A tier is a bot that continuously ties to anything near. This isn't a terribly difficult mutation to develop, since all you must do is store a number into .tie every cycle.
This novel adaptation has many uses in different environments. It simultaneously allows a bot to latch on to food, slow down, and gain space needed to reproduce.
In toroidal sims with gravity (creating a waterfall effect) it is useful to form either clumps of bots latched onto food or "flying" groups of 2-3 that can either float or rapidly fall.(Presumably we as humans could make use of this tie "wing ability" but as far as I know nobody's ever worked on it.)
In nontoroidal sims with gravity The Pit bots have an easy food supply but are initially unable to gain the needed space to reproduce. With ties they can form either a rolling mass of bots or a sort of platform arrangement both allowing them the opportunity to reproduce.
In a high Brownian motion environment they can use ties to fight the random motion they are subjected to. The ties allowed them to slow down and feed from the plants in a controlled manner.
Often times they learn to share their food, since no single bot could form the clump on it's own.