Recovery plan for the brush-tailed rock-wallaby - Department of ...

Approved NSW Recovery Plan Brush-tailed rock-wallaby
6 Ecology
6.1 Lifecycle
6.1.1 Reproductive biology
Females give birth to one joey at a time, after a gestation period of approximately 30 days (Close
1993). The young remain in the pouch for six months. After the joey first emerges from the
pouch, it spends a further 7–20 days in and out of the pouch. As BTRWs are crepuscular (that is,
most active at dawn and dusk), young may be left at dawn, dusk or at night in refuges while the
mother moves out to feed. Weaning is believed to occur 86 days after leaving the pouch, when
the joey is nine months old (Lee and Ward 1989). Sexual maturation of females occurs at 18
months, males at 20–24 months (Lee and Ward 1989, Rob Close pers. comm.). A recent study at
Hurdle Creek (Queensland) showed that most males did not begin to father offspring until they
reached a weight of 5.2 kilograms and that most breeding males fathered the offspring of
multiple females (Goldizen pers. comm.). Life expectancy in the wild is 5–10 years or more
(Eldridge et al 1988), and can be longer in captivity.
The minimum time between litters is likely to be 210 days (Lee and Ward 1989). The number of
joeys born per year is related to dominance. A study by Joblin (1983) found the dominant female
of a group produced 1.09 joeys per year, and the subordinate females produced 0.59 per year.
Reproductive success appears to be related to both the dominance rank of breeding females and
the habitat the breeding female occupies.
A study of BTRWs on Mototapu Island gave an average age to independence of 230 days, and a
mean of 1.35 joeys born per year per female (Bachelor 1980). Breeding seemed to have no
particular season and was continuous year-round, probably depending on food availability.
Continuous breeding has also been observed in the Shoalhaven Valley where young were seen to
vacate the pouch in January, March (twice), April, July, October and November (Susan
Robertson pers. obs. cited in NPWS 1999b). In contrast, a study in the Macleay River gorges
found that April had the highest birth rate, with most juveniles vacating the pouch during
August–November and dispersing during March and May (Joblin 1983). In the same area, Bayne
(unpub.) observed some year round breeding with a peak of large pouch young (when they are
most easily seen) in late spring, consistent with an autumn peak in birth rates. More recently, a
study of breeding patterns of females at Hurdle Creek (Queensland) found that more than 50 %
of females in the colony were giving birth to only one joey per year and therefore considered to
be breeding below their full potential. This is thought to be related to female condition, although
the study did not determine which factors were likely to be affecting their condition. Births in
this colony generally peaked in autumn. Young born in autumn or winter left the pouch during
spring or summer when food was most abundant, and had a much higher probability of survival
through to pouch emergence than those born during spring and summer (Goldizen pers. comm.).
Male BTRWs are thought to be polygynous (i.e. they have more than one partner at a time), with
a single male fathering offspring of other males (Goldizen pers. comm.). Female BTRWs are
serially monogamous, mating with a single male until he disappears from the colony, then
switching to another male (Goldizen pers. comm.). BTRWs often live in family groups of 2–5
adults and usually one or two juvenile and sub-adult individuals (Joblin 1983, Short 1980), but
also occur in male–female pairs (Bayne pers. comm.). Mating pairs have highly overlapping
home ranges, suggesting that matings occur in local breeding groups that contain a single mating
male and one or more breeding females which are close relatives. In the Hurdle Creek survey,
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