The sexual development of female mice is accelerated by exposure to an adult male or to male urine. The component of the urine responsible for this effect is androgen-dependent, heat labile, nondialysable, precipitatable with ammonium sulphate, and is not extractable in ether. These results indicate that the pheromone causing accelerated sexual development is associated with a protein component of male urine. Tests of the active fraction after digestion with proteolytic enzymes suggest that the pheromone may be a portion of a protein or a substance bound to a protein.
Female deer mice were exposed to a short day (SD, 6L:18D) photoperiod beginning during 1 of 3 stages of life. In the first experiment, exposure to SD during adulthood resulted in a minimal disruption of reproductive condition; many females bore 2 litters after the onset of this treatment. In the second experiment, females reared on SD from weaning matured normally, as measured by vaginal introitus; however, vaginal closure occurred in approximately one-half of these females by 9 weeks of age. In the third experiment, females were born of mothers housed on either an SD or a long day photoperiod, and were continued on the maternal photoperiod until 6 weeks of postnatal age. The SD photoperiod markedly inhibited reproductive maturation as measured by vaginal patency, ovarian weight, and uterine weight. A comparison of reproductive organ weights and vaginal condition provided evidence for the validity of the latter measure as an index of reproductive state. As assayed by the present testing procedure, the sensitivity of the reproductive system to photoperiod decreases as a function of age in female deer mice.
The sexual maturation of male prairie deer mice (Peromyscus maniculatus bairdii) that were reared with other males was inhibited in comparison with that of males reared in isolation. Inhibition occurred in males reared with nine or four males of the same age, as well as in those housed with one adult male. This phenomenon was observed after several periods of grouping, and consequently at several ages ranging from 5 wk to 4 mo. Females did not retard the sexual development of males. These results indicate that the sexual maturation of individual deer mice is modulated by the specific composition, rather than density, of the local group. Cole (1954), in his analysis of the influence of life-history traits upon population dynamics, found the age at which animals first reproduce to be particularly important. In species having relatively short prereproductive periods, a modest change in the age at first reproduction markedly alters the reproductive potential of a population. This age is not a fixed characteristic of a species, as Cole assumed for the purpose of his illustrative calculations. A number of reports document a delay of sexual maturation in populations at peak density compared with those in which numbers are still increasing (Christian, 1971;Krebs & Myers, 1974). Density-dependent retardation of sexual maturation has been advocated as a major mechanism in the regulation of population growth (Christian, 1971). Laboratory investigations are required in order to determine the nature of this influence of density upon the sexual maturation of individual members of a population.
The regulatory influence of conspecificsThis study was submitted by the first author to North Carolina State University in partial fulfillment of the requirements of the master's degree. We are indebted to P.
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