Ecological aspects affecting the use of feral pigs (Sus scrofa) as sentinels of TB (Mycobacterium bovis) in a high country farm habitat

Abstract

The successful use of feral pigs (Sus scrofa) as sentinels of bovine tuberculosis (Mycobacterium bovis) (Tb) in New Zealand wildlife, largely relies on two factors: the area typically traversed by feral pigs and the susceptibility of feral pigs to Tb. Therefore, knowledge of these ecological characteristics must be acquired for all key habitats where feral pigs may be used as sentinels of Tb.

Radio telemetry was used to gather location data for 27 feral pigs in typical South Island high country habitat over an 11 month period. Location data were used to assess the dispersal, home ranges and habitat preferences of the pigs.

Most feral pigs remained philopatric to their juvenile ranges, with only one male pig dispersing to a location approximately 4 km from the initial capture site. Total home range sizes varied from 176 to 1395 hectares (95% fixed kernel estimate) and did not appear to depend on the sex of an individual. Although variation existed in the seasonal home range size of individual pigs, no clear trend was evident for a predictable change in home range sizes between summer and winter seasons at the population level.

Both cover and slope influenced the relative use of different areas by feral pigs during the day, but aspect did not. However, it is likely that cover had a stronger influence than slope on the habitat used by pigs. The most preferred areas were rocky or riverbed, and dense scrub cover on slopes of 25 - 35°, although pigs were observed using all available habitat types throughout the study area.

The susceptibility of feral pigs to Tb is largely determined by their ability to find and scavenge brushtail possum (Trichosurus vulpecula) carcasses. The proportion of brushtail possum carcasses scavenged by feral pigs during winter in typical high country habitat was assessed by observing the fate of 40 possum carcasses placed at random locations in a 400 ha section of the study area. Feral pigs scavenged 32 carcasses within six weeks, and 39 of the 40 carcasses by the end of the study (after 12 weeks).

Strong evidence that feral pigs in typical high country habitat do not generally disperse long distances, and occupy seasonally stable home ranges of at least 176 ha, allows precise estimates to be made of the source of infection when a Tb-infected feral pig is located in such habitat. Knowledge that feral pigs prefer rocky habitats and dense scrub on 25°- 35° hill-slopes indicates where they are most likely be found during Tb surveys, and also where they are most likely to interact with Tb-infected animals. Evidence that feral pigs are likely to scavenge a high proportion of possum carcasses indicates that if a Tb-infected possum dies in a location where it is accessible to pigs, it is highly likely it will be scavenged by them within the time frame that Tb survives in the carcass.

The conclusion of this research is that feral pigs are likely to be an ideal sentinel of Tb in typical high country habitat.