Results: We found that the daily rhythm of body temperature is absent at birth but matures fully
during the first two months of life. The mature rhythm had a mean level of 38.3°C, a range of
excursion of 1.4°C, and was more robust than that of any mammalian species previously studied
(90% of maximal robustness). Sexually mature cows also exhibited a robust estrous rhythm of body
temperature. An elevation of about 1.3°C was observed every 21 days on the day of estrus. Small
seasonal variations in this pattern were observed.