Dragonflies and Starfish Live in Different Time: 237-Species Study Links Ecology to Visual Speed

A dragonfly hunting over a pond and a starfish crawling across the sea floor share a physical world but inhabit perceptually different ones. The dragonfly tracks prey moving at tens of meters per second, making corrections in fractions of a second. The starfish has no such demands. For decades, physiologists have assumed that such ecological differences drive corresponding differences in how fast nervous systems process sensory information - but the evidence has been fragmentary, limited to small groups of species or single taxonomic classes.

A study published in Nature Ecology and Evolution from Trinity College Dublin and the University of Galway provides the broadest test of this hypothesis to date, spanning 237 species across insects, birds, mammals, and fish. The findings confirm and extend a principle known as Autrum's hypothesis: that sensory systems evolve to match the pace at which an animal lives and moves.

Measuring the speed of sight

The researchers used a well-established metric called critical flicker fusion (CFF) - the frequency at which a flickering light is perceived as continuous rather than as distinct pulses. A higher CFF means the visual system can resolve faster changes in the environment.

In practical terms, humans typically have a CFF around 60 Hz. Many insects and birds operate at CFF values above 200 Hz - they can distinguish individual flashes of light that would appear as steady illumination to human eyes. For a species perceiving 200 distinct images per second, the world appears to move more slowly relative to their nervous system's processing speed. A fly facing a swatter experiences more time to respond than a human does.

The team analyzed CFF data across 237 species and tested how it related to four ecological variables: locomotion type, foraging strategy, body size, and light environment.

Flying is the strongest predictor

Of all the ecological variables examined, the ability to fly predicted CFF most strongly. Flying species had CFF values roughly twice as high as non-flying animals, even when other variables were statistically controlled. The navigation and collision-avoidance demands of three-dimensional flight at speed appear to have consistently driven the evolution of fast visual processing across widely separated lineages.

Foraging strategy provided the second major finding. Pursuit predators - animals that actively chase fast, agile prey - had significantly higher temporal resolution than species feeding on stationary or slow-moving food sources. A peregrine falcon or a cheetah must track a moving target and adjust course continuously. A limpet grazing on algae faces none of these demands.

Light environment also played a role: species active in bright daylight generally had faster visual processing than those adapted to dim or dark conditions. And in aquatic environments specifically, smaller and more maneuverable species tended to have faster vision than larger ones - the opposite of the terrestrial pattern, possibly reflecting the different predator-prey dynamics and obstacle densities in aquatic habitats.

"From a dragonfly tracking prey in mid-air to a starfish grazing slowly across the seabed, animals live in very different perceptual worlds," said lead author Dr. Clinton Haarlem, from Trinity's School of Natural Sciences and Trinity College Institute of Neuroscience. "Our results show that these differences are not random. Instead, they are closely linked to how animals move, hunt, and interact with their environments."

The cost of fast vision

High-speed visual processing is metabolically expensive. Neural tissue that operates at high frequency requires sustained energy input, and maintaining sensory systems capable of 200 Hz processing imposes real energetic costs. The pattern in the data - fast vision where it confers clear advantage, slower vision where it does not - is consistent with the expectation that evolutionary pressures balance the benefits of sensory acuity against its energetic cost.

This energy-cost framing also implies that species with fast visual systems are specifically adapted to fast environments. Remove them from those environments - or change those environments in ways that affect visual demands - and the adaptation may become less advantageous or actively costly.

Artificial light as a disruptive factor

The study raises a concern about artificial lighting that has not previously been framed in quite these terms. Species with very high CFF values may be particularly sensitive to flickering artificial light sources, including certain LED lighting systems and screens that flicker at frequencies undetectable to humans but potentially perceived as rapid pulses by high-CFF animals.

"These findings suggest species with fast visual systems may be especially vulnerable to flickering artificial lights," said Dr. Haarlem. "This could affect their hunting success, navigation, and impact predator-prey interactions, particularly in birds and aquatic predators."

The relationship between ecological context and sensory system design, demonstrated here at the largest scale yet tested, provides a framework for predicting which species face the greatest sensory disruption from human-modified environments - information that could improve how artificial lighting standards are designed near ecologically sensitive areas.