Pig behavior was recorded for 72 h continuously after mixing using a digital video recording system (Hikvision DS-2CE56C2P-IT3 3.6 mm; Hikvision network hard disk video recorder DS-7808HW-E1/M; Hikvision Digital Technology Co. Ltd., Hangzhou, China). The pigs in each pen were individually identified using a spray paint (7CF, Shenzhen Zhaoxin Energy Co., Ltd., Shenzhen, China) on the back and both sides of each pig’s body. Aggressive behavior score of each pig was assessed by observing the video and recording the frequency and duration of fighting behaviors of each pig within 36 h after mixing in each pen. For each aggressive event, the initiator of the fight, the frequency and duration of active attack, being bullied and standoff behaviors were recorded ( Table 1 ). A fighting behavior was defined as a fight or a displacement event when the physical contact of two individuals lasted for more than 3 s, the intervening period between fighting behaviors was at least 8 s [ 22 ]. In a fight, biting, pushing, and chasing was identified as the active attack behavior [ 23 ]. When the recipient pig suffers from biting and head-knocking performed by the aggressive pig and the recipient moves away without retaliation, it was identified as being bullied [ 24 ]. If the two pigs stood side by side, shoulder to shoulder, and one pig threw his head to the head or neck of the other pig, there was an aggressive interaction with no dominance sign produced by either pair member at any time, it was defined as a standoff event [ 25 ]. When a pig showed a submissive behavior, such as stopping its fighting, turning away from an attack, trying to flee or was displaced from the location, it was defined as a loser [ 26 ], and the other pig in the fight was defined as a winner [ 27 ]. If there was no clear outcome, the fight was designated as a draw [ 22 ].