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Credit: Sara Edwards

Why Do Predators and Prey Sleep at the Same Time?

We know a lot about the use of sleep — our brains make new connections, our muscles repair themselves — but not much about the why of sleep. Why don’t our brains make new connections continuously? Why not repair muscles during moments of mild activity? After all, it seems dangerous to sleep, right?

Christopher French, writing for The Scientific American, argues that sleep is less dangerous than feeding because it reduces your exposure to predators. Cows have to seek grass wherever it may be, which often is out in the open. However, French’s argument is counterintuitive precisely because the risks for sleeping are obvious: your eyes are closed, you have reduced alertness, and predators can stalk you. Sleeping makes you a “sitting duck.”

If you have to sleep, it also seems obvious to sleep at night. If you’re going to be a “sitting duck,” better to do so in the cloak of night. But why not feed at night, when you are harder to be seen? Regardless of the pros and cons of the what and the when of sleeping, the risks for prey and predators are different. For predators, sleeping doesn’t make them “sitting ducks,” which leads to the question driving this essay: Why do most predators and their prey sleep at the same time?

In a Google search for “predator prey sleep” I found one research paper, which took a stab at the relation between predator and prey sleep patterns, but did so with the use of a computer simulation. The researchers found that predation pressure affected sleep patterns in myriad ways, which is a way of stating the obvious. Perhaps we can find insight from another domain. Maybe game theory has some answers, specifically as it applies to war.

Predator and prey sleepovers strike me as being similar to laws of war, in that both seem absurd. Predators should attack sleeping prey; warriors should defeat their enemies totally. According to game theory, both players in war should be selfish and violent, even if peace and cooperation would be better overall for both parties. The potential value of conquest outweighs the moderate value of peace. The exception to this rule is iterative games. If you’re going to face your opponent after a fight, they might punish you for having been selfish in the previous round. An eye-for-an-eye eventually makes the world blind, so it’s good that we don’t do that.

Likewise, if lions hunted cows while they slept, cows would eat at night and develop night vision. Such an adaptation punishes lions, who then have to evolve night vision too, as well as extra dexterity, to hunt cows at night. Arms races like this are expensive, so it’s better if everybody conducts their business during the day. Since lions are going to be hunting anyways, and cows are going to be feeding anyways, why not make it efficient for everyone. Create a time for war, then lay down your arms and go to sleep.