Try to swat a fly. Even with a flyswatter. You will fail more often than you succeed.

You’re not slow. You’re not bad at this. The fly is living in a slower-motion version of the world than you are.

The Refresh Rate of Perception

Biologists have a clean way to measure how fast an animal experiences time. It’s called the critical flicker fusion threshold, or CFF.

Here’s the experiment. You flash a light at increasing speeds. At some point, the flashes blur together and look like a steady beam. The frequency where discrete flashes become continuous light is the CFF. It’s basically the refresh rate of an animal’s visual system.

A Hertz (Hz) is the number of times repeats per second. We will go into how a second is defined in the post on Day 8.

For humans, CFF is around 60 Hz. That’s why old TVs ran at 60 frames per second. Beyond that, you can’t tell the difference between flickering and steady. Fun fact, this is also why dogs, with a CFF around 75 Hz, were unimpressed with old CRT TVs. The 60 Hz refresh looked like a strobe to them.

For a housefly, CFF is around 250 to 300 Hz.

That means a fly’s brain is processing roughly five times as many visual frames per second as yours. When you swing a flyswatter at 5 meters per second, you see one smooth motion. The fly sees an event playing out across hundreds of crisp, individual snapshots. By the time your hand is halfway there, the fly has watched a slow-motion replay of your intent and made a decision about which direction to dodge.

You are not fast. The fly is.

Why It Varies

In 2013, a research group led by Kevin Healy at Trinity College Dublin published a paper that nailed down the pattern. CFF, they showed, scales inversely with body size and directly with metabolic rate. Smaller and faster-burning means a higher CFF, which means slower-feeling time.

The reasons are physical.

Smaller animals have shorter nerves. A fly’s visual signal travels micrometers from eye to brain. A blue whale’s motor command has to run thirty meters down an axon. Big bodies just can’t run fast neural loops, because the signal arrives too late.

And the energy cost is brutal. Maintaining a high refresh rate means firing photoreceptors over and over, pumping ions back across cell membranes, burning ATP. Small fast-metabolizing animals can afford that. Slow large animals can’t, and don’t need to.

So evolution settles each species at the CFF that fits its niche.

A Field Guide to Other Nows

A rough sense of how the rest of the animal kingdom experiences time:

• Housefly: ~250 to 300 Hz. Your hand moves in slow motion.
• Songbird: ~100 to 140 Hz. Necessary for darting between branches in a forest.
• Dog: ~75 to 80 Hz. Saw old TVs as strobes.
• Human: ~60 Hz. The baseline. Why film at 24 fps looks smooth to us.
• Cat: ~55 Hz. Slightly slower than us.
• Sea turtle: ~15 Hz. The world looks fine because it doesn’t need to move fast in it.
• Deep-sea fish: 10 to 15 Hz. Cold, dark, energy-scarce. Slow-mo for them.

Values compiled from Healy et al. 2013 and the Lafitte et al. 2022 systematic review of CFFs across 156 species.

Every species in this list is processing the same physical reality. They are just sampling it at radically different rates.

Predator vs. Prey

Who has a higher CFF, the predator or the prey?

The question matters because it shapes survival. A songbird being chased by a hawk that sees more frames per second than the hawk does has more reaction time. It picks up the swoop earlier and gets out of the way. Evolution rewards that across generations, and prey CFFs climb in response. Predators have to catch up. The chase pushes the numbers higher on both sides.

There’s a ceiling. CFF is expensive. Every additional frame a nervous system resolves costs energy, and each species can only afford to perceive as fast as its metabolism can fuel. The energy budget caps the arms race.

CFF also factors into communication, though we don’t fully understand how most animals communicate. But imagine a small, fast-perceiving animal signaling at frequencies higher than its slower predators can resolve. To the predator, the message is a blurred smear. To the recipients, it’s a clear sequence of flashes.

Encrypted messages, too fast for your enemies to read.

Subjective Lifespan

Here’s a thought experiment.

A mayfly’s adult life is a single day. A bowhead whale lives over 200 years. If you measure lifespan by sensory frames processed rather than clock time, you can imagine the gap closing. The mayfly burning hot and short, the whale burning slow and long, each living some comparable subjective stretch.

Has actually measured CFFs for mayflies or whales? I couldn’t find any definitive studies on the topic. The “all animals live equally long subjective lives” line you’ll find online, but not in a science backed study. Still, the question is fun to chew on.

I think the hypothetical whale still beats the hypothetical mayfly in terms of number of sesnory frames but, either way, it’s clear we need more studies on sensory frames and lifespan!

So What Is Your Now?

Your subjective present is roughly 1/60th of a second wide. That’s the slice of reality your visual system can resolve as a single moment. The concept of “now” depends on what animal we are talking about. The flow of time you feel is built out of these slices of sensory frames.

Tomorrow we’ll talk about how your brain stitches the frames together to create the present, and what happens when that machinery breaks.

Sources

• Flicker fusion threshold - Wikipedia
• Time perception - Wikipedia (covers species differences)
• Healy, K., McNally, L., Ruxton, G. D., Cooper, N., & Jackson, A. L. “Metabolic rate and body size are linked with perception of temporal information,” Animal Behaviour 86 (2013): 685–696
• Lafitte, A., Sordello, R., Legrand, M., Nicolas, V., Obein, G., & Reyjol, Y. “A flashing light may not be that flashy: A systematic review on critical fusion frequencies,” PLOS ONE (2022)

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