Responses of male brushtail possums to sterile females: implications for biological control

Summary

Immunocontraception is being considered as a potential method for the biological control of mammalian pests. Several studies have investigated or modelled its demographic consequences, but there have been few field studies of the possible effects of the presence of sterile females on local males. In New Zealand, the brushtail possum Trichosurus vulpecula is a widespread introduced mammal which predates heavily on native wildlife, such as eggs and young of birds. For this reason immunocontraception has been proposed as a possum control method.

In this study, female brushtail possums in two populations were surgically sterilized to study the potential effects of immunocontraception on body condition and local sex ratios.

Study area: The study was conducted in two mixed broadleaf podocarp forest fragments at Coatesville (36°44"S, 174°40"E) and Huapai (36°47"S, 174°45"E) near Auckland, New Zealand.

Live trapping: The study commenced in December 1994 at Huapai and March 1995 at Coatesville. Possum cage traps were set at intervals of 50 m on a grid covering an area of about 6 ha at both sites. During intensive breeding season sampling, traps were set for 2 nights/week for 6 weeks. Outside the breeding season, the traps were set for 2 nights/week for 2 weeks each month. The traps were baited with apples dusted with fruit-flavoured flour. Set traps were checked daily. On its first capture, a possum was anaesthetized then tagged with two uniquely numbered ear tags. Sex, maturity, weight and body measurements were recorded on the first capture of each trapping period. Males with a testis width exceeding 15 mm were recorded as mature. Females with an invaginated pouch were recorded as mature.

In trapping sessions during the breeding season, a vaginal swab was taken from each female on every capture and was smeared onto a microscope slide before the animal was released at the site of capture. All procedures performed were within animal ethics guidelines (Approval Number N420 University of Auckland). Swabs were taken to determine oestrus (see below).

Independent possums that had not been caught before were regarded as immigrants and classified as new animals in that session. Those caught within the trapping grids in more than one session, or three or more times within a session, were classified as residents. Possums caught only in the outer traps of the grids were regarded as 'peripheral' animals, with home ranges which presumably slightly overlapped the study sites.

Oestrus detection: Vaginal smears were taken to detect oestrus. The oestrous cycle of female possums is around 26 days during which they are in oestrus for 1 day only. Mating generally occurs 1–2 days after oestrus. Oestrus was assigned if sperm or large numbers of leucocytes were observed in a vaginal smear.

Sampling and treatment: Sampling comprised continuous trapping from March to July in 1995 to determine the mating period for both populations, followed by two 6-week sampling blocks in both 1996 and 1997. The first sampling block was timed to coincide with the period in which most possum matings occurred (mid-March to late April) and the second when most non-sterile females had pouch young (mid-May to late June). The treatment was conducted in January 1997 and consisted of the surgical sterilization (by tubal ligation under sterile laboratory conditions) of all mature female possums captured from half of each study site during that period. A sham sterilization, in which females were subjected to anaesthesia and an abdominal incision but not tubal ligation, was performed on all mature females captured from the other half of each study site at the same time. Ten mature females were sterilized at Huapai and six at Coatesville. Nine at Huapai and six at Coatesville were sham sterilized.

The trappability of possums varied markedly, with some captured every night in most of the trapping sessions, and others caught once only in some of the sessions. The 'new' and 'peripheral' categories may therefore have included some resident individuals with extremely low trappability.

Oestrus was detected in sterile female possums several weeks after most control (non-sterile) females were pregnant and had ceased mating activity. This indicates that female sterilization prolonged the mating season of the local possum population.

The body condition of males was significantly poorer in the presence of sterilized females in the winter post-mating period. In contrast, the condition of females showed no change following the sterilization treatment, either during the autumn mating period or in the winter post-mating period.

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.

Conclusions: The results for the control of possums by sexually transmitted immunocontraception implied by this study are: i) the reduced body condition of males in the presence of sterilized females might result in increased male mortality; ii) the attraction of males from surrounding areas to those containing sterilized females might facilitate the local spread of an immunocontraceptive agent.


Note: If using or referring to this published study, please read and quote the original paper. The original paper can be viewed at: http://www.blackwell-synergy.com/doi/abs/10.1046/j.1365-2664.2000.00546.x