In this blog post, we will examine the key logic and examples that demonstrate how certain cases of brood parasitism in birds—which are difficult to explain using Dawkins’ “selfish gene” theory—can be explained more convincingly through an extension of ESS theory.

Introduction

Anyone who has read Richard Dawkins’ book *The Selfish Gene* will know that it attempts to answer the question “Why does life exist?” from the perspective of genes. The author argues that the selfishness of genes shapes an individual’s behavior and presents brood parasitism (a parasitic reproductive strategy in which an organism lays its eggs in the nest of another species, causing that species to raise the offspring) as a prime example. Birds that practice brood parasitism appear to demonstrate the selfishness of genes because they can lay more eggs and spread their genes more widely by not raising their own young.
However, Dawkins’ explanation alone leaves several questions unanswered. If all genes are selfish, shouldn’t all birds adopt the brood parasitism strategy when there is no clear disadvantage to doing so? In reality, brood parasitism is observed only in certain species. In this text, we will use this question as a starting point to discuss an attempt to better explain the phenomenon of brood parasitism by extending ESS theory.

Dawkins’ Explanation of the Brood Parasitism Phenomenon

Dawkins explains the phenomenon of brood parasitism from the perspective of his core argument: “The selfishness of genes drives our selfish behavior.” Genes are replicators, and replicators evolve in a direction that maximizes the number of copies of themselves they leave behind. He views genes as “programming” an individual’s behavior as a means to preserve and propagate themselves through that individual. Therefore, an individual’s selfish behavior is explained as stemming from the gene’s selfish goal—namely, the desire to leave behind more copies of itself.
From this perspective, brood parasitism is plausibly explained as a strategy to increase one’s reproductive success by exploiting the care provided by another species. If brood parasitism allows for reduced rearing costs and the laying of more eggs, the gene will spread more widely; thus, from the perspective of genetic selfishness, brood parasitism is natural.

Contradictions in Dawkins’ Explanation of Brood Parasitism

However, there is an unresolved issue in Dawkins’ explanation. If genes are selfish, and if brood parasitism causes no actual harm, shouldn’t more species adopt this strategy? In fact, brood parasitism is observed only in certain species, and the majority of birds do not engage in it. This reveals a discrepancy between Dawkins’ general explanation and actual observations.
Furthermore, considering that the offspring of brood parasites may suffer a survival disadvantage because they are not directly protected, there is a conflict between the individual-level benefit and the offspring-level cost. Since Dawkins’s argument focuses primarily on the selfishness of individuals and genes, it fails to provide a sufficient explanation for why only some species adopt the brood parasitism strategy. Therefore, without rejecting the theory of gene selfishness, an additional theory is needed to explain why not all individuals follow the same strategy.

The Emergence of Evolutionarily Stable Strategies (ESS)

To address these questions, Dawkins introduces the concept of the Evolutionarily Stable Strategy (ESS) to explain why not all individuals within a species exhibit the same selfish tendencies. An ESS refers to strategy S in a population where, when most individuals use strategy S, no other strategy T can penetrate that population. In other words, if changing a strategy that is already widespread in a population does not increase an individual’s benefit, that strategy is stable.
The core of ESS lies in a complex cost-benefit analysis that goes beyond simple calculations of immediate gains. For example, while eliminating a competitor may seem advantageous in the short term, in the long term, another competitor may fill that void or reduce the overall benefit of the group. Due to these interactions, a specific mix of strategies can be stably maintained.
As a simple example, let’s assume there are two types of fighting strategies: “hawks” and “doves,” where the winner receives 50 points, the loser 0 points, a draw –100 points, and a long, drawn-out battle that wastes time –10 points. If everyone is a dove, the fight becomes a long-drawn-out ritual with minimal injury, resulting in a favorable average score. If a single hawk-type mutation appears in this group, that hawk earns high scores in every fight, increasing its gene frequency. However, if the entire group shifts to hawks, many severe injuries occur in clashes between hawks, causing the group’s average score to plummet. Thus, when the average score reaches a point where it is the same regardless of the mix of different strategies, that state becomes the ESS.
Therefore, from the ESS perspective, by considering group-level interactions and the balance between strategies—which cannot be explained by gene selfishness alone—we can understand why strategies such as brood parasitism are stably maintained only in certain species.

Application of Evolutionarily Stable Strategies (ESS) in Interspecific Competition

ESS theory explains why, when fighting strategies emerge within a species, a balance is reached based on whether each strategy benefits the individual according to the self-interest of genes—addressing questions such as why not all competitors are eliminated. Therefore, questions such as why brood parasitism appears only in some individuals can also be explained by extending ESS theory, since interspecific competition—viewed as a type of fighting strategy—exhibits patterns similar to intraspecific competition.
The first reason is that ESS theory does not deviate from the theory of “The Selfish Gene.” If ESS theory were based on premises entirely different from Dawkins’ theory, it would not be suitable for explaining brood parasitism. Looking at the calculations in Section 4, the average score for each individual is +6.25 points, which is much lower than the +15 points achieved when everyone adopts the dove strategy. Ultimately, even when considering only individual gains rather than overall gains, it would be more advantageous for everyone to follow the dove strategy; however, due to the “selfishness of genes,” individuals end up adopting strategies that do not allow them to pursue their own maximum benefit. This demonstrates that ESS theory can be applied to this problem, as it serves as an example of how different strategies are adopted due to the selfish nature of individuals.
Second, both the selection of the fighting strategy and the phenomenon of brood parasitism exhibit patterns of selection driven by the selfishness of genes. Therefore, if the selection of the fighting strategy follows ESS, this condition of selection based on the selfishness of genes increases the likelihood that ESS can also be applied to the problem of interspecific selfishness. In particular, since brood parasitism is a problem of interspecific selfishness, the extension of ESS theory can serve as a sufficient explanatory basis. However, for interactions that transcend species boundaries, such as parasitism and symbiosis, it is often impossible to alter the role of genes; consequently, there are aspects that are difficult to explain through gene selfishness, making it likely that the application of ESS will be limited to problems of interspecific competition.
Finally, when we examine various scenarios by extending ESS to interspecific problems, we find that ESS is smoothly applied and explains interspecific selfishness under the framework of “The Selfish Gene” theory. In this regard, the earlier question regarding brood parasitism can be sufficiently explained by the fact that a certain ratio is established when using ESS theory.

Calculating ESS in the Brood Parasitism Problem

As mentioned earlier, the brood parasitism phenomenon can be treated similarly to the hawk-dove problem. Therefore, just as we scored hawk and dove strategies to calculate ESS, let us score brood parasite and non-brood parasite strategies to calculate the ESS for the brood parasitism problem.
For simplicity, we assume that only brood parasite and non-brood parasite strategies exist. For example, we assign +50 points to a bird that has laid an egg in another’s nest, 0 points to a bird whose nest has been parasitized, −50 points to a situation where all nests are parasitized and thus unfit for rearing, and −10 points for the time wasted due to not laying an egg in another’s nest. Assuming these scores ultimately translate into reproductive success, we consider that the genes of individuals with higher scores are more likely to remain in the gene pool.
A population consisting entirely of non-brood parasites never engages in brood parasitism, so its average score is −10 points. If a brood parasite mutation appears in this group, that individual will certainly gain an advantage in non-brood parasite nests, scoring +50 points in every case, so the brood parasite gene will spread rapidly. However, if the entire population becomes exclusively brood parasitic, competition among brood parasites will frequently lead to situations where rearing is impossible, resulting in individuals receiving −50 points; in this case, the population’s average score becomes −25 points.
If a non-brood parasitic individual reappears in this state, the average score for non-brood parasitic individuals is −10 points, so the non-brood parasitic gene will increase. In this way, individuals adopt strategies that yield more points based on their own interests—that is, the selfishness of their genes—and an equilibrium is established at the point where the average scores of the two strategies become equal. Therefore, the phenomenon observed in reality, where not everyone engages in brood parasitism but rather stops at a certain proportion, can be explained from the ESS perspective.

Conclusion

As seen in the calculations in Section 6, we can conclude that the phenomenon of brood parasitism also settles at a fixed proportion when the ESS theory is applied. However, the calculations in Section 6 are a simplified example based on the book’s calculations. They have limitations, such as the difficulty of assigning precise scores and the lack of further refinement for various situations, making it hard to determine an exact ratio. Nevertheless, despite these limitations, extending ESS theory allows us to explain the brood parasitism phenomenon—which could not be fully accounted for by *The Selfish Gene* alone—at a specific ratio, thereby demonstrating its validity.
Furthermore, the application of ESS in this discussion is not limited to resolving the brood parasitism phenomenon; as mentioned in Section 5, it can be applied to various interspecific issues using the same framework. In other words, it is useful in that it complements and explains the contradictions in *The Selfish Gene* theory arising from interspecific competition from the perspective of ESS.