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Sculptures by Phyllis Bone

Kangaroo Left


Kangaroo Right

A Koala Skull
A Wild Koala

Within Australia there are a few marsupial species, which although classed as national icons are also pests. This status applies to grey kangaroos and koalas. There are also high numbers of brushtail possums which are classed as very serious pests in New Zealand. The control of these species is of great importance because of the damage they are inflicting upon both the local environment and other species. The main biological and ethical issues revolve around whether we are interested in controlling the populations of desirable species at an appropriate level, with an option for reversing the control in future years, or whether the species is undesirable and needs to be controlled indefinitely at minimum cost. Both the grey kangaroos and koalas are more important to the public than the brushtail possum, and hence the approach to control must be different.

Koalas and Grey Kangaroos The kangaroos, unharassed by predators and professional shooters, have reached densities of 370 per square km within the Australian Capital Territory. Here they are currently vying with both farmed and feral animals for the same vegetation. Koalas which have been introduced to Kangaroo Island off Southern Australia are also destroying the natural habitat. However, the pressure of public opinion has made the culling of these species politically unacceptable, and new methods of population control must be introduced.

An Eastern Grey Kangaroo
A Wild Brushtail Possum

Brushtail possums Population numbers of the brushtail possum are thought to have exceeded 70 million in New Zealand. Many of the more usual methods of fertility control such as surgical vasectomies or administration of a chemical contraceptive are not viable for controlling such large populations.

Immunocontraception The growing acceptance that so many current methods of population control are inhumane or simply not viable for controlling such large numbers of animals has lead to the development of a new form of population control. Immunocontraception is a method of stimulating an animal to produce antibodies against a reproductively important hormone or protein. If it is possible to induce an immune response in an animal which will hinder one aspect of their reproductive cycle, then this will effectively prevent pregnancy; or in some cases, depending on the target, terminate the pregnancy. This method of population control is also very appealing because of its ability to be species specific.
A problem when designing an immunocontraceptive is how to deliver it to the population. Once a vaccine has been developed, it can be administered within a laboratory or to a small population either via intramuscular injection or oral delivery. However, neither of these methods is any more efficient than a chemical (steroidal) contraceptive because the same contact with the animals is required. Immunocontraception can only become an effective form of population control if a delivery system which does not require contact with the animals is developed.

Delivery systems

  • Plant Delivery. The three marsupial species currently in need of control are all herbivorous, so one proposed delivery system is to insert an antigen into a readily grown plant such as maize or carrots, and then use these food stuffs to deliver the vaccine to the animal population.
  • Viral Delivery, using a crippled virus. In this case a virus which is specific to the target species would be engineered to carry the gene for the immunocontraceptive antigen. The virus would be able to infect the animal and stimulate the immune system to block reproduction; but it would be unable to spread between animals.
The current situation At the moment promising, preliminary laboratory trials have been undertaken by the Marsupial CRC on all three of the above pest species, using either whole sperm or zona pellucida as antigens. Work is also being undertaken to sequence genes found in the sperm and zona pellucida so that more specific antigens can be prepared in the future. Although many targets have been achieved, much more work needed before an effective method of population control will be produced.