Understanding the origin and function of introductory vocalizations before male zebra finch song

Abstract

Communication involves an exchange of information between the sender and receiver and for effective communication, animals have developed various displays. A number of these displays, in different species, begin with introductory gestures. An introductory gesture is not part of the main display and typically occurs at the beginning of the display. For e.g. Anolis lizards start their aggressive throat (dewlap) extension display with a few push-ups or tail flicks (Ord & Stamps, 2008) and frogs produce a single introductory note before their aggressive/advertisement vocalization (Schwartz & Wells, 1985; Wells & Schwartz1, 1984). Many different functions have been hypothesized for these introductory gestures, including “an altering function” before the main message (Richards, 1981), providing information for species recognition (Soha & Marler, 2000), and motor preparation for the main display (Rajan & Doupe, 2013). However, little is known about its properties, or origin, especially in the context of learned displays. In the course of my thesis, I have studied the properties and origin of the introductory notes before the song of the zebra finch, a well-studied Australian songbird. The song of the male zebra finch consists of a stereotyped sequence of sounds (syllables) that the young bird learns from a tutor. Song bouts begin with a variable number of short-duration vocalizations called introductory notes (INs). Previous work has suggested that these INs serve as “motor preparation” for song production (Rajan & Doupe, 2013). However, whether INs are learned or not, how they are produced by the brain, and what function they serve, remains poorly understood. Through my thesis work, I have addressed each of these questions. First, using different tutoring methods, I have shown that the acoustic properties of INs, and the mean number of INs before song, are learned from the tutor in the same way as song is learned. Interestingly, birds reared without exposure to INs also initiate songs with INs. INs produced by birds reared normally and INs produced by birds reared with songs without INs have some similarities. Specifically, in both sets of birds, INs are short duration syllables that repeat at the beginning of the song bout. Overall, these data demonstrate that INs, like songs, have both learned and innate components (Kalra et al., 2021). Second, to determine whether brain regions involved in song learning are also necessary for IN learning, I performed electrolytic lesions of the lateral magnocellular nucleus of the anterior nidopallium (LMAN). LMAN is necessary for learning of song in juvenile zebra finches (Bottjer et al., 1984; Scharff & Nottebohm, 1991). Lesions were carried out in 45 day old juveniles (n=16) and song and IN learning were assessed after birds became adults (> 90 days old). Half the birds (n=7) had lesions between 20 – 60% of LMAN while the rest had < 20% lesions (n=6). In birds with lesions of LMAN (20-60%), INs were acoustically less similar to those of the tutor. Mean IN number did not seem to be affected in these birds. Overall, these results suggest that LMAN may be involved in learning of IN acoustic structure and not number. However, the small sample size limits my conclusions. I have used song playbacks to assess the function of INs. Specifically, we asked if female zebra finches responded more to playbacks of song with INs. We did not find any difference in female response to songs with and without INs. These data suggest that INs do not have a role in female mate choice. Overall, the results of my thesis show that INs before zebra finch song are learned similar to song, possibly using the same brain pathways. They do not function in female mate choice and the learned aspects of INs may not have a motor preparatory function. Future work aimed at understanding how they are learned could provide new insights into how complex movements like song are initiated.