Survival of the Tamest : The Domesticated Phenotype in Red Junglefowl Selected for Tameness

Sammanfattning: Early animal domestication was probably associated with reduced fear of humans. Domestication is a process in which animals adapt to humans and the captive environment provided by humans. Selection on tameness as the only trait has previously been found to generate changes in other phenotypes in different animal species. These changes in the traits correspond to the domesticated phenotype, a set of traits that are common to domesticated species. In this thesis, I have focused on measuring a number of phenotypic traits in two Red Junglefowl lines selected for low and high levels of fear of humans in a number of generations. Furthermore, I have studied the correlations between fear of human score, and other traits in an F3 intercross of the selected lines. In total, the thesis consists of four papers investigating the effects of fear of human score on other phenotypic traits.In paper I, we found that basal metabolic rate (BMR), serotonin levels, feed conversion efficiency and boldness were affected by selection on tameness. Chickens from the low fear line had a higher BMR, tended to have a higher feed conversion efficiency and were bolder in the novel object test. Peripheral serotonin levels were also higher in the low fear males. Paper II investigated effects on other behaviours, besides tameness, in both chicks and adults. We found that low fear was associated with a general higher activity level and that high fear individuals were more intensive in their courtship behaviour compared to the low fear individuals. Paper III found effects of the selection on fear towards humans on both brain size and fear habituation. Brain size relative to body size was significantly smaller in the low fear line, with changes in specific brain regions likely as the main cause. Fear habituation, measured in a test with exposure to a novel frightening stimulus over two consecutive days, was more effective in chicks from the low fear line compared to the high fear line. We found no evidence of effects on conditioned place preference learning.In the final paper, paper IV, we generated an intercross line using our eighth generation of selected high- and low fear chickens. The paper compares fear of human score with behavioural as well as physical phenotypes. We found that low fear of human score was associated with a higher body weight, faster growth and less fearful behaviour in the open field test as well faster habituation in a test measuring fear habituation to a novel fearful stimulus. Brain size was also measured, and we found that low fear of human score was associated with a smaller brain relative to body weight in females and that this change in brain size was due to changes in specific brain regions, rather than changes in the brain in a concerted fashion.In summary, selection on tameness in Red Junglefowl has changed other phenotypes that were not intentionally selected upon. These changes correspond to the domesticated phenotype and are consistent with changes that have been suggested as happening in early domestication. Furthermore, the level of fear towards humans is correlated with a number of other phenotypes, showing that there is a possibility that pleiotropy or linkage may be behind these changes. Taken together, the results suggest that tameness could have been a driving factor of the domesticated phenotype in chickens.