A slavemaker ant raid in Colorado

Hi there--

Today, I've been "googling" red ants because last night in my backyard, I saw hundreds of red ants--not tiny--not the huge ones, marching across my backyard--in a 5" wide line, all carrying eggs. There were HUNDREDS of them--it took about a half an hour. Fortunately, I caught them in time to trace over to the leaders of the pack (not carrying eggs), and watched until the last ones came. They marched across my large backyard from a neighbors house, into the backyard of the neighbor on the other side!

It was fascinating and I'm thankful I saw it. My question is--what kind of ants would they be, and what were they doing? I'm a gardener, and I use no chemicals or pesticides--so I don't want to harm the ants--I am just very curious!

Thanks very much! Love your blog--it is fascinating! Found it by googling my ants!

Centennial, CO

Dear Shelly:

Thanks for your interesting question on ant behavior on a warm Colorado afternoon.

As you no doubt discovered in your search, there are a lot of different kinds of "red ants" in the world, but because of your clear description of the behavior, we can narrow it down considerably from all those hundreds of different sorts of ants that could qualify as red.

There are two most likely players in your area, a Polyergus species, or a member of the Formica sanguinea group of species; both in the broader category of slave-making, or dulotic, ants. These are ants that raid colonies of a related ant species and steal, not the eggs, but the pupae in cocoons (equivalent of a butterfly chrysalis) from the raided ant species. These mature into ants that constitute the primary work force of a mixed species ant nest population. The slave-makers themselves are quite expert at this brood-robbing, but often somewhat to completely (depending on the species) ineffectual at normal ant work, such as nest building, food gathering, and brood care. You might find it interesting, if you see this again, to follow the pupa-carrying ants home and see the amazing mixed species colony that results form this behavior.

This is one of several forms of social parasitism in ants, in which one species is dependent on one or more healthy colonies of a particular host ant species for its survival. Some lovely pictures of parasitic ants may be seen at the photography website of Myrmecologist Alex Wild:

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