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A Niche Industry

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Associate Professor Clare McArthur, from the University of Sydney’s School of Biological Sciences, outlines methods used to measure personality traits in brushtail possums and how those traits lead to different foraging behaviours within the species. The first test is the wiggle test or more formally the handling bag test, recording the time a possum spends wiggling while in a sack over one minute. This is repeated over several sessions. It turns out some possums consistently wiggle a lot and others not much at all: a measure of the personality trait of docility.

Another test is for the personality trait of boldness - measured by how much time a possum spends eating in a novel environment - reflecting its propensity to take risks. This is a consistent trait and some animals are bolder than others.
Because brushtail possums are arboreal, the tests are done using a modified bookcase (see left) and at night, when possums are active. The bookcase has a mesh front, red lights and holes cut into each level of the bookcase so the possum can move up and down. There is also a container of food on each level. Each possum is filmed for five minutes, and tested at least once again, at a later date.
This gives a measure for three personality traits: boldness, exploration and activity. Most possums go straight to the top level but many of them start exploring other levels, city and country possums alike. Bold animals feed more than shy ones. In the sampling done so far, these personality traits (such as boldness) vary among individuals but not between the sexes.

Next, Clare’s PhD student Valentina Mella explored how animals trade off food and fear. High quality food is often found in risky places and in safe places the only food left is low quality food, and the question was whether personality affected where individuals forage.     
Brushtail possums were individually identified and personalities assessed. Feeding stations were put out in the bush, some in risky areas, which for possums is feeding on the ground, where they are more prone to predators like foxes. Safe feeders were above the ground and the quality of the food inside them varied from high to low. This allowed individual possums to be tested on where it fed and how much it ate there. Interestingly the experiment did not bear out the expectation that bold animals would eat more in risky feeding sites. Both bold and shy possums got the same amount of food from risky sites but shy ones spent less time there and were more efficient in their foraging efforts. It was the bold ones that spent the most time there and were less efficient, probably because they did not see the site as risky. Monopolising the food source could be a strategy to keep others away, possibly preventing shy ones from coming in.

In terms of the foraging then, there are different ways of solving the same problem, demonstrating variations (niches) within a species. This has implications. It may well be why animals can survive in such dense populations, because there are different personalities within the species. It may also mean that striving to provide a landscape for a particular species needs to take into account those differences in personalities, rather than treating the species as a monoculture with a single ideal landscape.
Left: Possum feeding at ground feeder

Clare hopes to further explore the relationship between food and fear, particularly when food quality differs in its nitrogen content rather than by its plant toxins. This will allow insight into foraging over a much broader range of herbivores, grazers (grass-eaters) and browser/folivores (of trees and shrubs); rather than just the latter. It may well be that, like brushtail possums, foraging is a niche industry.
Left: Vigilant possum on ground

Associate Professor Clare McArthur was interviewed for A Question of Balance by Ruby Vincent. Dr McArthur provided all images. Summary text by Victor Barry, June 2015.

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