THE INFLUENCE OF LITTER SIZE AND LITTER ORDER ON LENGTH OF GESTATION AND EARLY POSTNATAL GROWTH IN THE GUINEA-PIG

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Abstract
SUMMARY: The influence of litter size and litter order on length of gestation and early postnatal growth of offspring has been examined on records of sixty-six female guinea-pigs which delivered 440 litters. It is shown that: (a) The association between foetal weight and length of gestation is not the same for different litter sizes: in small litters weight increases with length of gestation; in large litters weight decreases. It is suggested that weight influences time of parturition in large litters, but not in small ones. (b) Length of gestation increases with litter order. This result is shown to be independent of litter size and weight. (c) As litter size increases, weight of young at 28 days after birth decreases, but rate of postnatal growth (weight at 28 days expressed as a percentage of birth weight) increases. (d) Rate of postnatal growth (to 28 days) is lowest for first-born young, increases after the first birth, and falls again after the fourth.