Sexual selection is usually partitioned into two distinct categories: intrasexual selection and intersexual selection. Intrasexual selection results from the competition within one sex for access to resources like nesting sites, territories or mates. It favours traits that increase an individual’s competitive ability and therefore signal physical strength. On the other hand, intersexual selection relates to the efforts of one sex (usually the male) to attract members of the opposite sex and mate with them. Intersexual selection favours traits that signal a high phenotypic or genetic quality and therefore promise high benefits (direct or indirect) for the choosy sex. Alternatively, intersexual selection may also favour traits that are not related to quality but that exploit a pre-existing perceptual preference of the choosy sex (Ryan 1998).
I performed experiments to test whether song amplitude plays a role in intrasexual and/or intersexual selection. To look at the function of song amplitude in intrasexual selection, I carried out a playback experiment in the field where male chaffinches were exposed to high-amplitude and low-amplitude song (within the natural variation of song amplitude) of conspecific males (chapter 2). Territorial males approached the loudspeaker and dummy male closer and spent more time in their vicinity (within 5 m) in reaction to a simulated intruder singing high-amplitude songs than to an intruder singing low-amplitude
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songs. At the same time, territorial males sang less than in reaction to the low-amplitude playback.
The fact that I found differences in the behaviour of territorial males towards simulated high-amplitude as opposed to low-amplitude intruders indicates that the sound pressure level of territorial song indeed affects male-male competition. The reaction towards loud intruders appeared to be more aggressive, as territory holders approached closer and spent more time in the proximity of the simulated intruder. From a proximate point of view, a high-intensity song may be more efficient at stimulating the receiver’s sensory apparatus. A higher neuronal activity may trigger a stronger behavioural response. Ultimately, it is conceivable that a loud intruder signals a high resource holding potential (a measure of the “absolute fighting ability of the individual” [Parker 1974]), for example may be in better physical condition (see chapter 4), and therefore represents a bigger threat to the territorial male.
The costs of physical fighting are usually high in terms of energy expenditure, risk of injury and predation (Riechert 1988). Therefore, theoretical models and empirical evidence suggest that animals should choose their tactics during conflicts based on a conventional escalation sequence from low-cost to progressively more costly tactics (Parker and Rubenstein 1981; Enquist et al. 1990; Keeley and Grant 1993; Hack 1997). Low-cost tactics are used more often than high-cost tactics, which suggests that they may function as reliable signals to assess fighting abilities (Keeley and Grant 1993). Even if I could not observe any physical attacks during the playback experiment, social aggression may put a cost on loud singing because only high-quality individuals should risk escalating a conflict.
To test the potential role of song amplitude in intersexual selection, I performed an experiment on female song preferences, employing operant conditioning techniques (chapter 1). Song preferences in operant conditioning contexts predict preferences for live males (Holveck and Riebel 2007). My experiment revealed that females prefer high-amplitude over low-amplitude songs. Again, differences in neuronal activity due to a more intense stimulation of the female’s sensory apparatus may lead to such a differential behavioural response towards the two stimuli. In ultimate terms, females may get benefits from choosing loud males if sound pressure levels of songs correlate with male quality.
Considering the potential role of song amplitude in sexual selection, an important question to ask is to which extent potential sexual partners and conspecific competitors are able to estimate the song amplitude of a singer under natural conditions. From a physiological point of view, the intensity discrimination abilities of birds appear to be slightly inferior to human discrimination abilities. On average, an intensity difference between two
successive sounds must be 2–3 dB for a bird (1 dB for humans) to detect the difference, depending on species, sound frequency and absolute amplitude level (Hienz et al. 1980;
Dooling 1982). However, considering the large interindividual variation in song amplitude (Brumm 2009), avian intensity discrimination abilities are adequate to detect differences in sound pressure level between males. My experiment on the function of song amplitude in territorial interactions (chapter 2) showed that chaffinches have interindividual differences in song amplitude of up to 9 dB, and that they can perceive these differences.
As zebra finch males direct their songs over very short distances at females (Sossinka and Böhner 1980; Brumm and Slater 2006b), it is conceivable that females can accurately estimate the source amplitude level of a singing male. However, if birds communicate over larger distances, sound is attenuated and degraded depending on environmental factors like vegetation, air temperature and humidity (Wiley and Richards 1982; Brumm and Naguib 2009), which makes it difficult to estimate source amplitude levels. Nevertheless, birds may be able to estimate amplitude levels over considerable distances since there is evidence that they use sound amplitude to estimate the distance to a sound source (Nelson 2000; Naguib and Wiley 2001). Moreover, it should be kept in mind that females actively approach singing males when sampling potential mates (Bensch and Hasselquist 1992; Roth et al. 2009) and that male-male territorial interactions usually take place over short distances.
To conclude, I was able to demonstrate experimentally that song amplitude plays a role in both male-male competition and female choice and therefore may be subject to sexual selection. However, as I manipulated only amplitude, it is not possible to tell how important song amplitude is in relation to other song parameters like song rate or repertoire size. Future studies on sexual selection of birdsong may try to include manipulations of several song parameters to get estimates of their relative importance, rather than addressing each parameter in isolation.
A further aspect that requires to be addressed is the signaling value of song amplitude.
Most models of sexual selection, including the ‘good genes’ hypothesis (Hamilton and Zuk 1982) and Zahavi’s ‘handicap’ principle (Zahavi 1975), assume a correlation between a sexually selected trait and the genetic and/or phenotypic quality of its carrier. Does song amplitude honestly signal male quality? The following paragraph will address proximate mechanisms and the potential signaling value of song amplitude.