Mechanism of Male Choice: Sex Ratios

In this short chapter I want to discuss the role of sex ratios in choosiness. So far, we have mostly reviewed intrinsic reasons for male choosiness to be expressed such as male investment and female quality; however, sex ratios may also be important drivers of choosiness. Sex ratios are important in population biology and influence the evolution and structure of mating systems. Most important for the purpose of this book is that they can change quickly in time and space. Male and female choice are sensitive to such changes and can lead to situations where females are choosy when they are rare in a population but change to courtship and competition when males are rare. There are not many examples of this process, but there are likely some that have been overlooked. Interestingly, the majority of data on preferences are collected using binary choice tests, which almost always represent a 2:1 sex ratio. Furthermore, sex ratios do not take into account differences in mate quality, as all adult individuals are classified as either male or female without making any further distinction.

Flexibility of nuptial colouration in a unique ecotype of Threespine Stickleback (Gasterosteus aculeatus)

Nuptial colouration in animals may serve as a signal of competitor and/or mate quality during breeding. In many temperate fishes, nuptial colouration develops during discrete breeding seasons and is a target of sexual selection. We examine nuptial colouration and behaviour of a unique ecotype of Threespine Stickleback (Gasterosteus aculeatus Linnaeus, 1758), wherein males turn from dull brown-grey to pearlescent white during the breeding season. The main goal of this work was to determine the relative role of white colouration in intersexual competition and mate choice. In a combination of a field and laboratory work, we found that males are brightest white when engaging in courtship activities in the presence of a female; this indicates that white colouration may be primarily related to enhancing signalling during mate attraction. White colouration intensity increased as the breeding season progressed and may be related to an influx of conspecifics. Colour change from cryptic grey to bright white occurred rapidly (< 90 seconds) and may be deployed to enhance behavioural signals. We conclude that bright white colouration in the white ecotype is a potential signal of mate quality and may have evolved from a previously existing capacity for colour plasticity in common Threespine Stickleback.

Physical cues of partner quality

The dominant evolutionary theory of sexual attraction posits that attraction serves as a psychological mechanism for identifying healthy, fertile, and appropriate mates. According to this theory, humans and animals display cues that reflect their mate quality and are perceived as attractive by potential mates. There is evidence for such valid cues in human faces, bodies, and in non-bodily traits, which include adornments and items that signal provisioning ability, creativity, artistic skills, or conspicuous consumption. In this chapter, we discuss the evidence for the existence of these facial, bodily, and non-bodily cues, and for their role in communicating aspects of partner quality, including health, fertility, developmental stability, genetic quality, and potential for parental investment. We further discuss sex differences in the kinds of physical cues that men and women rely on in mate choice. We conclude by noting how central and evolutionarily important physical cues are even in contemporary sexual selection, and how the importance of physical cues of partner quality manifests in evolutionarily novel inventions such as physical self-enhancements, social media, and online dating.

Origins of music in credible signaling

Music comprises a diverse category of cognitive phenomena that likely represent both the effects of psychological adaptations that are specific to music (e.g., rhythmic entrainment) and the effects of adaptations for non-musical functions (e.g., auditory scene analysis). How did music evolve? Here, we show that prevailing views on the evolution of music — that music is a byproduct of other evolved faculties, evolved for social bonding, or evolved to signal mate quality — are incomplete or wrong. We argue instead that music evolved as a credible signal in at least two contexts: coalitional interactions and infant care. Specifically, we propose that (1) the production and reception of coordinated, entrained rhythmic displays is a co-evolved system for credibly signaling coalition strength, size, and coordination ability; and (2) the production and reception of infant-directed song is a co-evolved system for credibly signaling parental attention to secondarily altricial infants. These proposals, supported by interdisciplinary evidence, suggest that basic features of music, such as melody and rhythm, result from adaptations in the proper domain of human music. The adaptations provide a foundation for the cultural evolution of music in its actual domain, yielding the diversity of musical forms and musical behaviors found worldwide.

Giving Guys Get the Girls: Men Appear More Desirable to the Opposite Sex When Displaying Costly Donations to the Homeless

One of the evolutionary adaptive benefits of altruism may be that it acts as an honest (reliable) signal of men’s mate quality. In this study, 285 female participants were shown one of three video scenarios in which a male target took £30 out of a cash machine (ATM) and gave either a lot (£30), a little (£1), or nothing to a homeless man. The participants rated the male target on his attractiveness, their short- and long-term mate preferences towards him, and the degree to which they thought he was likely to possess various parenting qualities. The results showed that, regardless of whether the man was described as rich or poor, participants rated him as being more attractive when he donated money, but only when the donation was costly (£30). In addition, altruism was shown to be important in long-term, but not short-term mate choice, and displays of altruism were associated with positive parenting qualities. It is argued that displays of altruism act as a reliable (honest) mate signal for a potential long-term parental partner.

Origins of music in credible signaling

How did music evolve? We show that prevailing views on the evolution of music — that music is a byproduct of other evolved faculties, that music evolved for social bonding, and that music evolved to signal mate quality — are incomplete or wrong. We argue instead that music evolved as a credible signal in at least two contexts: coalitional interactions and infant care. We suggest that basic features of music, including melody and rhythm, result from adaptations in the proper domain of human music, providing a foundation that cultural evolution shapes into its actual domain.

Coevolution of song and egg coloration: multimodal mating signals?

The divergence of reproductive traits frequently underpins the evolution of reproductive isolation. One of the most enduring puzzles on this subject concerns the variability in egg coloration among species of tinamou (Tinamidae) —a group of birds endemic to neotropics (Cabot 1992). Specifically, some tinamous lay glossy and colorful eggs while others lay less colorful eggs. Here I tested the hypothesis that tinamou egg coloration is a mating signal and its diversification was driven by reinforcement. For most tinamou species, the male guard the nest that is sequentially visited and laid eggs in by multiple females. The colorations of the existing eggs in the nest could signal mate quality and species identities to the upcoming females to the nest, preventing costly hybridization, thus were selected to diverge among species. If so, egg colors should coevolve with the known mating signals as the tinamou lineages diverged. The tinamou songs are important mating signals and are highly divergent among species. I found that the egg luminance was significantly associated with the first principle component of the song variables among 31 tinamou species (after correcting for phylogenetic signal). Egg color and songs could be multimodal mating signals that are divergently selected as different tinamou species diverged. Mating signal evolution could be opportunistic and even exploit post-mating trait as premating signals.