Pre-orbital gland opening: part of sucking behavior in red deer (Cervus elaphus) calves

Opening of the pre-orbital gland in red deer (Cervus elaphus) was previously found to be associated with feeding and negatively associated with achieving satiety in bottle-reared red deer calves. We speculated that this acted as a signal from a calf to the mother that it was hungry. Thus, we focused on a possible association between the pre-orbital gland opening and the sucking behavior in red deer calves. We recorded 63 sucking attempts and 150 sucking bouts within a stable social group of 24 farmed red deer females with 19 calves ages 16 to 90 d. The probability of the pre-orbital gland opening or remaining open during a sucking bout rapidly decreased with the on-going sucking phase (P < 0.001), and it was less after than during a sucking attempt (P < 0.03). The proportion of open pre-orbital glands at the end of sucking depended on whether the nursing female (27.85%) or the calf (0%, P < 0.01) terminated the sucking bout. The open pre-orbital gland of the calf after the sucking bout significantly predicted a subsequent sucking attempt (P < 0.001). The probability of open pre-orbital gland was not significantly affected by the age of the calf, sex or birth weight, or age of the nursing female. In conclusion, our study provides evidence that the opening of the pre-orbital gland is one of the general patterns of calf sucking behavior, specifically that it reflects the level of satisfying calf needs.