Increased temperature during incubation increases the sociality of male broilers in later life

Egg incubation through artificial means is a highly controlled process, where eggs are held at a fixed air temperature of 37.6°C. Nevertheless, much research has been conducted on alternative temperature profiles to evaluate their influence on production and welfare traits. Most attention has been paid on increasing temperature as this had been shown to improve heat tolerance in later life. Limited research has been conducted on the effects on animal behavior, although the absence of negative emotions such as fearfulness and the presence of social expression is considered crucial for positive animal welfare as stated by the Welfare Quality® Protocol for Poultry. Therefore, the objective of the current study was to investigate the effect of two heat tolerance improving thermal treatments: (T1, +1°C and T2, +2°C from embryonic day 15-20) on fearful and social-related behaviors in later life. A total of 12 pens (4 per treatment) containing 13 to 14 chickens was reared for 42 days. During this rearing period, post-hatch fearful and social behavior were quantified using seven behavioral tests: an emergence test, a social reinstatement test, an isolation test, a novel arena test, a novel object test, a human stationary and a human approach test. These tests were chosen based on a thorough literature study and repeated 2 or 3 times. The research results showed that social-related behaviors were affected by the interaction of treatment and gender. Whereas in standard incubation, males have a lower social reinstatement tendency, both treatments changed their behavior. T1 males vocalized more than females during isolation (males: 73 ± 4 calls versus females: 38 ± 2 calls, P<0.001) and T2 males showed more social motivation during a social reinstatement test compared to C (T2: 121 ± 20 sec versus C: 66 ± 25 sec, P<0.05) and T1 (57 ± 27 sec, P<0.05). Finally, an effect of age was found with older birds being less fearful of a novel object and environment, but more fearful of an approaching human. Also, they seem more socially driven to contact conspecifics. However, the validity of these age results is questionable as habituation, changing sensory and/or cognitive skills, variance between repeats and possible reduced mobility at an older age could affect the behavioral response over time. In conclusion, the increased temperature during incubation can improve heat tolerance in later life, however, sociality is also affected. An increased tendency to reinstate contact with conspecifics is found, mainly in males.