This being a record of a digression kicked off by the stray query (snapped above) arising at the end of reference A1, aka B1. I thought that Gemini’s claim that foraging pigs could spit out the husks of the acorns that they were eating – partly because they were fibrous and indigestible, partly because they were full of tannins – was something that it should be possible to check, with the references he had provided as a starting point.
With such checking being an important part of building confidence in the information supplied by Google’s AI product Gemini.
A matter I had thought might keep me busy for the rest of the day. In the event, what with mission creep and one thing and another, more like a month.
The bottom line is that Gemini got this one right, albeit resting on a source I did not care to rest on.
There have been off-post digressions as well as in-post digressions. These are listed below. On the blog they are marked with the group search key ‘acornsk’.
Complete with some of Microsoft’s sometimes irritating blue underlines. Pushing us into some homogenous, mid-Atlantic version of English.
I started off with the pigs of the New Forest, eventually getting to what I think was Gemini’s source, reference B13, well written and nicely illustrated, with the relevant bit being snapped above. The Elaine Gill mentioned looks to be a respectable writer about horses. But this large website as a whole had no pedigree, an absence which was matter for reference A6.
I was not satisfied.
Then came the digressions, and after that I returned to the business of pannage, particularly as it is practised in the Iberian Dehesa (of reference B3) now, where it is a serious business, which the snap above and reference B14 suggest might be some hundreds of thousands of animals, rather than the hundreds of the New Forest.
Then came teeth, jaws and mastication, on which there was as a surprisingly large literature, some of it, for example, going deeply into the details, the dynamics of jaw movements.
I then I hit the bull’s eye at reference B11 and then reference B12 – actually paying for this last on the grounds that it was the finishing touch to the present endeavour – into which I had put a lot of time and effort. Reference 2009c in the long list from reference B11 above. A lot of this material is in English, but when you dig deeper, or further back in time, there is a lot more Spanish – which I do not do. But lots of provenance – much better in that regard than the material I have turned up on the New Forest pigs. But then, Iberian pigs is a much bigger business than New Forest pigs, with these last being more a curiosity than a business.
Reference B12 describes an experiment to measure the amount of waste involved in the husking of acorns by Iberian pigs, with the waste rates derived then feeding into calculations of the rate of conversion of acorn kernel into meat. The sort of calculations that livestock scientists are keen on.
While reference B11 claims that these Iberian pigs are the only pigs that can do this. Perhaps I should put them in touch with the New Forest people.
Weights of the parts of the experimental acorns are summarised in the snap above. To be compared with the 5g/acorn from a different – but unknown species of oak – that I recorded here at Epsom.
I am now satisfied that some pigs at least are capable of husking acorns before they eat them.
Oddly, there does not seem to be a clear relationship between the taste of the acorn – roughly correlating with the amount of tannin – and the amount of waste. On the other hand, there seems to be a stronger relationship between the taste of the acorn and the selection of trees under which to graze. Matters which I have not gone into.
Nor have I gone into the regression offered.
But I have read about commercial pressures which are gradually eating into traditional pig finishing (not rearing) in the Dehesa. Traditional might produce good ham and sausage, but it is also expensive compared to intensive finishing. Spanish consumers – like English consumers – are sensitive to price and there are clearly limits to what they will pay for a quality product: something packaged up to look like the quality product will often do.
Major digressions
Pannage, past and present: particularly the New Forest, Sardinia and the Iberian peninsular
The diet of pigs generally; extensive and – much more often – otherwise
Ruminants (for example, sheep, goats and cows) and non-ruminants (for example, pigs and horses)
Nuts: acorns, beech nuts, sweet chestnuts, horse chestnuts – and so on
Tannins: what they are and what they do; content and variation
Plant protection more generally
The place of oaks more generally
These are covered, in turn, in what follows.
Pannage
Pannage of pigs – more or less free-range foraging in open woodlands, often for both acorns and grass, perhaps also for roots – has been going on for a long time – with there being some evidence of it in Homer, say around 700BCE. It was well established in Kent at the time of the Norman Conquest and it still goes on in the New Forest, where it is just a relic of what went on in the Middle Ages. I have also read about wild pigs in Sardinia and domestic pigs in the southwest of the Iberian peninsula – the Dehesa of reference B3 – with Iberian pork being both highly valued and carefully protected, rather in the way of AOC wine in France.
You get pigs more or less all over the non-Muslim part of the world, and I dare say there is plenty of pannage outside of western Europe which I have not connected to. The snap above is taken from UN’s FAO.
Pannage appears to be a supplement, a way of finishing off pigs for slaughter, rather than a whole life activity. At reference B8 and elsewhere there is talk of the better flavour of acorn finished pork. But it is an expensive way to finish pigs compared, for example, to fattening them up in large sheds using feed from a factory. Or even compared with the pig sties of old.
Pigs were run in forests of the Kentish Weald from Roman times through to the eighteenth century and the Jutish forest was the subject of the book at reference B7. Part of this was landowners charging pig farmers for the use of their woodland pastures. And kings taxing the landowners for owning the pastures, with records of same to be found in the Domesday Book. However, the pigs were gradually pushed out by the adoption of more intensive, more productive ways of farming – but they have left their mark in settlement patterns and in many place names. Subjects about which the author of this book clearly knew a great deal – but he was possibly not that interested in the pigs themselves.
Pigs are run in the New Forest now, but one wonders if this is not more about maintaining the New Forest as a holiday destination than about pigs. For example, one of the reasons for running pigs is the need to manage down the numbers of acorns left lying about so as to reduce the risk of grass-starved but charismatic ponies eating too many of them.
Pannage past and present in the UK and Europe more generally is the subject of reference B8, the author of which has now moved onto poultry at Premier Nutrition: ‘Alex joined Premier Nutrition in December 2024, following a significant career in research and driving scientific direction in animal nutrition. Her extensive knowledge in emulsifiers, phytase and other feed additives that help to optimise diet formulations and production efficiencies whilst minimising environmental impact will help to support poultry customers’.
The diet of pigs generally
Pigs are omnivores and one of the reasons for their success is their ability to cope with a wide range of foods.
Until the fairly recent past, many country people here in the UK kept the odd pig for Christmas, feeding them mainly on scraps from the kitchen. A practise which, when extended to restaurants, cafeterias and canteens, gave us the term ‘pig swill’, followed by the derogatory ‘swill’, with its suggestion that pigs will eat anything. A practise which I think has been largely killed off by hygiene regulations – tainted, undercooked pork having once been the source of a variety of unpleasant parasites.
Earlier still, lots of people kept pigs in towns, presumably mainly feeding them on various kinds of left-overs, but this did cause problems (touched at the end of reference A7) and eventually they were squeezed out. To the point now where the vast majority of pigs are reared intensively in large sheds.
I then read that in western Canada, pigs are raised mainly on the products and by-products – for example, barley, wheat, canola meal (a by-product of rape seed oil extraction) and peas – of the large arable farms there. But pigs (as non-ruminants) are not keen on feeds with a high fibre content, such as whole oats.
There are lots of pigs and it is not surprising that there are lots of big companies offering pig feeds, which might either be delivered in bags or pumped into bins and silos. Reference 16 looks to be a typical website – but is not very forthcoming about what these feeds are made of – beyond the feed including stuff for animal health and behaviour, as well as for nutrition. Reference 17 is rather more informative, but has taken the trouble to copy protect that information. Not proof against the snipping tool though, so some snippets follow.
First selection.
Second selection. So mainly grain of various sorts, but also fish, meat, beans and dairy. Plus what are coyly called ‘non-nutritive additives’.
There are also providers catering to the organic farmer, but I have not looked into those.
While in Sardinia, a team analysed the contents of the stomachs of near 100 wild boars, hunted down during the acorn season, with some description of same to be found at reference A7. Its seems that while these boars were majoring on acorns, they were taking a variety of other stuff, animal and vegetable, including a lot of berries and a fair amount of sheep. I have read of pigs being opportunistic feeders and that certainly seemed to fit here.
Ruminants
Ruminants are large herbivores with multiple stomachs, notably deer, cows, sheep and goats. Also giraffes. But not horses or pigs. Or humans.
The first stomach, the rumen, is a large chamber in which food is fermented by bacterial action. Part way through this process the food – the cud – is taken back into the mouth and chewed again to stimulate further fermentation.
Fermentation includes breaking down the cellulose which is a major constituent of grass – important forage for many ruminants, in particular cows and sheep. Non-ruminants such as horses can deal with grass, but they are not so good at it – to the point where dogs, deer, rabbits, and foxes will eat their faeces – at some risk of also ingesting toxic micro-organisms of one sort or another.
The tannins in acorns disturb this fermentation, disturbance which might have good or bad effects on the animal concerned, depending.
The diet of Iberian pigs at pannage is mainly acorns plus grass with reference B11 telling us that: ‘… This means a daily intake of 1251 to 1469 acorns or 7.13 to 8.37 kg of whole acorn and 2 to 2.7 kg of grass, during 6.1 to 7.1 foraging hours…’.
Nuts
An achene is a small, dry, single-seeded, indehiscent fruit, meaning the fruit – the ripe ovary of a flowering plant – does not open to release its seed. Many of them have wings, in which case the ensemble is called a samara.
Nuts are large, dry, single-seeded, indehiscent fruits with hard shells. There are lots of them and many of them are eaten by humans. A lot of them by small vertebrates and some of them by other large omnivores and herbivores. Not many of these last eat nuts much at all. Some of these nuts are toxic in whole or in part
The incomplete and very selective table above gives something of the idea.
As far as I can make out, the only nuts eaten in any quantity by extensively reared pigs are acorns. Acorns are toxic if consumed raw in large enough quantities and are regularly responsible for the deaths of small numbers of free-grazing horses and cows.
Their name suggests that pignuts, which are nut-like while not actually being nuts at all, are eaten by foraging pigs and Caliban mentions them in Act II, Scene II of the ‘Tempest’, but I have not come across them in the present context, which is odd. Perhaps there are not enough of them about.
I associate to a childhood habit of cracking hazel nut shells with my teeth, a practise of which my dentist father strongly disapproved, concerned that I would break teeth. And I remember him explaining, in another context, that it was surprisingly easy to split teeth, rather in the way of a fresh log with an axe. Presumably teeth do break and omnivorous animals like pigs need to have a care.
Horse chestnuts are member of the small genus Aesculus, the plants and seeds of all of which are moderately toxic. There is a suggestion that Jōmon people of Japan used to eat horse chestnuts after attempting to leach out the toxins. That squirrels collect them but do not necessarily eat them and that deer, for some reason, can eat them without ill-effect. Matters on which the AI supplements to search can mislead and which I have failed to bottom out.
Chemical readers might care to look over reference 2. Too much for me.
Tannins
Tannins are to be found in lots of plants and have been used in tanning leather for a long time. They are defined chemically by their ability to bind to proteins and are of present interest because they are to be found in acorns and they interact with the digestive systems of pigs. Tannins in acorns are regularly responsible for the deaths of foraging horses and cows, although the numbers involved are quite small. Contrariwise, tannins also have medicinal uses and they are sometimes added to animal feeds. Much work has been done on them.
Abbreviations and words that crop up in the world of tannins include HT, CT, gall (for which see reference A2), tea, astringent (for which last two see among the minor digressions below), polyphenol and antioxidant. These last are very much to the fore in the places like Holland & Barrett. And some people see life as an ultimately losing battle between the forces of the oxidants and the forces of the antioxidants. In which connection, I was unable to source a free copy of reference B4, beyond the abstract below.
Autoxidation limits the longevity of essentially all hydrocarbons and materials made therefrom – including us. The radical chain reaction responsible often leads to complex mixtures of hydroperoxides, alkyl peroxides, alcohols, carbonyls and carboxylic acids, which change the physical properties of the material – be it a lubricating oil or biological membrane. Autoxidation is inhibited by additives such as radical-trapping antioxidants, which intervene directly in the chain reaction. Herein we review the most salient features of autoxidation and its inhibition, emphasizing concepts and mechanistic considerations important in understanding this chemistry across the wide range of contexts in which it is relevant.
Hydrolysable tannins (HT) are hydrolysed by weak acids or weak bases to produce carbohydrate and phenolic acids. With hydrochloric or sulfuric acids to produce gallic or ellagic acids. Condensed tannins (CT) are polymers formed by the condensation of flavans. They do not contain sugar residues. These are the two kinds of tannins which are important here.
Tannins are part of a complex, dynamic world and the tannin content of parts of plants varies a good deal: from species to species, from part to part, from plant to plant. According to position on the plant, the time of year and the time of day.
Roughly speaking, cows seem to be most vulnerable to tannin poisoning. Horses in-between and pigs least vulnerable.
It seems likely that one of the main functions of tannins in plants is to deter – or at least manage – predation by herbivores. The catch being that some of these last get a taste for them.
Reference B15 is an accessible source of further information. Starting off with the corrective snapped above. Mueller-Harvey goes on to explain that despite its bad reputation, there are plenty of good things about tannins and they can be useful as part of a regular feed. But a lot depends on the animal-plant pair concerned and the dose.
Plant protection
Most of what follows is taken from reference 5, produced under the auspices of what was the Overseas Development Administration (ODA). About herbivores (like cows) and omnivores (like pigs and ourselves) rather than anything smaller – where there is, nevertheless, plenty of action.
Plants need to protect themselves – particularly, their buds, leaves, young shoots, fruits and seeds – against attack by herbivores and others, while at the same time facilitating the interactions with same which support their reproduction and proliferation. Given that plants cannot move about much, they make more use of chemical defences than animals.
That said, plants can make predation more difficult by being out of reach, by having thorns and by being tough or otherwise indigestible.
Chemical defences range from mild toxins to fast acting poisons. Predators have some reciprocal defence in that many of these taste unpleasant, typically bitter. Some of these follow.
Phenols, including the tannins.
Toxic amino acids, including mimosine and canavanine. Mimosine, for example, interferes with cell division and can cause problems with livestock, particularly if plants like Mimosa pudica or Leucaena leucocephala (see below) are consumed in large quantities.
Cyanogenic glycosides, in particular hydrogen cyanide (HCN). HCN works by inhibition of cellular respiration and the anoxia that follows. The primary targets of cyanide toxicity in humans and animals are the cardiovascular, respiratory, and central nervous systems. The body, that is to say the liver, can deal with quite large amounts of HCN and to be fatal a large shot of it is needed. I read that 200 mg/m3 is likely fatal after 10 min, and 300 mg/m3 is immediately fatal, where I take the levels to be per cubic metre of the ambient air. This fatal level is maybe 10 times higher than that was sometimes found in the vicinity of cassava factories in Nigeria – cassava being a much consumed source of cyanide.
Alkaloids, among which we have quinine, strychnine, atropine, caffeine, nicotine and cocaine.
The wider role of oaks
What follows is taken from reference 6, another booklet produced under the auspices of the ODA.
What was newest to me in this was the importance of tree forage in dry places where the ground fodder – for example grasses – may be thin or absent.
I would only add that, as of 1993, this author wondered about the role of fungal contamination of acorns in their toxicity. Contamination which would, presumably be largely eliminated by washing and husking prior to grinding them into animal feed n factories.
Minor digressions
This being a note of various things that cropped up, things which caught my attention, but which were not necessarily very relevant to the matter in hand. In no particular order.
Peanut skins in feed. In the US at least, they get through enough processed peanuts for it to be worthwhile to use the nutrient-rich skins in animal feeds.
Machines for husking acorns. Acorns are sometimes used in animal feeds, for which purpose they are husked and ground. There are plenty of machines on offer to do the husking.
Sawdust in feed. Coarse sawdust is sometimes used in animal feeds.
Rooting. Pigs like to root around in the ground with their tough and muscular snouts, tusks if available, looking for roots, small animals and other stuff that they can eat. Depending on what else the land in question is used for, this can be a nuisance and in the New Forest pigs have to have nose rings fitted which slow this down. Reference 8 talks of nose-ringing too
Browsing in apple orchards. I think pigs were, perhaps are, run in apple orchards where there are fallers which are not fit for sale – but which the pigs will hoover up.
The workings of toxalbumin. Toxins manufactured by many plants which interfere with the basic cell metabolism of browsers. One molecule is enough to kill a cell, so you don’t need much of the stuff to do serious organ damage – or perhaps kill – providing they can break through the various barriers on the way to those organs from the alimentary tract. A saving grace being that they cannot multiply in the way of microbes.
Leucaena leucocephala. A small, fast growing tree, widely used around hot parts of the world as a fodder crop. One of the advantage of trees being their greater tolerance of water stress than the much smaller, herbaceous fodder crops. All the thing in the 1970s and 1980s.
Giraffes and knob trees. The (ruminant) giraffes of the Kruger National Park eat a lot of leaves and twigs from the knob tree, Acacia nigrescens, browse which contains plenty of tannins. I learn that tannin content can vary a good deal, even on an individual tree. For example, buds and young leaves contain more tannins than old leaves. Content can double between day and night. There is also a tricky interaction between browsing activity by giraffes and tannin content.
Psyllids. An ancient and rather primitive family of bugs, many of which specialise in eating (the sap of) one particular plant, mostly dicots, some of which are vectors for important crop pathogens. A serious pest for Leucaena leucocephala farmers in some parts of the world.
Holly oaks aka holm oaks. Large evergreen oaks, native to the Mediterranean region, important in the world of the pannage pig, for example in Sardinia and in the southwest of the Iberian peninsular. There is a very large one in Windsor Great Park. .
Classification of oaks. Quercus in a genus of between 400 and 500 species, in two sub-genii, Cerris and Quercus. The former contains three sections of which ilex is one, the latter five sections. The section ilex contains around 40 species – including the holm oak, Quercus ilex. There is something of a divide between Old World oaks and New World oaks, which survives into the current classification scheme. In addition to the four to five hundred species there are three to four hundred synonyms, relics of past taxonomic endeavours.
Tea contains plenty of tannin, as well as caffeine. Sometimes it makes me sleepy in the afternoon, rather than giving me a boost, as it does first thing in the morning. Maybe this is something to do with the tannins in the tea doing something in the afternoon – after a possibly substantial midday meal – which they do not do before breakfast.
Astringent, an old word originally meaning the ability to draw together soft organic tissues. Astringents used internally to reduce discharge of blood serum and mucous secretions, which can come from a sore throat, haemorrhages, diarrhoea, and peptic ulcers. Used externally to dry up various kinds of sores. Tannins are well known for their astringent qualities, with OED mentioning the root of the sea lavender in this connection, now known to be a good source of tannin. A word used more often in a figurative sense nowadays.
Quebracho. A common name used in south America for a number of trees distinguished by their very hard woods, with the name meaning ‘axe breaker’. An example of a old common name spanning a number of new botanical divisions. Arises in this context as tannins can be extracted from the heartwood of red quebracho (Schinopsis lorentzii) and white quebracho (Aspidosperma quebracho-blanco).
The pig cycle. At the time I was a student, something called the pig cycle featured in the teaching of economics. It was offered as an example of the way that perfect markets generate perfect sine curves. One of many hazards that the pig farmer had to contend with. See also cobweb theorem. See also commodities markets.
Conclusions
Gemini’s observation that pigs spit out the husks of acorns when eating them, was true to the extent that Dehesa pigs do this and New Forest pigs likely do this. I think he based the observation on a nature website which looks OK but which is short on provenance. The Spanish material, however, is much better in that regard.
It is also the case that he came up with this observation in seconds and it took me hours, if not days, to check it, even allowing for the various digressions. So, on the face of it, a considerable saving in labour.
But difficulties remain. One learns the fact ‘pigs can spit out the husks of acorns when eating them’, without necessarily having any background in either pigs or acorns. Knowing, for example, that while some pigs might eat acorns some of the time, most pigs do not eat them at all. It might be rather as if one knew that Dakar was the capital of Senegal without having a clue about either place - beyond knowing that they are places. Whether or not this matters depends on the use to which you intend to put this fact, the weight that you are going to put onto it.
This fact would be unlikely to be quite so isolated if you had learned it in the old-fashioned way. Which might, at least once upon a time, have involved going to visit some pigs or talking with a few pig keepers. Bearing in mind that this ‘old-fashioned way’ may never actually have existed, at least for most people, as before the advent of the Internet, very few people would have been able to find out about such a thing at all.
Then there is a degree of circularity in the foregoing in the sense that in my quest to validate something from Gemini – the husking of acorns by pigs – I have made extensive use of Bing, his Copilot assistant, Google, his AI assistant and of Gemini itself. But short of systematic and directed failure of these products, which seems very improbable, I do not think this disturbs the mainly positive conclusion.
I have been very struck by the importance of provenance in all this. Where has the information dug up by Google or Bing come from? Who wrote and where do they come from? Who is vouching for the quality of their work? With some of this being conspicuously absent from some of the superficially helpful information about pannage and pigs on offer.
I dare say plenty of academics and others worry about the erosion of the old system of stuffy academic journals with standards and peer review by the Internet. Problems which the likes of Google have been grappling with since they moved away from simply scoring a webpage for the purposes of search by its content, from taking it at face value. For myself, just the sort of thing that I think it worth paying the Financial Times for. Let’s hope that they are not abusing my trust.
I dare say also that this is a bigger problem in fields like cooking and countryside which attract plenty of amateurs – like myself.
Work in progress.
References B
Reference 1: https://psmv5.blogspot.com/2025/10/long-grove.html. The start point of the present excursion, in the middle of October. The start of what was intended to be a day trip to check up on Gemini.
Reference 2: Phenolics and polyphenolics in foods, beverages and spices: Antioxidant activity and health effects – A review – Fereidoon Shahidi , Priyatharini Ambigaipalan – 2021.
Reference 3: https://en.wikipedia.org/wiki/Dehesa.
Reference 4: Autoxidation vs. Antioxidants – the fight for forever – Helberg, Julian; Pratt, Derek A – 2021.
Reference 5: The use of trees by Livestock 4: Anti-nutritive factors – R T Paterson, Natural Resources Institute – 1993.
Reference 6: The use of trees by livestock 5: Quercus - Paterson R T, Natural Resources Institute - 1993.
Reference 7: The Jutish forest : a study of the Weald of Kent from 450 to 1380 A.D. - Witney, K. P - 1976.
Reference 8: Such as pigs eat: the rise and fall of the pannage pig in the UK – Alexandra L Wealleans – 2014.
Reference 9: not used.
Reference 10: https://psmv5.blogspot.com/2025/07/ham-sandwiches.html. Probably the Iberian pork in question.
Reference 11: Consumption of Acorns by Finishing Iberian Pigs and Their Function in the Conservation of the Dehesa Agroecosystem – Vicente Rodríguez-Estévez, Manuel Sánchez-Rodríguez, Cristina Arce, Antón R. García, José M. Perea and A. Gustavo Gómez-Castro – 2012.
Reference 12: Intrinsic factors of acorns that influence the efficiency of their consumption by Iberian pigs – Rodríguez-Estévez, A. García, A.G. Gómez-Castro – 2009.
Reference 13: https://www.wildlifeonline.me.uk/blog/post/pigging-out-in-the-forest-the-common-of-mast-in-britain.
Reference 14: https://dehesa-extremadura.com/en/la-dehesa/. More about the commercial side of things.
Reference 15: Unravelling the conundrum of tannins in animal nutrition and health – Irene Mueller-Harvey – 2007.
Reference 16: https://www.forfarmers.co.uk/pig.
Reference 17: https://www.pelletizermill.com/blog/pig-feed-formulation-and-ingredients/.
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