Ulans, AlexandraBrooks, George C.Jacobs, Leonie2025-01-152025-01-152024-07-302045-232210.1038/s41598-024-67965-z (PII)https://hdl.handle.net/10919/124200The objective was to assess the impact of environmental complexity on affective state (anxiety) in fast- and slow-growing broilers (Gallus gallus domesticus) as they gain weight. Six hundred fast-growing broilers (Ross 708; “fast-growers”) and 600 slow-growing broilers (Hubbard Redbro Mini; “slow-growers”) were raised in 24 pens with simple (standard; SE) or complex (permanent and temporary enrichments; CE) environments. Six birds/pen underwent the attention bias test on day 23 (fast-growers only), 28–29, 35–36, 42–43, and 56–57 (slow-growers only), with individuals only tested once (n = 576). Proportion of birds feeding, time spent vigilant and latencies to eat and step were recorded. Greater vigilance and longer latencies indicate more anxiety. Slow-growers fed more (p = 0.001), were less vigilant (p = 0.003), and stepped sooner than fast-growers (p = 0.007). For both strains, likelihood of feeding was unrelated to weight in SE, but decreased with increasing weight in CE (p = 0.048). Birds in CE stepped sooner than birds in SE (p = 0.030). Vigilance increased with body weight (p = 0.024). These results indicate that affective state (anxiety) can change as birds gain weight, depending on environmental complexity and genetic strain. Overall, slow-growers showed reduced anxiety compared to fast-growers, across housing treatments or weights.12 page(s)application/pdfenCreative Commons Attribution 4.0 InternationalAnimalsChickensBody WeightWeight GainBehavior, AnimalAnxietyEnvironmentMaleArticle - Refereedhttps://doi.org/10.1038/s41598-024-67965-z141Jacobs, Leonie [0000-0002-3799-5078]390803562045-2322