Preorbital gland opening in red deer (Cervus elaphus) calves: signal of hunger?

The opening of the preorbital gland in red deer (Cervus elaphus) calves has been hypothesized to be a signal to the mother that her calf is hungry. Closing of the gland should indicate that the calf has received a sufficient amount of milk, and thus the mother should stop the suckling. We tested the hypothesis that the calf signals hunger when its preorbital gland is open and signals satiation when the gland is closed. To test this, the behavior of eight bottle-reared calves was monitored for 6 mo. For each meal during this time period, opening of the preorbital gland was recorded before calves were offered the meal and after they ate. Satiation of the calf was estimated (not begging for more food after the meal = satiated; begging = not satiated). The data set contained 3,116 records of paired measurements of preorbital gland opening (before and after the meal). Calves were satiated after feeding for 90.15% of the meals. The opening of the preorbital gland was associated with feeding and achieving satiety. In most cases (77%), calves opened their preorbital glands before their meal. Eighty-five percent of hungry calves still had preorbital glands open after the meal, whereas preorbital gland opening was decreased (P < 0.03) up to 46.6% in satiated calves. Despite this, we found a large range in individual responses in preorbital gland opening before and after the meal, and also according to whether the calf reached satiety. The heavier calves in this study tended to have their preorbital glands open more frequently than the lighter ones, which may indicate the need for an increased amount of milk intake. There was no clear trend in the frequency of preorbital gland opening with age. In conclusion, preorbital opening is likely to be a signal of calf satiety, as suggested earlier. Nevertheless, preorbital glands were not always open when the calves were hungry and did not vanish during the feeding in all cases.