Abstract
During domestication animals have undergone changes in size of brain and other vital organs. We hypothesize that this could be a correlated effect to increased tameness. Red Junglefowl (ancestors of domestic chickens) were selected for divergent levels of fear of humans for five generations. The parental (P0) and the fifth selected generation (S5) were culled when 48–54 weeks old and the brains were weighed before being divided into telencephalon, cerebellum, mid brain and optic lobes. Each single brain part as well as the liver, spleen, heart and testicles were also weighed. Brains of S5 birds with high fear scores (S5 high) were heavier both in absolute terms and when corrected for body weight. The relative weight of telencephalon (% of brain weight) was significantly higher in S5 high and relative weight of cerebellum was lower. Heart, liver, testes and spleen were all relatively heavier (% of body weight) in S5 high. Hence, selection for tameness has changed the size of the brain and other vital organs in this population and may have driven the domesticated phenotype as a correlated response.
Highlights
Domestication has dramatically altered the phenotypes of animals due to selection on different traits chosen by humans
In the fear-of-human test conducted on S5 birds, S5 high had the highest score, S5 unselected intermediate and S5 low the lowest (F2.106 = 8.98; P < 0.001) (Fig. 1)
S5 High had a significantly lower body weight than the average of P0 (F1.85 = 19.51; P < 0.001), and S5 Low weighed significantly more than the average of P0 (F1.8 = 4.87; P < 0.03) (Fig. 2)
Summary
Domestication has dramatically altered the phenotypes of animals due to selection on different traits chosen by humans. It has been suggested that the occurrence of common phenotypes in domesticated animals might be a result of correlated responses to selection for one or a few traits, for example mediated by pleiotropy or linkage[4, 5] This hypothesis is largely supported by the famous farm-fox project, where silver foxes developed several dog-like traits after only few generations of selection for tameness only[5]. Since the common denominator for any successful domestication of animals is an initial reduction in fear of humans, the evolution of domesticated phenotypes may be driven by selection for tameness We have investigated this idea by selection of Red Junglefowl (Gallus gallus) for increased tameness, and measuring possible correlated effects. Our hypothesis is that selection for tameness may drive alterations in sizes of vital organs by means of correlated responses
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