Sexual Selection in Language, Music and Birdsong ΠCommon Themes and Issues
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SEXUAL SELECTION IN LANGUAGE, MUSIC AND BIRDSONG œ COMMON THEMES AND ISSUES MSc Dissertation Sarah Fisher August 2006 1 Introduction Sexual selection plays a major role in evolutionary biology, shaping some of nature‘s most spectacular beauties and intricate systems. The interplay between competition, display and choice is a major driving force in shaping the world around us. Human beings are very sexual creatures, and display competition, selectiveness and jealousy when it comes to our partners. To what extent did sexual choice shape our evolutionary history? Studies in the evolution of language have considered a role for sexual selection in the development of our most unique and mysterious trait, and more recently, in the emergence of music. However, the theories put forward are for the most part hesitant and conflicting. This study attempts to gather and assess some of these theories, while considering the insights available from another group of adept vocal learners and performers, the oscine songbirds. 2 CHAPTER ONE œ THEORIES OF SEXUAL SELECTION Sexual selection operates in two major ways œ firstly on traits that give advantage in intrasexual competition (for example antlers, claws), and secondly on traits that give advantage in choice by the opposite sex (for example bright plumage or attractive song) (Andersson 1994) Surprisingly, though both modes of selection were mentioned by Darwin in the Descent of Man (Darwin 1871), the latter was for the most part, disregarded until the mid to late 20th century. Runaway Sexual Selection R.A. Fisher (1915, 1930) is credited with the discovery of the —runaway sexual selection“ phenomenon œ a coevolution of male traits and female preferences. Fisher suggested that both the male trait and the female preference were selected. Females with adaptive preferences produce offspring that carry genes for a) the adaptive preference and b) the trait the preference is for. Therefore the trait and preference are genetically coupled through evolution. Fisher also noted that sexual selection might lead to sympatric speciation, as different trait/preference dyads escalated in different directions, ultimately segregating populations. 3 Fisher‘s work was apparently prompted by this intendedly sarcastic comment by Morgan (1903) œ —Shall we assume that still another process is going on, that those females whose taste has soared a little higher than that of the average … select males to correspond, and thus the two continue heaping up the ornaments on one side and the appreciation of these ornaments on the other? No doubt an interesting fiction could be built up along these lines, but would anyone believe it, and, if he did, could he prove it?“ Andersson (1994) suggests that sexually selected traits begin as having a selective advantage for the organism, e.g. a long tail conferring agility. The trait begins as a straightforward index of the individual‘s abilities. When it becomes attractive to the female (possibly as a trait they would like for their offspring) sexual selection begins to act on it and it becomes exaggerated. Mortality from environmental disadvantages of the trait (for example, increased predation, reduced movement, difficulty foraging) eventually halts the cycle. Therefore, sexual selection can act more freely on traits less likely to incur these problems. Song may be considered an example of one of these, as an individual can stop and start singing, whereas a physical, external feature, such as a bright tail, is a permanent fixture. Zahavi & Zahavi (1997) complain that in Fisher‘s model, traits are arbitrary and that their exaggeration is in fact useless and detrimental. Their only function is to 4 pander to female choice. Females continue to choose these decorated, worse- quality males only for the indirect benefits of passing on the display qualities to their offspring. They describe Fisher‘s situation as a —catch 22“ from which neither the males nor females can escape; a male without the hampering display feature may in fact be better quality, but he will fail to attract any mates. Similarly, the female who chooses a non-displaying male may have fit offspring, but they will themselves find it difficult to reproduce. Despite detriment to both parties, the system drives itself forward purely on its own momentum. Zahavi and Zahavi (1997) identify one major problem with the Fisher idea. Arbitrary traits may somehow become sexually attractive, but attraction is not the only function involved in sexual selection. Competition also plays a role. If individuals with the selected-for traits are in fact no fitter than those without, why do they consistently manage to deter their rivals? If the trait is in fact a burden on these individuals, they should be in a worse position than their undecorated contemporaries and be very likely to lose any competition they engaged in. It would only take one or two undecorated, undeterred males to mate with females and produce undeterred offspring, who would then proliferate in the population and outcompete the decorated opposition. The lack of substantiality behind the —signal“ would be unmasked. The Handicap Principle 5 The Handicap Principle (Zahavi and Zahavi 1997) was only widely accepted in the 1990s. —If we see a character which does not signal quality, then it must be a handicap. The handicap principle lies at the heart of evolutionary signaling, and must therefore play a major role in our understanding of it.“ -Grafen 1990, p.5 —The male, like a good salesman, does whatever he can to impress females, while the goal of the female, like that of a shrewd customer, is to check the merchandise and accept only proven quality.“ œ Z&Z 1997 Zahavi and Zahavi (1997)‘s Handicap Principle follows on from Fisher‘s observation that female choice shapes male traits. However, they also go on to suggest that the nature of the traits may be the reason females are selecting for them. Certain aspects of a male‘s physiology or behaviour act as signals to the female, advertising his quality. However, in order for the signals to be un- fakeable, they must also be costly. Though the Handicap Principle can be applied to many situations where signaling takes place between two individuals with opposing interests, the case of sexually selected signals is particularly complex and interesting. Individuals looking for a mate are interested in a wide range of qualities (unlike other signaling circumstances, where signals are often limited to messages about the 6 power of an adversary, or the potential escape-speed of prey). Sexual signals indicate the properties of a potential mate both in terms of heritable quality and care-giving ability. Traits such as bright plumage do not have any immediate use for the individual bearing them. On the contrary, they may draw the unwanted attention of predators. Yet this is precisely why, in Zahavi and Zahavi‘s estimation, they are so attractive. Colours attract rivals and predators and are therefore a handicap and an honest signal of resistance to these threats. A male who can bear such a burden and still survive must have a high level of agility and resilience. In order to keep his plumage bright, he must have a good diet. And since the coloured markings on each side of his body are symmetrical, he probably has a stable developmental history. Such tell-tale signs cannot be faked by a poor quality male œ his colours cannot be artificially brightened. A good male —wears“ colours well, whereas on a poor quality male the colours only show up the imperfections. Even if a poor quality male could artificially enhance his plumage (see Saino et al. 1997, detailed below) he would quickly fall foul of the predation risks incurred and be eaten, curtailing his newfound career as a sexually attractive suitor before he had a chance to make good on the mating opportunities his markings afforded him. It is only those who can overcome a handicap who have the ability to wear it. 7 Arcteid and Danaid butterflies secrete a chemical signal derived from a poisonous alkaloid compound found in plants. This signal indicates that when they were larvae, they were able to eat poisonous plants to no detriment. The alkaloids are also passed to female and offspring once the male takes a mate œ they are useful against predators. (Eisner and Meinwald 1984) Tern males feed their potential mate during courtship (Nisbet 1973), a transparent signal of their ability to collect food, and that that they are able to provide enough to share with the female and her potential offspring. Additionally, this courtship technique is favourable to the female in a proximate sense, because she gains a meal. The amount of food provided is a direct reflection of the male‘s abilities, so the signal acts as a reliable index. The signal is honest because the male can only bring as much food as he can catch œ he cannot —lie“ about his food-gathering abilities by conjuring an illusion œ the signal is made from the very substance it purports to. Food provision as a signal also limits his courtship to one female (unless he is a very prolific hunter). The table below summarises some forms of handicap and they properties they may convey. 8 Handicap Function Example A large protruding Illustrates agility and Peacocks (Petrie and tail stamina œ this trait Halliday 1994) affects balance and adds weight Brightly coloured Attracts predators, Red backed fairy wren markings thereby showing that (Karubian 2002) the individual can escape or outwit them*, shows that individual is healthy and able to maintain appearance through good nutrition Complex song Shows a wealth of time Frog (Ryan et al. 1982) and mental resources to dedicate to learning and practicing the song.