Can you see heat?
No, don’t be ridiculous
“Let’s get this right. Did you say see heat, or just feel it? No question of the latter, very useful around the kitchen, thank you. But I feel the hot-plate, I don’t see the actual infrared radiation”. Of course lots of animals possess the capacity to sense heat, not in a general way but with considerable acuity that is central to their lives. Vampire bats and bed-bugs in search of a blood dinner, beetles and other insects flying towards forest-fires – no, no, not for suicidal self-immolation, but to lay their eggs in the warm wood – and of course snakes like the rattlers and pythons that sense the infra-red radiation through special pits on their head. And why can’t they actually see the heat source? Now, I know you may not be a physicist but let me try to explain why this is impossible. Heard of the electromagnetic spectrum? Well, that’s a start. Now then visible light ranges from the ultraviolet (UV) towards the infrared, which we sense as warmth. UV has a shorter wavelength and packs more energy. That is why it is so dangerous, skin-cancers and other horrors. For vision to work the process of excitation in the retina needs a certain energy to trigger a signal and infrared radiation simply doesn’t pack the necessary punch. Infrared is invisible, OK?
Yes, of course
Let’s take a closer look at those infrared detectors in the rattlesnake Crotalus and their relatives (collectively the crotalids). They are a nifty piece of biological engineering, consisting of a deep pit divided by a very thin membrane with a tiny opening but otherwise serves to define an inner and outer chamber. This membrane is rich in blood vessels, packed with mitochondria and the nerve cells that actually detect the incoming infrared radiation. These pits are fantastically sensitive, easily capable of working at the level of thousandths of a microwatt.
It turns out that this pit is much more like an eye, specifically the so-called pinhole camera-eye, than you might expect. Pinhole eyes have no lens, and the apertural size of the opening to the pit means it has a poor focus, but the comparison still holds. Not only is it exceedingly sensitive, but its ability to detect thermal contrasts means that it sees cool areas as “shadows”. But there is more. The nerves that inform the brain about the infrared radiation feed into the same areas as the real eyes. In fact the snake has four eyes, two camera-eyes and two pinhole eyes, and because they all overlap the snake enjoys stereoscopic vision.
It all depends on the question
“What is it like to have the mind of a bat?” asked the philosopher Thomas Nagel. At first sight “seeing” infrared or echoes (as do bats and whales), let alone electrical fields (like many fish) seems utterly alien. But is it? Remember your eyes really aren’t like cameras at all. Yes, they usually have a lens, can focus and control the apertural size (the pupil), but the images are constructed in the brain. Second, the melding of the infrared signals and optical signals in the brain of the rattlesnake is by no means unique and abundant evidence exists for the combining of different sensory pathways. So for an animal to “see” it requires some sort of nervous system, but the information doesn’t need to be transmitted by the optic nerve. So too to detect the signal you need a protein receptor, but it doesn’t need to be the canonical rhodopsin.
Animals possess an extraordinary range of sensory receptors, each designed to receive one or other of the incoming torrent of sensations. So they can be in the form of molecules to taste, pressure waves as sound, or electromagnetic waves in the form of infrared radiation. So too animals have neural equipment to make sense of these sensations, but lurking behind all this is the Big Question. This is the one that leads to a shuffling of feet and awkward clearing of throats, the question of qualia and consciousness. The redness of the sunset, the taste of garlic, the sadness of G minor. Are these just neurological fictions?
Text copyright © 2015 Simon Conway Morris. All rights reserved.
Further reading
Bakken, G.S. and Krochmal, A.R. (2007) The imaging properties and sensitivity of the facial pits of pitvipers as determined by optical and heat-transfer analysis. Journal of Experimental Biology 210, 2801-2810.
Ebert, J. and Westholt, G. (2006) Behavioural examination of the infrared sensitivity of rattlesnakes (Crotalus atrox). Journal of Comparative Physiology, A 192, 941-947.
Goris, R.C. (2011) Infrared organs of snakes: An integral part of vision. Journal of Herpetology 45, 2-14.
Gracheva, E.O. et al. (2010) Molecular basis of infrared detection by snakes. Nature 464, 1006-1011.
Van Dyke, J.U. and Grace, M.S. (2010) The role of thermal contrast in infrared-based defensive targeting by the copperhead, Agkistrodon contortrix. Animal Behaviour 79, 993-999.
Learning to sing under water
Early whaling logs often referred to whales as ‘singers’. All whales use sounds for communication, and most use recognisable patterns, ranging from the haunting tones of bowhead whales to the fast echolocation. Bowheads and bottle nosed dolphins use ‘names’ to identify each other and their clan, have ‘conversations’, and copy each other’s calls.
Bowheads are second only in size to the blue whale, and are very long-lived. Stone harpoon heads, which went out of use at the turn of the 19th century, have been found lodged in the bodies of some individuals of today. Listen to their calls here.
Whales’ land-based ancestors made sounds as we do, using the throat and mouth. But try singing under water! Whales call during dives; whilst holding their breath, and with their mouths closed. Calling is essential for these highly social animals; this is evident because independently both baleen and toothed whales have evolved new ways to produce and transmit sound in the sea.
How are whale sounds made?
We produce sound in a tubular organ in our throats; the larynx. This organ is an air valve, closing off the lungs when we swallow, and controlling the outward flow of air. Sound is generated by vibrating paired membranes in the larynx; these ‘chop up’ the airflow into pressure waves. This is then modified by resonating in the chambers of the chest, nose and throat before being released through the mouth.
A humpback’s larynx does not sit across the windpipe. Instead it straddles the opening to an additional air sac located beneath the throat. These whales make sounds by passing air between this sac and the windpipe, vibrating the vocal folds of the larynx. These deep notes are modified and amplified by resonating in the laryngeal sac and nasal cavity, and are thought to be transmitted into the water through the sac, which vibrates the flexible throat pleats somewhat like a hi-fi speaker cone.
Dolphins have evolved a secondary set of vocal folds inside their nasal cavities, called ‘phonic lips’. These vibrate like our vocal chords and generating pressure waves.
Because during deep dives the air volume in their nasal cavities becomes very small, their clicks, rasps and whistles do not resonate in inner cavities. Instead, a lens of fatty material on the front of the skull known as the ‘melon’ acts as an underwater ‘speaker cone’, amplifying and transmitting these calls.
Why do whales ‘sing’?
The repetitive, phrased songs of humpback whales are a means of male display. Whilst both genders make sounds, only the males produce song patterns. They copy the signature calls of their clan, serenading unaccompanied females during seasonal migration and at the winter breeding grounds.
Their behaviour suggests that these breeding areas are a form of ‘floating lek’, where females ‘browse’ amongst available mates. Humpbacks also use other non-singing signals, e.g. when hunting together, females call to communicate with their offspring.
Bottle-nosed dolphins develop their own distinctive signature whistle in the first few months, and exchange these ‘names’ when meeting other dolphins. Females keep their whistle throughout life, whilst males change their signature calls when they move between social groups. Like baleen whales, they also communicate during foraging dives and to maintain social ties.
What can we learn from whale song?
Biologists classify ‘song’ as following ‘a repeating, predictable acoustic pattern’. Most songbirds mimic the calls of their peer group as juveniles. When they mature, this song pattern becomes ‘fixed’. Whales are unusual; they can copy and learn new sounds throughout their lives. Like us, they have a brain structure that continuously learns and can adapt to changes.
The diversity of signature calls and unique song patterns used by clans reveal that whales have a complex and sophisticated system of learnable language. Studying the language and culture of whale populations can thus provide clues about how own language and social complexity may have evolved.
Text copyright © 2015 Mags Leighton. All rights reserved.
References
Adam O et al. (2013) ‘New acoustic model for humpback whale sound production’ Applied Acoustics 74: 1182-1190
Cantor M and Whitehead H (2013) ‘The interplay between social networks and culture; theoretically and among whales and dolphins’ Philosophical Transactions of the Royal Society B 368: 20120340
Cholewiak D M (2013) ‘Humpback whale song hierarchicial structure; historical context and discussion of current classification issues’ Marine Mammal Science 29 3: E312-E332
Cranford, T.W., (2000) "In Search of Impulse Sound Sources in Odontocetes." In Hearing by Whales and Dolphins (Springer Handbook of Auditory Research series), W.W.L. Au, A.N. Popper and R.R. Fay, Eds. Springer-Verlag, New York.
Fitzgerald EMG (2006) ‘A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales’ Philosophical Transactions of the Royal Society B 273:2995-2963
Green SR et al. (2011) ‘Recurring patterns in the songs of humpback whales (Megaptera novaeangeliae)’ Behavioural Processes 86:284-294
Janik VK (2005) ‘Acoustic communication networks in marine mammals’ pp390-415 in Animal Communication Networks McGregor PK (Ed) CUP, Cambridge
Janik VK (2009) ‘Whale song’ Current Biology 19(3)R9-R11
Jensen FH et al. (2011) ‘Calling under pressure; short finned pilot whales make social calls during deep foraging dives’ Philosophical Transactions of the Royal Society B 278:3017-3025
King S L et al. (2013) ‘Vocal copying of individually distinctive signature whilstles in bottlenose dolphins’ Philosophical Transactions of the Royal Society B 280:20130053
Madsen PT et al (2002) ‘Sperm whale sound production studied with ultrasound time/depth-recording tags’ Journal of Experimental Biology 205: 1899-1906
Madsen P T et al. (2002) ‘Male sperm whale (Physeter macrocephalus) acoustics in a high latitude habitat: implications for echolocation and communication’ Behavioural Ecology and Sociobiology 53:31-41
Madsen PT et al (2012) ‘Dolphin whistles; a functional misnomer revealed by heliox breathing’ Biology Letters 8: 211-213
Mercado E and Handel S (2012) ‘Understanding the structure if humpback whale songs’ Journal of the Acoustics Society of America 132(5):2947-2950
Mercado E et al. (2010) ‘Sound production by singing humpback whales’ Journal of the Acoustics Society of America 127(4) 2678-2691 ***NB I am using the broader definition of ‘song’ as applied here by these authors. Janik (2009) suggests that only baleen whales are singers because of their more elaborated song patterns. However toothed whales also use patterns, even though they are clicks and whistles rather than resonating songs.
Møhl B (1999) ‘Dolphin hearing; relative sensitivity as a function point of application of a contact sound source in the jaw and head region’ Journal of the Acoustics Society of America 105(6); 3421-3424
Quick NJ and Janik VM (2012) ‘Bottlenose dolphins exchanges signature whistles when meeting at sea’ Philosophical Transactions of the Royal Society B 279:2539-2545
Reidenberg JS and Laitman JT (2007) ‘Discovery of a low frequency sound source in Musticetae (baleen whales); anatomical establishment of a vocal fold homologue’ Anatomical Record 290: 745-759
Reidenberg JS and Laitman JT (2008) ‘Sisters of the sinuses; cetacean air sacs’ Anatomical Record 291:1389-1396
Smith J N et al (2008) ‘Songs of humpback whales, Megaptera novaengeliae, are involved in intersexual interactions’ Animal Behaviour 76:467-477
Stafford KM (2012) ‘Spitzbergen’s endangered bowhead whales sing through the polar night’ Endangered Species Research 18:95-103