Contact rates between possums revealed by proximity data loggers

1. Information on close interactions between wild animals is difficult to obtain for cryptic species, but is important for understanding their social and mating systems as well as the dynamics of directly transmitted diseases.

2. The persistence of bovine tuberculosis (Tb) in New Zealand is enhanced by the presence of wildlife hosts for the infection, principally the brushtail possum Trichosurus vulpecula. Quantifying the relationship between the frequency of contacts and possum population density is important for understanding the dynamics of the disease and the potential effectiveness of different methods for disease control.

3. We used novel proximity data loggers to record contacts (< 40 cm distance) between individual male and female wild-living brushtail possums over 3 years at three different sites with a range of habitats and population densities.

4. Most contacts between male and female possums appeared to be sexual in nature, and contact rate was not related linearly to population density. Contacts occurred more frequently during the breeding season, when a female possum could interact with up to four males. However, even during this peak contact period the average contact rate was equivalent to only one contact every 2 days.

5. The total duration of contacts was significantly higher during the breeding season. During this time, contacts lasting for more than 1 min occurred episodically. A small proportion (2·3%) of the interactions lasted for 5–15 min, although the mean duration of contacts was only 18–26 s 24 h−1. Outside the breeding season, contacts were brief and infrequent.

6. The frequency of contacts showed a slight increase when the possum density increased sharply at the main study site, although the duration of contacts was unaffected. This increase in contact rate may have been the result of an influx of new animals causing perturbation to the existing population.

7. Previous paternity analysis using DNA profiling indicated that the possum mating system is polygynous, with males being promiscuous. Our data reveal that females are probably also promiscuous and that the mating system of possums may therefore be polygamous, including both polygyny and polyandry.

8. Synthesis and applications. Most contacts between possums occur in the peak mating season and appear to be related to mating and associated behaviour. There are some contacts outside the main mating season, but these are infrequent. The transmission of bovine Tb infection in possum populations may therefore occur by other routes in addition to direct contact. The non-linear (frequency-dependent) contact rate–density relationship recorded provides some insight as to why repeated population reduction is necessary to achieve lasting Tb control in possum populations. It also provides some support for the role that could be played by sexually transmitted, viral-vectored immunocontraception in the control of possum populations, either alone or in conjunction with more traditional control methods.