The Intrinsic Rate of Natural Increase of Irradiated Peromyscus in the Laboratory

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Abstract
This study was undertaken to determine the effect of continuous low—level exposure to gamma radiation on a population of small mammals (Peromyscus maniculatus sonoriensis) in the laboratory. The average dose rate was slightly greater than 1 roentgen/day. One group was exposed as adults, with irradiation beginning when they were weaned (age 3 to 4 weeks); another group was the offspring of the irradiated adults, hence these animals were irradiated since conception. Descendants from 3 separate field populations were present in each group. The intrinsic rate of natural increase, r, was increased in animals irradiated as adults (r = .3574), as a result of an elevation of the instantaneous rate of birth, b, and a reduction of the instantaneous death rate, d. Animals irradiated since conception showed a lower intrinsic rate of natural increase (r = .1897) than the controls (r = .3134) due to a lower b and an increased d. Life span was shortened by irradiation. The average life expectance at birth, eo, was 22.18 ± 1.54 months for control females, 19.61 ± 1.99 for the females irradiated as adults, and 15.62 ± 1.58 for females irradiated some conception. Male life expectancy was greater in each group. The animals that survived irradiation during the pre— and postnatal stages had greater further life expectancy than individuals of equivalent age groups that had been irradiated only as adults. Age specific natality, mx, was reduced by radiation, both by lowering the maximum number of young per female and by shortening the reproductive life spans of females. Reduction was greatest in the animals since conception. There was evidence of hyperovulation in the group irradiated as adults. There was variation among the 3 field populations represented, both in reproductive capacity of control animals and in sensitivity to radiation. Among controls, the population from the lowest field elevation had the greatest intrinsic rate of natural increase. This population showed the least sensitivity to radiation, judged by the change in r. The greatest sensitivity to radiation was in the population from intermediate elevations. Field populations of Peromyscus living in favorable environments might withstand the degree of reduction induced by this level of radiation exposure. Field populations living under less favorable conditions possibly could not.

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