Abstract
Research into the role of tryptophan (TRP) breakdown away from the serotonergic to the kynurenine (KYN) pathway by stimulating the brain-endocrine-immune axis system interaction has brought new insight into potential etiologies of certain human behavioral and mental disorders. TRP is involved in inappropriate social interactions, such as feather-destructive pecking behavior (FP) in birds selected for egg laying. Therefore, our goal was to determine the effect of social disruption stress on FP and the metabolism of the amino acids TRP, phenylalanine (PHE), tyrosine (TYR), their relevant ratios, and on large neutral amino acids which are competitors with regard to their transport across the blood-brain barriers, at least in the human system, in adolescent birds selected for and against FP behavior. We used 160 laying hens selected for high (HFP) or low (LFP) FP activity and an unselected control line (UC). Ten pens with 16 individuals each (4 HFP birds; 3 LFP birds; 9 UC birds) were used. At 16 weeks of age, we disrupted the groups twice in 5 pens by mixing individuals with unfamiliar birds to induce social stress. Blood plasma was collected before and after social disruption treatments, to measure amino acid concentrations. Birds FP behavior was recorded before and after social disruption treatments. HFP birds performed significantly more FP and had lower KYN/TRP ratios. We detected significantly higher FP activity and significantly lower plasma PHE/TYR ratios and a trend to lower KYN/TRP ratios in socially disrupted compared to control pens. This might indicate that activating insults for TRP catabolism along the KYN axis in laying hens differs compared to humans and points toward the need for a more detailed analysis of regulatory mechanisms to understand the role of TRP metabolism for laying hen immune system and brain function.
Highlights
Birds selected for egg-laying face many serious animal welfare problems
There was a significant effect of line [F(2,152) = 17.27, P = 0.001], where High Feather Pecking Behavior (HFP) birds had 15 % higher PHE/TYR ratios (0.74 ± 0.01 μmol/ μmol) compared to Low Feather Pecking Behavior (LFP) (0.63 ± 0.01 μmol/ μmol; t =-5.34; P < 0.0001) and 11% higher PHE/TYR ratios compared to unselected control line (UC) birds (0.65 ± 0.001 μmol/ μmol ± 0.005; t =-5.13; P < 0.0001)
There was a significant effect of genetic line [F(2,152) = 8.03, P = 0.0005], which could be attributed to lower TRP/Σ aromatic amino acid (AAA) ratios in UC birds (0.288 ± 0.002) compared to LFP birds (0.3 ± 0.004; t =-1.97, P < 0.05] and HFP birds (0.311 ± 0.004 t =-3.92, P < 0.0001)
Summary
Birds selected for egg-laying face many serious animal welfare problems. One of the most challenging issues is feather pecking (FP). FP in birds selected for egg laying takes place in a social context between two birds On commercial farms, these birds live in a densely populated environment with large groups of thousands of female individuals [7]. Social groups typically consist of one male with up to 20 females [8, 9] demonstrating the dramatic contrast in group size In these groups, it is unlikely for birds to adequately recognize/remember each other. FP behavior may naturally shape the social environment of the involved birds, where feather peckers may cause social stress and social stress causes FP within a group due to social instability [12, 13]. Feather peckers appear less social by displaying less motivation to join a group [15] and have pronounced stress reactions to increased social contact [16]
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