Rakali
What is a Rakali?
The Rakali (Hydromys chrysogaster), also known as the water rat, is a native mammal and an extremely versatile hunter, taking most of their food from the fresh or salt water environments. They are common to most coastal areas of Australia, and are often found quite far inland.
Rakali eating a crab
Rakali is one many Aboriginal names for this mammal - there are over 50 - but this is the name most comonly used. It's scientific name, Hydromys chrysogaster, means water mouse with golden belly, although this is not always an acurate description of the animal as there can be considerable variation in the coloring of these mammals.
They are also considerably bigger in size than a mouse. In May 2006, a male was weighed during a Bayside Rakali Survey which tipped the scales at 1.12 kilograms!
Description
Like a platypus, the rakali is highly adapted for aquatic life and is one of the most specialised amphibious Australian mammals.
Rakali have partially webbed hind feet, small retractable ears, and fur with a dense water-repellent underlayer. Their tail is often as long as the body and is thick and well covered by dark hair, usually with a prominent white tip and it acts like a rudder when the rakali are swimming or diving for food.
Their fur is very soft, and in the past the rakali was hunted to almost to the point of extinction to fuel the fur trade. Thankfully now the rakali is a protected animal within Australia and this practice has stopped. Fur colour ranges from black/brown with a golden or orange underside to grey/brown with a creamy white underside.
Natural Habitat
Rakalis can be found throughout much of Australia and Papua New Guinea where they can inhabit streams, rivers, wetlands, estuaries, bays and offshore islands in both fresh and salt water.
Rakali are very adept at swimming. The distinctive V shape that they produce in the water as they swim allows for easy identification, and the speed at which rakali swim is surprisingly quick; observers sometimes needing to walk very briskly to keep up with swimming animals!!!
Diet
Their diet consists of large aquatic invertebrates, small fish and even birds! Sometimes they can be seen scavenging fresh fish remains from fishermen’s catches.
Food is often carried to and consumed on a feeding platform close to the water’s edge. Remains of a meal consisting of inedible items such as shells and crustacean carapaces are left as ‘middens’ on these feeding platforms.
Rakali in St Kilda
The best time to view Rakali is around sunset, when they are quite active in their daily search for food.
Three sites have been surveyed by volunteers in the St Kilda area, representing three very distinct set of environmental conditions. Rakali have been observed at all three sites, although they appear to be absent from particular locations within some of these sites.
- The St Kilda Breakwater
The St Kilda breakwater is a marine habitat, sheltered to the north, but exposed to sea conditions on the south. Rakali burrow between large rocks that make up the breakwater (also a site of penguin habitation). The majority of observations at the St Kilda breakwater have been made around the sheltered north facing section, which has better visibility (generally calmer water, better lighting). A few interactions with little penguins have been observed at this site, with penguins tending to steer clear of the rakalis when possible. Rakali have taken some young chicks on the breakwater, and are suspected to have also taken some unprotected eggs.
Further away from the main pier and breakwater, rakali have been observed offshore at St Kilda beach, near the yacht club slipway. The jetty for the Williamstown ferry (approximately a quarter of the way up the pier) has also been a site that rakali have been observed at. The small boats harboured near the pier are often boarded by rakali, as evidenced by their feeding middens left on board. - Albert Park Lake is sheltered away from Port Philip Bay, and harbours several ecosystems including reed beds (pollution control ponds), small vegetated islands and concreted walls and jetties. Rakali bed amongst the vegetation and any infrastructure in the reed beds and islands, and burrow in pipes and cracks that are common through cemented areas. The water is typically brackish, varying in depth over the seasons.
Albert Park Lake appears to have several areas where rakali congregate, and other areas which are completely uninhabited by water rats. The two reed beds and areas in front of the rowing/boating clubs (near The Point restaurant and near YMCA rowing club) have had several sightings of rakali. Rakali have also been spotted boarding the boats that are moored in front of the boat clubs.
No observations of rakali have been made on the small islands themselves, although they have been seen swimming in the general direction of Gunn Island. The area from just east of the YMCA rowing club, around to the Carousel restaurant has not yielded any rakali sightings. - Elwood Canal passes through St Kilda, from a golf course to the bay. The concreted area of the canal amongst the suburban streets has been the focus of observations, as the vegetated part of the canal proves very difficult to spot rakali in. The concreted canal is littered with cracks and drain pipes that the rakali reside in. The water in the canal experiences extreme fluctuations, from being flooded to completely empty. Water varies from very salty to fresh.
Rakali have been observed the length of the Elwood canal. It is probable that some individuals may frequent the coastal area just outside of the canal, in addition to roaming the canal itself. Observations of rakali have been made up to Point Ormond, and it has also been reported that the rakali inhabit the area around the marina, although this has not been confirmed by Earthcare volunteers.
Most sightings along the Elwood canal have been near the Broadway bridge (where a large drain area is present), and near Marine Parade.
The great swimming that rakali possess allows them to swim quite large distances for considerable periods of time. In Albert Park Lake rakali have been observed to swim out around 50 m before heading back into shore, whilst they have been followed for lengths of greater than 100 m at both the St Kilda breakwater and Elwood canal.
The exceptional ability of rakali to dive has been well recorded. Dive times have been observed of up to 16 seconds, and several dives of varying time lengths during the one swim are not uncommon. Swimming in tight circles has also been observed, thought to be made during the rakali’s attempts to capture prey.
Population
Details of the sightings of rakali have been used to provide an estimate of the total number of individuals viewed on each given night. Some level of error in determining the exact number of individuals observed is apparent, due to the difficulty in differentiating individuals. During the 2003 surveys, the average number of sightings made on a given night was 11, and the average number of individuals per observation night was 5. This number declined during the drought though.
These numbers indicate a relatively high density of rakali in the St Kilda area, compared with studies from rural Victoria (Smales 1984; Gardner and Serena 1993), where study sites were larger.
As the rakali have not been captured and marked in any way, it was not possible to determine the sex or age of individuals. A lack of distinguishing features amongst the populations makes it difficult to differentiate between individuals.
Varying sizes of rakali were observed at all sites. Some rakali were clearly juveniles, indicating that successful breeding is occurring in the St Kilda area (most rakali reach adult size in a year) (McNally 1960).
Quite often large rakali were seen engaging in what appeared to be ‘playful’ interactions with smaller rakali. Whether this was between a male and a female, two rakali of the same sex, or an adult and a juvenile (or cases of both) is unclear. Intrasexual aggression between rakali has been reported, with other studies showing that although home ranges often overlap with rakali of all other age and sex classes, overlap with other adults of the same sex is less (Gardner and Serena 1993).
Due to the difficulty in differentiating individuals, it is not known whether the rakali in St Kilda tend to be quite territorial or not. Some locations had more rakali observations than others. Whether this was because the same rakali was always present, or a number of different individuals frequented the same location on different nights, is unknown.
More ‘positive’ (non-aggressive) intraspecific interactions were observed than ‘negative’ (aggressive) interactions. Most interactions observed occurred in the water; Some involved the sharing of food, however during most interactions no food was seen.
The nature of interactions between individuals may be courtship, fighting over mates, play, teaching young, territorial aggression or sharing/fighting over food.
Food
Common foods identified as being eaten by Rakali in the St Kilda area include:
- worms such as tube worms,
- crabs, the shells of which which are often found in middens left on rocks and boats by Rakali,
- small fish and yabbies.
The rakali has also been seen eating pieces of bread at Elwood canal and the St Kilda pier which had been discarded by humans.
Like to learn about Rakalis?
Join Earthcare St Kilda in monitoring the health of our local Rakali population. For further details, click here.
Visit www.rakali.com
