A small volume of calfskin, made somewhere between 1404 and 1438, covered in a script that appears in no other document on earth. Tonight we sit with the Voynich Manuscript: what it is made of, what is drawn in it, and what a century of codebreakers, linguists and translation software have failed to get out of it.

We cover the fourteen or fifteen calves that became its pages, the five different hands that wrote it, the plants that do not match any living species, the zodiac that begins in the wrong place, and the six-page foldout nobody can explain. We follow it from an alchemist in Prague who called it a sphinx taking up space in his library, through a Jesuit library in Rome, to a bookshop in Soho Square, to a shelf at Yale. And we go through the long line of people who announced they had read it, from a professor reading the cracks in dried ink, to the codebreakers of two world wars, to the machine-translation results of the last few years.

Tonight, a fish that stopped swimming.

The seahorse is a fish, though almost nothing about it cooperates with the idea. It has no scales, no tail fin, no teeth and no stomach. It stands upright in the water, moves at something like five feet an hour, and spends most of its life with its tail wrapped around a single stem of seagrass, waiting for something smaller than a grain of rice to drift past.

We look at why that tail is square rather than round, and what happened when researchers three-dimensionally printed both versions and crushed them. We look at how the slowest fish in the sea manages to catch one of the fastest escape artists in it, by carrying a pocket of still water just above the tip of its snout. We look at the pouch, and at what happens inside a male seahorse over thirty-four days when his own skin becomes something a research team was willing to call a placenta.

Along the way: the thirteen-million-year-old fossils from a Slovenian hillside that were already fully seahorses; the pygmy species found by accident in 1969, on a sea fan lying on a dissection table; and the sixteenth-century anatomist who looked at a ridge in the human brain, thought it resembled a silkworm, changed his mind, and named it the hippocampus instead.

The mate-for-life story is examined and mostly set down. The daily greeting is real. The rest the evidence has not settled.

No lesson in it. Nothing to carry into the morning.

The grid of rows and columns was invented, forgotten, and reinvented for thousands of years before anyone put it on a screen.

Tonight, a shape so plain you could draw it with a ruler: lines one way, lines the other, a small box wherever they cross.

It took a very long time to arrive. Scribes in Mesopotamia ruled columns into clay four thousand years ago, then set the idea down and forgot it, then found it again, over and over, for twenty-five centuries. It spent most of its life on paper spread across two facing pages of a ledger, where changing a single number meant an afternoon of arithmetic and so nobody changed anything at all.

We follow it from that clay through Venice and a friar who wrote down a method he had not invented, to a student watching a professor rub out a blackboard at Harvard in 1978, to the little program that sold two thousand dollar computers to people who only wanted the hundred dollar software. Along the way: a day in February 1900 that never happened and is still carefully preserved in almost every computer in the world, and a committee of scientists who renamed twenty-seven human genes because a spreadsheet kept reading them as dates.

No lesson in it. Nothing to carry into the morning. Just rows, and columns, and the small box where the two of them meet.

Get comfortable. Close your eyes. Relax.

For almost the whole of human history, people could not truly get clean with water alone, and nobody knew why. The answer is a molecule with two ends that want opposite things: a head that holds on to water, a tail that buries itself in grease. Tonight, the slow story of how people found that molecule thousands of years before anyone could explain it.

We start with the problem itself, and with surface tension, the reason plain water beads up and rolls off a greasy hand instead of soaking in. Then the trick: how a crowd of soap molecules wraps each speck of oily dirt in a shell the water is glad to carry away.

After that, the history. Ash and fat sitting side by side at every ancient hearth, and a discovery that must have been made over and over by people who never met. A recipe pressed into Babylonian clay before the pyramids were finished, for cleaning cloth rather than skin. The Egyptian medical scroll. The legend of Mount Sapo, told in full and then set aside, because the hill has never been found on any map. The Romans, who scraped themselves with oil and a curved blade and thought soap was something the Gauls put in their hair. Aleppo, olive and laurel, aged in cool cellars for most of a year.

Then a French surgeon named Nicolas Leblanc, who worked out how to make soda ash from common salt, never received his prize money, and set off an industry that made soap cheap and poisoned the air downwind of it until Parliament passed one of the first laws against air pollution anywhere.

A doctor in Vienna who noticed that mothers were dying on one ward and not the other, tried washing with soap, found it was not enough, and reached for chlorine instead.

And finally the twentieth century, when chemists learned to build the same two-ended molecule from petroleum rather than fat, changed one end of it, and quietly took over almost all the washing in your house. Shampoo, body wash, washing-up liquid, and most of the bars sold as soap are not soap at all. The plain bar in the dish may be the last true soap you own.

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The Slow History of Soap - Sources

Organised by section, in episode order.

How soap works: surfactants, micelles, surface tension

• Chemistry LibreTexts, "Application of Solubility: Soap" - https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map:Organic_Chemistry(Smith)/Chapter_03:_Introduction_to_Organic_Molecules_and_Functional_Groups/3.6:_Application_of_Solubility:_Soap
• Chemistry LibreTexts, "14.2: Soap" - https://chem.libretexts.org/Courses/can/CHEM_232_-Organic_Chemistry_II(Puenzo)/14:Biomolecules-_Lipids/14.02:_Soap
• ScienceABC, "Do Soaps Really Break Down Viruses? If So, How?" - https://www.scienceabc.com/pure-sciences/do-soaps-really-break-down-viruses-if-so-how

Saponification, and the accidental discovery from ash and fat

• Wikipedia, "Soap" - https://en.wikipedia.org/wiki/Soap
• Poleview Group, "A Little History Of Soap: Who And When Invented Soap" - https://www.poleviewgroup.com/news/a-little-history-of-soap-who-and-when-invente-78069433.html

Babylon: the clay cylinders, and soap for cloth rather than skin

• American Cleaning Institute, "Soaps & Detergents History" - https://www.cleaninginstitute.org/understanding-products/why-clean/soaps-detergents-history
• The Archaeologist, "The Invention of Soap in Ancient Babylon" - https://www.thearchaeologist.org/blog/the-invention-of-soap-in-ancient-babylon
• Origin Trace, "The History of Soap: From Ancient Grease to Modern Chemistry" - https://origin-trace.com/article/history-of-soap/

Egypt: the Ebers Papyrus

• American Cleaning Institute, "Soaps & Detergents History" - https://www.cleaninginstitute.org/understanding-products/why-clean/soaps-detergents-history
• Chagrin Valley Soap & Salve, "The Origin and History of Soap" - https://www.chagrinvalleysoapandsalve.com/blogs/idas-soap-box-blog/the-origin-and-history-of-soap

Mount Sapo: the legend, and why it is not history

• Wikipedia, "Mount Sapo" - https://en.wikipedia.org/wiki/Mount_Sapo
• Stenders Cosmetics, "The history of soap" (on the absence of any Mount Sapo in the Roman world, and sapo as a Germanic borrowing cognate with sebum) - https://www.stenders-cosmetics.com/en/articles/soaps-history/
• Roger Pearse, "Galen on the origins of soap, and the perils of hearsay" - https://www.roger-pearse.com/weblog/2015/01/12/galen-on-the-origins-of-soap-and-the-perils-of-hearsay/

Rome: the strigil, and Pliny on the Gauls

• Wikipedia, "Soap" (the strigil; Pliny describing sapo as an invention of the Gauls, used on the hair) - https://en.wikipedia.org/wiki/Soap
• Wikipedia, "Mount Sapo" - https://en.wikipedia.org/wiki/Mount_Sapo

Aleppo, and the medieval Mediterranean trade

• Wikipedia, "Aleppo soap" (the cure in subterranean chambers, six months to a year) - https://en.wikipedia.org/wiki/Aleppo_soap
• Aleplaur, "The History of Soap - A Laurel Journey From/To Aleppo" (al-Razi's Kitab al-Asrar and early soap recipes) - https://aleplaur.com/en/the-history-of-soap-a-laurel-journey-from-to-aleppo/

Nicolas Leblanc and the making of soda ash from salt

• Science History Institute, "Making the Process" - https://www.sciencehistory.org/stories/magazine/making-the-process/
• Britannica, "Nicolas Leblanc" - https://www.britannica.com/biography/Nicolas-Leblanc
• Wikipedia, "Nicolas Leblanc" - https://en.wikipedia.org/wiki/Nicolas_Leblanc
• EPFL Graph Search, "Leblanc process" (the 1783 prize, the Saint-Denis plant, 320 tons a year, the prize money withheld, and the older alkali sources: barilla, kelp, natron) - https://graphsearch.epfl.ch/en/concept/598272
• EBSCO Research Starters, "Leblanc Develops Soda Production" - https://www.ebsco.com/research-starters/history/leblanc-develops-soda-production

The alkali industry, its waste, and the Alkali Act of 1863

• The Chemical Engineer, "Nicolas Leblanc - Revolutionary discoveries" (waste tonnages, the 95 per cent requirement, air pollution traded for water pollution, and the Solvay process superseding Leblanc) - https://www.thechemicalengineer.com/features/cewctw-nicolas-leblanc-revolutionary-discoveries/
• Wikipedia, "Alkali Act 1863" - https://en.wikipedia.org/wiki/Alkali_Act_1863
• RSC Environmental Chemistry Group, "Galligu: An environmental legacy of the Leblanc alkali industry, 1814-1920" - https://www.envchemgroup.com/galligu-an-environmental-legacy-of-the-leblanc-alkali-industry-1814-1920.html
• Taylor & Francis, "Entrepreneurial Ventures in Chemistry: The Muspratts of Liverpool, 1793-1934" - https://www.tandfonline.com/doi/full/10.1080/00026980.2016.1279411
• Biology LibreTexts, case study on the Leblanc works and the 1839 lawsuit - https://bio.libretexts.org/Courses/Coastline_College/ENVS_C100:Environmental_Science(Hoerer)/02:_Environmental_Chemistry/2.04:Key_Elements_of_Green_Chemistry(Lucia)/2.4.01:_Key_Elements_of_Green_Chemistry/2.4.1.03:_Case_Study

Oliver Wendell Holmes, 1843

• Embryo Project Encyclopedia, "The Contagiousness of Puerperal Fever (1843), by Oliver Wendell Holmes" - https://embryo.asu.edu/pages/contagiousness-puerperal-fever-1843-oliver-wendell-holmes
• PBS NewsHour, "In 1850, Ignaz Semmelweis saved lives with three words: wash your hands" - https://www.pbs.org/newshour/health/ignaz-semmelweis-doctor-prescribed-hand-washing
• American Journal of Public Health, "Oliver Wendell Holmes (1809-1894) and Ignaz Philipp Semmelweis (1818-1865): Preventing the Transmission of Puerperal Fever" - https://ajph.aphapublications.org/doi/full/10.2105/AJPH.2009.185363

Semmelweis, Vienna, and why soap was not enough

• Science History Institute, "Ignaz Semmelweis" (he tried soap, water and a nail brush first, then chlorinated water, then chlorinated lime) - https://www.sciencehistory.org/education/scientific-biographies/ignaz-semmelweis/
• WHO Guidelines on Hand Hygiene in Health Care, "Historical perspective on hand hygiene in health care" - https://www.ncbi.nlm.nih.gov/books/NBK144018/
• Wikipedia, "Contemporary reaction to Ignaz Semmelweis" (his own account that washing with soap alone did not remove the smell) - https://en.wikipedia.org/wiki/Contemporary_reaction_to_Ignaz_Semmelweis
• National Geographic, "'Wash your hands' was once controversial medical advice" - https://www.nationalgeographic.com/history/article/handwashing-once-controversial-medical-advice
• EBSCO Research Starters, "Semmelweis Develops Antiseptic Procedures" - https://www.ebsco.com/research-starters/history/semmelweis-develops-antiseptic-procedures

Note on the mortality figures: sources give the gap between the two Vienna clinics variously as sixteen per cent against two, sixteen against seven, and ten against four. Because they disagree, the narration says only that the doctors' clinic ran several times higher, and states that the exact figures are contested.

Synthetic detergents: the First World War, Nekal, Dreft, Tide

• Britannica, "Soap and detergent - Early synthetic detergents" (the German wartime shortage, the Nekal alkylnaphthalene sulfonates, good wetting agents but only fair detergents) - https://www.britannica.com/science/soap/Early-synthetic-detergents
• EurekAlert / American Chemical Society, "Development of Tide laundry detergent receives historical recognition" (Dreft in 1933, Tide in 1946) - https://www.eurekalert.org/news-releases/825748
• Wikipedia, "Dreft" - https://en.wikipedia.org/wiki/Dreft
• Springer, "Synthetic Detergents" - https://link.springer.com/chapter/10.1007/978-1-349-00015-9_48

Why detergents work in hard water: carboxylate versus sulfonate

• No Brain Too Small, NCEA Level 3 chemistry notes on soaps and detergents (calcium and magnesium salts of sulfonates are soluble; detergents also work in acid conditions where soap does not) - http://www.nobraintoosmall.co.nz/students/science/NCEA_Level3/L3_Organic/pdfs/AS90730_soapsnotes.pdf
• Biard & Crockett, "Why Hard Water Doesn't Get Along with Soap" - https://www.biardandcrockett.com/blog/why-hard-water-doesnt-get-along-with-soap/
• Chemistry LibreTexts, "14.2: Soap" - https://chem.libretexts.org/Courses/can/CHEM_232_-Organic_Chemistry_II(Puenzo)/14:Biomolecules-_Lipids/14.02:_Soap

The foam on the rivers, and the switch to straight-chain tails

• Maine Department of Environmental Protection, "Foam, Lakes" (foam in rivers, sewage works and taps; branched ABS replaced by linear LAS) - https://www.maine.gov/dep/water/lakes/foam.html
• Council for LAB/LAS Environmental Research, "Environmental and other benefits of replacing ABS with LAS" - https://www.cler.com/environmental-and-other-benefits-of-replacing-abs-with-las/
• EBSCO Research Starters, "Soaps And Detergents" (propylene-derived branched chains; sewage enzymes unable to break them down) - https://www.ebsco.com/research-starters/science/soaps-and-detergents

Note on the 1965 date: the completion of the United States changeover from branched to linear chains on 1 July 1965 is taken from a single encyclopaedic source and should be treated as the weaker figure in this section.

The Dove bar of 1957, and what is no longer soap

• Wikipedia, "Dove (Unilever brand)" (launched in the United States in 1957; formulated from synthetic detergents to avoid leaving a bathtub ring) - https://en.wikipedia.org/wiki/Dove_(Unilever_brand)
• Sharecare, "What are synthetic detergents?" (syndet bars and soap-free cleansers) - https://www.sharecare.com/health/skin-cleansing-soaps/what-are-synthetic-detergents
• Botanie Soap, "What Is Syndet Soap? Syndet Bars vs. Traditional Soap" - https://botaniesoap.com/blogs/misc/syndet-soap
• ScienceABC, "What's The Difference Between Various Types Of Soaps, Shampoo And Detergents?" (shampoo as a detergent rather than a true soap) - https://www.scienceabc.com/eyeopeners/whats-the-difference-between-various-types-of-soaps-shampoo-and-detergents

The greasy coats of germs

• Global Handwashing Partnership, "How Washing Hands with Soap Destroys the Coronavirus" - https://globalhandwashing.org/how-washing-hands-with-soap-destroys-the-coronavirus/
• Virology Down Under, "Why does soap work so well on SARS-CoV-2?" - https://virologydownunder.com/why-does-soap-work-so-well-on-sars-cov-2/
• Mittal, Ni & Seo, "The Flow Physics of COVID-19" (soap dismantling the lipid envelope, with the debris trapped in micelles) - https://arxiv.org/pdf/2004.09354

A signal is falling onto the Earth tonight from sixteen billion miles away, and this is the story of how it gets here.

Voyager was not sent to leave the solar system. It was sent to look at moons. The path it took past Titan, the one moon in the solar system wrapped in a thick atmosphere, threw it up and out of the plane of the planets and settled the whole rest of its life. Its original mission ended in 1980. It was given a second one in 1990, finished that too, and is now doing the part nobody planned.

Tonight: the transmitter no more powerful than the bulb in a refrigerator. The seventy-metre dishes chilled almost to the coldest temperature there is, so their own warmth does not drown what they are straining to hear. Why every message is aimed not at where the spacecraft is, but at where it will be tomorrow. The two degrees of misaiming that silenced Voyager 2 for two weeks, and the five months Voyager 1 spent transmitting nothing at all before someone fifteen billion miles away worked out how to move a piece of its memory.

Nothing to follow closely. Nothing to carry into the morning.

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Sources for this episode:

NASA / JPL — Voyager mission overview https://voyager.jpl.nasa.gov/mission/

NASA — Voyager 1 mission page (launch, Titan constraint, VIM start date, 67,000 images) https://science.nasa.gov/mission/voyager/voyager-1/

NASA / JPL — Planetary Voyage (why the Voyagers were funded for Jupiter and Saturn only, and how the Titan flyby bent Voyager 1 out of the ecliptic) https://voyager.jpl.nasa.gov/mission/science/planetary-voyage/

NASA — The Interstellar Mission (heliopause crossings, 2012 and 2018) https://science.nasa.gov/mission/voyager/interstellar-mission/

NASA / JPL — The Spacecraft (3.7-metre high-gain antenna, 16 bits/sec uplink, 160 bits/sec downlink, RTG power decline) https://voyager.jpl.nasa.gov/mission/spacecraft/

NASA — Voyager mission blog (instrument shutdowns, power management, LECP) https://science.nasa.gov/blogs/voyager/

NASA / JPL — Voyager 1 Resumes Sending Engineering Updates to Earth (the failed FDS memory chip and the code relocation) https://www.jpl.nasa.gov/news/nasas-voyager-1-resumes-sending-engineering-updates-to-earth/

NASA / JPL — Voyager 1 Returning Science Data From All Four Instruments https://www.jpl.nasa.gov/news/voyager-1-returning-science-data-from-all-four-instruments/

NASA / JPL — Mission Update: Voyager 2 Communications Pause (the two-degree misaiming and the "interstellar shout") https://www.jpl.nasa.gov/news/nasa-mission-update-voyager-2-communications-pause/

NASA / JPL — What is the Deep Space Network? https://deepspace.jpl.nasa.gov/about/

NASA / JPL — The 70-metre antennas http://deepspace.jpl.nasa.gov/about/DSNComplexes/70meter/

NASA — Six Deep Space Network Antennas in Madrid Arrayed for the First Time https://science.nasa.gov/photojournal/six-deep-space-network-antennas-in-madrid-arrayed-for-the-first-time/

NASA / JPL DESCANSO — Voyager Telecommunications (technical monograph) https://descanso.jpl.nasa.gov/monograph/series13/DeepCommo_Chapter3--141029.pdf

NASA Technical Reports Server — Deep space communications antennas (received power density at Saturn-encounter distances) https://ntrs.nasa.gov/citations/19830028033

CNN — Voyager 1 will reach one light-day from Earth in 2026 https://www.cnn.com/2025/12/09/science/voyager-1-light-day-earth

You cannot see inside a sewn bag, which meant that for several centuries there was a great deal of money to be made by lying about what went into a mattress. In 1495 Parliament made it a legal matter. The forbidden list grew over the next hundred and thirty years into something close to a witch's cauldron: thistledown, cattails, scalded feathers, fen down, sand, gravel, hatmakers' wool, and any other corrupt stuff. This episode is about the London guild that sent officers out to open the ticking and judge the contents by smell.

No plot, nothing to follow. Close your eyes.

Idle Wonders is a nonfiction sleep podcast. New episodes weekly.

Every land mammal pays for speed. A dog trotting burns more than a dog walking; a horse cantering burns more than a horse at a walk. The rule is so reliable it is very nearly a law of nature. The kangaroo ignores it. Past a certain pace, a red kangaroo hopping faster consumes almost no extra oxygen at all. This episode is about where that free speed comes from, which turns out to be tendons behaving like a pogo stick, and about kangaroo reproduction, which is stranger than the hopping.

No plot, nothing to follow. Close your eyes.

Idle Wonders is a nonfiction sleep podcast. New episodes weekly.

-----------------------------------

SOURCE LIST The name, and the "I don't understand you" myth

• Macquarie Dictionary, "The origin of kangaroo — getting to the bottom of an Australian furphy" — https://www.macquariedictionary.com.au/the-origin-of-kangaroo-getting-to-the-bottom-of-an-australian-furphy/ Primary source for the Phillip Parker King 1820 word list, the "edible animal" explanation, and the count of kangaroo terms in the language.
• Wordorigins.org, "kangaroo" — https://www.wordorigins.org/big-list-entries/kangaroo Traces how the confusion spread, including the point that Guugu Yimithirr was not spoken near Botany Bay.
• Wikipedia, Guugu Yimithirr language — https://en.wikipedia.org/wiki/Guugu_Yimithirr_language Spelling and meaning of gangurru.
• Languagehat, "Kangaroo" — https://languagehat.com/kangaroo/ John Haviland's fieldwork; the point that a different animal would have produced a different English word.
• Timothy Allsworth, Medium — https://timswww.medium.com/the-kangaroo-the-australian-symbol-tims-weird-wonderful-world-1aac02e12b04 Banks's July 1770 journal entry; the 1898 correction attempt.

The tail as a working limb (2014 force-plate study)

Underlying paper: O'Connor, Dawson, Kram and Donelan, "The kangaroo's tail propels and powers pentapedal locomotion," Biology Letters, 2014.

• Journal of Experimental Biology summary — https://journals.biologists.com/jeb/article/217/21/3770/12775/The-kangaroo-s-fifth-leg The comparison to one human leg's mechanical work.
• UNSW Newsroom — https://www.unsw.edu.au/newsroom/news/2014/07/kangaroos-use-tail-as-fifth-leg Mitochondria in tail and hind-leg muscle versus front-leg muscle; front legs acting as struts.
• National Geographic — https://www.nationalgeographic.com/travel/article/140701-kangaroo-animals-hopping-mammals-australia Propulsive force rivalling front and hind limbs combined.
• ScienceDaily — https://www.sciencedaily.com/releases/2014/07/140701193308.htm
• O&P EDGE — https://opedge.com/news_2014-07-07_05/ Measurements taken in Sydney in 2001, analysed years later.
• Notre Dame, Biomechanics in the Wild — https://sites.nd.edu/biomechanics-in-the-wild/2019/03/05/secret-behind-kangaroos-tail/ Source for the roughly one-tenth vertical-support figure.

Breathing coupled to hopping

• Advances in Physiology Education — https://journals.physiology.org/doi/full/10.1152/advan.00050.2010 The mechanism, and the claim that a kangaroo uses less energy breathing while hopping than at rest.
• ScienceBlogs, Lifelines — https://scienceblogs.com/lifelines/2011/03/15/kangaroo-breathing
• Particle (SciTech WA) — https://particle.scitech.org.au/earth-water/how-do-kangaroos-breathe-while-they-hop/
• Phys.org — https://phys.org/news/2018-06-kangaroos.html Both cover James Wong's imaging and modelling work at UWA; this is the basis for the script's caveat that the mechanism is not yet fully measured.

Extinct sthenurine kangaroos

• Janis, Buttrill and Figueirido, "Locomotion in Extinct Giant Kangaroos: Were Sthenurines Hop-Less Monsters?", PLOS ONE, 2014 — https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0109888
• Brown University news release — https://news.brown.edu/articles/2014/10/kangaroos The 140-species comparison; the point that today's large kangaroos are the specialists.
• Australian Geographic — https://www.australiangeographic.com.au/news/2014/10/giant-kangaroo-ancestors-walked-not-hopped/ Broad hips, torsion-resistant ankles, the 240 kg figure.
• Sci.News — https://www.sci.news/paleontology/science-extinct-giant-kangaroos-walked-instead-hopped-02209.html Janis's own statement that trackways would strengthen the case.
• Live Science — https://www.livescience.com/48304-hopless-extinct-kangaroos.html

Molar progression

• AskNature, "Teeth Replace Themselves" (quoting Attenborough, 1979) — https://asknature.org/strategy/teeth-replace-themselves/ Closed roots, four pairs per side, only the front pair engaging, last molars in use around fifteen to twenty years.
• AskNature, "Teeth Specialized to Diet" — https://asknature.org/strategy/teeth-specialized-to-diet/ Confirms only the first two molars are in occlusion at a time in grazing macropods.
• The Conversation — https://theconversation.com/kangaroo-teeth-grow-forever-and-keep-a-record-of-their-owners-age-and-sex-232377 Molar progression as an ageing method; the parallel with elephants.
• Monash University — https://www.monash.edu/science/news-events/news/2024/teeth-tell-tales-new-study-reveals-how-kangaroo-incisors-indicate-age-and-sex

The six-to-eight-year typical wild lifespan appears in several of the general references below.

Heat, water and forearm licking

• AskNature, "Foreleg Licking Cools Skin" — https://asknature.org/strategy/foreleg-licking-cools-skin-2/ The superficial vessel network; licking as a major route of evaporative heat loss.
• Australian Wildlife Conservancy — https://www.australianwildlife.org/news-and-resources/news/beating-the-heat-how-australian-animals-stay-cool-in-a-sunburnt-country Cooled blood circulating to lower core temperature; digging to reach cooler soil.
• Frontiers for Young Minds — https://kids.frontiersin.org/articles/10.3389/frym.2024.1333636 Thin forearm skin over a large venous network.
• San Diego Zoo Wildlife Alliance fact sheet (western grey kangaroo) — https://ielc.libguides.com/sdzg/factsheets/westerngraykangaroo/characteristics Source for the "once dismissed as primitive, now understood as optimising" framing.
• Active Wild — https://www.activewild.com/red-kangaroo-facts/ Concentrated urine, night grazing, shade-seeking.

Hopping energetics: the treadmill work and the flat oxygen curve

• Dawson and Taylor, "Energetic Cost of Locomotion in Kangaroos," Nature 246, 1973 — https://www.nature.com/articles/246313a0 Full text PDF: https://courses.grainger.illinois.edu/me481/sp2021/Dawson1973.pdf The original paper. Note that Dawson and Taylor themselves reach for the pogo-stick comparison in their opening lines, and that they counted stride frequencies separately across speeds.
• Kram and Dawson, "Energetics and biomechanics of locomotion by red kangaroos," 1998 — https://www.sciencedirect.com/science/article/abs/pii/S0305049198000224 Constant metabolic rate from about 2 to 6 metres per second. Also the direct source for two other claims in the script: that uphill hopping is much more expensive than level hopping, and that red kangaroos prefer slower speeds that avoid high tendon stress.
• Clemente et al., comparative review of macropod locomotion, Australian Journal of Zoology, 2022 — https://connectsci.au/zo/article/69/4/136/43549/Understanding-Australia-s-unique-hopping-species-a Useful overview. Also flags a limitation worth knowing: these oxygen measurements have all been taken on treadmills.

A nuance on the flat curve. The 1973 data actually showed oxygen consumption falling somewhat as speed rose; the 1998 study found it essentially constant. Reviews summarise the pair as "little to no increase."

The ankle posture mechanism

• Thornton et al., "Postural adaptations may contribute to the unique locomotor energetics seen in hopping kangaroos," eLife — https://elifesciences.org/articles/96437
• eLife press summary — https://elifesciences.org/for-the-press/4ffacae7/kangaroos-fix-their-posture-to-save-energy-at-high-hopping-speeds
• preLights commentary — https://prelights.biologists.com/highlights/unlocking-the-secrets-of-kangaroo-locomotor-energetics-postural-adaptations-underpin-increased-tendon-stress-in-hopping-kangaroos/

Kram and Dawson looked for exactly this effect in 1998 and did not find it. The newer modelling work does. So this is an active disagreement in the literature, not a settled result, which makes the script's "not yet completely understood" line accurate rather than merely modest. That phrase traces to Thornton et al. 2022 and is quoted approvingly in the newer papers.

The pouch, the birth, and the climb

• Antimicrobial Protection of Marsupial Pouch Young (review) — https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5339227/ Lysozyme and dermcidin secreted by pouch skin; the pouch carries a broad bacterial load; newborns arrive without differentiated lymphoid tissue.
• The marsupial pouch: implications for reproductive success — https://www.researchgate.net/publication/250927274_The_marsupial_pouch_implications_for_reproductive_success_and_mammalian_evolution Pouch humidity control, and the point that both chemical protection and humidity may come from secretions or from the mother licking the pouch.
• ScienceBlog, on newborn sensory equipment and the crawl — https://scienceblog.com/t-kangaroo-joey-newborn-pouch-crawl-forelimbs/ Red kangaroo neonate around 750 milligrams after a 33-day gestation; forelimb climbing movements begin before birth; orientation via gravity, smell, temperature and touch rather than sight or hearing.
• Biology Insights, joey development — https://biologyinsights.com/what-is-a-joey-kangaroo-from-birth-to-independence/ Jellybean comparison, roughly two centimetres, under one gram, teat swelling, pouch duration.

Embryonic diapause

• Renfree and Shaw, "The enigma of embryonic diapause," Development, 2017 — https://journals.biologists.com/dev/article/144/18/3199/47979/The-enigma-of-embryonic-diapause The prolactin mechanism: suckling keeps prolactin high, which inhibits the corpus luteum and holds progesterone low; losing the pouch young removes the inhibition and a progesterone pulse reactivates the blastocyst.
• Renfree and Shaw, "Diapause," Annual Review of Physiology, 2000 — https://www.annualreviews.org/doi/pdf/10.1146/annurev.physiol.62.1.353 Nearly a hundred mammals across seven orders; widespread in rodents, kangaroos and mustelids; the roe deer is the single representative among the even-toed ungulates.
• ANU, female marsupial reproductive cycles — http://artserve.anu.edu.au/raid1/student_projects/kangaroos/r-cycles.html The seventy-to-a-hundred-cell unilaminar blastocyst, about a quarter of a millimetre across.
• Kangaroos at Risk, reproductive biology — https://www.kangaroosatrisk.org/biology.html Same cell count. Also the caveat below about which species actually do this.

The two milks

• Nicholas, "Asynchronous dual lactation in a marsupial, the tammar wallaby," 1988 — https://pubmed.ncbi.nlm.nih.gov/2456758/ The foundational result: two different milks from adjacent glands at the same time, with evidence that the glands are controlled independently.
• Wanyonyi et al., extracellular matrix regulation of asynchronous concurrent lactation — https://www.sciencedirect.com/science/article/abs/pii/S0945053X1300019X Early and mid lactation produce small volumes rich in carbohydrate and low in protein and lipid; late lactation produces large volumes rich in protein and lipid.
• ScienceDirect topic overview, tammar wallaby lactation phases — https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/tammar-wallaby Phase durations, and the point that permanent teat attachment lasts roughly the first hundred days.
• Stannard et al., "Marsupial and monotreme milk: a review of its nutrient and immune properties," PeerJ, 2020 — https://pubmed.ncbi.nlm.nih.gov/32612884/

A hibernating animal is not having a long sleep. Its brain activity does not look like sleep at all, and there is good evidence it builds up a debt of ordinary sleep while hibernating and has to wake in midwinter to pay it off. This episode is about what is actually happening underground: the arctic ground squirrel whose body drops below the freezing point of water without freezing, the four hundred years it took to work out what brown fat was for, the gut bacteria that let a squirrel rebuild its own muscle out of its own waste, and the two thousand years during which educated people believed swallows spent the winter in the mud at the bottom of lakes.

No plot, nothing to follow. Close your eyes.

Idle Wonders is a nonfiction sleep podcast. New episodes weekly.