A Human Voice Will Scare Mountain Lions And Liberate Mice

Whenever humans move into wildlife-rich areas, we know the lives of the wild animals change. We are big and noisy. We dominate their landscape, provide them with excess food, steal their habitat, fracture their home ranges, and outright kill them legally or illegally. Putting these previously investigated major acts of human interference aside, researchers in this experiment wanted to see if something more specific to a human, the sound of a human voice (think two hikers talking), would change the behavior of resident wildlife. 

In short, It did. When wildlife hear us, even top predators adopt prey behavior. Prey animals typically are fearful of predator vocalizations, and they will change their behavior due to fear. This experiment took place in the Santa Cruz Mountains of central California, where wildlife habitat exists close to urban and suburban development, and the largest native carnivore is the mountain lion. Medium-sized carnivores also living there include bobcats, striped skunks, and Virginia opossums. 

Researchers chose two one-square kilometer sites in these mountains, and for five weeks they played intermittent amplified recordings of people talking followed by five weeks of Pacific tree frogs calling. Tree frogs served as the control as they can normally be heard both day and night and are considered simply background noise; they are not predators, prey, or competitors of the studied species. For monitoring purposes GPS collars were placed on the mountain lions, and camera traps were used to follow bobcats, skunks, and opossums. Small mammal prey (deer mice, woodrats) behavior was also watched by supplying food patches and using live traps. 

When mountain lions heard human voices, they changed their usual paths and kept far away from the human voice. They entered the research area where the voices originated 30% less than when control tree frogs were vocalizing, and they gave the nearest speaker 29% more distance when they heard humans speaking. Human voice also caused mountain lions to move more cautiously. They decreased their average speed by 34%. Bobcats became more nocturnal – they decreased their daytime activity by 31%. Skunks dropped their overall activity level by 40%, and opossums ate significantly less, showing a 66% decrease in their ability to forage and find specially supplied food patches. In contrast, small mammals took advantage of predator absence. Deer mice increased the amount of space they used by 45%, while wood rats increased their food foraging intensity by 17%.

Certain wildlife will risk living closer to humans. Skunks and opossums show the trade-off – they are supplied more food (human garbage, feral cat, or outdoor feeding stations), but they experience more fear from chronic exposure to perceived risk from humans. Certain wildlife actually seek living closer to humans. Small mammals such as mice and rats enjoy the human shield effect – humans reduce their risk of predation. 

The study shows that it’s not just our buildings and roads that negatively affect wildlife, it is our sheer presence as individual humans that leads to widespread changes in wildlife behavior. These changes are already clear on a global scale as humans continue to move ever deeper into most wildlife habitats. 

This research showed that a landscape of fear exists solely from the feeling of risk created by an apex predator (humans) and can have far-reaching effects across wildlife communities. Animals are shifting to a more nocturnal existence; they are moving cautiously and fearfully, giving us humans plenty of room at the expense of their own space; they are foraging less and therefore eating less; and predator-prey systems are being disrupted. The authors believe that their work supplies evidence that many of the global changes in wildlife behavior can be explained in large part by fear of humans as predators, independent of our numerous other harmful impacts on the natural world. 

Those of us who work with injured or orphaned wildlife in wildlife rehabilitation, those of us who work with captive wildlife, and those of us who enjoy any outdoor activities especially in crucial wildlife habitats should be mindful of this experiment and the results. Even if we speak gently and softly, it is not comforting at all to wildlife. We are scaring them.