By Vidya Rajan, Columnist, The Times
This is the eighth and last of the “Senses” series where I will explore unusual senses that interrogate the environment, including magnetoreception, electroreception, astronavigation, polarized light perception, electrogenesis and electroreception, thermoreception, chronoreception, seismoreception and hygroreception.[1] These may well be called “sixth senses” and many organisms show sensitivities based on their umwelt or the unique ecosystem they each experience. There may be many more than those listed here; anecdotes reference elbows tingling when rain is anticipated, or one’s neck tingling when, unbeknownst, someone is staring at them. But their existence has not been established in controlled experiments, so I will not discuss them further.
Magnetoreception:
Sensing the Earth’s magnetic field is called magnetoreception, and its use as a navigational tool was suggested back in 1882 by Camille Viguier. The hunt has been on for a mechanism has been on ever since. In bacterial cells, crystals of the iron mineral, magnetite, were found to aligned with the field; magnetite has now been found in insects, molluscs, reptiles, fish and birds. Other experiments pointed to a photoreceptor in the eye called “cryptochrome”. To investigate, fruit flies were exposed to full spectrum light (300-700nm) and to light with the UVA-blue region (~420nm) blocked. In the presence of the UVA-blue region, flies were able to sense magnetic fields, but without it, they could not.[2] This indicates that the cryptochrome sensed UVA which was somehow involved in the perception of magnetic fields, at least in fruit flies.
Two experiments demonstrate this uncanny ability. The Lohmann laboratory in North Carolina at Chapel Hill placed hatchling loggerhead turtles in a ring harness and tethered them in a pool which was fitted with a magnetic system which could produce fields that matched different ocean areas. The hatchlings swam in directions that matched their migratory routes in the real world.[3] Since they were hatchlings, there was no indication of how they “knew” which way to go, and the detection organ is not known. The second experiment used homing pigeons. Pigeons has earlier been shown to have iron filings in their ears which produced a small electric current due to the Earth’s magnetic field, but this experiment showed their livers were also involved. The liver of pigeons has a high concentration of macrophages which contain iron-rich ferritin from engulfing broken-down red blood cells. The researchers took trained pigeons and treated half of them with clodronate liposomes which knocks out the liver macrophages. Then they released the birds to fly back home. On sunny days, they all got home (presumably using the Sun to orient themselves using a visual/non-magnetic navigation) but on cloudy days when magnetic orientation would be used for pathfinding, the clodronate-treated pigeons got hopelessly lost (until the Sun came out to light their way.)[4],[5]
On magnetically calm days, it turns out dogs also line up on a north-south axis to urinate and defecate, and they can use their internal compass to take short cuts. Plants also respond to extra strong and weak magnetic fields with anatomical anomalies. They typically do not have iron inclusions, so the mechanism still awaits elucidation.[6] Human magnetoreceptor abilities have been established in the laboratory and shown to stimulate alpha-waves in the brain[7] but, remember that Chinese-invented compasses helped navigation on featureless seas, not humans mounted on masts. So, carry a compass. And take the dog.
Astronavigation – navigating using the night sky – is not unique to humans; birds, bats, seals, moths and dung beetles use them too. It is thought that night-migration is preferred for lower numbers of predators and parasites, less competition from day-active species and for the cooler temperatures, especially in hot climates.[8] Stars are supposedly seen as point objects and migrate across the sky in a manner similar to the sun. Migrating animals like birds and bats and sea animals like seals calibrate their magnetic compasses at dusk and then continue their navigation using stars. The Indigo bunting uses stars up to 35o around the fixed center (Polaris, the North Star) to guide their migration in experiments under a planetarium dome. They may also use other natural cues (magnetic poles, polarized light) to fine-tune their direction. Dung beetles use the Milky Way as a guide for traveling long distances (relative to their size) and traveling back and forth between a dung heap and their home. Frogs released under a bright night sky traveled in the “correct direction and the opposite” rather than scattering in every direction, indicating they were orientating on an axis. Finally, moths appear to use the moon and stars, but their travel drifts, indicating that they do not recalibrate periodically or cannot use the rotation of the night sky in their mapping. Obviously good vision, including the ability to detect polarized light, and invisible direction finding along magnetic lines may be also be involved.
Beekeepers already know that bees sense direction with polarized light from the Sun. Not only that, they make allowances for the change in the Sun’s position throughout the day as they navigate. The stripes on a zebra apparently look different under polarized and non-polarized light. The stripes cause polarization of light in different directions such that biting horseflies cannot detect a flat place to land, or maybe they get confused by the high contrast between light and dark stripes.[9] The original paper does point out that it is not really possible to completely replicate horsefly vision using a polarized lens so they may be seeing other features as well. Anyway, I think that makes zebras kind of cool, and so I asked Claude (Anthropic) to make Figure 1. Don’t miss the sunglasses.
Figure 1: Left images: The effect of stripes on landing zones for biting horseflies. The white areas of the horses in the polarized image are potential landing spots as viewed by biting horseflies, whereas the black areas are not seen. The stripes reduce the landing spots as perceived by the flies. Image on left adapted from Reference [10] under fair use for educational purposes. Right: Cartoon summarizing the findings, made by Claude AI (Anthropic), showing the teal-colored areas oriented in one direction, indicating flatness for landing.
In terms of sensing electricity, there are two modes: some animals sense the fields put out by others (passive electroception), and other animals sense distortions in their own field due to the presence of other animals (active electroreception). In animals that live with the air around them, filiform (mechanosensory) hairs or antennae, and it is affected by atmospheric phenomena (Figure 2). This charge differential is also seen with spider webs (whose gain a slightly negative charge induced by positively charged insects flying by) and it is thought that Varroa may be electrostatically attracted to bees as well. Electroreception is an important sense in predatory animals for locating prey, and is used by knifefish, catfish, sharks, elephantnose fish, echidna, fishing bats, predatory wasps and, most famously, the duck-billed platypus.[11]
Figure 2: Electrical ecology in context with atmospheric phenomena showing bees’ positive charge and plants’ negative charge which causes electrostatic attraction of pollen towards the bee. Reproduced from Reference [12] under fair use for educational purposes.
Thermoception: The ability to sense heat is a survival skill to avoid being too cold or too hot. The function of thermoception is performed by ion channels called transient receptor potential (TRP) channels. Crossover between TRP channels causes some to perceive All organisms produce heat from their metabolism, and thermoception is the ability of some organisms to sense the presence of heat, which is just infrared radiation.[13] Predatory snakes such as pit vipers have “pits” lined with receptors to sense potential prey. But it is for more than hunting. Tellingly, heat sensation also works through TRP receptors which act as nociceptors (sensors for noxious stimuli) and can be triggered by both heat and painful stimuli.
Chronoception: Using biological clocks to anticipate regular variations in light, temperature and environmental conditions helps to maximize survival strategies like hunting, migration or reproduction.[14] All organisms have a circadian clock within their cells which tracks the 24-hour daily cycle with a variety of clock proteins whose abundance in the cell increases and decreases with that periodicity. The periodicity also translates to the organismal scale – some organisms have a “pacemaker” which regulates internal timing. In humans, the pacemaker is the superchiasmatic nucleus (SCN) which ultimately inhibits the pineal gland, reduces melatonin production, and causes wakefulness.
Echolocation: Also known as “biosonar”, this ability to send out sound waves and sense them bouncing off objects. It is found in bats and toothed whales such as dolphins and orcas who use it to find food and sense obstacles by sounding rapid clicks and then listening for the echo, which is processed into a 3-D map. They are sensitive enough to identify and target an object as small as a flying mosquito.[15] Note that the clicks are high-pitched with extremely short wavelength, so they are reflected more readily than low wavelengths which would pass over objects or be absorbed.
To finish, there is a joke that a senior naval officer during World War II was flummoxed when he heard that dolphins and bats used a version of sonar. “How is that possible?” he is supposed to have asked. “We just invented it!”
[1]. These senses can be shortened from the suffix -reception to -ception. The two forms are synonymous.
[2]. Gegear, R.J., Casselman, A., Waddell, S. and Reppert, S.M. (2008) ‘Cryptochrome mediates light-dependent magnetosensitivity in Drosophila’, Nature, 454(7207), pp. 1014–1018. doi: 10.1038/nature07183.
[3]. Lohmann, K.J., Putman, N.F. and Lohmann, C.M.F. (2012) ‘The magnetic map of hatchling loggerhead sea turtles’, Current Opinion in Neurobiology, 22(2), pp. 336–342. doi: 10.1016/j.conb.2011.11.005.
[4]. Lisowski, C. et al. (2026) ‘Homing pigeon navigation relies on superparamagnetic macrophages under overcast conditions’, Science. doi: 10.1126/science.ady2486.
[5]. Stokstad, E. (2026) ‘‘Mind-blowing’: iron-rich immune cells help homing pigeons navigate’, Science, 28 May. Available at: https://www.science.org/content/article/mind-blowing-iron-rich-immune-cells-help-homing-pigeons-navigate (Summary of the Lisowski paper in #5.)
[6]. Hart, V., Nováková, P., Malkemper, E.P., Begall, S., Hanzal, V., Ježek, M., Kušta, T., Němcová, V., Adámková, J., Benediktová, K., Červený, J. and Burda, H. (2013) ‘Dogs are sensitive to small variations of the Earth’s magnetic field’, Frontiers in Zoology, 10(1), 80. doi: 10.1186/1742-9994-10-80.
[7]. Galland, P. and Pazur, A. (2005) ‘Magnetoreception in plants’, Journal of Plant Research, 118(6), pp. 371–389. doi: 10.1007/s10265-005-0246-y.
[8]. Wang, C.X., Hilburn, I.A., Wu, D.-A., Mizuhara, Y., Cousié, C.P., Abrahams, J.N.H., Bernstein, S.E., Matani, A., Shimojo, S. and Kirschvink, J.L. (2019) ‘Transduction of the geomagnetic field as evidenced from alpha-band activity in the human brain’, eNeuro, 6(2), ENEURO.0483-18.2019. doi: 10.1523/ENEURO.0483-18.2019.
[9]. Caltech Magnetoreception Laboratory (no date) Human magnetic reception laboratory. California Institute of Technology. Available at: https://maglab.caltech.edu/human-magnetic-reception-laboratory/
[10]. Caro, T., Fogg, E., Stephens-Collins, T., Santon, M. and How, M.J. (2023) ‘Why don’t horseflies land on zebras?’, Journal of Experimental Biology, 226(4), jeb244778. doi: 10.1242/jeb.244778.
[11]. Otte, J. (2026). 11 Creatures that prey using electricity. [online] Animals around the globe. Available at: https://www.animalsaroundtheglobe.com/11-creatures-that-prey-using-electricity-6-330503/
[12]. Robert, D. (2024). Aerial electroreception. Current biology, [online] 34(20), pp.R1018-R1023. doi:10.1016/j.cub.2024.06.028.
[13]. Parrish, A.C. (2021) ‘Thermoception’, in The Sensory Modes of Animal Rhetorics. Cham: Palgrave Macmillan, pp. 145–163. PDF available at: https://link.springer.com/content/pdf/10.1007/978-3-030-76712-9.pdf
[14]. Kuhlman, S.J., Craig, L.M. and Duffy, J.F. (2018) ‘Introduction to chronobiology’, Cold Spring Harbor Perspectives in Biology, 10(9), a033613. doi: 10.1101/cshperspect.a033613.
[15]. Brinkløv, S.M., Jakobsen, L. and Miller, L.A., 2022. Echolocation in bats, odontocetes, birds, and insectivores. In Exploring Animal Behavior Through Sound: Volume 1: Methods (pp. 419-457). Cham: Springer International Publishing.
