Responses of Male Brushtail Possums to Sterile Females: Implications for Biological Control

1. Increasingly immunocontraception is being considered as a potential method for the biological control of mammalian pests. Although several studies have investigated or modelled its demographic consequences, there have been few studies of the possible effects of the presence of sterile females on local males. 2. In New Zealand, the brushtail possum Trichosurus vulpecula is an important mammalian pest, for which immunocontraception has been proposed as a control method. 3. We surgically sterilized female brushtail possums in two populations to study the potential effects of immunocontraception on body condition and local sex ratios. 4. Large numbers of leucocytes (indicating oestrus) were present in the reproductive tracts of sterile female possums several weeks after most control (non-sterile) females were pregnant and had ceased mating activity. This indicates that the sterilization of females prolonged the mating season of local possums. 5. The body condition of males was significantly poorer in the presence of sterilized females in the winter post-mating period following the treatment. In contrast, the body condition of females showed no change following the sterilization treatment, either during the autumn mating period or in the winter post-mating period. 6. More adult male possums were recorded at both study sites after the sterilization treatment, resulting in a change to the originally female-biased sex ratio. This may have been caused by the prolonged presence of oestrous females attracting males from surrounding areas. 7. The implications of our results for the control of possums by sexually transmitted immunocontraception are twofold. First, the reduced body condition of males in the presence of sterilized females might result in increased male mortality. Secondly, the attraction of males from surrounding areas to those containing sterilized females might facilitate the local spread of an immunocontraceptive agent. 8. We advocate further assessments of the effects of sterilization and/or immunocontraception on the population demography, individual condition and survival of pest animals.