The longevity and fertility of the Orkney vole, Microtus orcadensis, as observed in the laboratory.

Open Access
Publisher Website
Google Scholar
Abstract
SUMMARY.: An analysis has been made of some data relating to the longevity and fertility of the Orkney vole, M. orcadensis, as observed in the laboratory. The probability of surviving the first three weeks of life, based on a total of 1,358 live‐born young, of which 1,250 were still alive at weaning, was 0·9205. A total of 131 young of both sexes was observed between the ages of three and nine weeks, and of these 117 were alive at six weeks, and 114 at nine weeks of age. Combining the information for infant and juvenile mortality, some 20 per cent of live‐born young, taking both sexes together, died before reaching the age of nine weeks. The adult expectation of further life at the age of nine weeks, based on the life‐histories of sixty male and sixty female voles, was for males, 90·3±2·95 weeks, and for females, 97·3 ±4·34 weeks. Taking both sexes together, the average expectation of life at birth was about 1·6 years. . A total of 508 litters, consisting of 1,358 live young, waa recorded, giving a mean number of 2·67 young per litter. The largest number observed in any one litter was eight young. The mean number of young per litter, however, varied according to the age of the mother. The observed span of reproductive life for females lay between the ages of nine and 123 weeks. A female, if she lived throughout this span, might be expected on the average to give birth to a total of 12·9 littens, consisting of 17·0 daughters, if an equal sex‐ratio at birth is assumed. . The relation of fertility to the age of the mother was similar to that observed in other mammalian and some insect species, namely an initial risc to a peak, followed by a gradual fall as the age of the mother increased. The intrinsic rate of increase of a population subject to the observed life table and age‐specific fertility rates, was estimated to be, r=0·072 per head of population per week. A population increasing at this rate would double in numbers in 9·6 weeks. The longevity and intrinsic rate of increase of the Orkneg vole is compared with that of the mainland vole, M. agrestis. Judging from the available evidence, it appears that in the laboratory the Orkney vole is a longer lived animal, and its intrinsic rate of increase is less than that of agreatis. This difference is due mainly to the fact that the Orkney vole has fewer young per litter, and because the females reach their peak of productivity at a more advanced age. The expenses of this investigation were partly paid for hy a grant froin the Agricultural Research Council.