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参考文献2* 疑式原文
The Cost of Singing Softly
Given that soft song is indeed the display that most reliably predicts attack in song sparrows, what is to keep males from cheating in order to appear more aggressive? Again, to answer this question one must take another look at the handicap hypothesis, in which the costs of signals are crucial to enforcing their honesty. Scientists have proposed different types of expenses that could be incurred for aggressive signals, but most seem unlikely to apply to soft song. Energy costs, for one, are still unlikely because songs in general, and those of low intensity in particular, take very little energy to produce. The kind of developmental cost discussed earlier for song repertoire size also seems unlikely, because these costs are more likely to apply to signals established over the long term, rather than signals that change over the short term at will. A third possibility is what is called a “vulnerability cost,” where the manner
in which a signal is produced inherently makes the individual giving it more vulnerable to attack. A vulnerability cost might seem plausible for soft song if, because of its low amplitude, it can only be perceived by a receiver when the singer is close by, making it an unambiguous signal of proximity and thus vulnerability. However, because of the way that sound attenuates with distance, a signal that is soft when it reaches the receiver could be either a low-intensity signal produced near the signaler or a high-intensity signal produced farther away. By contrast, a signal that is loud when it reaches the receiver is actually less ambiguous in conveying that the signaler is close by.
We have argued that soft song is a reliable aggressive signal because it is an unambiguous and costly signal of attention, not of proximity or vulnerability. Soft song is usually produced quite close to the intended receiver, so that the receiver can both see and hear the singer. The listener then should be able to discern the volume at which the song is produced, and because of its low volume, soft song is unlikely to be audible to any other individuals. Thus by singing at low intensity near a rival, the singer is indicating that its attention is focused solely on that particular bird. Soft songs are also costly to the singer because they are unlikely to reach other targets. In the case of female receivers, the cost could be a diminished proclivity to mate, as we have shown that female song sparrows find soft song less attractive than songs sung at a normal broadcast amplitude. The more-significant cost of soft song, however, is probably that other male receivers fail to hear a response from the male being challenged, leading neighbors or other potential usurpers to be more likely to intrude on the softsinger’s territory.
A recent study of ours supports the idea that soft song is costly because it limits reception to a single intended receiver. In this experiment, we simulated singing interactions between an intruder and a territory owner, in which the latter sang softly or at a normal level. First we recorded the territory owner and mapped his territory; then we captured and held him temporarily.
While the owner was held, we placed two loudspeakers on his territory, and staged a virtual interaction by playing an intruder’s song through one speaker and the owner’s song from the other. The intruder song was recorded from a male holding a distant territory, and was always played at normal level. In half the trials, the owner’s song was also played at normal level, and in the other half the owner’s song was played at a level typical of soft song. The result was that intrusions by other male song sparrows were more common and more serious when the simulated owner sang soft songs than when he used loud song. Presumably, other males listening to the interaction from off the territory cannot hear the owner when he uses soft song, and thus cannot tell that he is countering the intruder. Thus by using soft song to signal his focused attention to one intruder, the territory owner sacrifices his ability to ward off other potential intruders.
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发表于 2018-7-11 14:44:46
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