Effects of Fertility Control on Behavior and Disease Transmission in Brushtail Possums

Abstract

Fertility control is currently under development for the control of brushtail possums (Trichosurus vulpecula), one of New Zealand's most serious vertebrate pests. Despite intensive research into various methods for achieving infertility, including immunocon‐traception and disrupting endocrine control of reproduction, researchers know little about the potential effects of these methods on the behavior of wild possums. We assessed the effects of surgically imposed sterility, either to block fertilization (tubal ligation) or to disrupt endocrine control of fertility (gonadectomy), by using radiotelemetry on the movement patterns and site fidelity of wild brushtail possums. In addition, we assessed the effect of gonadectomy on the transmission rate of a commonly occurring, directly transmitted pathogen in possums, Leptospira interrogans serovar balcanica (hereafter L. balcanica), to determine the effect of any behavioral changes on possum contact rates. Both tubal ligation and gonadectomy of females did not appear to have any appreciable effect on behavior, with sterilized females having space‐use patterns and fidelity to seasonal breeding ranges similar to those of fertile females. However, gonadectomy of male possums resulted in a significant reduction of 42% and 47% in the 95% and 70% isopleth seasonal breeding ranges, respectively. Furthermore, the transmission rate of L. balcanica in gonadectomized male and female possums was reduced by 88% and 63%, respectively, compared with that in fertile male and female possums. Overall, these results suggest that fertility control, either by blocking fertilization (e.g., immunocontraception) or by disrupting endocrine control of reproduction (e.g., gonadotropin‐releasing hormone vaccines), is unlikely to have an impact on social organization and behavior of brushtail possums in ways that may compromise the efficacy of fertility control for reducing population density. However, the reduction in the transmission rate of L. balcanica indicates that fertility control that interferes with endocrine control of reproduction is likely to reduce the contact rate between possums. This could have implications for the control of other wildlife diseases requiring direct contact for transmission.

Fertility control to mitigate human–wildlife conflicts: a review

“…For instance, the transmission coefficient of leptospirosis in possums was 28% higher in populations subjected to tubal ligation that leaves females cycling but unable to conceive, than in populations not subjected to fertility control (Caley and Ramsey 2001). In contrast, endocrine disruption caused by gonadectomy in possums decreased the leptospirosis transmission rate by 63-88% in sterilised female and male possums (Ramsey 2007).…”

Does contraceptive treatment in wildlife result in side effects? A review of quantitative and anecdotal evidence

“…In addition, hormonally sterilized female rats abandoned burrows more often, suggesting a change in dominance or social status, although overall habitat use was not changed (Jacob & Matulessy 2004). Male home range size may also be impacted, as found in brushtail possums (Ramsey 2007). Sterilization resulted in no difference in territory size, overlap or territorial behavior between controls and treated coyote packs (Bromley & Gese 2001).…”

“…Diseases could also be decreased in a treated population simply from lower densities of animals and healthier conditions (see body condition section). This decrease would be advantageous in certain pest species, such as the brushtail possum, which have a high population size, and are carriers of economically or ecologically important diseases, such as bovine tuberculosis (Caley & Ramsey 2001, Ramsey 2007. In addition, a potential decrease of brucellosis transmission is predicted in bison with GNRH treatment (Miller et al 2004), as anestrous females would be unable to spread the disease due to lack of offspring production.…”

“…While there has been little research that addresses these effects, most studies concur that disease transmission rates remain unchanged with contraceptive treatment (Kerr et al 1998, Williams et al 2007). However, the potential to reduce disease transmission exists (Ramsey 2007) and studies on rabbits showed a decrease in flea abundance (Twigg et al 2000, Williams et al 2007); possibly indicating a decrease in overall interactions between individuals. More research is clearly required.…”

Effects of a GnRH vaccine on the movement and activity of free-living wild boar (Sus scrofa)

“…No differences were observed in the crepuscular activities of untreated female eastern grey kangaroos (Macropus giganteus) compared with those of animals treated with the GnRH agonist deslorelin (Woodward et al 2006). No differences in space-use patterns and fidelity to seasonal breeding ranges were observed in ovariectomised brushtail possums (Trichosurus vulpecula) (Ramsey 2007) or surgically sterilised vixens (Vulpes vulpes) (Saunders et al 2002). The longterm effectiveness and potential side effects of GonaConÔ were recently evaluated in captive female wild boar (Massei et al 2012); 11 of 12 females did not give birth for at least 4 years (and some for 6 years) post-vaccination, but no effects were seen on bodyweight, haematology, biochemistry and on the social ranking of females that were housed together (there were no apparent ill effects of the vaccine on the single treated sow that did give birth or her piglets).…”