|Cheng, Heng Wei|
Submitted to: Aggressive Behavior
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2012
Publication Date: 3/5/2013
Publication URL: http://handle.nal.usda.gov/10113/56774
Citation: Dennis, R.L., Fahey, A.G., Cheng, H. 2013. Alterations to embryonic serotonin change aggression and fearfulness. Aggressive Behavior. 39:91-98. Interpretive Summary: Cannibalism and injurious pecking are major concerns in the egg production industry. Currently, hens are often beak trimmed, removing 1/3 to 1/2 of the upper and lower mandibles using a hot blade, to prevent or reduce injury and mortality from these injurious behaviors. However, the beak trimming procedure may introduce a separate welfare issue, causing pain to the trimmed birds. One potential solution is to reduce the birds’ motivation to initiate injurious pecking. However, the mechanisms of these behaviors are not well understood. Serotonin is a primary neurotransmitter involved in regulating pecking behaviors. In the present study we investigate the long term effects of a single dose of serotonin or a pharmaceutical agent that stimulates a serotonin receptor involved in aggression (5-HT1A receptor agonist) delivered to the egg immediately following lay. The eggs were then incubated and allowed to hatch, the resulting chicks were maintained to 18 weeks of age (sexual maturity). Aggression and fearfulness were measured at 9 and 18 weeks of age. Birds treated with excess serotonin in the egg were found to be less aggressive but have a greater fear response compared to untreated control birds. Birds treated with the pharmaceutical serotonin agonist in the egg showed no difference in behavioral response. Our data suggest that prenatal increase in the concentrations of serotonin has long-term effects on bird’s behaviors including aggression.
Technical Abstract: Prenatal environment, including maternal hormones, affects the development of the serotonin (5-HT) system, with long-lasting effects on mood and behavioral exhibition in children and adults. The chicken provides a unique animal model to study the effects of embryonic development on childhood and adolescent behavior. Manipulations to the egg can be made in the absence of confounding maternal effects from treatment. Eggs were injected with 50 ul of 5-HT (10ug/egg), 8-OH-DPAT (a 5-HT 1A receptor agonist; 16ug/egg), or saline on day 0 prior to the 21 day incubation (n=16 per treatment). Injections were performed at 0.5 cm below the shell. Behavior was analyzed at 9 weeks of age and again at the onset of sexual maturity (18 weeks). Birds treated with external 5-HT exhibited significantly less aggressive behaviors at 9 weeks compared to the birds from both 5-HT 1A agonist and saline groups (P<0.05), and at 18 weeks compared to saline control birds only (P<0.05). External 5-HT also increased fearfulness response (P<0.05) as tested by duration of tonic immobility. Increased degree of fluctuating asymmetry at 18 weeks in 5-HT treated birds (P<0.05) suggests that increased 5-HT concentrations in early embryonic stages creates a developmental plasticity in the morphology, which may be associated with the birds’ subordinate behaviors during agonistic encounters. However, agonism of 5-HT1A receptor does not significantly alter birds’ behavior or developmental fluctuating asymmetry compared with saline treated birds. These results indicate that prenatal increase in 5-HT concentrations has long-lasting effects on aggressive and fearful behaviors and morphological development in chickens. Serotonin and 5-HT agonist, 8-OH-DPAT, have different functions in embryonic development via different pathways.