Habitat use and responses to odours by rodents in New Zealand
Four species of introduced rodent live in New Zealand. No more than three species are sympatric here, although all four are in more tropical areas. All are pests for economic, public health and conservation reasons. The factors that might regulate the occurrence and interactions of rodents in New Zealand have not been identified and are poorly understood. A combination of laboratory and field studies was used to examine the interactions between rodents, especially the role that odours play in regulating interspecific interactions. Knowledge of rodent ecology and interactions is required to manage populations of these pests effectively. Snap trapping of Norway rats (Rattus norvegicus) and kiore (R. exulans) on Kapiti Island indicated that these species were associated with different habitats: most Norway rats were caught in steep, poorly-drained areas with tall vegetation while kiore were associated with flat, well-drained areas of low growing vegetation. Whilst more kiore were caught in grassland and kanuka (Kunzea ericoides) forest, kiore caught in kohekohe (Dysoxylum spectabile) forest were larger and apparently more productive. Radio-tracking of both species showed that kiore used smaller home ranges and denser habitats than Norway rats, and provided weak evidence that the two species avoided each other when sympatric. In 1996, rats were removed from Kapiti Island by poisoning using Brodifacoum. The ranging behaviour of lethally poisoned radio-carrying rats (n=9) was unaffected by ingestion of poison until shortly before their death when they stopped moving and became unresponsive. Laboratory experiments indicated that kiore might avoid the odours of other rats in a Y maze, but ship rats (R. rattus) and Norway rats were less responsive to heterospecific odours. Mice (Mus musculus) in laboratory trials could apparently distinguish the odour of rats from their own odour, but did not avoid rat odour. However, when presented with a caged ship rat in a test arena, mice were less active. Pairs of mice that had been exposed to ship rat odour produced larger litters than control mice, but this result needs to be replicated. The results suggest that rodents, particularly kiore, probably avoid heterospecifics, but this behaviour is not mediated by odours alone, rather by more direct interaction. Either competition or predation by larger rodents on smaller ones could explain this avoidance. Future studies should manipulate the presence and abundance of rodents to investigate competitive ability. Such studies would allow a clearer understanding of population regulation and the mechanisms of coexistence, which may in turn allow more efficient control of rodent pests. If rats could be shown to avoid the odour of carnivores, then their behaviour could be manipulated as part of a pest control strategy. Predator-naïve Norway rats showed an aversion to five of the six predator (canid, felid, mustelid) odours they were exposed to in a Y maze. Predator-experienced Norway rats and ship rats, and predator-naïve kiore showed no such aversion. Furthermore, free-living rats did not avoid feeding stations tainted with synthetic carnivore odours. Whilst some rats avoid predator odours, they are unlikely to be suitable for the widespread repellency of rats in New Zealand.
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