Biology 202
2003 First Web Paper
On Serendip

Bird songs continue to fascinate neurobiologists and neuroethologists because the development of song has been a popular model used to examine the role of environment on behavior. In most species, only male birds sing complex songs. Their vocalizations are the result of sexual dimorphism in the brain regions responsible for the production of song. However, this behavior is not genetically hardwired into the avian brain. Certain conditions must exist in order for male birds to successfully produce their species-specific song. Additionally, the neuronal circuitry and structure of the avian song system shows high levels of plasticity.

If the brain and behavior are indistinguishable, then the structural differences in the avian brain are responsible for behavioral differences across the sexes. Nottebohm and colleagues identified six anatomically distinct regions of the forebrain involved in the production of song, which are arranged into two independent pathways, the posterior pathway, which controls song production, and the anterior pathway, which controls song learning. The collective unit is typically referred to as the vocal control region (VCR) (1) (2).

Female birds sing rarely and this behavioral difference is reflective of the anatomy of the female avian brain. There are significant differences in the size of three neural areas involved in the production of song across the sexes, and a specific area, Area X, is present in the male and absent in the female. Additionally, the incorporation of radiolabeled testosterone in certain locations is different in males and females (3) (4).

Scientists have been particularly interested in the origin of the structural differences in male and female songbirds. Research has suggested the importance of gonadal hormones, specifically testosterone in the production of song. It was observed that castration eliminated all song production (5). Additionally, when testosterone levels are low, there is not only a decrease in the production of song, but also a decrease in the size of some nuclei involved in song production (6). Further support for the necessity of testosterone for song production was demonstrated by Nottebohm (1980) when he injected female birds with testosterone, which lead to the production of song (7). This research has interesting implications regarding anatomical changes that may occur when an organism is chemically imbalanced. Disruptions in chemical equilibrium may alter brain structure and subsequently influence behavior.

Nevertheless, not all research has supported the claim that testosterone is responsible for anatomical and behavioral differences between male and female songbirds. Contradictory results have been obtained in different species of birds, and no one has successfully induced feminine bird song behavior in male birds with the introduction of hormones. The role of hormones remains unclear at this time, however, the structures and circuitry involved in song production exhibits neuronal plasticity in many species and there is a relationship between gonadal hormones and sex-specific behavior.

Assuming functional circuitry, bird vocalizations come in many varieties, and range from several seconds to several minutes (8). The complexity of the avian vocal repertoire has allowed scientists to study the development of this behavior. Bird songs are often species specific, but there are also differences in the vocalizations within a species. If bird vocalizations were merely the result of genetics there would be little variation in the complexity or variety of songs. The individuality present across males of the species raised questions about the origins of the songs and factors responsible for creativity and individuality in song.

Thorpe and colleagues (1961) removed young birds from the nest immediately after hatching and raised them in captivity. When the males began to vocalize, their songs were significantly degraded compared to males in the wild. The songs were similar to the songs produced by wild males, however, they were simpler in form and there was little variation within or between individual birds (9). This experiment demonstrated that the variation in song observed in wild birds was not genetically programmed, but rather something that developed when an animal was in the wild and could hear the songs of other males in the species.

Marler and Tamura (1964) further examined the development of bird songs by examining the bird songs of finches. Finches have species specific songs, however, the songs differ slightly depending on their geographical niche. In essence, finches have dialects that are regionally specific. They removed young birds from the nest after hatching and found that all birds, regardless of their geographical location developed simplified versions of their regional song. The birds were unable to learn their region-specific songs because they were not able to hear males from their region sing it. It was determined that the critical period for acquiring song occurs during the first 3 months of life, which is before a bird can produce song. In actuality, a template of the song forms in as little as 20 days, song acquisition is often completed in approximately 35 days, and after 60 days of rehearsal the song becomes consistent. Even males reared in isolation can be trained to sing by playing a recording of their song as long as they are under 3 months of age. After 4 months of age, birds become unreceptive to training. This shows that there is a simple song that is processed at an early age and subject to modification during the earliest months of life. The young birds, once exposed to the song of adult males, store it until they begin to sing (10).

Further examination by Konishi (1965) demonstrated that a young bird that is deafened immediately after birth will sing, but is only capable of forming disjointed notes. The song of a deafened bird is even more severely degraded compared to a bird raised in isolation and would not be identified as the song of a particular species (11). This demonstrates that not only is auditory exposure necessary for the successful production of song, but auditory feedback is also necessary.

Song production is a complex behavior that suggests that the interaction of genetics and the environment is necessary for birds to successfully vocalize. Further study is necessary to fully understand the factors that are responsible for sexual differentiation of behavior and the neuroplasticity and sexual dimorphism seen in the avian brain. This model of animal behavior may contribute to understanding the differences between male and female brains in humans, and demonstrate how anatomical differences may be the cause of behavior differences across the sexes.

References

1) Song circuit diagram, a basic diagram of the song control circuit in the avian brain.

2) Songbird brain circuitry , created by Heather Williams at Williams College, this is a more detailed figure of brain circuitry.

3) Sex and the Central Nervous System , the web site to accompany a developmental biology text by Scott F. Gilbert.

4) Arnold, A.P. (1980). Sexual differences in the brain. American Scientist. 68. 165-173.

5) Thorpe, W.H. (1958). The Learning of Song Patterns by Birds, with Special Reference to the Song of the Chaffinch. Ibis, 100. 535-70.

6) Nottebohm, F. (1981). A brain for all seasons: Cyclical anatomical changes in song control nuclei of the canary brain. Science. 214. 1368-1370.

7) Nottebohm, F. (1980). Testosterone triggers growth of brain vocal control nuclei in adult female canaries. Brain Research. 189. 429-436.

8) Zebra finch song , created by Heather Williams at Williams College , includes audio clips of zebra finch songs.

9) Thorpe, W.H. (1961). Bird song. New York: Cambridge University Press.

10) Marler, P. & Tamura, M. (1964). Culturally transmitted patterns of vocal behavior in sparrows. Science, 146. 1483-1486.

11) Konishi, M. (1965). The role of auditory feedback on the control of vocalization in the white-crowned sparrow. Zeitschrift fur Tierpsychologie. 22. 770-783.

12) Hinde, R.A. (1969). New York: Cambridge University Press.

13) An Introduction to Birdsong and the Avian Song System. ,(HTML format) General information about birdsong and the avian song system from a special issue of the Journal of Neurobiology.


| Course Home Page | Course Forum | Brain and Behavior | Serendip Home |