By Vidya Rajan, Columnist, The Times
In the brilliant movie adaptation of the potboiler Jurassic Park by Michael Crichton,[1] there is an early gripping scene where the Tyrannosaurus rex’s enormous head is just inches away from the paleontologist-protagonist, who whispers to two children beside him that the predator could not detect them if they stayed perfectly still. I remember my outrage as a biologist – It was a predator, or at least a scavenger, for heaven’s sake, so what about its ability to smell? To hear? – but the plot line was that the money-grubbing lawyer was to be eaten and, so, off to the outhouse the T.rex went. Vision is just one of many senses engage with the surroundings and respond appropriately, that is, to stay, fight, or flee. Briefly, a “sense” is a biological tool that helps reveal information about the opportunities and dangers that an organism faces, and accurately sensing these is pivotal to surviving and thriving. In the following set of articles, I will examine specific senses independently, but in this article, I will describe some basic senses and their function with lighthearted/anecdotal examples.
All organisms, including single-celled ones, respond to their internal physiological conditions as well as their external surroundings. Internal sensing can be divided into “interoception” for physiological factors such as chemical balance and cell volume, proprioception for the relational location of parts of the body, and kinesthetic sense for movement. External sensing, “exteroception” uses a variety of senses: chemoreception (taste) to identify attractive or repulsive chemicals, thermoception for heat and spiciness, mechanoreception for stretch and touch, barometric reception for pressure changes, photoreception for light, echolocation, olfaction (smell, as distinct from taste), electroreception for sensing electrical fields, magnetoreceptors for sensing magnetic fields, nociception for pain, hygrosensation for humidity and, sound reception in hearing. Some of these senses are closely related; for instance, taste and smell are allied, as are temperature receptors with those for pain, and hearing is associated with pressure wave changes which is why you can both hear and feel the thudding of percussion through your feet on a dance floor. Although electromagnetic radiation (EMR) such as magnetic field lines and bioelectricity can be sensed by some animals, most others cannot detect EMR from the Sun, nuclear emissions or machinery, which can impact biological systems but cannot be detected by senses, thus requiring monitoring and exposure safety limits.[2],[3]
Senses help with communication – pollinators can find flowers through visual and olfactory cues; long-eared fennec foxes can hear prey moving under desert sand in the dark, sharks can smell faint chemical traces of blood and distress in the water, and many animals sing and dance to signal their openness to mating, including humans. The Guugu Yimithirr people are so attuned to cardinal directions that they have no words for “left” or “right”. Rather, they say instead, “Move a bit east”.[4] Some people claim to have “sixth senses” for spooky things like ghosts, or to optimize positive “vibes” through practicing Feng Shui, Vaasthu Sastra, or Reiki. Many nature observers who depend on the weather (historically farmers and sailors) swear by animals’ wisdom: cows lying down means it will rain, animals moving to higher ground means a tsunami is imminent, and famously, Punxsutawney Phil’s shadow (or not) predicting the arrival of spring.
Organisms that exist as single cells are usually suspended in a fluid environment, and solutes in the water are a measure of that organism’s ability to obtain food, and thereby regulate its growth and reproduction. The exteroception of attractive chemicals such as food and pheromones versus repulsive or toxic ones produce internal signals through molecules which regulate the physiological or genetic output and can guide the cell to accept or reject the substance. Another surprising sense some single-celled organisms have is the ability to sense light, but light has two uses, as a source of energy through photosynthesis, and for vision. Proper vision requires a brain for processing images, but sensing light and dark requires merely a light-absorbing pigment which triggers a mechanism that causes movement. Interoception is important for the cell to recognize its need for food, or for recognizing it is large enough to split into two cells. These are recognized by the state of binding of a key biomolecule such as the energy molecule, ATP, to a sensor molecule. Absence or low concentrations of ATP leave the sensor unoccupied, which could trigger, for example, random cell movement which might help it bump into food, or to deploy more receptors to search for food molecules. Cells also maintain tabs on their own cell size and the presence of abundant food causes them to grow and reach a tipping point where they divide into two, or sometimes more, cells.
In multicellular organisms, finding food becomes a matter of persuading all the cells of the organism to coordinate actions. Plants stay in one place and grow up towards light and down towards water. Fungi find a living or dead host to grow on and digest. Some animals allow food to come to them, which means they sit and wait for prey, but must accurately sense the prey’s approach using chemicals, vision, noise or pressure changes and unleash rapid trapping or killing motions using grabbing tools such as teeth or tentacles. For animals which pursue their prey, their senses have to be keen enough to detect prey that is either good at camouflage or fast enough to escape being caught. Many “prey” animals have collectivized for the benefits of a group being better at surveillance than a lone individual; this also benefits the predator because they can also collectivize to catch a slow or unfit member of a large group. To do this, they must quickly assess each individual’s fitness and communicate plans with sounds or actions. In response, herd animals such as antelope can signal fitness through dramatic leaps in the air as if to say, “I’m fit and will get away, so find someone else!”
Recently, the merger of two 30 solar mass black holes from 1.3 billion light years away created gravitational waves – a sort of pressure wave made up of electromagnetic radiation (photons). Although we do not have the ability to sense at the frequency of these waves, appropriately tuned instruments can help. The single black hole produced a “ringdown” – a which was picked up on 14 September 2015 and translated to pressure waves that our auditory senses could interpret. To hear what it sounded like, hit the play button on the LIGO Lab’s video below. The little “chirp” is all we can “hear” at our spatial and temporal distance of that release of energy equivalent to 4.3×1037 tons of TNT[5] or 3 of our Suns exploding simultaneously. Which other life forms sensed it? How long ago, and using which sense?
To address the question, “If a tree falls in a forest and no one hears it, does it make a sound?”, a sense-ible response would be ‘“Define “sound”.’, and sense-itive response would be: ‘Define “no one”.’
[1]. Michael Crichton also wrote several previous books under the pseudonyms John Lange, Jeffrey Hudson (ironically, after a seventeenth century dwarf, since Crichton was himself 6 feet 9 inches tall) and Michael Douglas (a portmanteau of his own and his brother’s first names). He also earned a MD from Harvard although he never practiced medicine, taught anthropology at the University of Cambridge, conceived the TV series ER, and worked as a scientist at the Salk Institute for Biological Sciences (where coincidentally, my husband worked in the same laboratory room many years later.) Crichton died at the age of 66 in 2008. From: Wikipedia Contributors (2019). Michael Crichton. [online] Wikipedia. Available at: https://en.wikipedia.org/wiki/Michael_Crichton. In 1994, Michael Crichton was the only person ever to top television, film, and book charts simultaneously (for ER, Jurassic Park, and Disclosure respectively). From: Interesting Literature. (2015). Five Fascinating Facts about Michael Crichton. [online] Available at: https://interestingliterature.com/2015/04/five-fascinating-facts-about-michael-crichton/.
[2]. Pirogova, E., Vojisavljevic, V. and Cosic, I., 2009. Biological effects of electromagnetic radiation. Biomedical Engineering, pp.87-106.
[3] Youvan, D.C., 2024. Beyond the Five Senses: The Potential Impact of Electromagnetic Field Perception on Human Capability and Evolution. (This is a published opinion and not a peer-reviewed paper. It is available at: https://www.researchgate.net/profile/Douglas-Youvan/publication/383661477_Beyond_the_Five_Senses_The_Potential_Impact_of_Electromagnetic_Field_Perception_on_Human_Capability_and_Evolution/links/66d5e0e1f84dd1716c790299/Beyond-the-Five-Senses-The-Potential-Impact-of-Electromagnetic-Field-Perception-on-Human-Capability-and-Evolution.pdf)
[4]. Note that the directions of the Guugu Yimithirr do not cleanly line up with specific points on the horizon like north, south, east and west, but rather to quadrants, and is slightly rotated clockwise from the traditional compass’s directions. From: Felton, J. (2025). The Guugu Yimithirr Language Is Notable For Using Cardinal Directions Instead Of ‘Left’ Or ‘Right’. [online] IFLScience. Available at: https://www.iflscience.com/the-guugu-yimithirr-language-is-notable-for-not-having-a-left-or-right-79588.
[5]. Wikipedia Contributors (2019). First observation of gravitational waves. [online] Wikipedia. Available at: https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves.
