Solveig Walløe Harpøth:
Behavioral capability and related brain structures has been linked many times. It is a relationship that may vary between individuals and species, depending on for example the level of sociality. This PhD-thesis investigates this relationship using parrots as experimental subjects. Parrots are among the most encephalized birds and posses excellent vocal imitation abilities. This along with their complex fission-fusion societies and thereby dynamic use of communication make parrots unparalleled as a model system for the neurobiology behind vocal learning. This PhD thesis is based on three independent studies where I 1) compare the level of vocal complexity (i.e. modification of the contact call in response to playback stimuli) with the social complexity of four different parrot species, 2) correlate the vocal modification ability of parrots with a brain region involved in vocal learning, i.e. the oval nucleus of the mesopallium, MO and 3) investigate the effect of long-term social and sound-isolation on the vocal modification ability and on the contact call of the peach-fronted conure, Aratinga aurea.
Article 1: The social complexity hypothesis states that with a complex social structure comes a need for higher communicative complexity. We compared four different parrot species, (peach-fronted conure, Aratinga aurea; cockatiel, Nymphicus hollandicus; peach-faced lovebird, Agapornis roseicollis; budgerigar, Melopsittacus undulatus) with different levels of social complexity. They had their vocal modification ability tested through semi-interactive playback experiments. We found that the species with the highest level of social complexity (i.e. peach-fronted conure) was also the species making the biggest change to the contact call in response to playback stimuli. The budgerigar appears to have the least complex social structure and it was also the species that showed the least change to the contact call. The cockatiel and the peach-faced lovebird both show intermediary social complexity, and they also showed intermediary changes to their contact call, compared to the peach-fronted conure and the budgerigar.
Article 2: It has been suggested that the size of various brain regions is important for behavioral capability and also the number of neurons have been suggested to be important. Here we correlate the vocal modification ability of the peach-fronted conure, the budgerigar and the peach-faced lovebird with a brain nucleus, MO, involved in vocal learning. We show that the species with the highest level of vocal complexity (i.e. the peach-fronted conure) was also the species with the largest volume of MO and the highest number of neurons in MO. The budgerigar had the smallest volume of MO and the least neurons in MO, whereas the peach-faced lovebird was in between the two other species in both volume and number of neurons. Further we found that the budgerigar showed sexual dimorphism in the volume of MO and also in the number of neurons MO, whereas the two other species did not. The budgerigar males learn calls more quickly than females and vocalize more. In the peach-fronted conure and the peach-faced lovebird both sexes appear to vocalize equally and have equal vocal modification abilities. Together the results indicate that volume and number of neurons appear to be important for the behavioral capability.
Article 3: Isolation has been shown to cause a behavior to develop abnormally or not at all but little research has investigated the effect of isolation on an established behavior. In this study we investigate the effect of long-term sound isolation on the vocal modification ability and on the contact call of the peach-fronted conure. The results are correlated with volume of MO and the number of neurons in MO. We show a significant decrease in the 25% and 50% frequency energy quartile throughout the sound-isolation period. Also, we compared the contact call of the sound-isolated individuals with four conspecifics of the same age kept in outdoor aviaries. We found the sound-isolated individuals to have longer call duration than the outdoor aviary individuals. We found that the sound-isolated individuals had a lower brain weight than the outdoor individuals. We did not find a difference in the volume of MO or in the number of neurons in MO between the two groups. Overall, the results indicate that being sound-isolated for a prolonged period of time has an effect on the acoustical appearance of the contact call of the peach-fronted conure, and on the brain.