Chemosensory learning in the chicken embryo.

Prenatal chemosensory learning has been demonstrated in mammals, fish, amphibians, and insects, but not birds, although there is evidence of the avian's ability to learn auditory stimuli before hatching. This paper examines how exposure to a chemosensory stimulus (strawberry) prior to hatching affects subsequent chemosensory preferences of newly hatched chicks. The chicks' preferences were assessed at 2 days after hatching using an "olfactory" preference test (strawberry-smelling shavings versus water-coated shavings) and at 4 days after hatching using a "gustatory" preference test (strawberry-flavoured water versus unflavoured water). Chicken embryos were exposed to strawberry from Day 15 to Day 20 of incubation by either presenting the odour in the air around the egg, rubbing it onto the shell, or injecting it into the air space. With no exposure to strawberry before hatching, strawberry was highly aversive to chicks after hatching. However, following exposure to strawberry before hatching, chicks expressed a greater preference for (or weaker aversion to) the strawberry stimulus. Chicks exposed to strawberry before hatching drank more strawberry-flavoured water and spent more time in a strawberry-scented area than chicks having no exposure before hatching. This change in preference was specific to the stimulus experienced before hatching and was present in the absence of any posthatching exposure to the stimulus. The results demonstrate that a chick's chemosensory preferences are changed as a result of experience with a stimulus before hatching and are suggestive of learning. The results, similar to those obtained in other animal groups, indicate the universality of "prenatal" chemosensory learning in the animal kingdom. A possible role of embryonic chemosensory learning for recognition is discussed.