Rakali are highly territorial, marking their home ranges with a strong scent resembling that of cat urine. Considerable fighting occurs in places where many animals co-exist, as evidenced by a high frequency of bite marks on the tail, hind feet or rump (Olsen 1980). Adults are mostly solitary when active, and it’s assumed that a female raises her offspring without any assistance from the father of the litter.
Relatively little is known about rakali spatial use and shelter sites, in part because these animals are very good at biting through and shedding radio-collars. The home ranges of an adult male and lactating female respectively encompassed at least 3.9 kilometres and 1.8 kilometres of a relatively small creek in Victoria (though measured only over a few days: Gardner and Serena 1995). By comparison, the home ranges of of three adult males tracked in southeastern Queensland were 0.9 to 2.2 kilometres long (Harris 1978). In northern Queensland, an adult male was documented to travel at least 3.0 kilometres (though 4.5 kilometres if he followed the most likely route) in a single overnight period (Vernes 1998). Similarly, the adult male tracked in Victoria traversed 3.1 kilometres of creek channel in just 5.5 hours (Gardner and Serena 1995).
Rakali occupy burrows located in creek and river banks, or shelter in large hollow logs near the water (Harris 1978; Gardner and Serena 1995). Unlike a platypus, a water-rat cannot maintain a stable body temperature in water below 25°C and therefore must periodically exit colder water in order to warm up (Fanning and Dawson 1980; Dawson and Fanning 1981; Gardner and Serena 1995). Radio-tracking studies have shown that platypus and rakali will use the same burrow, though not at the same time. For example, an adult female platypus occupied a burrow a few weeks after it had served as a nursery for a female rakali with young (Gardner and Serena 1995). This is actually not surprising given that these species are about the same size and typically occupy numerous burrows over time. It remains unknown if they are equally likely to dig a new burrow in the first place.
Photos courtesy of Con Boekel (top) and Carolyn Hall (bottom)
LITERATURE CITED
Dawson TJ and Fanning FD (1981) Thermal and energetic problems of semiaquatic mammals: a study of the Australian water rat, including comparisons with the platypus. Physiological Zoology 54, 285-296.
Fanning FD and Dawson TJ (1980) Body temperature variability in the Australian water-rat, Hydromys chrysogaster, in air and water. Australian Journal of Zoology 28, 229-238.
Gardner JL and Serena M (1995) Observations on activity patterns, population and den characteristics of the water rat Hydromys chrysogaster along Badger Creek, Victoria. Australian Mammalogy 18, 710-75.
Harris WF (1978) An ecological study of the Australian water-rat (Hydromys chrysogaster: Geoffroy) in southeast Queensland. MSc Thesis, University of Queensland.
Vernes K (1998) Observation of a long-range overland movement event by an adult common water rat, Hydromys chrysogaster. Australian Mammalogy 20, 409-410.