How animals perceive time Slo-mo mojo - Small creatures with fast metabolisms see the world like an action replay - FLIES live shorter lives than elephants. Of that there is no doubt. But from a fly’s point of view, does its life actually seem that much shorter? This, in essence, was the question asked by Kevin Healy of Trinity College, Dublin, in a paper just published in Animal Behaviour. His answer is, possibly not. Subjective experience of time is just that—subjective. Even individual people, who can compare notes by talking to one another, cannot know for certain that their own experience coincides with that of others. But an objective measure which probably correlates with subjective experience does exist. It is called the critical flicker-fusion frequency, or CFF, and it is the lowest frequency at which a flickering light appears to be a constant source of illumination. It measures, in other words, how fast an animal’s eyes can refresh an image and thus process information. For people, the average CFF is 60 hertz (ie, 60 times a second). This is why the refresh-rate on a television screen is usually set at that value. Dogs have a CFF of 80Hz, which is probably why they do not seem to like watching television. To a dog a TV programme looks like a series of rapidly changing stills. Having the highest possible CFF would carry biological advantages, because it would allow faster reaction to threats and opportunities. Flies, which have a CFF of 250Hz, are notoriously difficult to swat. A rolled up newspaper that seems to a human to be moving rapidly appears to them to be travelling through treacle. Mr Healy reasoned that the main constraints on an animal’s CFF are its size and its metabolic rate. Being small means signals have less far to travel in the brain. A high metabolic rate means more energy is available to process them. A literature search, however, showed that no one had previously looked into the question. Fortunately for Mr Healy, this search also showed that plenty of people had looked at CFF in lots of species for other reasons. Similarly, many other people had looked at the metabolic rates of many of the same species. And size data for species are ubiquitous. All he had to do, therefore, was correlate and repurpose these results. Which he did. To simplify matters he looked only at vertebrates—34 species of them. At the bottom end of the scale was the European eel, with a CFF of 14Hz. It was closely followed by the leatherback turtle, at 15Hz. Tuataras clocked in at 46Hz. Hammerhead sharks tied with humans, at 60Hz, and yellowfin tuna tied with dogs at 80Hz. The top spot was occupied by the golden-mantled ground squirrel, at 120Hz. And when Mr Healy plotted his accumulated CFF data against both size and metabolic rate (which are not, it must be admitted, independent variables, as small animals tend to have higher metabolic rates than large ones), he found exactly the correlations he had predicted. The upshot is that his hypothesis—that evolution pushes animals to see the world in the slowest motion possible—looks correct. Flies may seem short-lived to people, but from a dipteran point of view they can thus live to a ripe old age. Remember that next time you try (and fail) to swat one. From the print edition: Science and technology

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