Testosterone (T) is known to induce aggressive behavior, mainly in male animals. Subcutaneous implantation of T-filled silastic tubes, rather than intramuscular injection of T, is generally recommended for long-term treatment using exogenous T. However, the effect of T implantation on chicken aggressive behavior has not been investigated. In addition, the concentration of T required to induce aggressive behavior or whether rearing conditions such as isolated-or grouped-raising affect T-induced aggressive behavior in chickens is not known. The present study aimed to examine the relationship between the lengths of T-filled tubes, blood T concentration, and aggressive behavior in group-and isolation-raised male layer chicks. The testes were bilaterally removed and silactic tubes of various lengths filled with crystalline T were subcutaneously implanted at 14 days of age. A social interaction test was performed to quantitatively assess chick aggressive behavior at 32 days of age. Comb weight and size were used to assess the activation of endogenous androgen receptors. Total aggression frequencies (TAF) and aggression establishment rate (AER) were used to evaluate aggressiveness. Significant positive correlations (P＜0.001) were observed between the comb parameters and plasma T concentration. In the isolation-raised chicks, the TAF and AER were high irrespective of the lengths of the implanted T tubes or the corresponding plasma T concentrations. However, in the group-raised chicks, the AER tended to differ between the T-implanted aggressors (P＝0.0902), and the AER significantly increased with implantation of 1.0-cm-long T-filled tubes (P＜0.05), which corresponded to approximately 47 pg/mL plasma T concentration. These results suggest that both grouped raising and approximately 47 pg/mL plasma T concentration are required for the induction of T-dependent aggressive behavior, and that isolationinduced aggressive behavior is T-independent in male layer chicks.

Testosterone (T) is known to induce aggressive behavior, particularly in male animals. However, our recent results showed that a certain kind of aggressive behavior is T-independent; moreover, the role of T in chicken territorial and isolation-induced aggressive behavior has not yet been investigated. In addition, castration alone is insufficient to evaluate the role of T in aggressive behavior because we found that non-testicular T concentration, probably derived from the adrenal gland, in the blood of castrated chicks was low, but not zero. In the present study, therefore, the role of testicular T in chicken aggressive behavior was evaluated through castration, and the role of non-testicular T was assessed using the subcutaneous implantation of flutamide, a non-steroidal antiandrogen, in the castrated male layer chicks. Resident-intruder (R-I) and social interaction (SI) tests were used to quantitatively monitor territorial and isolation-induced aggressive behavior, respectively. Castration and drug implantation of the chicks were performed at 14 days of age. The R-I test was performed at 29 and 30 days of age, and the SI test was performed at 31 and 32 days of age. The total aggression frequencies (TAFs) and aggression establishment rate (AER) were used as indices of chick aggressive behavior. In the R-I test, castration significantly decreased the TAFs but the AER was not affected by castration or flutamide implantation. In the SI test, on the other hand, there were no significant differences in the TAFs, but the AER tended to increase in the intact chicks and decrease in the flutamide-implanted, castrated male chicks. These results suggest that the role of T in chicken aggression depends on the differences in social context of the behavior, and that both testicular and non-testicular T play an important role in the occurrence of isolation-induced aggression in male layer chicks.