Nociception in Chicken Embryos, Part III: Analysis of Movements before and after Application of a Noxious Stimulus

SIMPLE SUMMARY: Chicken embryos are currently being increasingly used in various areas of research but are frequently not covered by animal protection legislation. In the food industry, it is often even common practice to kill male embryos because they are of no economic use. In both cases, there is a lack of knowledge about the sentience of these chicken embryos, especially their ability to perceive pain. The purpose of this exploratory study was to investigate whether—and if so, on which developmental day—a chicken embryo shows a behavioral change or a specific response to a noxious stimulus, both of which would be indicative of functioning nociception. Two complementary approaches were used for the evaluation: human observation and computer-assisted evaluation using a markerless pose estimation software (DeepLabCut). Through a combination of both approaches, it became apparent that developmental day 15 was the earliest stage at which a response to the applied stimulus was detectable. This result thus represents a contribution to the future improvement of animal welfare as it suggests that from developmental day 15 a chicken embryo in the egg has the capacity to show a nocifensive reaction. ABSTRACT: Many potentially noxious interventions are performed on chicken embryos in research and in the poultry industry. It is therefore essential and in the interest of animal welfare to be able to precisely define the point at which a chicken embryo is capable of nociception in ovo. The present part III of a comprehensive study examined the movements of developing chicken embryos with the aim of identifying behavioral responses to a noxious stimulus. For this purpose, a noxious mechanical stimulus and a control stimulus were applied in a randomized order. The recorded movements of the embryos were evaluated using the markerless pose estimation software DeepLabCut and manual observations. After the application of the mechanical stimulus, a significant increase in beak movement was identified in 15- to 18-day-old embryos. In younger embryos, no behavioral changes related to the noxious stimulus were observed. The presented results indicate that noxious mechanical stimuli at the beak base evoke a nocifensive reaction in chicken embryos starting at embryonic day 15.