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Sweet smell of success

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Associate Professor Clare Macarthur, from the University of Sydney’s School of Biological Sciences, makes sense of how swamp wallabies use their noses to find the food they want to eat. She was working with PhD student Rebecca Stutz, on how to protect eucalypt seedlings from being eaten during revegetation projects, like the one at Booderee National Park. A native species of swamp wallaby was eating the eucalyptus seedlings.
Could associated plant refuges be the answer ... patches of plants that discouraged the animals from coming in the first place, protecting the seedlings. The flip side to this is the so called Shared Doom hypothesis. This means that seedlings placed among attractive or palatable plants will suffer, even if they are less attractive, because of that placement.
The researchers filmed the rock wallabies both day and night in grassy patches where eucalyptus seedlings had been planted. These grassy patches had also been worn down by local grey kangaroos so the seedlings were highly visible.
In one planting, local paper daisy plants, rarely browsed by rock wallabies, were used to shelter the eucalyptus seedlings. In another planting the seedlings were placed in natural vegetation. Both of these plantings were also filmed.
While it took wallabies longer to find seedlings among the paper daisies, they did eventually find them and ate them.
Clearly they were using smell to locate the plants so this led to further experiments. Some seedlings were chopped up and placed into plastic vials with holes in their lids. The vials were then buried in the ground, making them invisible. Other empty vials were also buried, making a patch of 10m intervals between the full and empty vials. Filming showed wallabies were more likely to visit and investigate the vials with chopped up eucalypt leaf, which was evidence that these animals could hone in on the eucalyptus seedlings on smell alone.
This experimental design was then repeated to test the ability of the wallabies to differentiate high from low quality food. Some vials were left empty and others were filled with either nutrient-starved seedlings or lush seedlings. The high quality seedlings (with more eucalypt and essential oils) were found first and investigated by the wallabies more often. Small chemical fibres were used as probes in the vials, capturing the molecules that the wallabies smelt. Incredibly, the wallabies were able to detect very low densities of evaporated oils, so low they did not show up after chemical analysis. This ability (that insects also have) means swamp wallaby noses are very sensitive and very useful. The essential oils are also toxic and make up a defence mechanism for the plant to ward off browsers. The wallabies, however, have managed to hijack that defence mechanism, using their noses to locate the plants, guided by those smells. The plants essentially advertise the fact that they are poisonous but that does not stop the wallabies from eating them because they naturally eat various poisonous food.
For revegetation projects the answer lies in finding ways to either trick the wallabies or to reduce their capacity to smell and then see the seedlings. Hiding the plants will help as the earlier experiments showed.
Clare Macarthur is looking to see if some kind of chemical camouflage could also assist. This would mean getting the odour profile of the seedling and finding ways to mask that profile or make it less detectable. This could be in the form of chemical placed around the seedlings as a protection for the essential oils odours. A protective vial that eventually evaporates and protects the seedling may well prove to be cheaper than other options like fencing off areas. Text: V.B. July 2015

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