Public display behavior

Epideictic behaviour is Wynne-Edwards’s hypothesis that animals gather conspicuously to register population density and regulate reproduction. Its importance lies in how it turns a familiar sight—starlings crowding together at an evening roost—into a proposed mechanism of group selection, and how the book tests that interpretation against an alternative: individuals may respond to density for their own reproductive advantage, even if the resulting behaviour resembles population-wide restraint.

The population-census hypothesis

Wynne-Edwards coined the term for the idea that mass gatherings can function as a population census. Huge flocks of starlings assembling at evening, or crowds of midges dancing over a gatepost, would not merely be aggregations with some other ecological purpose. On this account, their gathering helps individuals estimate how crowded their population is. The hypothesized estimate then informs their reproductive systems: when density is high, animals automatically reduce their birth-rates in the interest of the group.

The proposed mechanism need not involve conscious counting. Wynne-Edwards imagined an automatic connection between an individual’s sensory perception of population density and its nervous or hormonal systems. The book compares this to a thermostat requiring a thermometer: a system that adjusts reproduction to population conditions would need some way to register those conditions. Epideictic behaviour supplies that measuring function, with the gathering itself making density perceptible.

This proposal belongs to Wynne-Edwards’s broader account of population regulation. He argued that animals deliberately and altruistically reduce reproduction for the good of the group. In that framework, a census-like display is not an incidental feature of social life; it is part of the process by which individuals restrain their birth-rates and keep the population from growing beyond sustainable limits.

Plausibility and evidential limits

The hypothesis is presented as striking and, on its face, plausible. The connection between population density and reproductive restraint offers an apparent solution to the question of how an animal could respond to conditions affecting its group. Yet plausibility does not establish that the proposed behaviour occurs. The book says there is no direct evidence that any animal aggregation is epideictic, and that Wynne-Edwards’s larger theory rests on many examples that can also be interpreted in more orthodox selfish-gene terms.

The evidential distinction matters because observing animals in a crowd does not by itself reveal what the crowd is doing, what information its members use, or why reproduction changes. To demonstrate the proposed mechanism, evidence would have to connect the animals’ perception of aggregation to a change in reproductive output. The book makes this gap explicit rather than treating the vividness of the starling or midge examples as proof.

It nevertheless considers what evidence might look like. Suppose winter overcrowding were shown to reduce fertility in the following spring, and the effect were shown to result directly from birds hearing one another’s calls. An experiment might expose some individuals to a recording of a dense, very loud starling roost and others to a quieter recording of a less dense roost, then compare the number of eggs laid. If the birds hearing the louder recording laid fewer eggs, that would indicate that their calls functioned as an epideictic display in the proposed sense. This is a hypothetical test, not a report that the experiment was performed or that the result was observed.

A gene-centered alternative

Even if such a density-sensitive response were demonstrated, the book argues that it would not establish group selection. A female that can estimate local population density in winter might use it to predict how difficult it will be to find food for her young in spring. If the population is unusually large, laying fewer eggs could serve her own reproductive interests: rivals that fail to respond to the same warning may bear more offspring but ultimately rear fewer. The behaviour would reduce births in a crowded population, yet its evolutionary explanation would be individual advantage rather than altruistic restraint for the group.

The same alternative recasts population regulation as family planning. Parents are expected to optimize the number of offspring they can successfully rear, not to limit births for a public good. Too many eggs can bring costs in care and survival; too few can also reduce the number of surviving descendants. Density could therefore provide information relevant to an individual’s best reproductive decision without requiring an evolved intention to protect the population as a whole.

The book also develops a further consequence of this individual-centered account. If a bird benefits when rivals interpret the population as denser than it really is, it may pay to exaggerate the apparent number of birds. If starlings use the volume of noise in a winter roost as an index of density, an individual that calls especially loudly could make the group sound larger. The proposed tactic, named the Beau Geste Effect after a novel in which a unit of the French Foreign Legion used a similar deception, would encourage neighbours to lay fewer eggs than their true optimum. Its hypothetical advantage would come from reducing the number of individuals who do not carry the caller’s genes.

Meaning and significance

This comparison leaves the observable display and its possible function distinct from the evolutionary explanation assigned to it. A crowd might indeed transmit information about density, and an individual might indeed adjust reproduction after perceiving that information. Neither fact alone shows that individuals gather in order to serve the group. The same outcome—reduced reproduction when numbers are high—can arise from selection acting on individuals whose own reproductive prospects depend on food, offspring survival, and competition.

The book’s conclusion is deliberately qualified: epideictic displays may be a good idea, and Wynne-Edwards may have been right about the phenomenon while wrong about why it evolved. In that sense, the concept is both a proposed ecological mechanism and a test case in evolutionary reasoning. A pattern that appears to promote collective restraint need not be an adaptation for collective welfare; its apparent group benefit may emerge from the competing reproductive interests of individuals and their genes.