In contrast, programs in Tierra have little control over their longevity. As individual Tierran programs have no notion of energy levels, a separate `reaper queue' mechanism is employed to govern cell death. Programs can move up the queue if they cause error conditions during execution (see Section 3.2.1), but in general the probability of death increases with age [Ray 91]. The reaper queue therefore effectively imposes an upper limit on the lifespan of programs, whereas there is no theoretical upper limit in Cosmos.
Additionally, the energy token scheme in Cosmos introduces the idea of a competition for the available energy, which is missing in Tierra. Furthermore, if the parameter energy_collection_scheme is set to shared, cells may extract energy tokens from their neighbours. In this situation, a cell is a potential energy resource for other cells, and, if environmental energy were scarce, it would become advantageous for a cell to kill its neighbours by draining their energy. If cells could defend themselves against such attacks, some sort of coevolutionary process might arise from such interactions.