Vaginal temperature rhythms in sheep

Six ovariectomized adult sheep were tethered in a temperature- and light-controlled room and given feed and water ad lib. Vaginal temperatures were recorded at 10m intervals for 12 or 17d. Hourly means were calculated and subjected to statistical analyses by time series procedures. Initial analysis revealed a positive linear trend in most animals suggesting a low frequency rhythm with a period greater than the 17d experimental period. All subsequent analyses were, therefore, conducted on data adjusted for a linear trend. The periodogram, correlogram and spectral density analyses revealed a predominant circadian rhythm in 5 of the 6 animals. Ultradian rhythms, with periods of 12 and 8 hr, were also present in some animals. The magnitude of the rhymic change in vaginal temperature, as estimated from the amplitude of the circadian component, varied from 0.05° to 0.16°C. Calculation of the external acrophase indicated that the daily maximum temperature occurred between 08⁵¹ and 17²⁷.