A role for plasma aromatic amino acids in injurious pecking behavior in laying hens

Injurious pecking, including feather pecking (FP), is one of the most prevalent causes of mortality for commercial laying hens. The underlying biological mechanisms of FP are not yet fully understood, but they could be related to alterations in the serotonin (5-HT) and/or dopamine (DA) circuits within the brain. In the past, the central synthesis of 5-HT and DA was found to be influenced by the availability of their precursors, aromatic amino acids (AAA) such as tryptophan (TRP), phenylalanine (PHE), and tyrosine (TYR), in blood plasma, which are transported across the blood-brain-barrier into the brain. Because knowledge about plasma levels of AAA in laying hens is very limited, the present study compared the AAA profiles of a large sample of laying hens from two genetic lines: one selected for low mortality (LM) due to injurious pecking (n=129 birds) and one high production line (HP) selected for high egg-production only (n=132 birds). Head, comb, and feather covering were scored at the end of the experiment. Blood samples were collected at weeks 24 and 29 of age and were analysed for AAA using high performance liquid chromatography. Neither FP nor feather damage was observed in the present study, but aggressive pecking directed at the head/neck area occurred in several groups with an onset of this aberrant behavior between weeks 22 and 29. Eight HP pens and seven LM pens were affected by severe head/comb injuries inflicted via aggressive pecking. Therefore, our exploratory data analysis focused upon the possible interplay between the variability of our outcome measures (absolute levels of AAA in plasma as well as the ratios PHE/TYR and TRP/(PHE+TYR)) and the aggressive head/comb pecking as an expression of social stress within the pens. We found significantly lower TRP availability relative to PHE and TYR (TRP/(PHE+TYR) ratio) and higher TYR concentrations at week 24 in pens with an early onset of injurious aggressive behavior at weeks 22-23. This was most pronounced in the LM line, but at week 29, TRP availability normalized in both lines. It was furthermore evident that in LM birds, higher aggressive pecking activity per pen was associated with higher TYR levels (n=78 birds, r=0.643, p<0.001) and lower TRP/(PHE+TYR) ratios at week 24 (r=-0.541, p<0.001). In the HP birds, these associations were of lower strength and were negatively correlated (TYR: n=73, r=-0.308, p=0.005; TRP/(PHE/TYR) ratio: r=0.314, p=0.004). Our findings indicate that in LM birds, lower TRP availability at week 24 may be attributable to higher stress levels in pens where injurious aggressive pecking developed early on. These findings may lay the important groundwork for the analysis of AAA plasma levels as a useful avenue of research to investigate underlying physiological mechanisms of behavioral problems in laying hens.