Exploring the evolution of a trade-off between vigilance and foraging in group-living organisms

Abstract

Even though grouping behaviour has been actively studied for over a century, the relative importance of the numerous proposed fitness benefits of grouping remain unclear. We use a digital model of evolving prey under simulated predation to directly explore the evolution of gregarious foraging behaviour according to one such benefit, the 'many eyes' hypothesis. According to this hypothesis, collective vigilance allows prey in large groups to detect predators more efficiently by making alarm signals or behavioural cues to each other, thereby allowing individuals within the group to spend more time foraging. Here, we find that collective vigilance is sufficient to select for gregarious foraging behaviour as long there is not a direct cost for grouping (e.g. competition for limited food resources), even when controlling for confounding factors such as the dilution effect. Furthermore, we explore the role of the genetic relatedness and reproductive strategy of the prey and find that highly related groups of prey with a semelparous reproductive strategy are the most likely to evolve gregarious foraging behaviour mediated by the benefit of vigilance. These findings, combined with earlier studies with evolving digital organisms, further sharpen our understanding of the factors favouring grouping behaviour.

Keywords: anti-predator vigilance; genetic relatedness; group foraging; many eyes hypothesis; reproductive strategy; tragedy of the commons.

Figure 1.

Depiction of the disembodied simulation. Prey seek to forage as much as possible while avoiding being captured by the predator. If none of the prey in the group are vigilant, the target prey is captured 100% of the time.

Figure 2.

Treatment comparison when prey are forced to forage in groups. Both group homogeneity and a semelparous reproductive strategy select for high levels of vigilance. However, only homogeneous groups experience an increase in fitness as group size increases. By contrast, vigilance behaviour breaks down in larger, heterogeneous groups of semelparous prey. Error bars indicate bootstrapped 95% CIs over 100 replicates; some error bars are too small to be visible.

Figure 3.

Treatment comparison when prey can choose to forage in groups. Allowing prey to decide whether they wish to be in the group produces very similar results compared to when they are forced to group. In homogeneous groups, prey choose to spend most of their time in the group. However, grouping breaks down (alongside vigilance) in heterogeneous groups of semelparous prey. This occurs despite there being no direct penalty assessed for choosing to group. Error bars indicate bootstrapped 95% CIs over 100 replicates; some error bars are too small to be visible.

Figure 4.

Vigilance in prey with and without the option to forage in groups. In homogeneous groups, prey with forced and optional grouping evolve similar vigilance behaviours. By contrast, individualistic (non-grouping) prey evolve vigilance behaviours that maximize individual fitness. Meanwhile, individuals in heterogeneous/semelparous populations with the option to group evolve to be less vigilant than either of the other two treatments. Error bars indicate bootstrapped 95% CIs over 100 replicates; some error bars are too small to be visible. (a) Heterogeneous, iteroparous; (b) heterogeneous, semelparous; (c) homogeneous, iteroparous; and (d) homogeneous, semelparous.

Figure 5.

Fitness for prey with and without the option to forage in groups. In heterogeneous/semelparous groups, prey with the option to group have lower fitness than prey that are forced to group. Error bars indicate bootstrapped 95% CIs over 100 replicates; some error bars are too small to be visible.

Figure 6.

Grouping behaviours in prey experiencing grouping penalties. Even with a small grouping penalty (M=1.0), all treatments except homogeneous/semelparous no longer evolve grouping behaviour. Prey in the homogeneous/semelparous treatment only evolve slightly lower levels of grouping behaviour, even with extreme penalties to foraging in a group (M=1000). Error bars indicate bootstrapped 95% CIs over 100 replicates; some error bars are too small to be visible. (a) Heterogeneous, iteroparous; (b) heterogeneous, semelparous; (c) homogeneous, iteroparous; and (d) homogeneous, semelparous.