Szymon Marian Drobniak , Agnieszka Gudowska

Animals do not eat whatever food item they find. They usually balance the intake of key nutrients, for example, essential amino acids that cannot be synthesized by animals and must be provided in the diet. However, ability to gain optimal ratios, proportions, and amounts of nutrients may be hampered by a changing environment, competitive conspecifics or species, and predators. Here, we used an experimental system in which house sparrows (Passer domesticus) were fed diet with different amino-acids composition (the experimental diet had phenylalanine and tyrosine (PT) content reduced to 42% of the control diet). PT are precursors of coping hormones: dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline) which are involved mainly in the expression of stress and fear, but also learning and long-term memory formation. In line with this, birds fed PT-deficient diet learned to avoid unpalatable food markedly slower, coped worse with stress in the presence of a novel object and were much more aggressive towards other sparrows than control birds. Surprisingly, circulating amounts of the catecholamines in blood plasma were higher in PT-limited birds than in sparrows fed the control diet. This study provides the first evidence that different amino acids composition in the diet is associated with variation in behaviours and hormone levels in birds. We conclude that food, besides its nutritional function, seems to represent one of the main modulators of behavioural expression making a balanced diet crucial for survival. Moreover, the dependence of behaviour expression on diet poses interesting questions about the sources of animal personality.