Wynne-Edwards—Lack disagreement over animal population regulation

The disagreement between Wynne-Edwards and David Lack is a dispute about the evolutionary explanation for animal population regulation: does reproductive restraint evolve for the benefit of a group, or because it maximizes the reproducing individual’s surviving offspring? Both accounts can explain why animals do not reproduce without limit, and sometimes even predict similar outcomes. Their difference lies in what natural selection acts on and whose reproductive success the behavior ultimately serves.

Background: regulation and group selection

Wynne-Edwards, the leading advocate of group selection in the debate, proposed that individuals deliberately reduce their birth rates for the good of the group. Groups whose members restrain reproduction, on this account, are less likely to exhaust their food supply and become extinct than rival groups whose members reproduce rapidly. The idea gives population regulation an altruistic interpretation: individuals forgo potential offspring so that the population as a whole can persist.

The argument carries an evident appeal when applied to people worried about population growth, but the book distinguishes that appeal from evidence about how animal behavior evolved. Its starting point is that family planning in animals is not in dispute: species tend to have characteristic clutch or litter sizes, rather than producing an unlimited number of young. The question is why natural selection favors those limits. Wynne-Edwards answers that individuals restrain reproduction for the group; Lack’s account instead asks which reproductive strategy leaves an individual parent with the most offspring that survive.

Lack’s alternative: offspring that survive

Lack developed his alternative through work on clutch size in wild birds. Different species tend to lay different numbers of eggs: gannets and guillemots incubate one at a time, swifts typically lay three, and great tits lay six or more. There is variation within species, too: some swifts lay two eggs, while great tits may lay twelve. If clutch size is at least partly under genetic control, competing hereditary variants can favor different numbers of eggs.

At first, a variant for laying more eggs might seem certain to prevail. But that reasoning would imply that five eggs are better than four, ten better than five, and an unlimited clutch best of all. The contradiction exposes what the simple “more is better” view leaves out: bearing more young can reduce the resources available to care for each one. Lack’s central proposal is therefore an optimum clutch size for each species in a given environment. That optimum is not necessarily the number that best serves the population as a whole; it is the number that allows an individual to rear the most surviving offspring.

If three eggs are optimal for swifts, for example, a parent attempting to rear four may leave fewer descendants than one attempting three. With food spread among too many nestlings, fewer may survive to adulthood. The costs begin before hatching, in the allocation of resources to the eggs, and continue when parents provision the young. A parent’s food and effort are limited, so the number initially produced must be balanced against the care available afterward.

The account makes that balance concrete in the great tit, whose parent brings an average of one item of food to the nest every 30 seconds of daylight. Producing eggs, building and using a nest, incubating, and feeding young all draw on parental resources. If those resources are divided among too many offspring, the parent may rear fewer than if the initial clutch had been smaller. In Lack’s explanation, selection adjusts clutch size to make the most of what an individual female, or a pair, can provide—not to express altruistic concern for the group’s resources.

Territoriality and the red grouse

Wynne-Edwards extended his population-regulation account beyond clutch size, treating territorial behavior and dominance hierarchies as parts of a broader social system that regulates breeding. The red grouse provides a pointed case. These birds feed on heather and divide moorland into territories that appear to contain more food than their owners need. They fight over territories early in the season; later, losing birds may cease fighting, remain without territories, and mostly starve. Only territory owners breed.

That pattern can appear to support the idea that unsuccessful birds accept a social limit on reproduction. Yet the evidence described also shows that an outcast can breed if a territory owner is shot: the vacancy is promptly filled by a former outcast. The birds are physically capable of breeding, and their failure to do so is linked to access to territory rather than an absolute inability to reproduce.

The individual-selection interpretation offers a different reason why an outcast might stop challenging territory holders. Repeated contests consume energy and may be futile. If the chance of inheriting a territory when its owner dies exceeds the chance of winning one through a fight, waiting may be the better strategy for the individual. The same apparent restraint can thus be understood not as a sacrifice made to preserve the group, but as a response to the relative prospects of fighting and waiting.

Overcrowding: similar outcomes, different explanations

Overcrowding can reduce birth rates, but that observation alone does not decide between the two theories. In an outdoor enclosure with plenty of food, mice bred freely until the population levelled off. Females became less fertile as a consequence of overcrowding and had fewer young. The observed change is compatible with Wynne-Edwards’s explanation, but it does not establish that group benefit is the mechanism behind it.

Wynne-Edwards’s account treats high population density as a warning that food may become scarce. Animals, programmed for conditions in the wild, would respond by reducing their reproductive output to protect the group from over-exploiting its resources. The individual-selection account can reach much the same behavioral prediction by a different route. If crowding reliably signals a future famine, a female whose expected resources have fallen may do better by having fewer offspring. Rivals who fail to adjust could bear more young yet rear fewer of them.

The point of the comparison is not that the two explanations predict different birth rates in every circumstance. In this case they can predict the same reduction. They differ in the proposed evolutionary benefit: restraint serves the group in Wynne-Edwards’s theory, while in Lack’s framework it serves the reproductive success of the individual parent. An observed response to crowding is therefore not, by itself, proof of group selection.

Flocking, deceptive signals, and the debate’s conclusion

Wynne-Edwards proposed that large gatherings could help animals estimate population density. Evening flocks of starlings and clouds of midges dancing over a gatepost, in his account, function as population censuses: sensory information about density could be linked to reproductive systems, much as a thermostat uses a thermometer. He did not need to suppose that animals consciously counted their companions. The proposed process was automatic, mediated by nervous or hormonal mechanisms. The book notes, however, that there is no direct evidence that animal aggregations actually serve this purpose.

Even if animals used such gatherings to estimate density, the individual-selection explanation could still accommodate the behavior. A female that estimates how many members of her species are present during winter might use that information to anticipate food competition the following spring and reduce her clutch accordingly. But once density affects clutch size, an individual could benefit by misleading rivals about the number present. If the volume of a winter roost’s noise signals population size, a starling that sounds like more than one bird could encourage neighboring birds to lay fewer eggs than would otherwise be optimal for them. The proposed deception is called the Beau Geste Effect, after a novel in which a unit of the French Foreign Legion uses a similar tactic. Reducing rivals’ offspring could benefit the individual whose genes would otherwise be absent from those offspring.

This possibility reverses the apparent meaning of a population display. A behavior that Wynne-Edwards could interpret as a collective census might, in an individual-selection account, be exploited for competitive advantage. The book’s conclusion is that parental family planning is best understood as optimization rather than public-minded restraint: parents tend to have neither too many young nor too few, but the number that maximizes the offspring they rear. The clash between Wynne-Edwards and Lack therefore persists even where their accounts converge on the visible result. Similar population patterns can arise from different selection processes, and the outcome alone does not settle whose interests explain them.