In this section we introduce a new restriction on the ant programs, which we apply in addition to limiting the amount of food on the trail initially. However no significant increase in performance was found.
Instead of repeatedly executing each program until the ant has
made 600 moves or turns we limit the energy initially available to the ant but
reward its progress along the trail by increasing the energy available
to it.
As the trail is 144 squares long and has to be traversed within 600
time steps,
programs have to be fairly efficient,
consuming on average less than 
energy units per
square of trail traversed.
Figure 5 shows the energy consumption of
the evolved solution given in [Koza1992, page 154,]
and the example programs from Figs. 1 and
2.
It appears that the energy limit (solid horizontal line in Fig. 5)
provides less incentive to be efficient
to poor programs that only manage to follow the trail a short distance
than it does to better programs.
This suggests it provides less guidance at the begining of GP runs
than later.
However GP populations often appear to become more resistant to change
(possibly associated with programs becoming longer).
This in turn suggests the requirement for programs to be efficient
should be make itself felt at the start of each GP run.
We hoped that by penalising programs that consume much more than 4.2
energy units per trail square we would better guide the population.
We implemented this by limiting the each ant's initial energy to 25,
which we then increased each time the ant eats a food pellet,
up to the original limit of 600.
Each program is repeatedly executed until its ant has used as much
energy as it has been allocated.
The food pellets give the ant a multiple of 5 energy units.
With food pellets following gaps giving 
energy units
(cf. diagonal line Fig. 5).
Effort values when this guidance is used are given in Fig. 4 (plotted with squares) and Table 2 ``ramped energy''. Disapointingly these show no additional improvement compared to just limiting the amount of food ahead of the ant to five pellets.
Figure 5:
Energy limits and energy used as ants move along the trail.
Plots for an evolved solution and the two partial solutions shown in
Figs. 1 and
2.