Human activities including hunting and hiking are driving mammals around the world to be more active at night, when they’re less likely to run into people, according to a new study. The consequences of this shift are still unclear, but scientists suspect it could threaten the survival of several animal populations.
Researchers analysed 76 published studies that monitored the activity of 62 mammal species, including some that are mostly nocturnal by nature, on 6 continents. They compared the night-time activity of each species during periods of time or in regions with high human disturbance, such as during hunting season or in areas rife with roads, with their night-time activity during periods of time or in regions with low human disturbance. The findings1, published on 14 June in Science, show that most mammals become on average 20% more active at night in response to higher levels of human disturbance.
Animals such as coyotes (Canis latrans), which typically split their activity evenly between day and night, conduct nearly 70% of their activity at night in hiking areas near California’s Santa Cruz Mountains. Some cases are more extreme, says lead author Kaitlyn Gaynor, a wildlife ecologist at the University of California, Berkeley. For example, sable antelopes (Hippotragus niger), which are mostly up and about during the day, shift about 50% of their activities to night hours in Hwange National Park, Zimbabwe, a natural reserve and game-hunting park.
Although there have been isolated studies that looked at animal activity in the presence of people, this is the first large-scale survey that quantifies how mammals shift their activity patterns in time, says Marlee Tucker, who studies broad ecological patterns at the Senckenberg Biodiversity and Climate Research Centre in Frankfurt, Germany. The next step, she adds, is to understand the consequences of this shift for the animals’ reproduction and foraging.
The finding confirms previous assumptions that animals tend to avoid people, probably because they perceive them as a threat2, says Chris Darimont, a conservation scientist at the University of Victoria in Canada. But the study also produced some surprising results, he adds. One of them is that non-lethal activities including hiking and agriculture elicit the same responses from mammals as lethal activities such as hunting. “It doesn’t matter whether we’re having a picnic in a park or cutting down trees, the wildlife around us perceives us as a risk to their survival,” Darimont says.
Although being more active at night might help mammals to reduce lethal encounters with people, it could also have detrimental effects, says Ana Benítez-López, a conservation biologist at Radboud University in Nijmegen, the Netherlands. Many carnivores hunt by sight, so they’re most successful during the day, when they can see well, she says. If the animals have to shift their activities to night hours, they might be less successful. And when species don’t feed or mate properly, that can have detrimental effects on their long-term well-being, Benítez-López says.
Because of the potentially negative impacts of a more nocturnal lifestyle, it might be helpful for people to restrict certain recreational activities to specific hours of the day, Gaynor says. Similar strategies already limit activities during certain times of the year: for example, some rock-climbing routes in New York’s Adirondack Mountains are closed during the peregrine falcon (Falco peregrinus) breeding season. Gaynor notes that it’s important that people bear in mind how human activities might sway animal behaviour. “Just because we don't see wildlife on a daily basis, it doesn’t mean it’s not out there.”
Gaynor, K.M., Hojnowski, C. E., Carter, N. H. & Brashares, J. S. Science 360, 1232–1235 (2018).
Frid, A. & Dill, L. M. Conserv. Ecol. 6, 11 (2002).