Probiotic supplementation affects fear-related behaviours and cognitive judgement bias in early-stressed chicks
Probiotic supplementation with *Pediococcus acidilactici* was found to mitigate acute early stress and improve fear-related behaviors and cognitive judgment bias in commercially hatched laying hen chicks, suggesting a potential role in enhancing poultry welfare despite the underlying mechanisms remaining unclear.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
The connection between the gut and the brain is a two-way street that runs deep in the animal kingdom. Just as stress can upset a stomach, the community of tiny bacteria living inside an intestine can influence how an animal thinks, feels, and reacts to the world. This communication network, often called the gut-brain axis, helps regulate everything from digestion to fear responses. In farm animals, where stress is a common part of daily life, scientists are increasingly interested in whether feeding beneficial bacteria, known as probiotics, can help calm the mind as well as the gut. If the right bacteria can soothe a stressed animal, it could offer a natural way to improve the lives of billions of birds raised for food, reducing the anxiety and behavioral problems that often plague them.
Researchers at Linköping University in Sweden set out to test this idea using young laying hens. They focused on a specific type of stress that happens before the chicks even leave the hatchery. In commercial operations, chicks are often exposed to harsh disinfection processes, such as formaldehyde fumes, and are handled roughly during sorting and transport. This early trauma can leave them fearful and anxious for weeks. The scientists wanted to see if adding a specific probiotic, a strain of bacteria called Pediococcus acidilactici, to the chicks' feed and water could act as a buffer against this stress. They gathered 80 chicks, splitting them into four groups to compare different scenarios. Two groups came from a standard commercial hatchery where they experienced the usual stressful routine; one of these groups received the probiotic supplement while the other did not. The other two groups were control chicks hatched in a quiet, controlled environment at the university, with one group receiving the probiotic and the other receiving regular feed.
The team tracked the chicks from their first day of life until they were six weeks old. They collected fecal samples to see what bacteria were living inside the birds and ran a series of tests to measure how the chicks behaved. These tests included placing a chick in a large, empty room to see how quickly it would start moving, showing it a new object to see if it was afraid, and separating it from its friends to measure how much it cried out in distress. They also used a clever test to gauge the birds' mood, presenting them with an ambiguous image that was half chick and half owl to see if they would interpret it as a friend or a threat.
The results showed that the commercial hatchery environment did indeed change the chicks' internal biology and their behavior. On their first day, the chicks from the hatchery had a different mix of gut bacteria compared to the calm, university-hatched chicks, with the hatchery birds showing a more diverse community, likely because the sterilization process cleared out some of the usual bacteria and left room for new ones to grow. By the time the chicks were six weeks old, the probiotic supplement had successfully established itself in the gut of the hatchery birds that received it, appearing in 80 percent of those samples.
When it came to behavior, the probiotic seemed to help the stressed chicks cope. In the open field test, where birds are placed in a large, unfamiliar space, the chicks that had received the probiotic supplement started moving much faster than those that did not. They were also more active overall, exploring their surroundings with less hesitation. This was particularly true for the hatchery birds that got the probiotic, whose behavior began to look more like the calm, university-hatched birds than their stressed, untreated counterparts. However, the effect was not uniform across every test. When a new object was introduced, the probiotic-supplemented hatchery birds showed a different pattern, suggesting that while the supplement reduced general fear, it did not simply make the birds fearless in every situation.
The most striking finding came from the test of the birds' emotional outlook. When presented with the ambiguous image that was part chick and part owl, the birds that received the probiotic were quicker to approach it. In animal behavior studies, approaching an uncertain situation is often seen as a sign of optimism, while avoiding it suggests pessimism or fear. The probiotic-fed chicks seemed to interpret the uncertain image more positively, a shift that suggests their emotional state had improved. Interestingly, the supplement did not change the birds' weight or their immediate physical stress hormone levels in the blood, indicating that the benefits were likely related to how their brains processed stress rather than just their physical growth or immediate hormonal spikes.
The study concludes that adding this specific probiotic to the diet of commercially hatched chicks can help buffer the negative effects of early-life stress. While the exact mechanism of how the bacteria talk to the brain remains a mystery, the evidence suggests that Pediococcus acidilactici can help young birds feel less fearful and more optimistic. This points to a promising future where simple dietary changes could significantly improve the welfare of poultry, offering a natural way to ease the transition from the hatchery to the farm and ensuring that these birds live with less anxiety and more confidence.
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