Dear Straight Dope:
A summer infestation having turned the long lip of my kitchen counter into an ant version of Route 66, I began to ponder: just how far away can an ant nest be located from a food source? At some point, the ants' foraging activities have to use up more energy than they're able to haul back home, right? I began the process of outlining the standard assumptions--average ant weight of 1 mg, each ant might carry ten times as much . . . at this point the laziness factor kicked in and I thought I might have better luck with this problem, not to mention fewer mistakes, if I employed the services of your Science Advisory Board. So how about it?
Robert Knecht Schmidt, Cleveland, Ohio
You ask a good question, although the energy calculation you were trying to do doesn’t enter into the answer at all–an ant colony can have a column of workers going to a food source even in the unlikely event individual ants lose energy on the deal. (I say unlikely because a forager doesn’t use much energy, and can carry quite a lot.) After all, the incoming energy is going into new brood, and if the colony is starving, any food coming in is better than none at all, even if workers have to die to obtain it.
The number of foragers that show up at a resource depends very little on distance, and mostly on how good and how large the resource is. An ant that brings back high-quality provender lays a chemical trail that prompts other ants to follow it. Each additional ant that finds high-quality food at the end of the trail will add more chemical on its way back, regardless of distance, so if there’s a lot of the resource, the trail will become very strong and stay in place for a long time. When the resource starts to run out, the ants returning empty-handed (as it were) do NOT reinforce the chemical trail. Eventually the trail evaporates and ants stop using it in search of food.
Any resource within the colony’s foraging radius is fair game. The foraging radius is largely hardwired into each species, modified by factors such as resource density (when resources are denser, foragers find food faster, and don’t need to go as far from the colony); colony size (when there are more foragers, they spread out over a larger area); and colony need (if they’re hungry, more ants are sent out to find food, which spreads them out more); and, perhaps most importantly, territorial limits (if ants run into a neighboring colony, borders are established). In practice, the wanderings of most ant colonies are well within the foraging radius, which represents an upper limit.
That’s the nutshell version, at least. I have colleagues who have devoted their entire professional lives to just this sort of question, and in the process have only begun to unravel the complexities of how it all works. You might look for Bert Holldobler and Edward O. Wilson’s excellent book The Ants (1990) if this sort of thing interests you.
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