Coping with strangers: how familiarity and active interactions shape group coordination inCorydoras aeneus

Abstract

Social groups whose members have had sustained prior experience with each other frequently exhibit improved coordination and outperform groups whose members are unfamiliar with one another. The mechanisms by which familiarity assists coordination are not well known. Prior social experience may simply allow individuals to learn the behavioral tendencies of familiar group-mates and coordinate accordingly. In the absence of prior social experience, it would be adaptive for individuals to develop strategies for coping with unfamiliar others to minimize the disadvantage of unfamiliarity. To explore the dynamics of familiarity in shaping group behaviors, we used a highly social catfish, Corydoras aeneus, that utilizes a distinctive, observable tactile interactions. Here we describe this tactile interaction behavior, physical “nudges” that are deployed to initiate group movements and maintain contact with group-mates during group movements. We then report the results of two experiments exploring the relationship between nudges and coordination. First, within triplets of two familiar and one unfamiliar individual, we found no individual differences in nudging rate based on familiarity. Despite all individuals interacting at similar rates, however, unfamiliar individuals failed to coordinate as well as their familiar group-mates, and were more frequently absent from group movements. Second, comparing pairs of familiar with pairs of unfamiliar fish, there was no difference in the level of coordination between pairs. Instead, we found that unfamiliar pairs exhibited significantly higher nudging rates, suggesting that unfamiliar pairs could compensate for their unfamiliarity by nudging more frequently. In contrast, familiar individuals coordinated with comparatively little nudging, presumably because they were experienced with each other. Overall, these results suggest that nudges can be used to improve coordination of group activities, but that their usage is reduced in the case of familiar individuals, implying that these potential signals may be costly.

The role of tactile interactions in flight responses in the Bronze Cory catfish (Corydoras aeneus)

“…Fishes that do not interact tactilely are not known to bump into one another, even during large, dynamic shoaling movements (Katz et al, ), and none of the Bronze Cory catfish in this study (or in any of our previous studies) collided with objects in their enclosures or the walls of their tanks, even when placed in novel environments (as were the fish in this study). While it cannot be excluded with absolute certainty that some tactile interactions were the result of unintentional collisions between individuals, we have also shown in previous studies that the frequency of interactions is affected by familiarity in a foraging setting, which adds support to our interpretation that nudging is a directed, intentional interaction between individuals (as in Riley et al, ). Consequently, we are confident in our assessment of nudging as a discrete social interaction that individuals utilize in a variety of contexts, with the current study focusing on its role in the context of group flight responses.…”

“…The nature of this behaviour, a tactile nudge, perhaps lends itself to maintaining cohesion, as the initiator must be in such close physical proximity as to touch the receiver of its nudge (for an example of this behaviour, please see the video in the Appendix S1). Although the current study does not definitively elucidate a causal link between nudging and increased cohesion, our previous work showed that, under nonthreatening conditions, unfamiliar fish achieve high levels of cohesion through increased rates of nudging compared to familiar fish, which exhibit high cohesion with comparatively low levels of nudging (Riley et al, ). This suggests that nudging is a directed social behaviour that is influenced by familiarity and context, and not a mere consequence of proximity.…”

“…The fact that some of these nudges resulted in “recruitment” in the sense that the receiver had been stationary prior to the nudge and then initiated a flight response following the nudge implies that nudges can alter the behaviour of receivers and potentially alert them to the presence of a threat. Nudges were also associated with coordinated movements in pairs of fish when not exposed to threats (Riley et al, ). The consistency in nudging in triplets during exploration/foraging and while responding to potential threats that we report here suggests that nudging is a behaviour that is useful for coordination under different conditions and that nudging more frequently during exploration/foraging may contribute to a group's coordination during responses to a potential threat.…”

“…We are confident that, on the whole, these nudges represented directed interactions from the initiator to the recipient rather than accidental physical contact because our previous work has shown that nudges are associated with group coordination and allow unfamiliar individuals to coordinate with their group-mates as effectively as familiar individuals (Riley, Gillie, Johnstone, Boogert, & Manica, 2018). In particular, unfamiliar pairs show increased nudging and coordinate at similar rates to familiar pairs; pairs show increased nudging rates while engaged in coordinated movements as compared to when not coordinating; finally, unfamiliar group members in a triplet tend to spend more time alone (i.e., not in proximity with the other two members of the triplet that had previously been familiar with one another), but unfamiliar group-mates who initiate more nudges tend to exhibit higher cohesion with their groupmates (beyond the effects of proximity) (Riley et al, 2018). For a full ethogram detailing our scoring procedures and assessment criteria for nudging during our familiarity and coordination study, see Riley et al (2018).…”

“…In particular, unfamiliar pairs show increased nudging and coordinate at similar rates to familiar pairs; pairs show increased nudging rates while engaged in coordinated movements as compared to when not coordinating; finally, unfamiliar group members in a triplet tend to spend more time alone (i.e., not in proximity with the other two members of the triplet that had previously been familiar with one another), but unfamiliar group‐mates who initiate more nudges tend to exhibit higher cohesion with their group‐mates (beyond the effects of proximity) (Riley et al, ). For a full ethogram detailing our scoring procedures and assessment criteria for nudging during our familiarity and coordination study, see Riley et al (). This body of work and our field observations have led us to hypothesize that this active physical contact between individuals might be a mechanism by which groups coordinate responses to predators.…”