Ants, Food Raids & Evolution

Image #1 Army Ants reproduced with permission from “Army Ants: Nature’s Ultimate Social Hunters” by Daniel J.C. Kronauer; Image #5 painted clonal raider ants photograph by Daniel Kronauer. Credit: #1: Daniel J.C. Kronauer, #5: Daniel Kronauer

It is rainy season, therefore the season for starting the growth cycle of some plants, in Costa Rica. It is always ant season here. Some of the trees we planted last year, mostly citrus varieties but also pomegranate, have become feasting locations for ants who devour their leaves and haul them off.

My assumption, seeing this constantly during the 25 years since we moved to Costa Rica, has always been that ants are primarily vegetarians So, today a bit of ant-wonk from a team of scientists at Harvard University, summarized on Phys.org’s website, to correct my assumption (the video alone is worth visiting the source article):

How army ants’ iconic mass raids evolved

Army ants form some of the largest insect societies on the planet. They are quite famous in popular culture, most notably from a terrifying scene in Indiana Jones. But they are also ecologically important. They live in very large colonies and consume large amounts of arthropods. And because they eat so much of the other animals around them, they are nomadic and must keep moving in order to not run out of food. Due to their nomadic nature and mass consumption of food, they have a huge impact on arthropod populations throughout tropical rainforests floors.

Their mass raids are considered the pinnacle of collective foraging behavior in the animal kingdom. The raids are a coordinated hunting swarm of thousands and, in some species, millions of ants. The ants spontaneously stream out of their nest, moving across the forest floor in columns to hunt for food. The raids are one of the most iconic collective behaviors in the animal kingdom. Scientists have studied their ecology and observed their complex behavior extensively. And while we know how these raids happen, we know nothing of how they evolved.

A new study in Proceedings of the National Academy of Sciences led by Vikram Chandra, postdoctoral researcher, Harvard University, Asaf Gal, postdoctoral fellow, The Rockefeller University, and Daniel J.C. Kronauer, Stanley S. and Sydney R. Shuman Associate Professor, The Rockefeller University, combines phylogenetic reconstructions and computational behavioral analysis to show that army ant mass raiding evolved from a different form of coordinated hunting called group raiding through the scaling effects of increasing colony size.

The researchers discovered that the ancestral state to army ants’ mass raids is the rather different-looking group raids that their non-army ant relatives perform. The evolution of mass raids from group raids happened tens of millions of years ago and the transition from group raids to mass raids is perfectly correlated with a massive increase in colony size…

Read the whole article here.

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