The effects of reducing population density on contact rates between brushtail possums: implications for transmission of bovine tuberculosis

1. Interactions during mating are thought to be an important mechanism for transmission of tuberculosis (Tb) Mycobacterium bovis in the brushtail possum Trichosurus vulpecula. However, little information is available on the frequency of contacts between males and females in oestrus during the breeding season, and the relationship between mating contacts and population density. 2. We used radio-telemetry to record contacts between male and oestrous and non-oestrous female possums, and determined paternity of offspring using DNA analysis. This was repeated following the removal of c. 70% of the resident possums to determine the effect of reducing density on the contact rate. 3. We could not detect any significant differences in the contact rate between oestrous and non-oestrous females and males, either before or after the density reduction, even when paternity was positively identified from DNA analysis. This suggests that actual mating contacts could not be distinguished from other agonistic or affiliative contact behaviours. 4. Despite this, the relationships between male-female and male-male contact rates and population density were non-linear convex-up, implying that the contact rate during the breeding season did not decrease in proportion to reductions in density. This appeared to be driven by the enlargement of male ranges and a corresponding increase in male overlap of female ranges following the density reduction. 5. The form of the contact rate function will influence predictions of disease spread in epidemiological models for Tb in wildlife. This has major implications for the development of tactical approaches to disease management based on such models.