Early growth and development of roof rats, Rattus rattus L.

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Abstract
Thirteen litters of R. rattus bred in the laboratory were weighed and measured daily, and subjected to a variety of developmental tests from birth to 18 days of age. Curves of weight and length vs. age are presented; from birth some rats weighted nearly twice as much as others, and were corresponding larger. Variability in weight was largely a consequence of differences between litters; mean litter weight was negatively correlated with litter size. Weight is thus of limited use in the field as a guide to the age of young rats, while body measurements and particularly developmental indices had much narrower confidence limits. Litter size averaged 7.2 with equal numbers of males and females; litters were born at about montly intervals. About 6% of pups died between birth and weaning. The chronology of development of R. rattus is also described in detail. Ligher rats generally developed more slowly than their heavier conspecifics. The pattern of development of R. rattus was remarkably similar to those of a variety of other ecologically diverse Rattus.