Fear of the Dark

Imagine a scary setting. Whatever kind of environment would make the hair on the back of your neck stand up straight and give you the feeling that someone–or something–is watching you, imagine it. I'll make a bet that the majority of these scenes are set sometime during the night. And why not? To some extent, everyone has a fear of the dark. Nyctophobia, or a deep and severe fear of the dark, is one of the more common phobias in humans. Where does this fear come from?

There is a lot of evidence to suggest that a fear of the dark is deeply rooted in our evolutionary history. 4.5 million years ago, the early ancestors of Homo sapiens were not the masters of the planet like the species is today. Instead, animals like Australopithecus afarensis were prey for bigger, stronger, faster animals. One advantage our ancestors had were that they were social animals, relying on each other to watch out for danger. This was an effective strategy, but only during the day. At night, their vision failed them. That meant they were vulnerable to attack. Truly, it was never a fear of the darkness itself, but the dangers the darkness concealed that fueled fear. The advent of anthropogenic fire certainly helped protect and defend us, but only if one stayed close to the flames. Eventually hominids started to build shelters, which added another layer of protection from both the elements and the predators.

The Tsavo Lions exhibit at the Field Museum in Chicago, IL
Photo credit: Jeffrey Jung

Even today, in areas where lions and humans live near each other, the risk of of a human being attacked by a lion is roughly 60% higher after 6 pm, according to a study published in 2011. This is partially due to the fact that lions generally hunt at night; the reasons for this are a combination of lower temperatures and an inherent advantage that predators have over their prey. One of the most vicious and notorious lion attacks were those committed by the Tsavo Lions, who preyed on the workers of the Kenya-Uganda Railway in 1898. These two lions were bold and their attack style was unprecedented: they would come into camp at night and drag workers from inside their tents. Any attempts to stop the lions from entering the camp with fire or fencing were thwarted by the lions. Workers left the construction site in large numbers and all progress on the railroad halted until British Lt. Col. John Henry Patterson hunted and killed both of the lions in December 1898.

The movie poster for the 1996 film about the
Tsavo Lion attacks in Tsavo, Kenya.

After their deaths, the legend of the Tsavo Man-Eaters grew. Patterson took their skins and made them into floor rugs, as was the style of the time. In 1924, the rugs were sold to the Field Museum of National History in Chicago, IL for a sum of $5000 (about $73,400 in today's money). The skins were in poor condition from a quarter century of being walked on, but now the skins and the skulls of the lions are on display in the lower level of the Field Museum. Isotopic analysis of the keratin in the bones and hair suggest that one lion ate the equivalent of 10.5 humans, and the other ate the equivalent of 24.2 humans. From Patterson's personal journals, the Tsavo lions killed between 28 and 31 people. The discrepancy in the numbers may reflect people the animals are known to have killed, but not necessarily eaten. The story of the Tsavo Lions continues today in modern entertainment, and appears in video games, movies, and books. My favorite is The Ghost and the Darkness, a 1996 movie starring Val Kilmer as Patterson.

What makes lions so adept at hunting at night? One of the many tools available to lions is their night vision. Cats need just 1/6th of the light humans need to see, giving them a significant advantage in low-light environments. Their eyes are also better at focusing that light more effectively than ours are. The curved cornea and large lens allow the cat eyes to take in all available light, and they also have a higher number of rods that are sensitive to dim light in their eyes than humans do. Cats also have a special layer in their eyes that hominids lack, called the tapetum, which directs light to sensory cells that bathes the retina in 50% more of the available light than the cells would receive otherwise. While a cat cannot see in fine detail or in as many colors as we can see, their eyes are designed to hunt, particularly at night. It's no wonder that cats are likely apex predators in their habitats, considering this and all their other tools!

Eye structure and function in cats by K. N. Gelatt,
seen in Merck Veterinary Manual.

The night is not just a place where scary things hide; it can hold discovery as well. It is needed to study distant stars and to see the Milky Way from Earth's surface. It is also associated with rest, sleep, and safety, a time when families gather in their homes and prepare for the next day. With all the artificial light available to humans now, true darkness can be hard to find, at least in the United States. Light pollution is a problem that affects migrating animals and can affect a human's ability to sleep soundly. Minimizing the disruption to our bodies' rhythms is why companies like Apple have "night modes" for their technological products. I myself have a rule that I am not allowed to look at my phone or computer screens after 9:30 pm, or else I sleep poorly and can hardly think the next day.

Perhaps the lesson here is that everything, even light, has a dark side. That nervousness I feel walking alone at night is just a normal evolutionary response that served my ancestors well, but it never stopped me from Trick-or-Treating after sunset when I was younger. There are still dangers in the dark, of course, but at least they are (probably) not lions anymore!

Sources:
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0022285
https://www.sciencedirect.com/science/article/pii/S0149763410001399 
Kerbis Peterhans, J. C. & Gnoske, T. P. (2001). "The science of 'man-eating' among lions Panthera leo with a reconstruction of the natural history of the 'man-eaters of Tsavo'". Journal of East African Natural History. 90 (1): 1–40.
http://www.pnas.org/content/106/45/19040
The Man-Eaters of Tsavo and other East African Adventures by Lieut.-Col. J. H. Patterson