A second case of meat farming

July 9, 2011 • 5:30 am

This observation was apparently presented at the Evolution 2011 meetings in Oklahoma, but I missed it.  Via Alex Wild at Myrmecos comes a report from New Scientist of the discovery of only second species (besides H. sapiens) that raises animals for meat.

It’s an ant!  To be precise, it’s a species (it’s not clear if it’s more than one) in the genus Melissotarsus.  And the work is apparently that of Scott Schneider, a graduate student in biology at the University of Massachusetts at Amherst. (All quotes below from the New Scientist piece.)

Lots of ants practise a rudimentary form of agriculture. Some are gardeners, gathering leaf fragments on which they cultivate a crop of tasty fungus. Others are dairymaids, “milking” the sweet excretion known as honeydew from aphids, scale insects and other related insects.

But the Melissotarsus ants of continental Africa and Madagascar are special. If biologists’ best guess proves correct, these ants raise their insect herds for meat, not milk – the first example of meat farmers other than humans. And that’s not all. The insects they cultivate may be the best example of true domestication outside of our crop plants.

You have to know what you’re looking for to even see Melissotarsus. The ants – barely 3 millimetres long – live most of their lives within the intricate gallery systems they excavate in and under the bark of trees. They’re such committed burrowers that their second pair of legs points up, not down, so they can get a foothold in the tunnel roof as well as the floor. They share their galleries with several species of armoured scale insects, so-called because most species secrete a tough, waxy scale that covers and protects them.

Note the position of the second pair of legs in this photo from Myrmecos and the following one from the Encyclopedia of Life:

Melissotarsus sp. (photo by antweb.org via Myrmecos)

If you have house plants, you’ve probably been plagued by scale insects (order Hemiptera: true bugs!), which often have a hard shell or cottony outer covering to protect them from predation. Here’s another photo, by the indefatigable Alex Wild, which he captions as “Camponotus rosariensis ant tending scale insects for honeydew. Notice that the younger scale insects have legs. These first instar scale are more mobile than the older instars, which settle down once they find a good spot for feeding.”:

Most ant/scale insect/honeydew systems are evolved mutualisms (or “symbiosis”), in which the ants tend the scale insects, protecting them from predation, while they eat the honeydew (the not-completely-digested sap exuded by the scale insects).  Both species benefit, and some of these associations are at least 15 million years old. It’s clear that the ants have evolved special behaviors to “farm” these insects for the exudate, but I’m not sure whether the scale insects themselves have evolved to make them more attractive to the ants (if both species evolved, it would also be a case of “coevolution,” but I’m not sure about that here).

In the case of the Melissotarsus species, the scale insects “tended” by the ants don’t produce honeydew, nor do they have an edible wax exudate.  The idea is that the ants are raising the scale insects as livestock, completely analogous to the way humans raise cows:

So what do the ants get from all their work housing and protecting the scale insects? Almost the only remaining possibility is that the ants sometimes make a meal of the insects themselves, Schneider reported at a recent meeting of the Society for the Study of Evolution in Norman, Oklahoma. No one has yet caught Melissotarsus in mid-munch, partly because the ants like their privacy and quickly seal off any peepholes into their galleries. Next year, however, Schneider will measure stable isotopes in the ants’ bodies, which will indicate whether their diet is mostly plant or animal in origin.

Now they haven’t yet seen the ants eating the scale insects, nor has this observation yet been published in the scientific literature, so we have to take it as provisional.  (Presentations at meetings don’t count, at least in evolutionary biology, as “publications” because they’re not peer-reviewed). That’s why the “farming idea” is characterized by New Scientist as “biologists’ best guess.”  The authors really do need to observe the scale insects being nommed, which is hard to do because the ants + scales are secretive.

But if the “meat farming” idea is true, you might well ask why haven’t the scale insects, who seem to gain nothing but death from this interaction, evolved defenses against the ants?  I can think of two answers.  First, maybe the scale insects really do benefit from being farmed.  Perhaps, though kin selection, a “herd” of scale insects is actually a related group, with the eaten individuals “sacrificing themselves” for the good of their kin, who are protected from predation by species other than the “farmer” ants. This makes the testable prediction that the “herd” consists of related individuals, and the harder-to-test prediction that the genes of those individuals being “farmed” leave more copies of themselves than the genes of scale insects who aren’t farmed. (It’s not clear if the scale insects occurs in groups that aren’t farmed by ants.)  That would also suggest that the scale insects may have some yet-undescribed adaptations for being farmed.

Alternatively, it may simply not have been possible for the scale insects to evolve defenses.  After all, evolution isn’t perfect: every successful parasite represents a failure of adaptation by its host.  If the genetic variation for protection hasn’t arisen (or can’t arise) through mutation, then there’s no counter-evolution.

h/t: Matthew Cobb

28 thoughts on “A second case of meat farming

  1. Or as a speculative long shot: maybe the scale insects are parasitising the ants.

    i.e. they’ve found a way to manipulate the ants’ herding behaviour so the ants protect them without getting anything in return.

  2. “why haven’t the scale insects, who seem to gain nothing but death from this interaction, evolved defenses against the ants?”

    Defense against what?

    Why haven’t cows evolved defenses against humans? Humans facillitate cows’ reproduction; that’s all that matters from an evolutionary standpoint. Presumably, the same goes for the bugs.

    1. I think domestication has something to do with it. Humans have somewhat impeded natural selection in cows and replaced it with our own guided selection (i.e., selective breeding.)

    2. Yes, why would they evolve defenses against what keeps the population live?

      But on the other hand, maybe they do. It depends on the ants’ behaviour. Do they tend equally to all, or do they discriminate? If the former, being an unattractive meal would be beneficial for reproduction. If the latter, it would be detrimental

  3. In what sense are the scale bugs “herded”? How do they end up under the control of the ants?

  4. If the ants are raising the scale insects as livestock, completely analogous to the way humans raise cows, could that even include selective breeding by the ants that prevents any undesirable characteristics, such as defenses, from evolving?

  5. According to some arguments (See Budiansky’s Covenant of the Wild) animals ‘choose’ domestication, so it is in the interest of the genes to be propagated and once that happens the animal has filfilled its purpose. So it could be that the scale insects genes (which remind me of barnacles) get an advantage that they would not otherwise have.

  6. Perhaps the scale insects are simply harvested after they have reproduced, which would preclude the putative effects of natural selection. If there are no barriers to reproduction (i.e., competition for mates, food, etc.), if everything is provided to them, then what selective pressures remain?

    Please forgive the gruesome analogy, but if humans harvested only post-menopausal women, would human evolution be affected? I suspect not.

    1. Sexual selective pressures and social dominance.

      Even if you have “enough” to reproduce, there is always “more” to reproduce more.

      And menopause could be later.

  7. Perhaps the scale insects have adapted, produces a succulent meat so delicious and filling that the ants can only eat a few of them before becoming completely satiated.

    Then again, you may as well ask what the Coe gains from its relationship with man. Effectively cows have been selectively bred by humans, regardless of the evolutionary needs of the cattle. The same has probably occurred for the scale insects. Perhaps the ants even cull the less docile ones?

  8. …every successful parasite represents a failure of adaptation by its host.

    I like this last point.

  9. Fascinating.

    [naturalistic fallacy] Oh noes, farming is natural! What will the vegetarians say?[/naturalistic fallacy]

    ants + scales are secretive.

    Well, at least the common scales are. … No, wait, that is secretionary. D’oh, I swear english is greek at times!

  10. Quoth Coyne: “every successful parasite represents a failure of adaptation by its host”

    This is by no means certain. In fact, it’s not true in at least some cases.

    A few years ago, medical research was reported about a correlation between infestation by intestinal worms and the incidence of inflammatory bowel disease. Some clever GI doc had noticed that in cultures where intestinal parasites were common, IBD was uncommon, and vice versa.

    An experimental therapy was tested on IBD patients, in which their bowels were inoculated with a species of parasite that does not infest humans. I don’t remember the exact results but iirc the majority of the patients showed great improvement in their bowel condition. The worms died out after a while, being unable to reproduce in the human GI tract.

    This all sounds to me much like symbiosis with benefits to both intestinal worms and to their human hosts. Darwinian mutualism, or co-evolution, perhaps.

      1. From what I recall, the trials involved very small numbers of hookworm. Larger numbers would cause pathology, particularly in undernourished populations. The worms are actually living off the blood of the host and “die out” because they were irradiated. Otherwise they would start as cycle of transmission and re-infection. It would be interesting to know what came first, the immunosuppressive effect or the parasitism

  11. The more I think about it…it seems to me that the relationship between humans and livestock is something akin to symbiosis.

    Modern livestock wouldn’t last even a single generation without support from humans. The wild turkey is one of the hardest prey animals for human hunters to catch, but even a three-toed sloth could catch a domestic turkey if it were so inclined. The situation is the same with other animals, though perhaps not to quite such a famously obvious extreme.

    So, these are animals which have prospered astoundingly well compared to their undomesticated cousins. Where herds of buffalo once stretched to the horizon and flocks of pigeons darkened the skies, now European cattle and chickens dominate the landscape. Though the lives of the individual animals are cut short, their genes and the species as a whole have flourished in unprecedented ways.

    And the price the species pay is to have vast numbers of individuals get eaten.

    From the other side of the coin…while one can debate the wisdom of depending upon these animals for sustenance, there can be no debate about the fact that the human species currently is and has been for ages utterly dependent on them. If every cow, chicken, and pig were to be instantly and magically removed (as opposed to somehow phased out) from the human food chain, the effects on the human population would be catastrophic.

    So: is it a predator / prey relationship? No; the preyed-upon species gain far more from us than mere culling of less-fit genes. Parasitic? Again, no: these species most certainly gain a great deal.

    What’s left but symbiosis?

    Or, if it somehow doesn’t qualify as symbiosis…is there another term already, or do we need to invent one, or…?


    1. Agreed.

      But there is also the fact that the original populations diminished because of us; the result can also be terned enslavement. (I dunno if that is a biological description, I would rather think not.)

      1. Domination? Usually you’d speak of the dynamics internal to one species or even one pack of a species, but really, is there a difference that’s relevant?

  12. Seems like there’s still a lot of information yet to be found out about this fascinating coexistence. I looked in vain for info about the scale insects’ life. Are all instars as well as adults found in the galleries? Presumably, they’re feeding on the sap of the host trees; would it be possible to have the host plants take up some sort of marker that would then be passed to the scale insects that could then be looked for in the ants, as a sign of predation? Could the scale insects be producing excess eggs, some of which are being eaten by the ants? Or perhaps, two kinds of eggs–“food” eggs & reproductive eggs?

    Great story!

  13. “…the discussion of determinism versus nondeterminism of human behavior…” is an interesting issue. My limited reading on the subject suggests there is nothing to discuss except the extent, that research has shown both to exist, varying from one individual to another depending, to some degree on “will” or “won’t” power.

    At the end of the Space Shuttle program my thoughts are that we move space exploration to unmanned programs and use the funds saved to fund major explorations of comprehensive inner space. My guess is we would find more to baffle than enlighten.

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