Some aspects of the reproduction, growth and development of the rice field rat, rattus argentiventer Robinson & Kloss ( rodentia: muridae)

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Date
1979-08
Authors
Yuet Ming, Lam
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Abstract
Studies on the rise field rat R.• argentiventer were carried out in the double-dropped (Bumbong Lima, Bukit Merah, Permatang Pasir) and the single-cropped (Kota Kuala Muda) rice fields. Its population structure varied considerably throughout the year, greatly influenced by the state of the crop and this was observed to be more pronounced in the single cropped area. On the average females were caught more frequently than males in the ratio, male to female, 1 t 1. 78. Young rats predominate during and after the breeding seasons. Studies on the reproductive conditions of males and females showed that the weight of the rats on attaining sexual maturity was 140 gm (corresponding . HBL-class 175 mm) and 60 gm (HBL-class 125 mm) for males and females, respectively. Males in the single-cropped area attained a greater size upon sexual maturity when compared to those of the double-cropped areas but this difference was not observed in the females. Reproduction of argentiventer was sharply bimodal in the double-cropped rice fields and unimodal in the single-cropped rice field. The breeding season was found to correspond very closely with the reproductive and ripening phases of the paddy during which litters were produced. The mean number of embryos per female was 11.04 and 12.50 in the double· 8lld single-cropped ar~as, respectively. Intra-uterine mortality was 2.92 per cent in the double-cropped areas and 1.55 per cent in the single-cropped area. The number of ova ovulated, number of embryos and litter size were found to be positively correlated to the size of the female. The distribution of embryos between the two horns of the uterus were equal with no marked tendency for all to develop on one side. Mean litter size was 10.33 and 10.38 in the double and single-cropped areas, respectively. The most common litter size in the doublepped areas was 9 and the more common litter sizes in the single-cropped area were 11 and 12. The sex ratio in litters from the double-cropped areas, male to female, was 1. : 1.41 and in the single-cropped area 1.04 : 1. In the rice fields !• argentiventer was found to be r-selectecl, typified by high reproductive rate, large litter size, early sexual maturity and short life-span of usually less than a year. Tl\e reproductive JOtential of females in the double-cropped areas .was estimated to be 4. 9 times that of the single-cropped area and in the double-cropped areas productivity was 81 young per female per year compared ~ 17 in the single-cropped area. Reproduction, growth and development of !.• argentiventer was studied in the laboratory. Females were polyoestrous and bred throughout the year.. The oestrous cycle was 4. 3 days and females exhibited pOStpartum oestrus. Mean gestation period was 21.4 days. Mean litter size was 7 and the sex ratio, male to female, was 1.09 : 1. The litter size increased initially up to the 4th litter after which it declined gradually. The average number of litters per female per year was 7 and mean number of young per female per year was 50. Birth of the young was by head or breech delivery. ~re-natal and post-natal growth and development of the young was studied. Embryos were visible on the 6th day of gestation and pregnancy could be determined by palpation on the 9th day. Growth of the embryo was most rapid from 15-21 days gestation and embryos could be sexed by anogenital distance from 18 days gestation onwards. There was sexual dimorphism in the size of the young at birth and subsequent growth and sexual maturity. Morphological and behavioural changes were described from birth till 60 days of age. Males have scrotal testes starting at 26 days of age but only became sexually mature at 90 days old. Females have perforated vagina at a mean age of 33 days and could conceive at a mean age of 49 days. Growth characteristics of males and females were described from birth to 24 months of age. Both male and female exhibited the greatest growth (mm/day) in the first month after birth and the greatest weight gain (gm/day) in the first two months after birth. A model· for the growth of males and females in the laboratory from 1-44 weeks was developed.
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Rattus argentiventer Robinson & Kloss , (Rodentia: muridae)
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