Morning lark or night owl? Prevailing ideas of mammal activity are outdated

Squirrels are plentiful when the sun’s out, while rats are a more common sight after dusk. Often, the former are described as diurnal, and the latter nocturnal. But for many animals, those labels are due for a revision. Most mammals’ daily activity cycles aren’t quite so cut and dry, according to a new study of global wildlife camera data. The rhythms of many species are more varied and flexible than previously thought, and those rhythms are shifting, in response to humans. 

The research, published February 26 in the journal Science Advances, combines nearly 9 million recorded observations into one of the most complete ever analyses of the timing of mammalian activity. It upends many commonly held assumptions about wildlife habits, and has consequences for conservation.

“You see all these terms in the literature. This species is nocturnal, this one is diurnal. It sounds very authoritative, but we weren’t so sure based on our collective field experience,” Kadambari Devarajan, an ecologist and conservation biologist who co-led the study as part of her post-doctoral fellowship at the University of Rhode Island, tells Popular Science. “Anecdotally, it seemed like there was a lot more plasticity,” particularly in response to urbanization and human development. So, Devarajan and her collaborators took on the mammoth task of checking for themselves. 

The four core team members worked with hundreds of other researchers around the world to compile data from 200 camera trap projects across 38 countries. They organized and assessed this data based on factors like time of day, species, location, and daylight length given the date and geography. Then, they searched previously published research for mentions of each species’ established activity pattern: nocturnal, diurnal, crepuscular (most active at dawn and dusk), or cathemeral (active both day and night).

Comparing the observational analysis with the research review revealed a significant mismatch. Just 39 percent of the 445 species captured on camera were accurately described in the previously published research. Common raccoons, for instance, are cataloged as nocturnal in the scientific literature, but hundreds of observations from around the Americas showed instances of raccoon activity in the day, night, and twilight. In many locations, the furry masked bandits tended towards nighttime movement, but in others they spanned the 24-hour schedule. 

Wider-ranging species tended to show more varied patterns. Between 60 and 73 percent of the species recorded in more than one project showed signs of at least two types of daily rhythms: some mixture of daytime, night, and twilight activity. Geography of any given population also had an influence: the farther animals were away from the equator, the more likely they were to be active during the day. Smaller species also tended to be more nocturnal overall, and increased daylight hours were correlated with more daytime animal activity. 

Amazingly, not one species included in the dataset showed an exclusively crepuscular pattern of dawn and dusk activity, as defined in the study, despite many animals being characterized as such in prior research. For example, white-tailed deer are generally described as twilight specialists, but according to the new photo evidence, they tend more towards intermittent bouts of both daytime and nighttime activity across much of their range. In some places, they seem to be active only during the day or only at night.  

Finally, where human environmental impacts like habitat loss and urbanization were more pronounced, animals were more likely to shift their schedules and display greater flexibility. In most instances, increased human influence tended to push wildlife towards nighttime activity, in agreement with past research. Striped skunks, gray foxes, and American porcupines– along with more than a dozen other mammals were more likely to be nocturnal in highly developed areas. A handful of animals, like the common tapeti (a species of South American rabbit) also demonstrated the opposite effect–becoming more active during the day. All of the study  results can be explored in detail via a publicly shared online tool. 

“The ambition of what they did in terms of the number of observations and also how they set out to test these typical classifications is novel and important,” says Cole Burton, a conservation biologist and associate professor at the University of British Columbia who wasn’t involved in the new research. 

Burton has previously conducted large camera trap studies of wildlife, including one assessing how animals responded to human activity amid pandemic lockdowns. Though more narrow in scope, his own work also showed that many mammals are flexible about their activity timing. 

In light of the new research, he wonders if there’s any point in strictly defining mammalian patterns at all. “Maybe we should be moving to a more continuous understanding–a spectrum–rather than trying to put the animals in these neat little boxes.” Individual wildlife populations might be better understood via timetable than a label, Burton suggests. 

The idea goes beyond just terminology. Decades of  previous scientific research has incorporated these apparently oversimplified animal activity categories into ecology models, to help scientists grasp animal’s niches and interactions with each other. The new study implies species’ previously understood ecological roles may not be so concrete and contained.

“Based on this camera trap data set, those old classifications often don’t hold,” says Burton. “That is a really important result for people to be aware of, especially those who are using those kinds of traits in their models.”

Then, there are the ramifications for wildlife itself. “A lot of conservation studies have focused on spatial distribution patterns, but the temporal component may be just as important,” says Devarajan. “When an animal is active and when it needs access to certain resources could be critical for protecting a species.” 

As the study title says, it’s not just where the wild things are, but when. Everything from train schedules, traffic patterns, trash collection, and public park hours could either lead to human-wildlife conflict, or be adjusted with conservation in mind. Getting a better grasp on wildlife habits could allow land managers to preserve time slots as well as spaces for species to thrive undisturbed. 

[ Related: Mountain lions in Los Angeles become night owls to avoid humans. ]

Yet first, more research is needed. Though sweeping and impressive, both Devarajan and Burton note the new study has limitations. The researchers didn’t look directly at human activity to determine its influence on animals, using development and urban density as an imperfect proxy instead. The study is one of the first times scientists have tried to quantify each type of animal tendency, but in doing so, their definitions may have been overly restrictive.Their camera trap dataset is incomplete; it doesn’t account for every region around the world and is biased towards dry season observations because of the difficulty of maintaining a camera through monsoons. These gaps mean they didn’t directly model the effect of seasonal change for any of their data– despite the fact that animal activity is known to shift throughout the year. 

Finally, the study doesn’t offer any insight into how these apparent shifts in animal activity are impacting wildlife. Follow-up research comparing specific populations of a species over time will be needed to understand if changing rhythms is a useful adaptation or detrimental in the long-term. “What we really need to know is, is this working out for them,” Burton says. “Or is it too much of a cost?” 

“Despite mammals being relatively well studied, we need to maintain a certain humility. There’s a lot we don’t know,” says Burton. “I feel like animals are working hard to adapt to us and how we’ve changed the environment. Being aware of that and trying to understand that helps us help them be successful in those changes, so they actually lead to coexistence.” 

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