Shining Light On The Laying Hen Brain: The Effect Of Light During Incubation Depends On Cognitive 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 stimulation via the eyes. This unequal stimulation results in increased lateralization of the brain, which is the specialization of brain hemispheres on certain functions. In current commercial practice, chickens are incubated in complete darkness. Information about the effect of light during incubation on laying hen cognition is scarce, although stronger effects are expected in hybrids coming from white eggs than brown eggs. In this experiment, we therefore incubated eggs of ISA Brown (ISAb) and Dekalb White (DW) layer hybrids (N = 1200) either in a green light-dark cycle (light), or in complete darkness (dark). At hatching, brains were collected from 80 individuals. Tyrosine hydroxylase and DoubleCortin levels were investigated, but no significant differences were found between incubation treatments or hybrids. Then, cognition tests were performed on the surviving females. In a detour test (N = 198), light-exposed chicks showed 24% more lateralized behavior (i.e., turned left from the obstacle) than chicks incubated in the dark, regardless of the hybrid. In a 5-day holeboard test (N = 78), adult hen performance results were inconsistent. Finally, during a social recognition test (N = 76), ISAb-dark showed significantly greater interest for the familiar hen, whereas DW-light showed greater interest for the unfamiliar hen. To conclude, light during incubation affected the brains and cognition of laying hens at different life stages post-hatching, although the effects were not consistent across tests or hybrids.