Naturespeak A naturalist's view of the world

June 23, 2008

A Love Triangle

Filed under: Uncategorized — wykes @ 6:25 pm

Animal husbandry is a wonderful thing.  Although some wives out there might believe that this is a marriage reference, I am actually referring to the care and feeding of animals by humans (still the same thing according to those same wives).  Domestic animals have long been a crucial part of the human condition.  We raise sheep, cattle, pigs, and chickens and they (willingly or not) provide us with things like wool, eggs, mutton, beef, leather and “the other white meat.” It would be normal to assume that this kind of relationship is strictly confined to the human species, but such is not the case.  Ants practice husbandry also. They have, in fact, been doing it much longer than we have.

  Wood Ants, those marvelous creatures that build solar powered nest mounds, are sophisticated aphid farmers (looks like they have us beat on two counts!).  I recently came upon one of their work farms while walking a woodland trail.  The ant factory farm was being conducted on the leaves of a sapling Aspen (see here).  Near the base of each leaf -where the stem joins the flat green pasture of the blade- herds of tiny aphids were being tended by three or four ants.  The black & red farmers looked like giants compared to their miniscule charges (see above), although they are only about 10 mm in length. The aphids came in a variety of sizes ranging from pinhead proportion to about 1 mm. in length.

  The aphids act like tiny milk cows in this scenario. They suck on the juicy plant sap using straw-like mouths and eject waste fluids in the form of sweet honeydew droplets. The dairymen ants milk this sweet substance as a much needed food source. Unlike their human counterparts, wood ants need only to lightly tap their “cows” with an antenna to induce them to release their product into their open jaws. The tree farm is a constant hub of activity as ants return to, and arrive from, the ground colony and the stationary aphids keep pooping up those sugar drops.

  It’s easy to see the plus side of this relationship as far as the ants are concerned.  As omnivores that make a living foraging for all manner of plant and animal food, their only reliable carbohydrate source is honeydew. A colony can consist of up to half a million individuals – that’s a lot of mouths to feed.

  What’s in it for the aphids?  Fortunately, the ants forgo the opportunity to eat them outright and act instead to protect them from predators. By shepherding their herds, the ants keep spiders, wasps, and other ants out of the pasture tree.  Angry Wood Ants are not to be trifled with.  They have the ability to squirt formic acid onto their opponents- sometimes as much as several feet away. This talent is where the term “piss ant” comes from. As a result, the aphids can reproduce in relative peace and live long happy lives as fat sap-sucking bags of insectitude. All they need to do is act naturally while their guardians get pissed off.

   With both sides benefiting, this whole thing might be called a symbiotic relationship. There is, however, a third party in this affair as well.  The aspen tree is the unwilling third part. As the primary blood donor, you’d think the tree would suffer from this controlled aphid invasion. These are, after all, the same aphids that suck the life out of garden plants and are major pests for humans engaged in plant husbandry.  In the case of the trees, however, it has been revealed that ant farm “infested” trees tend to be a bit healthier than thier non-infested counterparts. Caterpillars and other leaf eaters are discouraged by the ant activity- a benefit to the tree that outweighs any sap loss. Colonized oak trees actually produce healthier acorns, for instance. They don’t produce more acorns, but have fewer problems with the weevils and borers who normally destroy the nuts. 

  What we have here is something called “indirect mutualism” – a love triangle in which everybody is happy in the relationship.  Humans don’t do well when love triangles are involved, so I guess the ants have us beat on all fronts.

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