Kangaroos are marsupials and belong to the Family Macropodidae (i.e. big feet) that is grouped with the Potoroidae (potoroos, bettongs, rat-kangaroos) and Hypsiprymnodontidae (musky rat-kangaroo) in the Super-Family, Macropodoidea. This comprises around 50 species in
The Rock-wallabies (Petrogale spp.) is the most diverse genus amongst the living macropods with 16 species ranging from 1 to 12 kg in size. They are found across mainland Australia and on some recently separated offshore islands but not on the Bass Strait Islands, Tasmania or New Guinea. The species diversified from a common ancestor about 4 million years ago and their closest affinity to other macropods is with the Tree-kangaroos. Diversification of species occurred in two waves. The first gave rise to the Short-eared Rock-wallaby, the Monjon, the Narbelek, the Yellow-footed Rock-Wallaby and the Proserpine Rock-wallaby. The second was about a million years ago and lead to species that are not all morphologically distinctive like those along the Queensland seaboard. All Rock-wallabies favour habitat with rocky outcrops and slopes, cliffs and gorges or are found on boulder piles and escarpments especially in the wet-dry tropics. Their ability to scale precipitous rock faces in leaps that appear to defy gravity comes from adaptations to the feet and tail. The feet are short relative to the majority of macropods that inhabit flat ground. The pads are thick, spongy and highly granulated so that they compress on the rock surface and maximise grip. The tail is long and cylindrical with little taper and great flexibility. The tail acts as a counterbalance and rudder in rapid hopping across uneven surfaces and allows changes of direction in mid-air.
Petrogale assimilis ('similar rock-weasel')
Bowling Green Bay National Park, Queensland
The Allied Rock-wallaby is widespread in north-eastern Queensland and is found offshore on Palm and Magnetic Islands. Bowling Green Bay National Park is 28 km south of Townsville and is formed by rugged mountains (including Mt Elliot at 1342 m) running down to a coastal plain of wetlands, saltpans and mangroves. A central feature is Alligator Creek with a large formal campground with toilets. There are three other remote bush camping sites and one main walking track of 17 km return to Alligator Falls. The Allied Rock-wallabies can be viewed around the campsites along with Agile Wallabies and occasional Rufous Bettongs. Some tame individuals can be seen on Magnetic Island off the coast of Townsville.
The Allied Rock-wallaby is part of a complex of species that includes the Sharman's Rock-wallaby, the Unadorned Rock-wallaby and the Mareeba Rock-wallaby. These species are not easily distinguished except by the shape and number of chromosomes. The genera tourist does not have a cytologist's kit to make such distinctions and so locality is the best guide to determine which species you are seeing. As is typical of Rock-wallabies, males (4.7 kg) are larger than females (4.3 kg). Freshly moulted individuals are typically greyish on the back but as the year advances the colour changes to pale through to dark brown. The underside of the the body and limbs is lighter and typically a sandy brown. In some individuals there may be a pale cheek stripe, with indistinct and patchy markings behind the shoulders and a dark dorsal stripe along the crest of the head. The paws and feet are dark and the tail darkens towards the tip which has an indistinct brush. Some individuals, particularly in the northern part of their range, have a whitish to greyish tail tip.
The biology of most macropods is poorly known but a few species have attracted the curiosity, intellect and field-based skills of a team of researchers (mainly PhD students). The Allied Rock-wallaby is one such species and a number of PhD studies were conducted by a team known as the "Black-rockers" from James Cook University. The Allied Rock-wallaby is widespread but its preference for the cliffs, rocky slopes and outcrops and boulder piles of rock-wallabies in general results in a fragmented distribution. The rocky habitat is usually embedded in tropical woodland and open forests where the rock-wallabies often forage at night. They are best seen in the evening when they make this transition from rock shelters to more open foraging habitat but they may 'sun-bake' at other times in cold weather.
Individual rock-wallabies have a high fidelity to their rocky shelter area and a small home range that expands from the plenty of the wet season to the paucity of the late dry season. Thus home range shows an elasticity based on the quality, quantity and distribution of food across a generalist diet. The wet-dry tropics challenge species because rainfall is concentrated in only one part of the year and they annually face a seasonal drought. Like much of Australia the onset and offset of the wet and dry periods is variable and the precipitation received is unpredictable. The Allied Rock-wallaby copes by eating new grass after rain and a general diets of browse and small herbs (forbs). This is supplemented by high quality items like fruit, seeds and flowers and includes animal matter like insect larvae.
The Allied Rock-wallaby is a continuous breeder but recruitment may be cut back by extended drought (poor/short wet seasons). Gestation is 29-34 days with a post-partum oestrus. Pouch life is 6-7 months and young-at-foot are weaned at about 11 months of age. Females are sexually mature around 19 months and males a little later at 24 months. Since individually identified Allied Rock-wallabies have been studied in the long-term, longevity has been extended out to about 13 years of age. Behavioural observations suggested that females are monogamous with one or two associating with a single male. However, paternity testing through genetic studies identified extra-pair copulations and so females are not entirely faithful to one male and males are likely to take any mating opportunity.
The behaviour of the Allied Rock-wallaby has been comprehensively studied and revealed long-term, stable relationships between an adult male and one or two females. This was manifested by close association and allo-grooming (often mutual), the sharing of daytime shelters, the defence of the shared area especially the shelter and foraging close-by in the same home range. These long-term bonds are leaky as genetic studies revealed extra-pair paternity.
Barker SC (1990) Behaviour and social organisation of the allied rock-wallaby Petrogale assimilis, Ramsay, 1877 (Marsupialia: Macropodidae). Australian Wildlife Research 17, 301-311.
Clancy TF, Close RL (1997) The Queensland rock-wallabies. An overview of their conservation status, threats and management. Australian Mammalogy 19, 169-174.
Delaney R (1997) Population dynamics of the allied rock-wallaby Petrogale assimilis: Implications for conservation. Australian Mammalogy 19, 199-207.
Delaney R (1997) Reproductive ecology of the allied rock-wallaby, Petrogale assimilis. Australian Mammalogy 19, 209-218.
Horsup A (1994) Home range of the allied rock-wallaby, Petrogale assimilis. Wildlife Research 21, 65-84.
Horsup A, Marsh H (1992) The diet of the allied rock-wallaby, Petrogale assimilis, in the wet-dry tropics. Wildlife Research 19, 17-33.
Spencer PBS, Marsh H (1997) Microsatellite DNA fingerprinting confirms dizygotic twinning and paternity in the allied rock-wallaby, Petrogale assimilis (Marsupialia: Macropodidae). Australian Mammalogy 19, 279-280.