Theories of sexual selection assume that predation is a restrictive, simplifying force in the evolution of animal display characters and many empirical studies have shown that predation opposes excessive elaboration of sexually selected traits. In an unexpected turnaround, I show here that predation pressure on neotropical, weakly electric fish (order Gymnotiformes) seems to have selected for greater signal complexity, by favouring characters that have enabled further signal elaboration by sexual selection. Most gymnotiform fish demonstrate adaptations that lower detectability of their electrolocation/communication signals by key predators. A second wave phase added to the ancestral monophasic signal shifts the emitted spectrum above the most sensitive frequencies of electroreceptive predators. By using playback trials with the predatory electric eel (Electrophorus electricus), I show that these biphasic signals are less detectable than the primitive monophasic signals. But sexually mature males of many species in the family Hypopomidae extend the duration of the second phase of their electric signal pulses and further amplify this sexual dimorphism nightly during the peak hours of reproduction. Thus a signal element that evolved for crypsis has itself been modified by sexual selection.

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In a field study, we show that a young song sparrow (i) selects his songs from three or four older birds who have neighboring territories, (ii) preferentially learns song types that these tutor neighbors share, and (ui) ultimately sets up his territory next to, or replaces, one of these tutor neighbors. The consequence of this song learning strategy is that the young bird's song repertoire represents the "logical intersection" of the song repertoires of his tutor neighbors. We argue that this repertoire is optimally designed for mimicry (sounding like your neighbors) and for communication between neighbors (song sparrows address or reply to a neighbor with a song they share with that neighbor).Song learning in passerine birds is a selective process in which the young bird retains in his final, adult repertoire only a fraction of the many song types to which he is exposed (1). Despite considerable theoretical interest in the design and function of song repertoires (2-6), however, little is known about the variables determining which of the "tutor" songs are selected for the song repertoire, other than that conspecific songs are preferred over heterospecific songs. To test the hypothesis that social variables are the key determinant of song selection, we studied song learning in a free-living population df song sparrows (Melospiza melodia).In this study, we tested a model of song learning derived from three sets of observations. First, in many (but not all) songbird species, birds share their song types with neighbors, with the resemblances in some cases being so close as to Suggest that one bird learned the song type from the other (7). Second, young male song sparrows, starting in the summer of their hatching year, "float" on the territories of several adjacent territorial males and eventually (usually by the following summer) try to set up a territory in this floater range (see refs. 8 and 9; unpublished observations). Third, in song sparrows (and many other songbirds) the early part of a bird's life (especially his second and third months) is critical in the formation of his song repertoire (10). Since we have never observed a male song sparrow to add of drop a song type between his first breeding season and subsequent years, we assume that a.young song sparrow's repertoire crystallizes sometime in his first year of life, possibly as early as his hatching summer. Putting these observations together, we hypothesized that during this floater period the young bird learns the song types of some or all of the territorial males in his floater range. To test this hypothesis, we attempted to trace the song tutors for a sample of young male sparrows from our study population.In this paper, we follow conventional usage and call the bird from whom the song was learned the tutor (the learner is the student). We do not mean to imply by these terms that the older bird actively teaches the younger bird (although he may) or that the younger bird is a passive learner (indeed our evidence suggests quite otherwise Wash-...

The gymnotiform electric fish Brachyhypopomus pinnicaudatus communicates with a sexually dimorphic electric waveform, the electric organ discharge (EOD). Males display pronounced circadian rhythms in the amplitude and duration of their EODs. Changes in the social environment influence the magnitudes of these circadian rhythms and also produce more transient responses in the EOD waveforms. Here we show that injections of serotonin produce quick, transient, dosedependent enhancements of the male EOD characters similar to those induced by encounters with another male. The response to serotonin administered peripherally begins 5-10·min post injection and lasts approximately 3·h. The magnitude of the response to serotonin is tightly associated with the magnitude of the day-to-night swing of the circadian rhythm prior to injection. Taken together these findings suggest that the male's social environment influences his response to serotonin by altering the function of some part of the downstream chain between the serotonin receptors and the ion channels involved in control of the EOD waveform. Although chronic activation of serotonin circuitry is widely known to elicit subordinate behavior, we find that 5-HT initially increases a dominance signal in these fish. These findings are consistent with the emerging view that serotonin facilitates different adaptive responses to acute and chronic social challenge and stress.

Previous theory and research have suggested that bird species with song repertoires in general, and song sparrows (Melospiza melodia) in particular, cannot readily discriminate between the songs of neighbors and strangers. In a recent study (Stoddard et al. 1991) we showed that song sparrows can in fact discriminate neighbors from strangers on the basis of song. In this study we sought to demonstrate that song sparrows can make the finer discrimination between individual neighbors and that they can do so on the basis of a single song type. We compared the response of territorial males to song playback of neighbors and strangers at three locations : the neighbor's regular boundary, the opposite boundary, and the center of the territory. The birds showed strong neighbor-stranger discrimination at the regular boundary but not at the opposite boundary, nor in the center of the territory. The differences in song discrimination between different boundary locations indicate that song sparrows associate particular songs with particular territories, effectively discriminating between individual neighbors on the basis of song. Song repertoires themselves do not interfere with neighbor recognition to the extent originally postulated. As speakers are moved inside the territory from the border, however, the degree of discrimination diminishes. We believe that differences in speaker placement may have contributed to the variability in neighbor-stranger discrimination observed in previous studies of the song sparrow and perhaps other repertoire species as well. This interpretation is consistent with data from another song sparrow population showing that half the territory takeovers are by immediate neighbors.