Perennial parthenogytes and sired sisters of a common brood, the worker ant seems the most unlikely entity to be influenced by sexual/natural selection. The complexity of ant society, though, depends on the ability of each individual worker to predictably participate in structurally-stabilizing societal routines.
The colony is disturbed. Three fingers wrap around the underside of a flat sandstone, exposing a brood of ant larva. Within seconds open-jawed ants, mandibles spread for the pinch, pour out of covert caverns in numbers and with movement that is startling to the unsuspecting toddler. Soon a dozen ants cling to his fingers, a dozen more dart up his hand, and dozens more rush to the site of the disturbance. In the shadow of the stone can be seen dozens more ants carrying the uncovered pupae and larva to underground safety. A parent wipes the ants off of the toddler's hand and several ants dye in the process.
How did the ants mobilize so quickly? Was there a sub-perceivable siren like a war trumpet calling the colony to action? A similar ability to communicate is evidenced in ant trails.
Notice an ant trail. Often ending at a choice item of caloric density, the ants closely follow a narrow trail--sometimes single file. Try this at home: find an ant trail, locate an open spot between ants on the trail, rub a finger or a napkin over the gap with enough friction as to spread a hypothetical grease smudge. Now observe. The ants will arrive at this gap in the trail and stop. They will veer away from the direction of the trail, trying to pick up where it resumes. Ants on both sides--coming and going--will do this until eventually the trail is reestablished. This process may take several minutes.
Why is it that ants are able to follow a trail so closely? What do the above portraits show? Ants are efficient communicators. Many ant species use hormone-rich liquids to communicate. A disturbed ant disperses an aqueous alarm. The receptor ants pass along the liquid alarm--often depositing the liquid on the ground for passerby ants to pick up the message. Similarly, an ant scout discovers a sugary spill on the sidewalk. She makes her way quickly back to the colony while she unnoticeably deposits a liquid trail, Hansel style, to direct her sisters back to the feast.
The general absence of reproduction among worker ants (there are exceptions--vestigial evidences of the former evolutionary ability of ants to individually reproduce) makes the worker an elusive candidate for natural selection. Yet the influence of natural selection acts on the individual ant. The next installment will explore more of the connections between ant behavior, natural selection, genes and memes.