An Adaptive Dynamic Model of a Vigilance Game Among Group Foragers

In group foraging animals, vigilance tends to decrease as group size increases. A forager in a group receives a vigilance benefit not only when it is being vigilant itself but also when a group mate is being vigilant. The many eyes hypothesis supposes that individuals exhibit lower vigilance in larger groups because of this. However, changes in safety resulting from the vigilance benefit conferred by group mates can change the decision to join or leave a group so as vigilance changes because of changes in group size, group size may also change in response to changes in vigilance. Additionally, individuals may have poor information about the vigilance strategies of their neighbors. We present a game theoretical model of vigilance that incorporates dynamic group sizes and does not require behavioral monitoring of the vigilance strategies of others. For systems at equilibrium, maximum vigilance decreases with increased group size. Furthermore, by varying intraspecific competition we show an inverse relationship between group size and vigilance. Thus, we provide a mechanism in support of the many eyes hypothesis from an evolutionary game theory perspective and conclude that variation in intraspecific competition and its effect on group size may be responsible for the relationship.

Organization enhances collective vigilance in the hovering guards of Tetragonisca angustula bees

One benefit of group living is vigilance against predators. Previous studies have investigated the group size effect, where individual vigilance decreases as group size increases without reducing the overall ability of the group to detect predators. However, there has been comparatively little research on whether the positioning of individuals can improve the collective vigilance of the group. We studied the coordination of vigilance and its effect on predator detection in the eusocial bee Tetragonisca angustula. Nests are defended by hovering guards that detect and intercept intruders before they reach the nest entrance, in addition to those that stand upon it. We show that hovering guards are positioned nonrandomly, with a strong tendency for equal numbers on both sides of the entrance. This organization increases the collective vigilance of the guard group, as groups distributed in an even ratio, either side of the entrance, have a greater collective field of view than groups that deviate from an even ratio. Finally, we use a bioassay to show that when guards are on both sides of the entrance, their ability to detect intruders before they reach the entrance increases. Overall, our results provide strong evidence that vigilance is coordinated and that this improves nest defense. Although other group-living animals are often selfish in their individual vigilance behaviors and face competing time constraints such as foraging, the altruistic nature of eusocial insect workers has probably facilitated the evolution of coordinated vigilance, as documented here in T. angustula.

Movement patterns in Entomopathogenic nematodes: Continuous vs. temporal

To exploit resources, animals implement various foraging behaviors to increase their fitness. Entomopathogenic nematodes are obligate parasites of insects in nature. In previous studies, entomopathogenic nematodes were reported to exhibit group movement behavior in the presence and absence of insect hosts. However, it was not determined if group movement is continuous or temporal. For example, nematode movement behavior upon emergence from the host might start out in an independent fashion prior to aggregation, or group movement may be exhibited continuously. In the present study, we explored the propensity for innate group movement behavior of two insect parasitic nematodes in two families and genera: Heterorhabditis indica and Steinernema carpocapsae. We hypothesized the nematode populations would initially move independently from their origin and then come together for group movement. Movement patterns were investigated in sand when nematodes were applied in aqueous suspension (via filter paper) to a specific locus or when the nematodes emerged naturally from infected insect hosts. To compare nematode movement behavior over time and space, nematode dispersal was monitored at three distances (2.5, 4.5 and 8.0 cm) from the center (origin) and at two different time periods, 2 days and 3 days after nematode addition. We discovered that nematode dispersal continuously exhibited an aggregative pattern (independent movement was not observed). Results from both nematode species as well as the host-cadaver and filter paper (aqueous nematode suspension) application methods indicated a continuous aggregative pattern. The discovery of continuous aggregative movement patterns in steinernematid and heterorhabditid nematodes elucidates further the complexity of their foraging behavior and may serve as basis for exploring foraging behavior in other host-parasite systems.