I’m writing this on a Tuesday, at 2:26 p.m. Minutes ago, it was 9 a.m., or so it feels; back then, I was enjoying the delusion, refreshed each morning, that I’d accomplish what I needed to do today. I still might — there are hours left in the workday — but I’m wiser than I was when I woke up five hours ago: it’ll be 7pm soon, the day definitively in tatters. Another of Time’s routine beatdowns.
One advantage of being a cat, or a stingray, is not having to think about time this way (and, by extension, death). But are they entirely free from the temporal plane? Do they perceive it in any way? Do some species perceive it more acutely than others? For this week’s Giz Asks, we reached out to a number of experts to find out.
Andrew Jackson
Associate Professor, Zoology, Trinity College Dublin, whose research interests lie in understanding ecological systems from an evolutionary perspective
The concept of time is, to begin with, philosophical, and tricky. I’m interested not so much in the passage of time, per se, as in the speed at which animals perceive the instantaneous world — how events change in the world around you, how you’re able to perceive and react to those changes.
One way to measure this, in a laboratory setting, is to sit a human down and flash a lightbulb in front of them; you speed up the flashing lightbulb until they don’t see it flashing anymore, until it becomes one constant light source. And you can effectively pose the same experiment with animals.
There’s an enormous range in this ability in the animal kingdom. At the very lowest end, you’ve got the deep-sea marine isopods, wood lice, which are enormous and can see only four flashes every second. At the upper end of the scale, there are flies capable of seeing 250 flashes per second. Do they perceive time differently? I don’t know. But certainly their view of the world happening around them is incredibly different. (Humans are somewhere between these two on the scale.)
What this means is you can have two animals sitting beside one another, one seeing all these little details, hyper-sensitive to all these minute little changes, the world flying around them, and meanwhile the other is basically living in a completely different temporal niche, living in a slow-placed, kind of lazy world, completely oblivious to all of it.
A favourite example of mine is that some of the swordfish species, just before they go hunting, will increase the blood supply and heat to their brain and their eyes, the consequence of which being that they can dramatically increase their temporal perception rate. So you have this reasonably warm-blooded, fast-moving superpredator, its visual speed jacked up to its absolute maximum, diving down into the deep and meeting these slow, sluggish animals. For the latter, it must be like dealing with a visitor from another dimension.
Edward A. Wasserman
Professor, Psychological and Brain Sciences, University of Iowa, who has studied the conceptual abilities of pigeons
Little mystery surrounds most sensory systems in animals. Just as we do, animals perceive lights, sounds, and smells with dedicated distance receptors in their eyes, ears, and noses. The perception of time is far more enigmatic. A light, a sound, or a smell can each be perceived to last for 4 seconds. Indeed, a 4-second interval might begin with a flash of light and end with a burst of sound. Without any specialised bodily receptor for the perception of time, how can animals mark its passage?
Of course, how animals perceive time presupposes that they do. You don’t have to take my word for it. Extensive laboratory research with mice, rats, pigeons, and monkeys clearly documents this perceptual ability.
One elegant way to explore this matter is to present stimuli of various durations and ask animals to make one response when the duration (say 1 second) of a light is perceived to be “short” and to make a second response when the duration (say 4 seconds) of the same light is perceived to be “long.” Correct responses are rewarded with food, whereas incorrect responses are not. Rats readily learn this task. They also show reliable transfer of the temporal discrimination when a hissing sound is substituted for the light. It’s as if both sensory systems have access to a common brain system.
A further fact increases the intrigue. Brighter lights or louder sounds are perceived to be longer than dimmer lights or quieter sounds. This finding suggests that this brain system actually scales stimuli from multiple sensory dimensions in terms of their magnitude or intensity. So, it seems that the original question “How do animals perceive time?” has now evolved into a question: How do animals (and people) perceive magnitude?” Answering that question poses a daunting challenge for science.
Frans de Waal
Professor of Psychology and Director of the Living Links Centre for the Advanced Study of Ape and Human Evolution at Emory University
People always assume that animals live in the present, that they’re captives of the present, and we’re the only species that is not.
There is now increasing research on “time travel” in animals. Can they think back to particular events in their life, can they think forward to a future?
There is good evidence suggesting that great apes and some corvids (crow family) can do both.
For example, in experiments chimpanzees save tools that they can use only later on, and in the wild they collect grass stems and carry them to termite mounts over long distances, where they will then use them, which suggests that they planned this all along.
Knowledge of past events has also been tested by giving apes a problem that they have seen only once three years before to see if they recall what to do. They do.
Kevin Healy
Head of the Galway Macroecology Group at the National University of Ireland
First off, all animals can certainly perceive time in some sense. As time is simply just the rate at which things happen, by sensing how the world changes around them via hearing, seeing, smelling etc. an animal is perceiving time. What’s interesting is that these senses determine the ability of an animal to perceive time. To understand this, let’s use vision as an example. Visual systems sense the environment through photosensitive cells in the retina, such as rods and cones, firing when photons hit them. When one of these cells fire, they are recording some piece of information about the outside world, such as the existence of a bright light. Now let’s imagine some change in the environment, such as the light flashing on and off. For the visual system to perceive this change, the photosensitive cells need to fire and then recharge again before the next photon hits. If they don’t recharge in time, they don’t perceive that the light flashed on and off.
This example, of a light flashing on and off, is what is used by scientists to measure how animals perceive time and is called the critical flicker fusion effect. Critical flicker fusion is the frequency at which a flashing light is no longer perceived as flashing to an observer. Animals that can perceive time in fine detail can see lights flash on and off at rapid frequencies, while those with lower time perception abilities would only see such flashing light as a constant light.
For example, humans can see flashing lights up to frequencies of 60 Hz. Lights that flash at higher frequencies, such as AC light fixtures that flash at 200Hz +, just look like constant light sources to us. However, other animals have much higher prerational abilities compared to us. For instance, some blowfly species have critical flicker fusions of 300Hz, meaning they can see how the world changes around them at 300 frames per second. Other animals are much slower. Starfish (yes, they have eyes) can see less than one frame per second, while some deep-sea fish can perceive less than 10 frames per second. For these animals, much of the world around them would be perceived as motion blur (think of the blur when looking out a car’s side window). Hence, their perception of time is likely to be slow, matching their own slow lifestyles.
In general, we find that animals with fast paced lives, such as birds that catch their prey on the wing, have fast eyes and detailed perceptions of time, while animals with slow lives, such as deep-sea fish, have slow eyes. Humans, meanwhile, are somewhere in the middle, between cats (55Hz) and dogs (75Hz).
Unfortunately, while it’s impossible to say what it must feel like to perceive time like a fly or a starfish, we can at least study the limitations of their sensory systems, which helps us to understand the limitations of their time perception.
Andrew Beale
Postdoctoral Scientist, MRC Laboratory of Molecular Biology, Cambridge
I don’t know whether animals perceive time in the way we do or not, but their bodies and physiology do operate according to time and a “clock” — the circadian clock. This isn’t a clock like a ticking clock, but rather a complex set of reactions and interactions between and within cells of the body. It’s sometimes called the molecular circadian clock or the molecular clock. These reactions operate with a timing of approximately 24 hours (hence the name ‘circa’ = about, “dian” = day), and result in circadian rhythms — patterns of activity in anything from sub-cellular function to whole body patterns of sleep and wake, for example. The “clock” is set by light and the external day and night cycle (via special cells within the eye that detect the brightness of light but don’t form an image in the brain) and the result is that physiology is coordinated with the outside world. This means that our bodies do things at the appropriate time (like secrete a sleepy hormone, melatonin, in the evening when people are going to sleep).
When we think about human patterns, we can see these circadian rhythms in lots of things — sleep and wake; patterns of alertness or cognitive ability; the ability to process alcohol (try drinking at lunchtime vs. in the evening); and of course jet lag, where we suffer from misalignment of our internal circadian clock with the external day and night. But animals have these clocks, too, even animals that live underground, and so animals, like humans, “perceive” time at the physiological level. My paper published in Nature Communications in 2013 on the Mexican blind cavefish was one of the first to definitively show that cavefish have circadian rhythms by showing that the molecular part of the circadian clock is still functional, even though these fish have been isolated away from the day and night cycle for hundreds of thousands of years. A number of other cavefish species from other parts of the world also have circadian rhythms, but these have often been altered in some way during the course of evolution — the Somali cavefish, Phreatichthys andruzzii, has a circadian clock and shows circadian rhythms, but it has lost all sensitivity to light, so its “perception of time” in a circadian sense is probably more to do with temperature cycles or cycles of the availability of food. A Chinese cavefish shows some aspects of a functional circadian clock where some genes important in the clock are expressed. An Indian cave fish, Nemacheilus evezardi (also known as Indoreonectes evezardi), a cave loach found in the Kotamsar/Kotumsar Caves in India, shows some aspect of circadian rhythms in behaviour.
So multiple evolutionarily independent cave fishes all maintain a circadian clock to some extent, despite the absence of all external cues for the passage of time. So while it is important for us on the surface to coordinate our behaviour and physiology with the day and night, the results in cavefishes point towards the fundamental importance of maintaining a time aspect to an organism’s internal physiology, whether or not this aligns with the outside world (which the cavefish don’t have). So, whether there is a conscious perception of time, I don’t know, but there is definitely a physiological perception of time in most, if not all, organisms on the planet.
Gabriele Andreatta
Postdoc Fellow, Biology, University of Vienna
First of all, what time?
Time perception is inextricably linked to reference events, such as sunrise, sunset, full moon, new moon, seasonal changes. Yet, time can be perceived through getting hungry between meals, entering puberty, ageing, etc. Both are very important aspects, and the major events in animals´ life are orchestrated by the cross-talk between the two “time-perceiving systems.”
To keep track of the environmental rhythms mentioned before, animals possess a species-specific array of photoreceptors (opsins and cryptochromes) whose activation depends upon both light intensity and specific wavelengths. As dusk, dawn, summer days, and winter days have different features, animals can discriminate between these periods, regulating their behaviour accordingly.
However, although the underlying mechanisms are less understood, animals can also use other environmental information, such as temperature oscillations, tidal rhythms, and food availability, to regulate their biology.
This reliable information about time is used to entrain molecular oscillators called endogenous clocks, which coordinate body rhythms with environmental rhythms. Interestingly, studies in a variety of models have shown that these molecular oscillators continue to function robustly even in the absence of the entrainment cue, at least for a certain period of time. A famous example is the circadian clock, in which molecular dynamics oscillate with a period of ~24 hours, timing, for instance, sleep-wake cycles and activity patterns.
From studies on flies and mice, we know that a clock ticks in almost every organ, helping to orchestrate tissue-specific functions. However, they are all synchronised with the environment by a master clock which resides in the brain.
Other clocks with different periodicity have been discovered, each evolved to coordinate animal life with a different environmental cycle such as tidal, lunar, or annual rhythms. Unlike the circadian clock, the molecular machinery of these oscillators still remains elusive, but several models are emerging to fill this gap.
For instance, the marine bristleworm Platynereis dumerilii possesses circadian and circalunar clocks, which have been shown to communicate with each other to regulate behavioural and reproductive aspects with both daily and lunar cycles.
On the other hand, a complex endocrine network allows the “perception” of time based on physiological and developmental changes affecting our body. Drosophila larvae kept in constant darkness for their entire development still metamorphose and emerge as flies. Similarly, bristleworms reared with no moonlight stimulus still reproduce and complete their life-cycle. What they are lacking is a rhythm, a coordination with the environment which in nature would be necessary to maximise their chances to survive and reproduce. However, in a certain moment of their life, these animals “know” it is the right time to move on with their life-cycle, leaving one developmental stage for another. Similarly, humans enter puberty only when our body is ready to sustain this energy-demanding transition. In all these cases, the brain is constantly updated with information about body and organ growth, as well as energy storage, to better “decide” when the right time has come. Finally, it is difficult to not mention ageing in this context. Although the perception of the associated events is indirect, all the possible genetic, cellular, and metabolic processes involved are interpreted in our bodies, too.
At the end of the day, it is just a matter of planning. This is what time is about, regardless of whether time is measured with the alternation of environmental conditions or by “feeling”our bodies change over the course of our life.
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