textS2: Evolution of foraging and vigilance parameters (original) (raw)

In Figure 1 we show the evolved vigilance and foraging parameters in populations of both solitary (blue) and grouping (orange) foraging for different predation risk (d P). The main result is that when vigilance p V evolves (A: d P = 5 − 7; Figure 2A, main text), the vigilance angle a V (D) evolves to its maximal value, to ensure the predator is seen when it approaches from behind. Vigilance duration t V evolves to its minimal value (G), so that scanning for the predator occurs "briefly and often", rather than "rarely and long" (stars show where vigilance did not evolve). Moreover, the time that foragers wait before resuming foraging after fleeing from a predator t P (see Figure S1E, in Text S1), evolves to be greater than about 50 minutes (I), which ensures that the predator is out of detection range and does not immediately return when foraging is resumed. With respect to foraging parameters, we find that when vigilance evolves, there is a switch from having low move probabilities and long move distances, to high move probabilities and short move distances (compare no predator and d P = 9 with d P = 5 − 7 in B and C). Long move distances evolve so that searching for food in the same place is avoided [1], however this can also be achieved by repeating movement with a greater move probability p M. In combination with vigilance, the latter option has preference because this enables more regular scanning for the predator during movement. When vigilance disappears in large groups (d P = 0), again long move distances evolve. Otherwise, as we found previously without vigilance [1], the move angle a M does not evolve (E), and the food scan angle a F evolves to around 250 degrees, which is where food item detection is maximal. Moreover, movement duration t M (H) and foraging duration t F (not shown, but results identical to those of t M), evolve to minimal values. Qualitatively, foraging is therefore the same as without vigilance or grouping.