Fertility control of wild mouse populations: the effects of hormonal competence and an imposed level of sterility

We report on a study of confined populations of wild mice in which 67% of f emales were surgically sterilised to simulate the possible effects of ferti lity control on population dynamics. Social structure can influence the bre eding performance of female mice and, as this may be hormonally controlled, we examined whether the maintenance of hormonal competence by sterilised f emale mice was necessary to achieve a significant decrease in population si ze. We compared two methods of surgical sterilisation - tubal ligation, whi ch leaves the animal's reproductive hormone regulation intact, and ovariect omy, which disrupts the normal regulation of the hormones of the pituitary- ovarian axis. There was no difference in the population sizes produced by t he two methods of sterilisation and thus the maintenance of hormonal struct ure is unlikely to influence the population's response to fertility control . If anything, the population response to the presence of hormonally compet ent but sterile females was different from that expected - populations with tubally ligated females had slightly higher growth rates, recruitment of y oung, and breeding performance, than populations with ovariectomised female s. The 67% level of infertility amongst females in the population successfu lly reduced population size and growth rate when compared with unsterilised populations. This reduction in population size was not related to the leve l of sterility imposed. Compensation occurred through improved breeding per formance of unsterilised females, particularly in the tubally ligated popul ations.