Why?

A captive breeding and management program was required for the Brush-tailed Rock-wallaby (BTRW) as there were perceived to be too few animals remaining in the wild and too few populations to ensure the species would survive long term.  In addition, their breeding-rate is less than one young per year which makes it difficult to maintain positive population growth in the face of an array of natural and introduced predators and competitors.  Introduced predators and competitors can be controlled around the remaining colonies, however, small populations are also at great risk from fire, disease, drought, annual fluctuations in sex-ratio and an array of other factors periodically affecting the survivorship of young.  A captive-breeding program isolates the animals from many of these factors that may have a negative effect on the survival of both adults and young.  In the case of the BTRW it also provided species managers with the opportunity to try and manipulate breeding in order to try and increase the number of young produced by each female above the one young born and weaned annually, under natural conditions. 

 Cross-fostering

For this species a technique was optimized, which had not been previously used in the recovery of any other threatened marsupial species.  This technique is termed cross-fostering.  Cross-fostering refers to the transfer of a pouch young from a threatened species into the pouch of a surrogate mother from a similar common species.  This has two main benefits for the endangered species:

(i) it frees the mother from the burden of carrying a young to term (seven-eight months in the case of the BTRW) and

(ii) it allows her to cycle again, mate and produce another pouch young, which can in turn be fostered off to a surrogate mother.

 When?

Cross fostering trials started in 1997 at Tidbinbilla Nature Reserve using Tammar Wallabies as surrogate mothers.  The Tammar Wallaby was selected as a potential surrogate species, as it is common in captivity; a great deal was known about its reproduction; it can be maintained in large breeding groups; females are a similar size to the BTRW females and they are generally quiet by nature.  Young BTRW’s as small as one gram and as large as 100 grams were transferred during these trials.  These trials were complete in 1998.  However, Tammar Wallaby females only cycle between January and the end of June and so a second surrogate species was required that cycled year round.

The Yellow-footed Rock-wallaby (YFRW) was chosen for this role as it is a close relative of the BTRW; females are around the same size in both species; female YFRW’s breed year round and they can also be maintained in large breeding groups.  Cross-fostering trials between BTRW and YFRW commenced in 1998 at Adelaide Zoo and were completed in 1999.

Overall, approximately 80% of young transferred during these trials to either surrogate species survived to weaning.  Problems were encountered bringing wild BTRW’s into close confinement at Adelaide Zoo with mothers throwing their young within the first month following birth.  This was overcome by transferring the BTRW pouch young at 10-14 days of age into the pouch of their foster mother.

 The breeding program officially got underway in 2001.  Young are now routinely transferred at 10-14 days of age.

 To help ensure that the genetic diversity of the BTRWs in captivity reflects what is in the wild, trials were also conducted to adapt cross-fostering techniques from wild mothers.  In order to succeed the Recovery Team had to develop and trial techniques for maintaining tiny pouch young in an incubator away from the pouch during their long trip from the upper Snowy River Gorge to their waiting surrogate mother in captivity.  These techniques were successfully established and three pouch young have now been removed from their mothers pouches in Eastern Victoria, flown by light plane to S.A, to be reared by tammar and Yellow-footed rock-wallaby foster mothers in Adelaide.

 Who?

Several institutions are involved in the BTRW cross-fostering program.  These include five intensive breeding institutions: Adelaide Zoo and the Waite Animal Facility in South Australia; Waterfall Springs Wildlife Sanctuary in Gosford, NSW and the Tidbinbilla Nature Reserve, an institution with both intensive breeding facilities and large semi-captive enclosures for holding animals prior to reintroduction.