Recent research has sparked a significant re-evaluation of Charles Darwin’s theory of sexual selection, particularly in relation to peacocks. A study led by Dr. Rama Shankar Singh, affiliated with the Centre for Peace Studies at McMaster University, reveals that peahens choose mates based on a combination of size, vigour, and beauty, rather than beauty alone, as Darwin proposed over 150 years ago.
Darwin introduced his theory of sexual selection in 1871 in his work, The Descent of Man. He suggested that female preference for ornamental traits, such as the peacock’s long tail, drove the evolution of these features. However, findings from recent research provide evidence that challenges this notion, demonstrating that the development of the peacock’s tail is explained by a simple developmental rule that accounts for its symmetry and complexity.
Darwin perceived the peacock’s extravagant tail as maladaptive; it seemed too impractical to be a beneficial trait in the context of natural selection. He famously expressed his dismay, stating, “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!” His view was rooted in two assumptions that the new research calls into question.
The first assumption was that maladaptation could not arise from adaptation. The study indicates that while the peacock’s long tail may hinder predator evasion, it can also enhance mating success. After the mating season, peacocks shed their long tails, indicating a trade-off between survival and reproductive success.
The second assumption was that peahens appreciate the tail’s beauty similarly to humans. Darwin believed that birds possess an innate sense of beauty, leading to an emphasis on aesthetic traits in mate selection. Yet, the extensive research conducted over the past three decades has found no conclusive evidence supporting the hypothesis that peahens select mates based on the eye-spots on the tails.
Dr. Singh’s investigation into the peacock’s long tail began unexpectedly, stemming from his work as a fruit fly geneticist. His analysis of museum specimens led to two crucial discoveries. He observed that the zigzag arrangement of feather follicles results in the tail’s remarkable symmetry and complexity. This arrangement is the densest known form of spherical packing, showcasing how such patterns can yield aesthetic beauty in nature.
Furthermore, the research revealed that peahens perceive tail size and eyespot colour as a single trait. The correlation between the size of the tail and the number of eyespots means that female peafowl do not evaluate these features separately. Instead, they respond to the overall appearance of the tail, integrating both size and coloration into their mate selection process.
Dr. Singh argues that this understanding diminishes the need for a separate theory of sexual selection. He points out that the processes of mate choice and sexual selection are integral to the broader framework of natural selection. This perspective aligns with earlier views, including those of naturalist Alfred Russel Wallace, who contended that sexual selection should be viewed as a component of natural selection rather than a standalone theory.
Darwin’s motivations for advocating his sexual selection theory were multifaceted. He sought an explanation for the evolution of secondary sexual traits that often appear exaggerated, particularly in birds like peafowl. Additionally, he aimed to address human racial differences, proposing that standards of beauty varied among races, which he believed contributed to social isolation.
Despite the passage of time, the controversy surrounding sexual selection persists, as evidenced by Dr. Singh’s findings. While the concept of mate choice is widely accepted, the theory of sexual selection itself remains contentious. This research encourages a critical re-examination of long-held beliefs and underscores the complexity of evolutionary processes in nature.
This study was supported by funding from the Natural Sciences and Engineering Research Council of Canada. The findings contribute to a growing body of evidence that challenges traditional views of mate selection and emphasizes the intricate relationships between traits in the animal kingdom.
