Natures wonders: ants forage by individual memory (Introduction)

by David Turell , Saturday, September 28, 2019, 18:50 (1881 days ago) @ David Turell

We know ants act as a colony by standardized individual actions as in bridge formation. Ant colony memory exhibits the same general mechanism:

https://aeon.co/ideas/an-ant-colony-has-memories-that-its-individual-members-dont-have?...

"Like a brain, an ant colony operates without central control. Each is a set of interacting individuals, either neurons or ants, using simple chemical interactions that in the aggregate generate their behaviour.

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"Past events can alter the behaviour of both individual ants and ant colonies. Individual carpenter ants offered a sugar treat remembered its location for a few minutes; they were likely to return to where the food had been. Another species, the Sahara Desert ant, meanders around the barren desert, searching for food. It appears that an ant of this species can remember how far it walked, or how many steps it took, since the last time it was at the nest.

"A red wood ant colony remembers its trail system leading to the same trees, year after year, although no single ant does. In the forests of Europe, they forage in high trees to feed on the excretions of aphids that in turn feed on the tree. Their nests are enormous mounds of pine needles situated in the same place for decades, occupied by many generations of colonies. Each ant tends to take the same trail day after day to the same tree. During the long winter, the ants huddle together under the snow. The Finnish myrmecologist Rainer Rosengren showed that when the ants emerge in the spring, an older ant goes out with a young one along the older ant’s habitual trail. The older ant dies and the younger ant adopts that trail as its own, thus leading the colony to remember, or reproduce, the previous year’s trails.

"Foraging in a harvester ant colony requires some individual ant memory. The ants search for scattered seeds and do not use pheromone signals; if an ant finds a seed, there is no point recruiting others because there are not likely to be other seeds nearby. The foragers travel a trail that can extend up to 20 metres from the nest. Each ant leaves the trail and goes off on its own to search for food. It searches until it finds a seed, then goes back to the trail, maybe using the angle of the sunlight as a guide, to return to the nest, following the stream of outgoing foragers. Once back at the nest, a forager drops off its seed, and is stimulated to leave the nest by the rate at which it meets other foragers returning with food. On its next trip, it leaves the trail at about the same place to search again.

"Every morning, the shape of the colony’s foraging area changes, like an amoeba that expands and contracts. No individual ant remembers the colony’s current place in this pattern. On each forager’s first trip, it tends to go out beyond the rest of the other ants travelling in the same direction. The result is in effect a wave that reaches further as the day progresses. Gradually the wave recedes, as the ants making short trips to sites near the nest seem to be the last to give up.

"From day to day, the colony’s behaviour changes, and what happens on one day affects the next. I conducted a series of perturbation experiments....After just a few days repeating the experiment, the colonies continued to behave as they did while they were disturbed, even after the perturbations stopped. Ants had switched tasks and positions in the nest, and so the patterns of encounter took a while to shift back to the undisturbed state. No individual ant remembered anything but, in some sense, the colony did.

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"...the larger the magnitude of the disturbance, the more likely older colonies were to focus on foraging than on responding to the hassles I had created; while, the worse it got, the more the younger colonies reacted. In short, older, larger colonies grow up to act more wisely than younger smaller ones, even though the older colony does not have older, wiser ants.

"Ants use the rate at which they meet and smell other ants, or the chemicals deposited by other ants, to decide what to do next....It is likely that colony behaviour matures because colony size changes the rates of interaction among ants. In an older, larger colony, each ant has more ants to meet than in a younger, smaller one, and the outcome is a more stable dynamic."

Comment: As with bridges, automatic individual reactions make the whole colony operate as a unit.