Fear is complicated. This is probably why biologists have only begun to scratch the surface of how being scared affects wildlife. When biologists study animal populations, simplifying these complex populations into an easy to understand equation can help better understand these populations as a whole and within each component. For example, consider a population of gray squirrels living on a college campus. They are well fed by dining hall trash, so some squirrels will immigrate in for a better squirrel life (Fig. 1). These squirrels may have a lot of babies. Others might not be so lucky and are picked off by a hawk or cat. Some others may leave for greener pastures and emigrate out. This sets the stage for the simplest way to track a population: Immigration + Births - Deaths - Emigration. A huge focus for study in biology has been death by predation – a.k.a. being eaten. Famous studies such as the Lynx and Hare relationship in Canada have tracked the interplay between predators and their prey . But only recently have scientists recognized the power of fear. Fear effects, also called non-lethal effects, are how the mere presence of predators alters animal behavior . Here are four ways that fear affects wildlife:
Fig. 1 – A Duke University squirrel, surely pursuing a degree in “Acorn Studies” (source: Ildar Sagdejev via Wikimedia).
Fear takes up time.
In the business world, time is money. In the world of wildlife, time is also a form of currency. There are only 24 hours in a day, and for animals, those hours must be split “strategically” between sleep, eating, mating, parenting, and keeping a watchful eye for predators. The balance might be different for a Northern Cardinal that lives in a dense forest patch than one that lives on the edge of a forest (Fig. 2). The dense forest may be a safer habitat, so that Cardinal can slack off a bit on guarding against predators and spend more time eating or parenting. The other (edgier) Cardinal, in the meantime, is constantly looking over their shoulder waiting for a perched hawk to attack from above (Fig. 3). That Cardinal might spend less time foraging or may not venture out as far into the open to get the best food. These subtle “decisions” are fine-tuned over evolutionary time to maximize survival and reproduction, the combination of which we call an animal’s evolutionary fitness.
Fig. 2 – Bright red with an orange bill is a great strategy for attracting mates…and hawks as well! (source: Dick Daniels via Wikimedia).
Fig. 3 – This Cooper’s Hawk is not a sight a Cardinal wants to see (source: Kally Colgan Azar via Flickr).
Fear leads to different parenting decisions.
Parenting in the animal kingdom is an ultimately selfish endeavor. The name of the game is evolutionary fitness, and the balance is between survival and reproduction. Sure, a desert-dwelling Kangaroo Mouse could try to produce as much young as possible, but what is the point if they cannot possibly feed all of them? And at what point does parenting become so costly that the mouse may not even have time to feed or take care of themselves (Fig. 4)? Fear may result in minimal parenting, or even an adjustment to the amount of eggs they lay. This has been well documented in songbirds . Even a slight adjustment to the amount of nest predators in a habitat, and birds will reduce the number of eggs they lay, and even how often they feed the nestlings. If there is a good change that the nestlings will be eaten, why bother investing a ton of effort into them? Hedging their bets, they may invest the bare minimum to get some nestlings grown up and out of the nest, while not sacrificing their own condition or chance of survival. These adjustments are often less extreme or non-existent in species that have few offspring and take care of them for a long time, such as whales or primates such as gorillas.
Fig. 4 – With so many hungry nestlings, who even has time to feed themselves? (source: Kati Fleming via Wikimedia).
Sometimes fear is social.
What is better than going to a scary movie with a group of friends? And why do we mock the stupidity of the characters on screen when they decide it is a brilliant idea to “split up?” We all have this instinctive feeling that the world is less dangerous in numbers. This fear causes flocking behavior in birds, herding behavior in mammals, and schooling behavior in fish . There are two main reasons we think fear causes grouping: (1) you are less likely as an individual to be eaten if you are in a crowd, (2) hundreds of eyes searching for predators are far better than two. Fear results in some incredible behaviors in animals, and if you think about it, there’s a good possibility it played a role in the development of human societies for the mutual protection large groups offer. Fear has also resulted in the development of complicated communication systems. For example, a small songbird, the Tufted Titmouse, has a series of calls that can distinguish between types of predators and the level of risk that the predator poses at any given moment . Biologists have discovered that this communication system is about one third the complexity of the English language! Dozens of species understand what the Titmice are saying, from White-breasted Nuthatches to Eastern Chipmunks, and whole flocks of birds form around this tiny loquacious sentinel (Fig. 5). All of this came about because several species are scared of the same things: snakes, hawks, owls, and cats.
Fig. 5 – This little bird has a lot to say (source: Jocelyn Anderson via Wikimedia).
One species’ fear can alter whole ecosystems.
Fear is so powerful we can see its consequences on an ecosystem-scale. The most famous (though scientifically controversial) example is when wolves were reintroduced to Yellowstone National Park . Humans had previously hunted wolves to local extinction in Yellowstone, so Elk were able to roam without fear. They roamed, and they roamed, and they ate every sapling in sight! The landscape changed – most notably, aspen tree growth was suppressed. When wolves were reintroduced by the park, that all changed. Elk began avoiding “dangerous” areas, such as valleys and river banks. The aspens grew back, and the whole structure of the ecosystem began to change (Fig. 6). This allowed for other species to thrive, including small mammals and nesting birds. One species’ fear caused this massive shift in a short time span, showing the power of the mere presence of predators on a landscape.
Fig. 6 – Without wolves around, they have little to fear (source: DHeyward via Wikimedia).
Despite these amazing findings, the true effects of fear are still being debated in the scientific community, mainly because it is tricky to disentangle the indirect effects of fear from the direct effects of animals being eaten . Regardless of this debate, it is clear that predators are even more important to ecosystems than we once thought. Not only do they go around eating things, they scare the living daylights out of them as well! As we continue to tinker with ecosystems and drive certain apex predators to extinction while reintroducing others, we should keep in mind the sheer power of fear, and its consequences on wildlife.
 Zanette, Liana Y., Aija F. White, Marek C. Allen, and Michael Clinchy. “Perceived predation risk reduces the number of offspring songbirds produce per year.” Science 334, no. 6061 (2011): 1398-1401.
 Peers, Michael JL, Yasmine N. Majchrzak, Eric Neilson, Clayton T. Lamb, Anni Hämäläinen, Jessica A. Haines, Laura Garland et al. “Quantifying fear effects on prey demography in nature.” Ecology 99, no. 8 (2018): 1716-1723.
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