Epideictic behaviour

Epideictic behaviour is Wynne-Edwards’s proposed explanation for certain animal aggregations: crowds, flocks, and shoals might provide individuals with information about population density, which in turn could regulate their reproduction. The hypothesis makes mass gathering part of a population-control mechanism, but the account distinguishes its plausibility from its evidential standing and shows that a similar outcome could arise through individual reproductive interests rather than restraint for the group.

The proposed census

Wynne-Edwards coined the term for deliberate massing that would, in his hypothesis, facilitate a population census. Huge evening flocks of starlings and midges dancing over a gatepost serve as examples of aggregations he interpreted in this way. He did not propose that animals consciously count their fellows. Instead, he imagined an automatic nervous or hormonal mechanism linking perceived population density to the reproductive system, much as a thermostat depends on a thermometer to register conditions.

The idea belongs to a broader account of animals limiting their birth-rates when population density is high. If reproduction is restrained for the group’s benefit, a mechanism must provide information about how crowded the population is; epideictic behaviour supplies a possible means of obtaining it. In this explanation, aggregation is not merely a by-product of animals living together but a display with a proposed regulatory function.

Evidence and competing explanations

The hypothesis is presented as plausible on its face, but its evidence is described as weak: numerous examples can be interpreted in Wynne-Edwards’s terms, yet they can also be explained along more orthodox selfish-gene lines. The account reports no direct evidence that any animal aggregations are epideictic. The term therefore names a proposed function, not an established finding that animals gather in order to conduct a census.

A hypothetical starling experiment clarifies what evidence might support the claim. If winter overcrowding reduced fertility the following spring, and that effect resulted specifically from birds hearing one another’s calls, researchers could compare individuals exposed to recordings of dense, loud roosts with individuals exposed to quieter, less dense roosts. If the first group laid fewer eggs, the calls would qualify as an epideictic display under the hypothesis. This is offered as a possible test, not as a reported experiment or result.

Interpretation through individual selection

Even if such a connection between calls and reproduction were demonstrated, the account argues, it would not by itself establish group-benefit restraint. Genes associated with producing a larger family than an animal can support would be penalized when their carriers ultimately rear fewer young. An individual that responds to signs of crowding by reducing reproduction could therefore achieve a better result for its own reproductive prospects than rivals who continue to produce more offspring. Density-sensitive reproduction might consequently yield a population-level effect resembling Wynne-Edwards’s proposal while arising through selection on individual interests.

Epideictic behaviour thus marks a distinction between observing a collective pattern and establishing its evolutionary explanation. A flock may be interpreted as a population census, but that interpretation requires evidence that aggregation cues affect reproduction; even then, the same causal link may fit a different account of selection. The concept’s importance lies in making that explanatory question explicit, while the discussion leaves the proposed census unconfirmed.