Circadian rhythms are 24 hour cycles of physiology and behavior that evolved for adaptation to changes in environment and physiological state. We hypothesize circadian rhythms adjust to metabolic demands of lactation. Our objective was to determine if circadian rhythms of behavior, body temperature, cortisol and melatonin were different between ewes suckling twins and singletons after two weeks of lactation. Two groups of ewes (n = 5 ewes/group) suckling singleton or twin lambs were acclimated to sampling protocol for 5 d beginning 7 ± 1 d postpartum. After acclimation, behavior was measured (i.e. suckling, eating, drinking, lying down) every 10 min over 24 h. During the following 24 h period, milk, saliva, and rectal temperature were collected every 4 h. Salivary cortisol and melatonin levels were measured with ELISA. Singleton (4.1 ± 1.1 kg) and twin (5.2 ± 1.2 kg) lamb weight and ewe body condition score (BCS: 2.8 ± 0.2 and 2.7 ± 0.2, respectively) were not different (p > 0.05) at birth. At 9-12 d postpartum singleton (9.5 ± 2.6 kg) and twin (8.2 ± 1.5 kg) lamb weight and ewe BCS (2.9 ± 0.2 and 2.8 ± 0.2, respectively) were not different. However, total offspring weight per dam was greater in twin (15.5 ± 2.3 kg) than singleton (9.5 ± 2.6 kg; p < 0.05) group. Average daily gain for individual twins was roughly half of singleton, and net daily weight gain was not different, suggesting metabolic demand on ewes suckling twins was not significantly different from ewes suckling singletons. Lack of difference in suckling behavior score was consistent with lack of difference in metabolic demand between the groups of ewes. Salivary cortisol levels, melatonin levels, and rectal temperature measurements were not significantly different between groups. Circadian rhythms of standing, lying and eating behaviors were similar among groups. Ewes in the twin group ate more frequently during light phase suggesting that slightly higher metabolic demand of suckling twins may be met by increasing consumption alone in the first 2 weeks of lactation.
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Vol. 85 • No. 4