The Kroot are expert jungle-fighters and trackers, who can be found fighting as mercenaries across the Ultima Segmentum and beyond. The majority of Kroot warriors fight as mercenaries in the armies of the Tau. Their integration into the Tau empire requires them to provide troops to the Tau military, and furthermore attempts to prohibit them from fighting alongside the armies of other races.

Kroot evolution depends on their absorbing the genetic traits of other races, selectively inheriting the most desirable. They do this through eating specific prey animals to ensure that the next generation take on certain characteristics of that animal. Unfortunately, the Tau insistence that the Kroot fight exclusively for them would lead to a disastrous stagnation, as they have absorbed the traits of most of the creatures from within the Tau region. To collect as wide a range of characteristics as possible, they secretly despatch entire armies of mercenaries to fight alongside other races in order to expose themselves to creatures and environments not found in Tau space.

The result is that each of these mercenary bands develops separately to the mainstream of Kroot society. When they periodically return to the Kroot homeworld of Pech, they bring with them a wealth of new traits to be absorbed by the race at large. These itinerant bands often appear radically different from the standard Kroot, having absorbed all manner of outlandish genetic data.