I’m looking at the fish in the mirror: a tail of social signaling

Just as we exploit social media to self-promote, find mates, and flaunt social status, animals use visual, olfactory, auditory, or mechanical displays to communicate with one another. Like a Facebook status, these displays often communicate some internal attribute about the organism. A well-known example of this type of signaling is the vibrant tail-feathers of the male peacock. Males with iridescent tails signal to females that they are good mates, while those with less impressive tails signal that they may be unfit mates [1]. However, if some traits can easily signal that an organism is desirable, why don’t all organisms just display the desirable trait regardless of their true quality? In other words, how is the honesty of a signal maintained? Why don’t animals lie all the time?

A male peacock displaying his iridescent feathers. Image from Wikimedia commons.

This question is exactly what Bachmann et al. tried to figure out in their 2017 study [2]. They reviewed two possible hypotheses for what maintains honest signals. The first hypothesis is termed the costly signaling hypothesis and can basically be broken down like a cost-benefit analysis. The backbone of this hypothesis is that producing or displaying a signal must be costly in some way. If you are a fit organism, then the benefits of producing the signal will outweigh the costs. If you are an unfit organism, however, the costs of producing or displaying the signal may outweigh the benefits. In this way only fit organisms display the signal that indicates their internal attribute (e.g. mating potential or dominance status), thus maintaining the honesty of the signal. The second hypothesis is called the social policing hypothesis, where the community at large polices and punishes individuals who display dishonest signals. While both of these hypotheses have merit, not a lot of work has been done on how to determine which mechanism maintains honesty in a system. Most studies up until this point simply assumed that signals were costly, and instead focused on figuring out the origins of said costs. In their study, however, Bachmann et al. test the mechanisms responsible for maintaining honest signals in a system.

Before performing any experiments, Bachmann et al. first proposed a framework for identifying the mechanisms responsible for maintaining honesty in a system. This framework has two essential parts: 1) determine the signal and what it communicates, and 2) determine if society punishes those who display a dishonest signal. If dishonest signals are punished then social policing is responsible for maintaining the honesty of that signal. If dishonest signals go unpunished, however, Bachmann et al. argue that the honesty of that signal must be maintained via other mechanisms (i.e. the costly signaling hypothesis). As a proof of concept for this framework, the authors proceeded to use these steps to determine what mechanism maintains honest signaling in a group of fish called Princess of Burundi cichlids (Neolamprologus brichardi).

The facial stripe of a Princess of Burundi cichlid (Neolamprologus brichardi). Image from Wikimedia commons.

These cichlids are a great study system for animal communication, because they are highly social and adhere to a strict hierarchy. This means that individuals in a group are constantly communicating with one another to perform tasks and maintain the group dynamic. In the first set of experiments, Bachmann et al. set out to determine what visual signals these cichlids use, and what those signals communicate (part one of their framework). They first measured the conspicuousness of the cichlid facial colors–known as their facial mask—and predicted that a dark-colored, highly conspicuous mask would be used for communication, while a light, inconspicuous mask would most likely not be used for communication. They found that the facial mask, specifically the stripe of the mask, was highly conspicuous and most likely used in communication. They next asked what the facial stripe communicates. Bachmann et al.  hypothesized that the stripe is used to communicate during territorial disputes. To test this, they staged territorial disputes between cichlids, and measured the conspicuousness of the facial stripes before and after. They found that while both cichlids began a dispute with dark facial stripes, the loser of the dispute would immediately pale their facial stripe. This indicates that a cichlid’s facial stipe provides a real-time status update on their current motivation to fight. With the signal and meaning in hand, Bachmann et al. next attempted to complete the second part of their framework: determining if society punishes those who display dishonest signals.

Bachmann et al. hypothesized that if social policing maintained honesty of signals, then fish that lie about their status (displaying either a darker or paler stripe than their actual aggressive intent) should be punished by the community. They investigated this by measuring and comparing levels of aggression (quantified by number of aggressive strikes, and latency to strike) towards three different groups of fish:

Group Treatment What this Means
Liar Group 1 Painted facial stripe to be darker Fish is less aggressive than its stripe indicates
Liar Group 2 Painted facial stripe to be paler Fish is more aggressive than its stripe indicates
Honest Group (Control) Facial stripe left un-manipulated Fish is as aggressive as its stripe indicates

To measure levels of aggression between these three groups, the authors utilized a special kind of assay called a mirror image stimulation (MIS) assay. MIS assays measure the aggression of a fish using its own mirror image instead of using another fish as an opponent. These assays are used, because they ensure that a fish and its opponent (its mirror image) are equally matched in every way (e.g. size, posture, displays, color, etc.). Bachmann et al. measured aggression for each fish twice using MIS assays. Once where they painted a fish’s facial stripe to be either paler or darker (liar groups 1 or 2) and once where they left the strip un-manipulated (honest group). They then compared the levels of aggression each fish displayed during its honest trial to the levels displayed during its liar trial. They found that aggression was increased in both liar groups compared to the honest group. Essentially, fish evaluated whether the facial stripe of its opponent (its mirror image) matched its posturing/behavior. If the fish identified a mismatch—such as in the liar groups—they attacked much more. This indicates that social policing is playing a role in maintaining this signal!

These findings have implications on a few different scales. First, Princess of Burundi cichlids use social policing to maintain honest signals of aggressive intent. This is pretty interesting, because it implies that the real-time feedback on aggression is necessary for this group. The authors didn’t delve into the ecological reasons for this, but it seems logical that it may have something to do with maintaining the hierarchy structure within this group.  Second, this study provides an easy to use framework for determining the mechanisms behind honest signals. Hopefully, this means that future studies can utilize the framework to determine what maintains honest signals in other organisms. Collecting this type of empirical evidence could eventually help determine how prevalent social policing is.  Finally, these finding are generally important for evolutionary biology, because it suggests that forms of social selection may be a powerful force akin to sexual or natural selection.

[1]      A. Loyau, D. Gomez, B. Moureau, M. Théry, N. S. Hart, M. Saint Jalme, A. T. D. Bennett, and G. Sorci, “Iridescent structurally based coloration of eyespots correlates with mating success in the peacock,” Behav. Ecol., vol. 18, no. 6, pp. 1123–1131, 2007.

[2]      J. C. Bachmann, F. Cortesi, M. D. Hall, N. J. Marshall, W. Salzburger, and H. F. Gante, “Real-time social selection maintains honesty of a dynamic visual signal in cooperative fish,” Evol. Lett., pp. 269–278, 2017.



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