|Ecology and ecophysiology of the lace monitor (Varanus varius) in a temperate environment|
|Applied Ecology Research Group
University of Canberra
Between November 1997 and September 1998, the space, food, energy and water resources used by a population of lace monitors (Varanus varius) at Lake Burrendong, were examined over an entire active (September - April) and inactive season (May - August). A total of 33 monitors were examined, of which 23 were radio-tracked for periods up to 11 months.
Radio-tracking provided new information on the spatial ecology of V. varius. During summer (December to February) V. varius moved often, over large (184.5 ha), overlapping home range areas. In the intermediate seasons of spring (September to November) and autumn (March to May), lace monitors moved less often and utilised less than 39% of their summer home range. Finally, in the cold winter season (June to August) many lace monitors did not move at all and most used less than 5% of their summer home range. The thermal environment, and probably reproductive status, and food availability affected V. varius use of space, and the importance of these factors varied seasonally.
V. varius used the surrounding macro habitats in proportion to availability. The open woodland habitat was used most often and the home ranges of all monitors contained at least two of the four habitat types described. Monitors did not prefer one vegetational type to another and this may be because of the active and rather generalist catholic diet that was described for this species in the present study. Conversely, this analysis detected strong and consistently non-random patterns of roost site selection by all radio-tracked lace monitors. Lace monitors selected dead trees rather than live trees, larger trees rather than smaller trees, and trees with hollows over those without hollows. These results suggest that lace monitors may depend on the hollows of standing trees as refuge sites for sleeping and inactive periods.
The use of energy and water by V. varius was seasonally dependent and paralleled changes in the thermal environment, and possibly reproductive status and the availability of prey. In the early summer, C02 production and water influx rates were relatively high (0.122 MI C02 g-1 h-1 and 20.3 ml H20 kg-1 day-1), but they declined substantially during the mid summer (0. 111 MI C02 g-1 h-1 and 15.4 ml H20 kg-1 day-1), late summer (0.080 ml C02 g-1 h-1 and 12.6 ml H20 kg-1 day-1),and autumn (0.049 MI C02 g-1 h-1 and 9.0 ml H20 kg-1 day-1) seasons. The summer/autumn difference represented more than a 2-fold reduction in energy expenditure and water flux. However, within the confines of season this study showed that individual V. varius could reduce their energy and water budget by fourteen fold during the summer period, simply by regulating the proportion of days spent abroad. The Monitors maintained body mass in all seasons. Thus, this work suggested that lace monitors regulated their energy and water budgets in a manner that presumably enhanced survival and probably reproductive success.
V. varius were opportunistic consumers of a wide variety of prey, including carrion, mammals, birds, reptiles and invertebrates. This work suggested that V. varius opportunistically exploit the local conditions (through dietary shift) and that different populations of the one species may show intra-specific variation in diet.
In conclusion, the findings of this study have made an important contribution to both the ecology and ecophysiology of V. varius. This study has improved our understanding of, and ability to predict, the detailed requirements of large varanid predators and scavengers. More generally, the present study demonstrated the feasibility of radio-tracking for monitoring the behaviour of reptiles, and the potential usefulness of such research for conservation planning. Consequently, this project provided basic ecological information which has direct practical application in the design of conservation/management strategies and reserves.
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