaneous ovulating guinea pig, it manifested the phenomenon in an erratic manner, suggesting that it is an induced ovulating hystricomorph rodent. Both species manifested a two-phased oestrous cycle: a phase of sexual activity, which was represented by an open or secretion-sealed vagina in the grasscutter and an open vagina in the guinea pig; and a phase of sexual inactivity, which was represented in both species by an inaccessible vagina, closed off the exterior by an epithelial membrane.

• Above all, the induced nature of the grasscutter's ovulatory mechanism would ease its breeding by most farmers since they do not need to know the phase of the oestrous cycle to successfully breed their grasscutters.
   

Acknowledgements

The authors thank the Japan International Cooperation Agency (JICA) and the Noguchi Memorial Institute for Medical Research for funding the study. The authors also thankfully acknowledge the technical assistance of Messrs Emmanuel Atta Tioh and David Appiah for the care and management of the grasscutters.
 

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