Shining light on laying hen cognition: the effect of light during incubation depends on task and hybrid.

The influence of early life on animal welfare later in life is increasingly recognized as important. In chickens, a promising early-life intervention is the exposure to light during incubation. Due to the position of the embryo in the egg, the two brain hemispheres receive different amounts of light. This unequal exposure results in increased lateralization of the brain, which is the specialization of brain hemispheres on certain functions. In current practice, chickens are incubated in complete darkness. Broilers incubated in darkness have poorer cognitive abilities compared to light-incubated broilers. Information about the effect of light during incubation on laying hen cognition is scarce. In this experiment, we therefore incubated eggs of ISA Brown (brown) and Dekalb White (white) layer hybrids either in a green light-dark cycle (light), or in complete darkness (dark), resulting in a 2x2 design with four treatment groups: brown-light, brown-dark, white-light, white-dark. The individual birds underwent three cognition tests. At 4 weeks old, in a detour test, the chicks could access a mirror by passing a transparent barrier on either the left or right side. After six trials per chick, a lateralization index was calculated. Light chicks were 24% more lateralized than dark chicks (p(p=0.96) or interaction effect (p=0.83) on chick lateralization. A 5-days holeboard test conducted when the hens were 21 weeks old revealed no treatment effects on hen working memory (p=0.09). Finally, at 24 weeks old, hens were given the choice between a familiar and an unfamiliar hen in a social recognition test. White-light hens approached the familiar hen faster than white-dark hens on days 1 and 2 (p5 (pthe familiar hen (p=0.17). Brown hens spent 14% more time close to the familiar hen than white hens did on day 2 (p(p=0.06), though the tendency was mainly seen in the brown hens. To conclude, light during incubation affected laying hen cognition at different life stages, though the effects were not consistent across tests or hybrids