Associations Between REM Sleep-like Posture Expression and Cognitive Flexibility in 2-Month-Old Japanese Black Calves

Sleep supports brain development and adaptability, yet its relationship with cognitive performance in farm animals remains unclear. This study examined the association between a behavioral indicator of rapid eye movement (REM) sleep, the REM sleep-like posture (RSLP), and cognitive ability in Japanese Black calves. Twenty two-month-old calves (11 males and 9 females; 88.0 ± 11.0 kg) were continuously video-recorded for 48 h to quantify daily RSLP time, frequency, and bout duration. Calves then performed associative and reversal learning tasks using color cues paired with rewarding (sweet) or aversive (acidic) fluids. Daily RSLP time, frequency, and bout duration were 220.9 ± 67.3 min/day, 34.9 ± 10.0 bouts/day, and 6.8 ± 2.1 min/bout. Fifteen calves (7 males and 8 females) reached the associative learning criterion, while five (1 female and 4 males) did not. No association was found with associative learning, whereas calves with longer daily RSLP time required fewer rounds to reach the reversal learning criterion (r = -0.56, <i>p</i> = 0.030), indicating greater cognitive flexibility. These findings suggest that longer REM-related sleep may support adaptability in calves. Monitoring RSLP could provide a non-invasive indicator of welfare and cognitive capacity, supporting precision management to improve comfort and productivity of calves.