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I’ll stand guard for you!

Article: Brandl SJ, Bellwood DR (2015) Coordinated vigilance provides evidence for direct reciprocity in coral reef fishes. Nature Scientific Reports 5:14556 doi:10.1038/srep14556


Understanding Coordinated Vigilance

Cooperation is a key trait that we, as humans, have been taught from a young age. In general, humans follow the model “tit for tat” or the expectation that if we perform an altruistic act, another person will perform an altruistic act in return. For example, giving blood. We voluntarily take time out of our day to donate our blood, with no expectation of an immediate reward. But, in the future, if we are ever in need of blood, we expect the help of generous blood donations made by others. This ‘reciprocal altruism’ is hard to come by in other animals, however. Reciprocal exchange has been witnessed in several species, such as the interspecific cleaning behavior of cleaner wrasses, but these behaviors are based more on by-product mutualism, having an immediate reward to both parties, rather than a true altruistic act with a future pay off. True reciprocal altruism needs a suite of complex cognitive abilities and many animal species have been thought to lack the appropriate social and cognitive skills to perform reciprocal altruism. First, you must recognize an individual as your partner, then you must have the capacity to recall their previous actions, and finally you must have the ability to perform a costly action to yourself that benefits this individual, based on the expectation that this will entail a future reward for yourself.

A few highly social bird and mammal species have shown evidence of true reciprocal altruism, and now fishes are thought to perform this behavior as well. In this study, Brandl and Bellwood set out to monitor rabbitfishes and quantify their behavior with one another, to truly assess whether their behavior exhibits that of reciprocal cooperation.


What was observed

fig 1

Fig 1. Angles were assessed based on a vertical line perpendicular to the substratum using a protractor superimposed on the screen. 90° denotes a horizontal position parallel to the substratum (a), while 30° mark a head-down position (b), and 150° mark a head-up position (c). The head-up position (ranging from ~90° to 180°) was identified as a vigilance position. Illustrations are original drawings by Simon J. Brandl.

Scuba divers set off to observe and videotape four species of pairing rabbitfishes on the Great Barrier Reef. The divers were specifically looking at the pair’s behavior and orientation as they actively swam, hovered, foraged, and interacted with other fishes (Fig. 1).

In all four rabbitfish species, it was found that one pair member would assume a stationary, upright position in the water (a 90° angle), while the partner would feed. The feeding individual would often be found penetrating deep into crevices, obstructing its visual field (Fig. 2). In other words, one partner stays on guard while the other one feeds. They are literally watching each others back! The paired fishes would alternate between foraging (eating) and the vigilance position (guarding position) and with one fish on guard, the foraging fish can focus on eating, have more consecutive bites per foray and go deeper into crevices to forage.

fig 2

FIg 2. The foraging individual (in the head-down position) feeds in cracks and crevices in the substratum, while the vigilant individual is positioned in the water column with its head up. Note the obstructions to the visual field of the forager, suggesting high vulnerability to predation and the unobstructed field of perception of the vigilant fish. (a) Siganus corallinus, (b) S. vulpinus, (c) S. doliatus, (d) S. puellus. Photographs taken and owned by Jordan M. Casey, who gives permission for their publication under an Open Access license.

The upright position of the vigilant fish enables a greater ability to detect predators compared to a foraging fish. Less than 1% of the total behaviors studied were aggression behaviors towards other pairs, and pair bonds appeared to be relatively stable, suggesting there would be no need to continuously look for a new partner. It was also found that the vigilant individual was generally in charge, determining where they went and when they moved foraging positions. Approximately 95% of all cases were when the foraging fish responded to actions or potential warning cues generated by the vigilant individual. The frequent occurrence of fin-flicks prior to abandoning the vigilant position indicates intentional communication between the two fish. While the significance of fin-flicks remains to be determined in rabbitfish, fin-flicks generate an acoustic signal, which means communication between the forager and the vigilant fish can happen even when visual contact is limited.





So, do rabbitfish really perform reciprocal altruism?

After observing this obvious cooperation behavior within rabbitfish pairs, the question remains: Is this simply a by-product mutualism/pseudo-reciprocity or do rabbitfish pairs satisfy the requirements of direct reciprocity? For the rabbitfishes observed in this study several key characteristics stand out to support the requirements of true reciprocal altruism:

-The rabbitfish pairs were stable and cooperation is likely to continue between the same individuals.

-A higher food intake is more probable within a paired rabbitfish than a solitary rabbitfish, due to the fact that a paired individual can forage longer because his partner is watching his back.

-The constant alteration between foraging and standing guard between the individuals in a pair show a balanced partnership.

All of these observations are consistent with direct reciprocity and suggests that the complex cognitive and social skills assumed necessary for reciprocal altruism are evident in rabbitfishes, which may be advantageous, but not necessarily essential, to the evolution of fishes. These findings will most likely ignite efforts to better understand fish species and could shift the way we study them, now knowing that they do in fact have the capacity for complex social behaviors.



  1. […] interested in other posts about mutually beneficial relationships among fish, you should check out yesterday’s post! Rebecca Flynn I am a recent M.S. graduate from the University of Rhode Island, where I studied […]

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