Home range

As part of his study of the ecology of bare-nosed wombats, McIlroy attached small radio-transmitters to several wombats, each transmitter

Figure 5.13 This wombat drank for eight minutes without pausing.

having a separate frequency. The transmitter was attached to the wombat by a simple chest harness and an aerial loop around the neck, and Mcllroy was able to receive signals from the transmitters on a modified walkie-talkie set used in conjunction with a hand-held directional loop antenna. The maximum range for reception was about 300 metres when the wombat was above ground and 150 metres when it was down a burrow. By rotating the antenna until he received a signal and then finding the direction from which the strongest signal came, Mcllroy was able to pinpoint the wombat's position. With experience, he was also able to determine whether or not the wombat was moving, to estimate its speed if it was, to determine whether or not it was feeding and how intensively, and to locate a wombat's position in its burrow.

A wombat can cover a considerable distance during a night's grazing and travelling. Mcllroy radio-tracked one wombat for eight hours one night, during which it covered a distance of about three kilometres, and I have followed a wombat for nearly four kilometres before it disappeared into a burrow. In areas where there are several burrows close together, as often happens along creek banks, and there is a good supply of suitable grasses nearby, a wombat will still wander for one to two kilometres in the course of a night's grazing.

The area over which an animal normally travels in pursuit of its routine activities is, by definition, its home range. In the case of the wombat - and the situation is similar in many other mammals - the home range includes the wombat's burrows, feeding places, rubbing posts, dust-bathing patches and so on. These places are connected by a network of paths regularly used by the wombat.

By radio-tracking, McIlroy was able to estimate the home range sizes of six adult wombats, and he found that the size and shape of a home range mostly depended on the pattern of distribution of used burrows and suitable feeding areas. For example, where suitable burrow sites were in forest that was close to pasture or other good feeding areas, the home range was as small as about five hectares; but when a wombat regularly fed in an area at some distance from its burrows, the range was as large as 23 hectares.

The size of a home range may increase or decrease under different seasonal conditions. During winter and in periods of drought wombats may extend their home ranges to take in more distant feeding areas. In the summer of 1982-83, when severe drought conditions prevailed in the coastal forest, all the wombats I was studying increased their ranges considerably. One was regularly found grazing in a clearing nearly one kilometre from its usual home range.

A bare-nosed wombat's home range appears to have a core area - a part of the home range that is used more intensively than the rest. The term 'home range' is often used synonymously with 'territory', but the two are not necessarily the same. More often the territory is a smaller area, situated within the home range, which the animal will defend against the intrusion of others of its kind. The bare-nosed wombat does not appear to consistently defend any particular part of its home range, not even the core area. Sometimes it will chase another wombat out of a burrow, but on another occasion it will share it with one, or even two, others. At times, too, a wombat will aggressively defend a certain favoured feeding area. This seems to happen more often during periods of food scarcity, such as winter or drought. When grass is growing abundantly, the same wombat in the same feeding area will tolerate another grazing only a few metres away.

Southern hairy-nosed wombats also have a core area - the grazing halo around the warren, which may extend for up to 0.7 kilometres from the warren. Unlike bare-nosed wombats, they will defend this core area as a territory, chasing away any wombats from other warrens. The warrens are surrounded by defecation wallows and rubbing posts which are all marked regularly with faeces. Graeme Finlayson and several colleagues also used radio-tracking to monitor the activities of 16 adult southerns. They found that these wombats used up to five warrens but they generally centred their activity around one or two large, multi-entranced warrens. Their home range size varied from 1.3 to 4.8 hectares and the home ranges of both males and females usually overlapped considerably.

Northern hairy-nosed wombats have a home range of about six hectares but in the wet seasons this is halved and the wombats are only active for about two hours per night. Johnson and Crossman studied 28 northerns for a year and found that the feeding ranges of wombats of the same sex did not overlap but the ranges of females overlapped those of one or more males.

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