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We predicted that the proportion of suckling attempts rejected and terminated by the mother would be greater for female foals than male foals, based on parent offspring conflict theory and on the assumption that throughout the study, all zebra mothers were in good condition because of captivity. We presumed that an increasing rate of suckling terminated or rejected by a mother would indicate a decreasing effort by the mother to invest in her offspring. We observed foals of captive plains zebras at the Dvur Kralove Zoo, Czech Republic. We found that the probability of successful suckling tended (slope = 0.0016; Z = 1.78; P = 0.074) to increase with increasing age of the female foals, but decreased (slope = -0.0018; Z = -2.51; P = 0.012) with increasing age of the male foals. The proportion of suckling bouts terminated by the mother decreased (slope = -0.0077; Z = -4.27; P < 0.0001) with increasing age of the female foals, but not the male foals (slope = -0.0005; Z = -0.34; P = 0.732). Our results indicate that conflict between mothers and female foals was less than that between mothers and male foals. The observed sex differences in termination and rejection of suckling bouts could be explained by the different behavior of the male and female foals, or by the selective maternal investment. Finally, we revealed no significant effect of herdmates on suckling behavior. Key words: mother-offspring interaction, parental investment theory, plains zebra, sex difference, zoo doi: 10.2527/jas.2009-1982

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. Key Words: Cervus elaphus, Hunger, Preorbital Gland Opening, Red Deer Calves, Satiety, Signal

The opening of the preorbital gland of red deer (Cervus elaphus) calves has been previously associated with feeding and satiety. However, it has been suggested to be most likely affected by some other factor or factors, possibly by excitement of the calf. If so, a calf should open its preorbital gland while being exposed to any stressful procedure. The hypothesis was tested that the preorbital gland is closed in a relaxed calf, whereas it is opened in a stressed calf. Preorbital opening was observed in 41 newborn red deer farm calves during a regular daily routine consisting of a search for newborn calves, their inspection, weighing, and painful marking with an ear tag. The openness of the preorbital gland (preorbital gland closed or opened) was recorded just before manipulation of a lying calf (i.e., in a calm calf) and then during the manipulation (i.e., in a distressed calf). Before manipulation, in all but 3 calves (7.3%, all of which were males), the preorbital gland was closed. All calves observed (100%) opened their preorbital gland during their manipulation, at least by the time the ear was punctured by the ear tag. The proportion of individuals with an open gland was lower (P [less than or equal to] 0.001) before than during manipulation (7.3 vs. 100%, respectively). Hence, openness of the preorbital gland in newborn red deer calves was associated with a stressful manipulation by the humans, which suggests that it may be a simple and easily recognized indicator of calf stress. Key words: Cervus elaphus, excitement, preorbital gland opening, red deer calf, stress