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Medieval bestiaries describe a creature, a type of desert-dwelling ant that digs for gold. It was also said to be the size of a fox, but I'll ignore that in this question.

Why would ants unearth pieces of gold? What evolutionary purpose would this behavior serve?

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    $\begingroup$ I'll not find it quickly but one of Orson Scott Card's spin-off short stories from the Ender World universe is describing gold-digging insects (bugs, these were not ants). They were though artificially altered (normally they were digging and "digesting" some other mineral that was becoming their shell material). $\endgroup$ – Ister Feb 25 at 8:57
  • $\begingroup$ A gold-digging ant? Well, I suppose for the same reason anyone else does - to obtain access to money and social standing to which they weren't born through a monogamous (if relatively temporary) relationship, but without the negative consequences and social approbation inherent in direct prostitution. See related terms such as "kept ant", "trophy hive", etc, etc. $\endgroup$ – Bob Jarvis Feb 27 at 3:33
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    $\begingroup$ I was working for a company (I don't want to name them as there is no publicly available source) - they designed and manufactured ESP-s. In the early product versions the housing was pre-mold (there were small openings on it). They got back some defect devices from the field in South America, the problem was, that some gold bonds (wires) were missing. They could not find the reason, until they found a dead ant inside one of the returned devices. Apparently some small south american ants are able to eat gold wires... $\endgroup$ – G. B. Feb 27 at 6:12

13 Answers 13

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Why would ants unearth pieces of gold? What evolutionary purpose would this behavior serve?

It makes their chimneys work better.

The workers gather the gold and bring it to the colony. The soldiers use their large powerfull mandibles to shape the soft gold into roughly flat flakes which they distribute around the nest site.

The gold's reflection of the sun's light and heat in an upwards direction has two effects:

  • It heats the chimneys, encouraging convection and allowing air circulation around the ant's farmed food source - the fungus below.

enter image description here

Attribution BBC 2019

  • It reflects the heat off the surrounding ground, allowing it to be cool - enabling the underground tunnels used for farming the food fungus to expand and grow and thus the colony to be bigger and more succesfull.

enter image description here

Attribution: Darwin's Toolkit by UW–Madison CALS 2019

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    $\begingroup$ +1 For not derailing the question by assuming it wasn't actually ants. It's also a good answer. $\endgroup$ – Cyn Feb 24 at 21:19
  • $\begingroup$ @Cyn Thanks, was tempted to synonymise "ant" with "Umpa-Lumpa" or "mine worker", but there's lithium in the water here. $\endgroup$ – Confounded by beige fish. Feb 24 at 21:24
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    $\begingroup$ fossil hunters actually check ant mounds for fossil teeth because ants prefer he hardest material they can find to make their mount tops out of, in many places that equals fossilized teeth. so it is definitely possible for them to at least use gold ore for better conductive ability in cooling tunnels. $\endgroup$ – John Feb 25 at 1:32
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    $\begingroup$ Also, as gold is one of the most malleable metals, if any type of small animal wanted to build something out of metal by chewing it, it would be probably gold. Or lead, but lead would poison them, so they stick to gold. $\endgroup$ – vsz Feb 25 at 6:45
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    $\begingroup$ The problem with this is that the ants might use any other material that has similar heat/light reflection properties instead, if it is more plentiful than gold. $\endgroup$ – cowlinator Feb 27 at 1:19
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They don't value the Gold - They expel it from their burrows

What use is a shiny metal to an ant? They don't make artefacts or have currency. They don't have an aesthetic sense apart from food.

The ants line their burrows with a sticky substance they produce from their rear ends. Obviously a lump of gold is an obstruction to the building of their underground kingdoms.

They discard these annoying lumps of useless metal by bringing them to the surface and abandoning them there. If humans remove this refuse then so much the better.

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    $\begingroup$ Boom. Might not require them to seek out and dig up gold, but it’s what an ant would likely do, especially if the gold doesn’t react with something the ants need it to react to. $\endgroup$ – Joe Bloggs Feb 24 at 20:26
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    $\begingroup$ But then wouldn't the ant expel all obstructive substances such as metals / rocks / ores etc. therefore not really sorting out the gold. $\endgroup$ – colmde Feb 25 at 10:16
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    $\begingroup$ @colmde - That's a fair point. The thing is that it is humans that notice the phenomenon. Ants in non-gold-bearing areas won't be noticed by people. Only the ants that fortuitously live in gold-rich deserts will become a source of human fascination. $\endgroup$ – chasly from UK Feb 25 at 10:28
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Electrical conduction

Gold is a good conductor. Perhaps these ants use electrical pulses (rather than pheromones) to communicate in their nests. So they move the gold so that there are paths wherever they need them. They might also evolve special "router" ants that serve as junctions between paths. So the central pheromone unit (CPU) can send pulses that only reach specific sets of ants. Each ant could be identified by an instead of pheromone (IP) number.

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    $\begingroup$ Some types of ants seem to be attracted to electricity. $\endgroup$ – jamesdlin Feb 25 at 2:18
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    $\begingroup$ Gold is usually preferred over other materials because it's very inert - it won't rust or corrode. So a moist, acidic, or otherwise chemically reactive environment would help to justify the ants' preference. And acid batteries would make excellent power sources! Central pheromone unit! Pure poetry. What if... the circuitry forms a complex computer system, and the colony is actually run by a sentient AI?! Then AI would stand for Ant Intelligence. I will see myself out. $\endgroup$ – BoomChuck Feb 25 at 4:36
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The ants are giant packrats!

The gold digging ants are not typical ants, http://blogs.getty.edu/iris/the-monstrous-ant-of-the-medieval-bestiary/

There are also ants that, according to some bestiaries, live in Ethiopia or India, are the size of dogs, and dig up gold from sand, guard it, and pursue anything that tries to steal it, especially greedy humans. Artists depicted these ants not as larger versions of the familiar-looking insects, but more like actual dogs.

enter image description here

http://www.terrierman.com/goldenant.htm

The mountain ant

In ancient Persian the word for marmot was "mountain ant". And the mountain ants do indeed dig up gold on occasion.

https://www.nytimes.com/1996/11/25/world/himalayas-offer-clue-to-legend-of-gold-digging-ants.html

Now a team of explorers says it has solved the puzzle. The explorers believe they have pinpointed the land of the legendary gold-digging ants and the people who profited in one of the most inaccessible regions of the Himalayas along the upper Indus River.

They say the outsize furry ''ants,'' first described by Herodotus in the fifth century B.C., are in fact big marmots. These creatures -- Herodotus calls them ''bigger than a fox, though not so big as a dog'' -- are still throwing up gold-bearing soil from deep underground as they dig their burrows. Most important, the explorers say they have found indigenous people on the same high plateau who say that for generations they have collected gold dust from the marmots' work.

Here is a colony of these big marmots.

marmots https://www.kathmanduandbeyond.com/feeding-himalayan-marmots-ladakh-india/

But these marmots don't care about the gold. They just bring it up. Why would a rodent care about shiny gold nuggets enough to hoard them and even fight for them? There is one rodent that does. Packrats!

https://nuggetshooter.blog/2018/01/31/pack-rat-gold/

That morning as the horse and mule were loaded Tucker noticed a hole in his saddlebag with gold dust spilling from it and cussing that rat for chewing into the bag he began unpacking to save his precious gold, but it was gone…Only the fine gold that hadn’t spilled from the pouch into the saddlebag after being chewed apart by the packrat remained. All of the nuggets were gone as well as his pocket watch, and other objects small enough for the critter to haul off.

So: your "ants" are colony dwelling, burrowing rodents with a packrat-like habit of hoarding neat things, especially shiny things. In the above linked article, the prospector who lost his gold to the rat spent several days digging into burrows trying to find the one where his gold was, without luck. The article concludes by speculating that in an area naturally rich in gold nuggets, resident packrats might accumulate nuggets over time, giving rise to nugget troves in ancient rodent dens.

I am not sure a marmot would charge a guy with a sword like the ones above are doing but I would not want to test them. They are pretty big one at a time, marmots.


@chasly from UK pointed out that this answer does not cover the evolutionary angle; true. And a downvote for this lack - horrors! I was unable to find any thing written about why packrats might favor shiny things for their nests. I have taken this speculation on myself.

  1. Packrats like to incorporate shiny things into their nests - this is well known.

  2. Packrats like to use old nests if they can find them. It makes sense - if a nest has long been inhabited that means it is a good place for packrats. Plus older nests are more substantial, in part because of...

  3. Amberrat.

https://www.flickr.com/photos/centralaustralia/8419933870 amberrat Rats and especially packrats urinate onto their nests. Over time the accumulated urine can crystallize, cementing the nest material together into shiny masses called amberrat. These cemented nests are durable and of use to paleontologists, as the preserved materials record eras past.

Thus we have selective advantage for rats that like old nests, and old rat nests tend to be shiny. This could mean a selective advantage for rats who like shiny nests. Such rats will make their nests shiny if given materials to do so because they like shiny nests. Gold nuggets are definitely shiny. Thus - selective advantage for "ants" that seek out gold and incorporate it into their nest.

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  • $\begingroup$ This is one most excellent piece of research! $\endgroup$ – AlexP Feb 24 at 20:46
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    $\begingroup$ Excellent and fascinating piece of research! It gives a possible origin of the myth but doesn't actually answer the question as asked though. $\endgroup$ – chasly from UK Feb 24 at 20:58
  • $\begingroup$ I love the photo of the fat-ass marmots! Heh! $\endgroup$ – Fattie Feb 25 at 14:37
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    $\begingroup$ This is a good answer but OP specifically said "It was also said to be the size of a fox, but I'll ignore that in this question." $\endgroup$ – Captain Man Feb 26 at 17:46
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I cant find a good source, but gold found in streams are supposed to be created by bacteria that are allergic to gold. They use mechanisms to make the water stop retaining the gold so it drops to the floor of the riverbed.

Your ants have larvae that are allergic to gold. They might not die from it, but its not good for them. The ants have evolved to actively dig up the gold and drag it to specific area's where larvae will not be living, and the ants who do the digging and dragging will avoid contact with larvae and ants that care for them.

Edit: found source. https://www.businessinsider.com/bacteria-creates-microscopic-gold-nuggets-2013-2?international=true&r=US&IR=T

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They live on a planet close to an X-ray star. A layer of heavy metals in their integument gives them some resistance to radiation.

They were engineered this way by a race that lusts for gold. The ants actually eat it, dissolve it in an internal equivalent of Aqua Regia, and plate it out on their integument.

The original species was much less discriminating and would use lead, cadmium, platinum, osmium -- almost any heavy nucleus.

Periodically the engineers land and release a pheromone that brings the ants to collection jars where they are flash burned, and the metal taken for further processing.

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  • $\begingroup$ this is pretty much "damned clever" ! $\endgroup$ – Fattie Feb 25 at 14:38
  • $\begingroup$ If all the ants that respond to the pheromone are burned, when a queen eventually develops that ignores the pheromone, that colony will thrive and create new colonies. Maybe you should use a reward instead so this doesn't happen. If the ants brought the gold to the jars in exchange to a reward like food, it would be beneficial for those colonies to provide gold, and they would out-compete any other ant colonies who don't trade for food. If they shed the integument periodically, they could bring the shed parts to trade. $\endgroup$ – John Locke Feb 27 at 1:15
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Sex

First, I'll assume that, as per other answers, we're not actually talking about ants, but rather large(ish) mammals.

Rather than take the packrat route, let's go with bowerbirds. The males build elaborate courtship structures, and decorate them with colorful objects. The satin bowerbird, for instance, is partial to blue.

So, if a species of "ants" has females which are partial to shiny, males will collect and display shiny objects - such as gold nuggets. Furthermore, deep digging which brings up pieces of gold will also be selected for.

It's amazing the number of behaviors which can ultimately be explained by sex.

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  • $\begingroup$ Agreed. It's amazing how well known natural selection is, and sexual selection is almost unknown, even though it's more important in some cases. For instance, humans almost all the time survive until breeding age, but having children and spreading their genes is completely different matter. $\endgroup$ – Nyos Feb 25 at 23:03
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    $\begingroup$ @Nyos - My own school days are long gone, but I remember in grade school reading about the Irish Elk, and being singularly unimpressed by the explanation of their enormous antlers as some sort of genetic overshoot. It was sexual selection all the way, baby. $\endgroup$ – WhatRoughBeast Feb 26 at 0:10
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    $\begingroup$ OP specifically said "It was also said to be the size of a fox, but I'll ignore that in this question." Based on that I think the OP is specifically interested in what we understand are ants, not large ants, not large mammals, just ants. $\endgroup$ – Captain Man Feb 26 at 17:47
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Gold helps with their Farming

They don't bring gold to the surface and "throw it outside", but they could bring it into their fungus farms, where it could either:

  • Repel harmful bacteria or organisms that would otherwise harm their farms.
  • Attract & grow beneficial bacteria or organisms.

Some metals in our world exhibit an oligodynamic effect ("a chemical substance or microorganism intended to destroy, deter, render harmless, or exert a controlling effect on any harmful organism by chemical or biological means" [1]), including gold.

Apparently stainless steel is terrible at inhibiting bacteria, while brass would be much safer, in a hospital for example.

Some ants in our world have been farming fungus for millions of years, and apparently some people are wondering how they manage to avoid devastation by disease or parasites (like what happened to the Big Mike bananas).

And so in this other world there could be ants that farm a particular type of crop that either thrives on or around gold, or the gold repels a particularly nasty organism that would otherwise devastate their crops.

[The gold might not even affect their crop directly, but could be a link in a chain, like preventing a bacteria that would feed another fungus, that would feed another organism or predator, that would destroy the ants' crop or the ants themselves. Imagine a truffle-hunting pig (or elephant) that destroys everything in it's search.]

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It's a result of Darwinian evolution. Ant colonies that expelled gold to the surface, for whatever reasons, naturally attracted humans to them. Humans have a proclivity for carrying food and water with them when they journey through the desert. Gold-laden dirt would be relatively heavy, so humans intending to collect the dirt and return it to civilization for refining would tend to leave an equivalent amount of mass behind. This jetsam would frequently contain consumables such as food and water that would no longer be needed due to the interrupted travel plans of the fortunate discoverer. Those ant colonies would be more likely to successfully spread deeper into areas of desert that would otherwise be too inhospitable for them.

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  • $\begingroup$ Not sure who downvoted this, especially without leaving a reason. I see no reason some sort of symbiotic relationship couldn't have evolved, where a humanoid species who covets gold discovers some ants who coincidentally happen to be expelling gold chunks from their tunnels, and then cultivate and feed and spread these ants (destroying nests that churn up useless crap, effectively selecting for gold-farming). Even after the civilization is long gone, without a specific and highly aggressive predator the species will likely continue on for millennia. $\endgroup$ – Doktor J Feb 26 at 22:25
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Dummy gold war

They are exceptionally clever ants. When they find gold, they plant it on the anthill of they enemy and wait for a human to see it and destroy their enemy.

Very effective.

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    $\begingroup$ Humans are common enough around the globe, I'm almost kind of surprised no animal has evolved something like this already if it wasn't for the fact evolution normally moves so slow, and drawing humans attention at all is probably worse than drawing them more to your enemy colony. $\endgroup$ – liljoshu Feb 24 at 21:12
  • $\begingroup$ @liljoshu some animals have "evolved" to get help from humans, not in the limited "destroy my enemies" way, but in the "give me shelter & warmth & food" way, like dogs & cats $\endgroup$ – Xen2050 Feb 25 at 6:03
  • $\begingroup$ Quotation marks on the "evolved" part in @Xen2050's comment, since that development has been heavily guided by human hands through selective breeding. $\endgroup$ – Kapten-N Feb 25 at 7:38
  • $\begingroup$ @Kapten-N True, especially for dogs, but there was probably a "0-day exploit" of wild animals hanging around people for food (table scraps, bones, mice/rats). And cats still look & act a lot like their wild counterparts, aside from slightly less biting & scratching they may not have been bred to be very different, and domestic cats can still basically "run away" & go feral again easily, I doubt a chihuahua would fare well on it's own. [Original quotations were because it's unclear if learning a new trick to make others fight is really "evolving"] $\endgroup$ – Xen2050 Feb 25 at 8:28
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    $\begingroup$ en.wikipedia.org/wiki/Vavilovian_mimicry is when "self-domestication" happens in plants. Also see things like antibiotic resistance, pesticide resistance, and the ever-growing list of rat poisons that no longer work to poison rats. $\endgroup$ – Ben Barden Feb 26 at 18:12
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Medieval bestiaries describe a creature, a type of desert-dwelling ant that digs for gold. It was also said to be the size of a fox, but I'll ignore that in this question.

Their large size is the key.

These very large ants will only dig their mounds out of the sides of mountains, hills and cliffs. The geographical area contains a large deposits of quartz over several square miles, and there is less chance of a tunnel collapse inside a quartz mountain.

Gold is found in quartz deposits, and the ants can dig out the gold far easier than quartz because it's a soft metal.

So where there are ants, there is gold by the entrance to their quartz caves.

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Ants (of any size) communicate largely through pheromones - everything from mating to directions. As it happens, these ants also communicate through pheromones, but something about properties unique to gold either scrambles or removes the pheromone signals. Instinctively realizing that gold, therefore, is quite dangerous, the ants would thus attempt to remove the gold from their burrows, and depending on the strength of the effect, they may even hunt for gold and collect it to keep from being influenced.

Normal ants often have "garbage rooms" where they store their dead, as well as other garbage produced by the hill and its occupants, which would make a fine place to store excavated gold. They may even try to protect this gold, as their instincts have taught them that other creatures can use the gold to disorganize them.

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  • $\begingroup$ This answer sounds good, but it seems very unlikely. If I'm not mistaken, gold is a pretty inert material, i.e. it doesn't really react with most other substances. Thus it shouldn't really have any effect on ant pheromones either. $\endgroup$ – Kapten-N Feb 25 at 7:44
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It is possible to train ants. For example, these articles mention doing this incidentally to their main line of research:

Desert ants do not acquire and use a three-dimensional global vector

Ants on stilts

Training them to collect things is admittedly a bit of a further stretch, but it isn’t totally outlandish.

By the way, the legend stretches way back into antiquity, long before the middle ages, to at least AD 79, in Pliny’s Natural History Book XI, section XXXVI:

The horns of an Indian ant fixed up in the Temple of Hercules were one of the sights of Erythrae. These ants carry gold out of caves in the earth in the region of the Northern Indians called the Dardae. The creatures are of the colour of cats and the size of Egyptian wolves. The gold that they dig up in winter time the Indians steal in the hot weather of summer, when the heat makes the ants hide in burrows; but nevertheless they are attracted by their scent and fly out and sting them repeatedly although retreating on very fast camels: such speed and such ferocity do these creatures combine with their love of gold.

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protected by L.Dutch Feb 27 at 2:49

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