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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...

Five Japanese macaques and five other Old World monkeys were trained to discriminate among field-recorded Japanese macaque vocalizations. One task required discrimination of a communicatively relevant acoustic feature ("peak"), and a second required discrimination of an orthogonal feature of the same vocalizations ("pitch"). The Japanese animals more proficiently discriminated the peak feature when stimuli were presented to the right ear (primarily left cerebral hemisphere), as opposed to the left ear (primarily right hemisphere). In discriminating the pitch feature, the Japanese animals either showed (i) a left-ear processing advantage or (ii) no ear advantage. The comparison animals, with one exception, showed no ear advantage in processing either feature of the vocalizations. The results suggest that Japanese macaques engage left-hemisphere processors for the analysis of communicatively significant sounds that are analogous to the lateralized mechanisms used by humans listening to speech.