You can tell a lot about an animal by its faeces. Red-necked wallabies take this to a whole new level, using faecal cues to avoid their enemies.
Most people encountering a fresh pile of faeces would delicately step over and be on their way, but ecologist John Sharp doesn’t mind stooping for a closer look.
"I became a bit of an expert on faeces," he said. “Most kangaroo pellets are larger, they look closer to a wombat pellet. They're bulkier and slightly squarer, whereas wallaby pellets are more like elongated tubes."
Perhaps too much detail for most, but if you're a grazing herbivore then paying close attention to faeces could save your life. John's research, published last month in Proceedings of the Royal Society of London B, shows that red-necked wallabies use faecal cues to avoid their two major enemies: predators and parasites.
Red-necked wallabies (Macropus rufogriseus) can have up to 22 different parasite species squirming inside their digestive tracts. These are fine in low numbers, but too many parasites can sap nutrients and compromise the immune system. They spread from animal to animal via what’s called the faecal-oral route: larvae hatch inside faeces and wriggle onto the surrounding grass, where they wait to be eaten.
The best way for a wallaby to avoid new parasites is to avoid grazing near faeces, but this isn’t always possible. Luckily, there are other ways to minimise your infection risk. Wallabies share very few parasites with grey kangaroos, so when given a choice they should prefer to graze near kangaroo rather than wallaby faeces.
This is exactly what John's research found. On two grassy runways in Victoria’s Grampians National Park, John and his University of Melbourne colleagues laid out a grid of feeding trays. Each tray contained crushed corn contaminated with either wallaby or kangaroo faeces.
The results were pretty definitive: "We found that wallabies ate more and spent longer at trays that had kangaroo faeces – trays that were a lower gastrointestinal parasite risk," said John.
It seems the wallabies, after taking a sniff of each tray, could easily detect and avoid the more parasite-risky food option. John thinks that they use odour cues from the faeces to determine which species deposited them.
"We assume it is through scent, and there's a lot of research on how herbivores recognise faecal cues through scent. But it could be something else, like sight."
This is an impressive enough skill, but the wallabies’ powers of faecal detection proved to be even more nuanced. In a second experiment, John collected faeces from dogs that had been exclusively fed a single type of meat: either wallaby, kangaroo, possum, sheep or rabbit.
“We had the same sort of feeder trays,” he said, “and then we associated those feeder trays with predator cues [faeces] on little plastic lids placed a metre away from the tray.
"I was interested in how herbivores could use their knowledge of what a predator has been eating to make a judgement call on how dangerous the predator is."
We already know that some animals can infer from faeces if a predator has been eating a vegetarian or meaty diet. But John's wallabies took this ability even further – they could distinguish between carnivorous diets and avoid those that were more risky.
“We recorded the wallabies’ approach behaviour, all the vigilance behaviours they exhibited, how long they stayed at the feeder trials, and also how much they ate,” said John. “The main result is that they reacted far more to dogs that had been eating kangaroos and wallabies, than they did to dogs that had been eating other mammals.”
This makes perfect sense from a wallaby’s point of view. If a predator has traces of a macropod in its faeces, then it is perfectly capable of catching you – and should be regarded with higher caution.
John suspects that the wallabies are sniffing out sulfurous metabolites produced during meat digestion, but he hasn't yet shown this experimentally. Chemical analyses of the different predator faeces were inconclusive, making the wallabies’ real-time discrimination even more impressive.
For most of us, faeces are a waste product, but for foraging herbivores they are an invaluable source of data on the threat posed by predators and parasites. When the study’s two main experiments were combined, the wallabies didn't seem to prioritise one threat over another – they avoided parasite-risky faeces just as diligently as they did predator-risky faeces.
This confirms that predators and parasites are an ever-present danger in the lives of herbivores, and both equally important in shaping an animal’s behaviour.
“It’s often called the 'landscape of fear',” said John. “That’s predators, parasites – everything that can affect and be harmful to a herbivore or a foraging animal.
“This study shows us a bit more about how they react to this landscape of fear, and how all the time they’re processing and assessing the risk that’s posed to them by foraging in a certain location.”