Death by parasite or by predator: It's an herbivore's dilemma.

When foraging for food, herbivores have good reason to fear lurking predators, but the dangers presented by parasites can be just as deadly. Parasites colonize skin, fur or innards and can sap herbivores of nutrients, impact their chances of reproduction and, in some cases, even control their minds.

Little is known, however, about how plant eaters manage these joint threats. Is it worse to stray into a predator’s realm or risk being invaded by blood-sucking nematodes? Or is it better to avoid both at the expense of eating?

Researchers from the University of Melbourne set out to find answers. The team traveled to an airstrip in Grampians National Park in southeast Australia, where wallabies and kangaroos often gather to graze.

The researchers used domestic dog feces to represent mortal danger, since many members of the canine family are wallaby predators. Dingoes—now extirpated from the region—used to be the wallabies’ main foe, while today their arch nemesis is the red fox. The team fed dogs a specialized meat diet consisting of wallaby, kangaroo, sheep, possum or rabbit, and then collected the mutts’ feces following each meal.

Next, they placed several feeders around the airstrip with a small amount of one of the poop samples near each container. Settling back, the team watched what 21 wallabies made of the food offerings.

The animals approached the feeders 97 times but generally avoided eating near the feces from dogs that ate either wallabies or kangaroos, their close relative. The smell of this poop put them on high alert. Wallabies were more hesitant to approach and spent more time scanning for danger around these feeders, causing them to eat less overall compared to wallabies eating near more benign poop samples.

In a second experiment, the researchers measured the wallabies’ aversion to parasites, lacing the feeders with feces from fellow wallabies or kangaroos. Wallabies can be colonized by up to 22 gastrointestinal parasites, many of which are transmitted via feces from infected wallabies, which contain parasitic eggs or larvae. Seemingly aware of this risk, the 19 wallabies tested consumed five times more food from feeders with kangaroo poop than ones with wallaby poop.

Combining the experiments, researchers placed wallaby, kangaroo and dog feces near several fresh food containers. Wallabies again ate the least at feeders positioned near the poop of wallaby or kangaroo-eating dogs. They similarly ate less at containers polluted with wallaby rather than kangaroo feces.

Surprisingly, parasite aversion was just as strong as predator avoidance, and wallabies were even more repulsed by feeding at sites where both of these conditions were met. Their results were published today in the journal Proceedings of the Royal Society B.

The cues wallabies use to make these calls remain unknown, but the researchers write that wallabies clearly “make impressive use” of smelly hints to weigh risk.