Fertility control of common brushtail possums, Trichosurus vulpecula, in free-ranging populations

© 2011 Dr. Muhammath Siyam SubairThe common brushtail possum, Trichosurus vulpecula, is a serious ecological and agricultural pest in New Zealand, where it was introduced, and is very abundant in many Australian cities. Management of overabundant native species is contentious, with wildlife managers increasingly using fertility control as a humane option. My study developed three DNA vaccines based on coat protein 4 (CP4) and vesicle associated protein 1 (VAP1), previously shown to reduce fertility by ca 40% for a mean period of 1.4 years and 1.5 years respectively. CP4 and VAP1 cDNA were amplified by PCR, separately cloned into pcDNA3.1 plasmid vector, transformed into Escherichia coli, confirmed by sequencing, purified from endotoxins, and verified by in vitro expression in mammalian cells before use. The DNA vaccines were pcDNA3.1-CP4, pcDNA3.1-VAP1, a combined vaccine of the two (pcDNA3.1-CP4 + pcDNA3.1-VAP1), and a control vaccine (pcDNA3.1 blank plasmid). The effect of vaccines on animal behaviour was tested both on captive and free-ranging possums. Compared to controls, immunization with CP4 or the combined vaccine resulted in significant behavioural changes related to social rank and number of dens used in captivity and home range areas, long-range movements and den types used in the wild. The combined vaccine, the most effective vaccine, caused the highest degree of changes, while VAP1, the weakest contraceptive produced the lowest degree of changes. The social rank increased significantly in pcDNA3.1-CP4 and combined vaccine-immunized possums compared to controls. However, the dominance behaviour was positively correlated with weight gain. The absence of weight gain in wild females in my study after treatment means that dominance patterns in free-ranging possums are unlikely to change as a result of this factor after vaccination. Similarly, while immunized females used a higher number of dens compared to controls in captivity, the number of dens used by free-ranging females did not change between groups following treatment. Both pcDNA3.1-CP4 and combined vaccine-immunized females undertook significantly higher proportion of long-range movements and had a larger home range area compared to controls. These movements were associated with the absence of dependent young. No evidence of dispersal or mortalities was found