Jennifer K. Martin

Resource distribution influences mating system in the bobuck (Trichosurus cunninghami: Marsupialia)

Mammalian mating systems are thought to be shaped by the spatial distribution and abundance of key resources, which in turn influence the spacing behaviour of individuals. In particular, female home range size is predicted to reflect the availability of key resources. We documented the availability and distribution of food and shelter resources for two neighbouring populations of bobucks, or mountain brushtail possums, Trichosurus cunninghami, that were characterised by different mating systems: our “forest population” was socially monogamous, whereas the “roadside population” was polygynous. Both silver wattle, Acacia dealbata, the main food resource for bobucks, and den-trees, which provided shelter, occurred at significantly higher density at the roadside site. The pattern of distribution of these two resources also differed between the sites. Both food and den-trees were scattered evenly throughout the roadside habitat. In contrast, den-trees were located predominantly at one end of the forest site, while silver wattle trees were located at the other. There was no significant difference in the amount of silver wattle, or in the number of den-trees, located within the home ranges of individual females at the two sites. However, forest females had home ranges, on average, almost three times the size of those of roadside females. At the roadside site, the size of female home ranges varied inversely with the density of silver wattle, indicating that these females ranged over as large an area as necessary to gain access to sufficient silver wattle trees. There was no such relationship among forest females. These populations provide a clear example of resource distribution determining female home range size. This influenced the number of female home ranges a male’s home range overlapped with, which in turn determined the mating system. Such clear links between resource availability and mating system have not previously been established in a marsupial.

Long-term pair-bonds without mating fidelity in a mammal

Most mammals are polygynous and are characterised by male-biased sexual size dimorphism. One hallmark of mammalian monogamy is the lack of such dimorphism. Bobucks, or mountain brushtail possums, Trichosurus cunninghami, lack sexual size dimorphism; however, few behavioural data exist for this species. We studied the mating system of a bobuck population in south-eastern Australia. Adult bobucks were strongly paired: pair-members had exclusive access to a suite of den-trees and overlapped in home range on average by 70%. Pair-members rested together in the same tree-hollow on approximately 70% of days during the breeding season and 47% of days during the non-breeding season. While active, pair-members remained within approximately 8 m of one another during the breeding season and within 31 m of one another during the non-breeding season. Females established pair-bonds at 2-5 years of age; pair-bonds ended only as a result of the death of one pair member. However, molecular paternity analysis established that 35% of young were the result of extra-pair copulations (EPCs). This is the first study to provide strong evidence of long-term pair-bonds in a marsupial and raises questions about the relative benefits of pairing to males and females in this population.

Den-use and home-range characteristics of bobucks, Trichosurus cunninghami, resident in a forest patch

Detailed knowledge of how individuals use space when active and while sheltering is crucial to understanding the habitat requirements of a species. I present the first home-range estimates for bobucks, Trichosurus cunninghami, that are based on both nocturnal and diurnal radio-tracking fixes. I tracked 37 individuals (14 adult females, 14 adult males, three subadult females and six subadult males) between mid-1999 and late 2003 in a forest patch in the Strathbogie Ranges, south-eastern Australia. I collected a total of 9562 diurnal fixes (mean 309 fixes per adult) and 5211 nocturnal fixes (mean 169 fixes per adult). All individuals used multiple den-trees; adults used a mean of 7.2 den-trees per individual. Adult bobucks of both sexes had a mean home-range size of 6.0 ha. There were no significant differences in the mean number of den-trees used or in the mean home-range size of adult males and females. Subadults used significantly fewer den-trees and had significantly smaller home ranges than adults. This study demonstrates the importance of large and long-term datasets in accurately determining the habitat requirements of a population.

A cost-effective and informative method of GPS tracking wildlife

Abstract. In wildlife research, our ability to GPS track sufficient numbers of individuals is always limited by cost, which restricts inference of species–habitat relationships. Here, we describe the modification and use of a relatively new and inexpensive off-the-shelf GPS device, to provide detailed and accurate information on the movement patterns of individuals (mountain brushtail possums, Trichosurus cunninghami), including how movement varies through time, and how individuals interact with each other. Our results demonstrated that this technology has enormous potential to contribute to an improved understanding of the movement patterns and habitat preferences of wildlife at a fraction of the cost of traditional GPS technology.

Temporal dynamics of helminth infections in eastern grey kangaroos (Macropus giganteus) in Victoria

Abstract. Parasite infection is increasingly recognised as a factor shaping the population dynamics, life history and behaviour of hosts. However, before the impacts of parasites on wildlife hosts can be investigated, seasonal patterns in host exposure to parasitic agents must be determined. We examined infection patterns at three sites in Victoria, and combined field experiments and observations to construct a generalised life cycle of the helminth community in eastern grey kangaroos (Macropus giganteus). Kangaroo populations in Victoria had very similar helminth communities, with 20–25 species detected at each site. Despite examining relatively few hosts in this study, at least 87% of all gastrointestinal helminths were recovered according to bootstrap estimates. The prepatent period of infection in eastern grey kangaroo nematodes was at least 3 months, and faecal egg output showed a distinct seasonal pattern, with a peak in egg counts from October through to January each year. Data from one site indicated that faecal egg counts were influenced predominantly by the abundance of a single nematode species (Pharyngostrongylus kappa), despite adults accounting for only 7% of the total nematode burden. This highlights the problems associated with using faecal egg counts to estimate nematode burdens in this host. Contamination of pasture plots showed that nematode eggs take ∼14 days to larvate once deposited, and that autumn rains likely triggered emergence from faecal pellets. The abundance of infective larvae in the environment therefore appears to be closely tied to environmental conditions, with a peak in infection of hosts in the winter months.

Anthelmintic Treatment Does Not Change Foraging Strategies of Female Eastern Grey Kangaroos, Macropus giganteus.

Large mammalian herbivores are commonly infected with gastrointestinal helminths. Heavily parasitised hosts are likely to have increased nutritional requirements and would be predicted to increase their food intake to compensate for costs of being parasitised, but experimental tests of the impacts of these parasites on the foraging efficiency of hosts are lacking, particularly in free-ranging wildlife. We conducted a field experiment on a population of free-ranging eastern grey kangaroos (Macropus giganteus) to test this prediction, removing nematodes from one group of adult females using an anthelmintic treatment. We then carried out observations before and following treatment to assess the influence of parasites on foraging behaviour. Contrary to our predictions, the manipulation of parasite burdens did not result in changes in any of the key foraging variables we measured. Our results suggest that despite carrying large burdens of gastrointestinal parasites, the foraging strategy of female kangaroos is likely be driven by factors unrelated to parasitism, and that kangaroos in high nutritional environments may be able acquire sufficient nutrients to offset the costs of parasitism. We conclude that the drivers of forage intake likely differ between domesticated and free-ranging herbivores, and that free-ranging hosts are likely more resilient to parasitism.