Effect of lower temperature stimuli during incubation on fear and social-related behaviours in broilers

Temperature is the most influential environmental parameter during egg incubation affecting embryo development, hatchability and post-hatch performance of broilers. Industrial incubation operates to control the temperature and humidity around the eggs closely based on predefined setpoints and keep fluctuations to a minimum. On the other hand, during brooding, the mother hen regularly leaves the nest for self-maintenance resulting in a striking difference between the constant industrial versus fluctuating natural incubation temperature profiles. Previous studies on ducks and laying hens have found fear and/or social behaviours to be affected by temperatures below recommended values during incubation, a similar result could be existing in commercial broilers. Therefore, this study examines the effect of lowering machine temperatures during incubation on behaviour. Three incubation treatments were defined with a constant machine temperature of 37.6 °C on embryonic day (ED) 0–13 and 19–21 with the machine temperature differing on ED 13–18: (1) Control (C; 37.6 °C), (2) non-optimal Incubation Treatment (IT; 36.6 °C), (3) Natural Treatment (NT, 30 °C for 30 min per day). Post-hatch fear and social behaviour were quantified using six scientifically accepted behavioural tests: three fear tests (novel object on day (d) 1 and d8, tonic immobility (d40), inversion (d42)), two social tests (isolation (d19) and social reinstatement (25)) and one social/fear test (emergence (d18 and d20)). For the emergence test, wherein the time taken for an individual chick to emerge from a T-shaped box was measured, a treatment effect was found on the latency to emerge (P < 0.05) with C chicks emerging more often and faster than IT chicks. As no differences for IT were found in other social tests, the emergence suggest IT chicks to be more hesitant towards novel environments. No differences were present between NT and C or NT and IT. For the isolation test, where an individual chick is visually and audibly isolated for 3 min and the number of vocalisations was measured, an interaction was found between treatment and sex (P < 0.001). Females vocalised more than males in IT and C, but this was not the case for NT. NT male chicks could thus be considered more sensitive to social separation, which could be explained by the higher impact of early stress, caused by the severe temperature drops in NT, having a stronger impact on the anxiety behaviour of males compared to females in later life.