Space use by animals on the urban fringe: interactive effects of sex and personality

Personality traits shape individual perceptions of risks and rewards, and so, should affect how animals value and use their environment. Evidence is emerging that personality affects foraging, space use, and exploitation of novel environments such as urban habitat. But the influence of personality is also hypothesized to be sex-dependent when primary motivation for space use differs between sexes, as often occurs in polygynous species. We tested the influence of personality traits, interacting with sex, on space use by the polygynous common brushtail possum, Trichosurus vulpecula, in an urban-woodland boundary in Sydney, Australia. We quantified personality traits, including exploration, using behavioral assays in an artificial arena. We also GPS-tracked free-ranging individuals, and measured range size, core area: home range, and proportional urban range. We found that personality traits affected space use either as a main effect or, as predicted, an interaction with sex. More exploratory animals, regardless of sex, had higher core area: home range ratios and proportionally larger ranges within urban habitat. However, less exploratory females yet more exploratory males had larger ranges. Our findings provide new insight into movement ecology by demonstrating, for the first time, the sex-dependent influence of personality. The demonstrated influence of personality on urban use by possums also suggests a personality filter for wildlife, as populations transition into urban areas. Finally, as individuals at the interface between urban and natural habitat are also a conduit between the two, a corollary of our findings is that there may be personality-mediated spread of disease across this boundary.

Estimating disease survey intensity and wildlife population size from the density of survey devices: Leg-hold traps and the brushtail possum

Wildlife disease surveillance requires accurate information on the proportion of managed populations sampled or their population density, parameters that are typically expensive to measure. However, these parameters can be estimated using spatially explicit modelling of capture probabilities, based on the distribution and deployment times of capture devices, given accurate information on the relationships between these variables. This approach is used in New Zealand's surveillance programme aimed at confirming areas free of bovine tuberculosis (bTB¹) in brushtail possums (Trichosurus vulpecula). However, there is uncertainty about the accuracy of the underpinning parameters characterizing possum trappability (g), given the distance between where a trap is placed and the possum home range centre. Sampling intensity (SI: the percentage of the population sampled during a population survey) and sigma (σ; 95% home range radius/2.45) were measured, using leg-hold traps deployed under a set protocol to standardize survey effort, at four sites containing previously radio- and GPS-collared individuals. Those data were used to derive an estimate of the nightly probability of capture of possums in a trap set at their home range centre (g₀). Those estimates were compared to the standard assumptions currently used as defaults in the day-to-day approach used by bTB managers. Home-range size (and therefore σ) varied widely between sites (range 3.6-49.4 ha), probably largely in response to differences in possum density. Field measured SI also varied widely between sites, and was closely positively correlated with home range size (R² = 0.967; P =  0.017); wide-ranging possums were more trappable than sedentary ones. We found that g₀ was inversely related to σ, but the magnitude of increases in g₀ with declining σ appeared to be insufficient to compensate for the fewer places at which each possum could be trapped when those home ranges were small. SI was, therefore, not constant across sites where a standard survey effort was applied. The assumed relationship between g₀ and σ in the current spatial model may, therefore, need reassessment. The management implication of these result is that the sampling effort required to attain a target sampling intensity is dependant on the target animal density, and for bTB management of possums in New Zealand, is under-estimated by the current default parameters in a model of freedom-from-disease for higher density possum populations.

Brushtail possum (Trichosurus vulpecula) social interactions and their implications for bovine tuberculosis epidemiology

The brushtail possum is the main reservoir of bovine tuberculosis in New Zealand. Disease prevalence is generally higher in males than in females. This has conventionally been assumed due to greater infection rates of males, but recent work has raised the hypothesis that it may instead be driven by survival differences. With bovine tuberculosis transmission among possums most likely occurring between individuals in close proximity, here we analyse social networks built on data from wild possums collared with contact loggers inhabiting a native New Zealand forest, to investigate whether there is mechanistic support for higher male infection rates. Our results revealed that adult female possums were generally just as connected with adult male possums as other adult males are, with male–female connection patterns not being significantly different. This result suggest that the new ‘survivorship’ hypothesis for the sex bias is more likely than the conventional ‘infection rate’ hypothesis.

Spatial utilization predicts animal social contact networks are not scale-free

While heterogeneity in social behaviour has been described in many human contexts it is often assumed to be less common in the animal kingdom even though scale-free networks are observed. This homogeneity raises the question of whether the patterns of behaviour necessary to account for scale-free social contact networks, where the degree distribution follows a power law, i.e. a few individuals are very highly connected but most have only a few connections, occur in animals, or whether other mechanisms are needed to produce realistic contact network architectures. We develop a space-utilization model for individual animal behaviour to predict the individuals' social contact network. Using basic properties of the χ² distribution we present a simple analytical result that allows the model to give a range of predictions with minimal computational effort. The model results are tested on data collected in New Zealand for the social contact networks of the wild brushtail possum (Trichosurus vulpecula). Our model provides a better prediction of network architecture than other simple models, including a scale-free model.