SEX DIFFERENCE IN THE SIZE OF THE NEURAL SONG CONTROL REGIONS IN A DUETTING SONGBIRD WITH SIMILAR SONG REPERTOIRE SIZE OF MALES AND FEMALES

Previous studies have suggested a causal relation between sex differen ces in behavior such as singing and sex differences in the size of bra in areas such as the forebrain song control areas of songbirds. In the present study we show that the size of the forebrain vocal control ar eas nucleus hyperstriatalis ventrale pars caudale (HVC) and nucleus ro bustus archistriatalis (RA) and its neuron numbers are about twice as large in males as in females of the African dueting bush shrike Laniar ius funebris. However, song types are of similar complexity (number of elements per song type, physical properties of elements) in both sexe s, and repertoire size does not differ between males and females. Furt hermore, in captivity male and female shrikes are able to learn the sa me song types. This demonstrates for the shrike that sex differences i n the size of vocal control areas and in its neuron numbers do not pre dict the type of sex-typical vocal behavior; This result is supported by a statistical comparison of the sex differences in HVC size, RA siz e, and song repertoire size of all songbird species studied to date. S ex differences in species in which only the males sing are indeed larg er than in species in which the females also sing; in songbird species with singing females, however, the sex differences in HVC and RA volu me appear to be independent of the vocal repertoire size of females. T he songbird model therefore does not support the notion that sex diffe rences in area size and neuron number explain sex differences in a beh avior that occurs in both sexes. Furthermore, in the shrike, neuron so ma size is similar in males and females in the song motonucleus hypogl ossus pars tracheosyringealis (nXllts) and in the premotor nucleus RA, but is sexually dimorphic in the higher vocal center HVC. Thus, male and female shrikes produce songs of similar complexity with different neuron phenotypes.