Seasonal changes in the neural attributes of brain nuclei that control song in songbirds are among the most pronounced examples of naturally occurring plasticity in the adult brain of any vertebrate. The behavioral correlates of this seasonal neural plasticity have not been well characterized, particularly in songbird species that lack adult song learning. To address this question, we investigated the relationship between seasonal changes in gonadal steroids, song nuclei, and song behavior in adult male song sparrows (Melospiza melodia). At four times of the year, we measured plasma concentrations of testosterone, neural attributes of song nuclei, and several aspects of song structure in wild song sparrows of a nonmigratory population. We found seasonal changes in the song nuclei that were temporally correlated with changes in testosterone concentrations and with changes in song stereotypy. Male song sparrows sang songs that were more variable in structure in the fall, when testosterone concentrations were low and song nuclei were small, than in the spring, when testosterone concentrations were higher and song nuclei were larger. Despite seasonal changes in the song nuclei, the song sparrows continued to sing the same number of different song types, indicating that changes in the song nuclei were not correlated with changes in song repertoire size. These results suggest that song stereotypy, but not repertoire size, is a potential behavioral correlate of seasonal plasticity in the avian song control system. Key words: androgen; seasonal plasticity; bird song; motor stereotypy; song sparrow; song repertoireMost temperate zone vertebrates breed seasonally. Seasonal changes in brain structures that control reproductive behavior have been reported in numerous vertebrate species (Nottebohm, 1981;Buijs et al., 1986;Boyd and Moore, 1991;Wade and Crews, 1991;Hofman and Swaab, 1992;Skene et al., 1992;Senthilkumaran and Joy, 1993;Lee et al., 1995). Seasonal changes in neural attributes of brain nuclei that control song in songbirds are perhaps the most striking example of naturally occurring plasticity in the adult vertebrate brain (Nottebohm, 1981).Several attributes of song nuclei change seasonally: (1) overall size (Nottebohm, 1981;Arai et al., 1989; Kirn et al., 1989;Brenowitz et al., 1991; Rucker and Cassone, 1991;Smith et al., 1995); (2) size, density, and number of neurons (Brenowitz et al., 1991;Johnson and Bottjer, 1995;Smith et al., 1995); (3) dendritic and synaptic morphology (DeVoogd et al., 1985;Clower et al., 1989;Hill and DeVoogd, 1991); and (4) incorporation and survival of new neurons (Alvarez-Buylla et al., 1990;Nottebohm et al., 1994). Similar changes in the size of song nuclei have been reported both in captive songbirds in which photoperiod and/or testosterone (T) were manipulated to mimic seasonally changing environmental and hormonal conditions (Nottebohm, 1981;Brenowitz and Arnold, 1985;Arai et al., 1989) and in free-ranging, wild songbirds experiencing natural seasonal changes in environmental cue...

The evolution of communication behavior, like that of all traits, is shaped both by external forces of natural and sexual selection and by internal forces of development and physiology. Complexity in both the function and production mechanisms of communication signals makes them a powerful model for understanding evolutionary interactions within and across levels of biological organization (Ryan, 2005).Electrocommunication in weakly electric fish is an outstanding model for integrating mechanistic and historical biology (sensu Autumn et al., 2002) because these behaviors are diverse across species, are easily recorded and analyzed, and are controlled by a well-characterized neural circuit. The family Apteronotidae is particularly well-suited for a comparative approach because it has the highest species diversity among Neotropical electric fish (Crampton and Albert, 2006). Apteronotids continuously emit a quasi-sinusoidal voltage signal or electric organ discharge (EOD) that has two functions. Nearby objects locally distort the electric field generated by the EOD, and by detecting these distortions with their electroreceptors, fish can electrolocate. The fish can also use their EODs to communicate by detecting the interactions of their own EOD with those of other fish. Individual apteronotid fish maintain an extremely stable EOD frequency (EODf) (Bullock, 1970;Moortgat et al., 1998) and waveform (Rasnow and Bower, 1996). In addition to using the baseline EOD frequency and waveform as communication signals, fish also modulate the frequency and amplitude of the EOD during social interactions Electrocommunication signals in electric fish are diverse, easily recorded and have well-characterized neural control. Two signal features, the frequency and waveform of the electric organ discharge (EOD), vary widely across species. Modulations of the EOD (i.e. chirps and gradual frequency rises) also function as active communication signals during social interactions, but they have been studied in relatively few species. We compared the electrocommunication signals of 13 species in the largest gymnotiform family, Apteronotidae. Playback stimuli were used to elicit chirps and rises. We analyzed EOD frequency and waveform and the production and structure of chirps and rises. Species diversity in these signals was characterized with discriminant function analyses, and correlations between signal parameters were tested with phylogenetic comparative methods. Signals varied markedly across species and even between congeners and populations of the same species. Chirps and EODs were particularly evolutionarily labile, whereas rises differed little across species. Although all chirp parameters contributed to species differences in these signals, chirp amplitude modulation, frequency modulation (FM) and duration were particularly diverse. Within this diversity, however, interspecific correlations between chirp parameters suggest that mechanistic trade-offs may shape some aspects of signal evolution. In particular, a consistent trade-of...

Numerous animal lineages have expanded and diversified the opsin-based photoreceptors in their eyes underlying color vision behavior. However, the selective pressures giving rise to new photoreceptors and their spectral tuning remain mostly obscure. Previously, we identified a violet receptor (UV2) that is the result of a UV opsin gene duplication specific to Heliconius butterflies. At the same time the violet receptor evolved, Heliconius evolved UV-yellow coloration on their wings, due to the pigment 3-hydroxykynurenine (3-OHK) and the nanostructure architecture of the scale cells. In order to better understand the selective pressures giving rise to the violet receptor, we characterized opsin expression patterns using immunostaining (14 species) and RNA-Seq (18 species), and reconstructed evolutionary histories of visual traits in five major lineages within Heliconius and one species from the genus Eueides. Opsin expression patterns are hyperdiverse within Heliconius. We identified six unique retinal mosaics and three distinct forms of sexual dimorphism based on ommatidial types within the genus Heliconius. Additionally, phylogenetic analysis revealed independent losses of opsin expression, pseudogenization events, and relaxation of selection on UVRh2 in one lineage. Despite this diversity, the newly evolved violet receptor is retained across most species and sexes surveyed. Discriminability modeling of behaviorally preferred 3-OHK yellow wing coloration suggests that the violet receptor may facilitate Heliconius color vision in the context of conspecific recognition. Our observations give insights into the selective pressures underlying the origins of new visual receptors.

The song control nuclei of songbirds undergo pronounced seasonal changes in size and neuronal attributes. The mechanisms by which seasonal changes in environmental variables such as photoperiod mediate seasonal changes in these brain regions are not known. Manipulations of photoperiod and/or testosterone in captive songbirds induce seasonal changes in the size of song nuclei comparable to those observed in wild songbirds. It is unclear, however, whether the effects of photoperiod on the song nuclei are mediated by testosterone or by steroid‐independent mechanisms. We independently manipulated photoperiod and testosterone in castrated male Gambel's white‐crowned sparrows (Zonotrichia leucophrys gambelii) to determine the contributions of steroid‐dependent and ‐independent actions of photoperiod to seasonal changes in the size and neuronal attributes of song nuclei. Testosterone implants increased the size of several song nuclei, regardless of photoperiod. Photoperiod exerted small but significant steroid‐independent effects on the volume of the higher vocal center and the size of neurons in the robust nucleus of the archistriatum. Photoperiod also modulated the effect of testosterone on the size of area X; testosterone treatment had a more pronounced effect on the size of area X on short days than on long days. These results suggest that although testosterone is the primary factor mediating seasonal changes in neural attributes of the song nuclei, photoperiod may act via mechanisms that are independent of steroid levels to supplement or modulate the actions of testosterone. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 426–442, 1997.