Gut Microbes and Tryptophan Metabolism Linked to Aggression in Stressed Chickens

Chronic stress does more than stunt the growth of farmed chickens—it appears to rewire their gut bacteria and blood chemistry in ways that make them measurably more aggressive. That is the central finding of a new multi-omics study published in the journal Stress Biology, in which researchers at Nanjing Agricultural University traced a molecular chain of evidence connecting stress hormones, cecal microbes, circulating metabolites, and hostile behavior in broiler chickens. By combining 16S rRNA gene sequencing of gut bacteria with untargeted plasma metabolomics and behavioral testing, the team has assembled one of the most detailed pictures yet of how the microbiota-gut-brain axis may drive aggression in poultry.

The experiment began with fifty one-day-old yellow-feather broilers, housed in standard cages under a controlled 16-hour light and 8-hour dark cycle and fed a commercial diet. At 28 days of age, the birds were sorted into body-weight-matched pairs and randomly assigned to two groups. One group received daily subcutaneous injections of corticosterone—the principal stress hormone in birds—at a dose of 4 milligrams per kilogram of body weight for seven consecutive days. The control group received injections of a 15% ethanol solution at the same volume. This corticosterone regimen is a well-established way to simulate chronic stress in poultry, allowing researchers to isolate the hormonal component of stress without confounding factors such as heat, crowding, or transport.

The physiological consequences were swift and clear. After 35 days, corticosterone-treated broilers had significantly lower final body weights and reduced average daily gain compared with controls. Paradoxically, they ate more: average daily feed intake rose significantly, and the feed-to-weight ratio worsened, meaning the stressed birds converted feed into body mass far less efficiently. The authors suggest this pattern reflects a metabolic shift under chronic stress, with energy diverted toward fat deposition through increased proteolysis and gluconeogenesis—essentially, the birds were burning muscle to fuel a stress response while consuming more feed overall.

Behavioral testing, conducted on days 35 and 36 using video-recorded encounters between unfamiliar birds from the same treatment group, revealed a striking escalation in aggression. Corticosterone-exposed chickens pecked their opponents significantly more often, and the total number of aggressive acts rose sharply. The effect spanned the social hierarchy: both dominant birds—identified by how frequently their opponents avoided them—and subdominant birds showed elevated aggression, though dominant individuals displayed the strongest increases, including more twisting attacks. This aligns with earlier observations that dominant animals may have aggression reinforced through mesolimbic reward circuitry, and with prior reports that prenatal corticosterone exposure and social stress both heighten injurious pecking in hens.

Crucially, the team also measured serotonin, the neurotransmitter long associated with behavioral inhibition. Chronic corticosterone significantly reduced serotonin concentrations in the hypothalamus—a key brain region for stress regulation—while paradoxically increasing serotonin levels in the plasma. Dopamine levels in the plasma, hippocampus, and hypothalamus remained unchanged. Because more than 90% of the body’s serotonin is produced by enterochromaffin cells in the gut, and because peripheral serotonin cannot cross the blood-brain barrier, this split pattern points to a gut-centered mechanism: stress may boost peripheral serotonin synthesis while starving the brain of the tryptophan precursor it needs to maintain central serotonergic tone. Low central serotonin activity is consistently associated with heightened aggression and impulsivity across species, from fish and rodents to humans.

Sequencing of the cecal microbiota added a microbial dimension to the story. Overall diversity metrics told a subtle tale: seven alpha-diversity indices showed no significant differences between stressed and control birds, and principal coordinate analysis based on Bray-Curtis distances found their community structures broadly similar. Firmicutes and Actinobacteria dominated both groups, with Faecalibacterium, Bifidobacterium, and Lactobacillus as the most abundant genera. But linear discriminant analysis effect size screening identified 29 differentially abundant taxa. Stressed birds carried more Shigella, Holdemania, and Clostridium, and fewer Desulfovibrio, Cellulosimicrobium, Enterococcus, Coriobacteriaceae, and Bacteroidales.S24_7. The message is that chronic stress reshapes the gut ecosystem in targeted ways rather than broadly collapsing it.

The metabolomics analysis, performed on plasma using ultra-high-performance liquid chromatography coupled to an Orbitrap mass spectrometer, identified 281 metabolites, of which 53 differed significantly between groups—23 upregulated and 30 downregulated under stress. Pathway enrichment pointed to tryptophan metabolism, pantothenate and CoA biosynthesis, and beta-alanine metabolism as the most affected networks. Tryptophan and its derivatives were upregulated in the plasma of stressed birds, even as hypothalamic serotonin fell. The researchers propose that immune activation under chronic stress may divert tryptophan away from serotonin production and into the kynurenine pathway, whose rate-limiting enzymes, IDO and TDO, are highly responsive to inflammatory cytokines. Kynurenine pathway metabolites such as quinolinic acid are themselves neuroactive and have been implicated in behavioral disorders.

Spearman correlation analyses then wove the threads together. Aggressive behavior correlated positively with the genus Clostridium and negatively with Actinomycetales and Coriobacteriaceae—relationships the authors note had not been described before. Clostridium, in turn, correlated positively with indole, L-tryptophan, and butyryl-L-carnitine, while Actinomycetales and Coriobacteriaceae tracked with beneficial compounds such as niacinamide and salicylic acid. Of 31 metabolites linked to the three key microbial taxa, 18 were also associated with aggression, including indole, L-tryptophan, biochanin A, niacinamide, leucodopachrome, and pyroglutamic acid. The mechanism may run through the vagus nerve: Clostridium species are known to metabolize tryptophan into neuroactive indoles and tryptamines that stimulate vagal afferents, and microbial metabolites such as kynurenine and indole derivatives are ligands for the aryl hydrocarbon receptor, a regulator of intestinal immune tone that can further bias tryptophan toward the kynurenine route.

The findings carry immediate practical weight for an industry where aggression translates directly into economic loss—non-beak-trimmed hens, for instance, show roughly a tenfold increase in mortality from aggressive pecking. The authors outline nutrition-based interventions supported by prior work: ensuring adequate dietary tryptophan, with commercial target tryptophan-to-lysine ratios around 16 to 19 percent, can support serotonergic tone and reduce abnormal pecking; probiotics such as Lactobacillus rhamnosus and Bacillus subtilis have been shown to mitigate stress-linked injurious behavior; and sodium butyrate, a microbial postbiotic, counters corticosterone-induced oxidative stress. The team is careful to stress that correlation is not causation, and that the observed taxa-metabolite-behavior links should be validated under different housing systems, stocking densities, lighting programs, and diets. Future experiments using fecal microbiota transplantation from donors with divergent stress phenotypes, gnotobiotic colonization, and targeted tryptophan manipulation could establish whether these gut communities truly transfer aggressive tendencies—a question made more plausible by mammalian studies in which microbiota transfer has conveyed anxiety- and depression-like behaviors between animals.

Subject of Research: Gut microbiota and plasma metabolite mechanisms underlying stress-induced aggressive behavior in broiler chickens

Article Title: Integrative analysis of gut microbiota and plasma metabolites reveals mechanisms underlying aggressive behavior in chronically stressed broiler chickens

Article References: Wu, X., Zhang, J., Ren, H., Cheng, X., Gao, J., & Ma, W. (2026). Integrative analysis of gut microbiota and plasma metabolites reveals mechanisms underlying aggressive behavior in chronically stressed broiler chickens. Stress Biology, 6(1), Article 30. https://doi.org/10.1007/s44154-026-00297-2

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00297-2

Keywords: gut microbiota, broiler chickens, chronic stress, aggressive behavior, tryptophan metabolism, serotonin, corticosterone, metabolomics, microbiota-gut-brain axis, Clostridium, animal welfare, poultry science