Posted by: Rob Viens | March 16, 2012

Darwin meets the Leafcutters

On March 16th Darwin awoke from his night at the Hotel d’Universe and went out collecting. (Alas, no record of how he slept that night but I don’t think he’d slept on land since the camp on St. Jago so it must have been pretty good.) That morning Darwin writes:

“I took a long walk & collected a great number of plants & insects; it was a fine glowing day; but it is quite delightful to find, so contrary to what I had expected, that the heat by no means incapacitates one for exercise.” (Mar 16)

Darwin loved insects, and at some point in the future I plan to discuss his love of beetles, but today our thoughts turn to ants.  His diary does not mention the encounter, but Darwin’s zoological notebook has a detailed description of the ants that he saw in droves in Bahia (sometime after March 10th):

“On first entering a Tropical forest one of the most striking things is the incessant Labour of the Ants.— The paths in every direction are traversed by hosts of them carrying parts of leaves larger than themselves & reminding one of the moving forest of Birnam in Macbeth” (Zoological Journal from the Beagle)

Darwin is describing one of the most interesting and unusual societies in the tropics – leafcutter ants. Heck some would say that these are some of the most interesting societies in the world, even when compared with human society.  What species Darwin saw is unclear – there are 47 different species of leafcutters organized into two genera (Atta and Acromyrmex). All of these species are found in the New World, so they would have been unlike anything he had seen before.

Video of leafcutter ants from National Geographic Video (Leafcutter ants are show for the first minute or so – you can see then marching starting at about 45 seconds):

OK – here is the really cool thing about leafcutters. Leafcutter society practices a type of agriculture!  Seriously – they bring all those leaves back to the colony where they chew them up and turn them into mulch to fertilize their fungi gardens.  The ants take care of their crops by feeding them, keeping them safe from pests, and even using antibiotics chemicals to kill microbes that might harm the fungus. Interesting, the antimicrobial secretions the ants use on their fungal gardens come from a bacteria that grows on the ant (Actinobactera). They even adapt – if they bring a leaf that turns out to be toxic to the fungus, they spread the word, and the colony stops collecting that type of leaf. When a new queen leaves the colony she actually takes some of the fungi in a special pouch in order to seed a new crop when she forms a new colony. Ultimately this fungal crop is used as a source of food for parts of the colony. Seriously – I’ve known about leafcutters since the time I worked at the zoo, but they still continue to amaze me! The natural world is truly amazing – and Darwin was experiencing it all first hand. I can only imagine his reaction had he seen them cultivating crops!

Darwin also observed other species of swarming ants.  The following description almost surely refers to another species (it does not sound like the behavior of leafcutters, who I believe are also a “red” ant):

“Some of the smaller species migrate in large bodies.— One day my attention was drawn by many spiders, Blattaæ & other insects rushing in the greatest agitation across a bare bit of ground.— Behind this every stalk & leaf was blackened by a small ant: They crossed the open space till they arrived at a piece of old wall on the side of the road.— Here the swarm divided & descended on each side, by this many insects were fairly enclosed: & the efforts which the poor little creatures made to extricate themselves from such a death were wonderful surprising.— When the ants came to the road they changed their course & in narrow files reascended the wall & proceeding along one side in the course of a few hours (all when I returned) they all had disappeared.— When a small stone was placed in the track of one of their files, the whole of them first attacked it & then immediately backed retired: it would not on the open space have been one inch out of their way to have gone round the obstacle, & doubtless [continued at (a) on back of page] if it had previously been there, they would have done so.— In a few seconds another larger body returned to the attack, but they not succeeding in moving the stone, this line of direction was entirely given up” (Zoological Journal from the Beagle)

E.O. Wilson, one of my personal living scientific heroes, spent a career studying ants.  He estimates that there are as many as 10,000,000,000,000,000 (10 quadrillion) ants alive on the Earth at any one time, and that there combined biomass is equal to the total biomass of the human race (probably 10x the biomass of the human race in Darwin’s time).  So it is not surprising that Darwin ran into a few ants on his journey!

Dinoponera australis – a really big ant from South America (from antweb.org):

Dinoponera australis

Darwin seems to take it easy again today – “In the middle of the day [he] went on board the Samarang & dined there.” Then he hung out with his new (and amusing) friends (though no mention of our friend Captain Paget).  Still on the Samarang later in teh day Darwin notes:

“I spent most part of the evening with the Mids; & such a set of young unhanged rogues the young ‘gentlemen’ are, is sufficient to astonish a shore-going fellow.” (Mar 16)

Before he could get into too much trouble the Beagle returned (about 9 pm) and Darwin returned to his own hammock. Upon returning he learned that he got off lucky:

“It was a piece of high good luck that I remained on shore during the two days: the ship rolled & pitched so much, that the greater part of the junior officers were sick.” (Mar 16)

I’m guessing he appreciative of the time away from the ship even more when he heard that. Only a couple days left in Bahia – then it is off to Rio and an extended shore leave. (RJV)

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