Mitchell, Duncan

The chapter, "The thermal physiology of the ruminant fetus" was written by the listed authors including Alida Faurie (Douglas College Faculty). This volume contains 26 chapters in sections on regulation of feed intake, rumen microbiology and fermentation, nutrient absorption and splanchnic metabolism, tissue maintenance and utilization of endogenous body reserves, tissue growth, reproduction, pregnancy and lactation, ruminant physiology and genetics, and host resistance to parasites and pathogens. The chapters are the plenary papers presented at the IX International Symposium on Ruminant Physiology in Pretoria, South Africa during October 1999. This chapter discusses the fetal body temperature homeostasis, feto-maternal thermal relationship, birth-related changes in body temperatures, and preterm labour and abortion in ewes.

Maternal and fetal body temperatures were measured in five Boer goats, of mean mass 64 +/- 8 kg, using temperature-sensitive radiotelemeters implanted intra-abdominally. Body temperatures were recorded every 5 min. Throughout the last month of gestation, fetal temperature was approximately 0.6 (o)C higher than that of the mother, in normal laboratory conditions (ambient air temperature: 21-24 (o)C). This feto-maternal temperature difference between the goat fetus and its mother is similar to that found in other mammals, including sheep. When the pregnant goats were subjected to short-term heating and cooling, the difference between maternal and fetal body temperatures changed. Thus the mean difference between fetal and maternal body temperatures decreased from 0.4 to 0.2 (o)C during 2 h of heating, while it widened from 0.3 to 0.7 (o)C during 6 h of cooling. These data support the idea that the fetus is thermally protected from excursions of body temperature during changes in the mother's thermal environment. Reports of goat stock losses and abortions during cold spells in their natural habitats may be the result of more severe and/or prolonged cold exposures that not only adversely affect fetal or maternal body temperature, but also influence other aspects of metabolism.

The fetus is thermally clamped to its mother, and susceptible to infectious agents arising in maternal or amniotic tissue. We review the consequences of maternal fever and infection for the fetus. Possible consequences include noxious fetal hyperthermia, cytokine-induced damage to tissues and premature labour or abortion. [ABSTRACT FROM AUTHOR]

We measured core body temperatures of three pregnant ewes and their fetuses by means of chronically implanted temperature-sensitive data loggers. In standard laboratory conditions (dry-bulb temperature 20–22°C) core body temperatures of pregnant sheep and their fetuses during the last trimester of pregnancy demonstrated a constant feto-maternal temperature gradient, and a daily thermal circadian rhythm for the mother and fetus with a small amplitude (<1°C). When the same pregnant ewes were transferred to a natural field habitat where dry-bulb temperature varied between 7°C and 27°C in a 24 h period, there was a marked increase in the amplitude of ewe nychthemeral temperature rhythms (∼2°C). There was similarly an increase in fetal temperature rhythms, but the variations were smaller. Immediately after birth in the field, the core temperature of newborn lambs fell by up to 4.5°C. Thereafter, newborn lambs increased their core body temperatures and appeared to regulate core temperature and daily temperature rhythms at the same levels that they were as fetuses. Only in the second month post-birth do lamb core temperatures and temperature rhythms begin to resemble those of adult sheep. We have shown that fetal thermal protection, which is evident in ewes housed in in-door conditions, persists in natural field conditions also, where fetuses appear to be more protected from night-time falls- than daytime rises in maternal core temperature.

We exposed Dorper-cross ewes at 􏰀120–135 days of gestation to a hot (40°C, 60% relative humidity) and a cold (4°C, 90% relative humidity) environ- ment and to treadmill exercise (2.1 km/h, 5° gradient) and measured fetal lamb and ewe body temperatures using pre- viously implanted abdominal radiotelemeters. When ewes were exposed to 2 h of heat or 30 min of exercise, body temperature rose less in the fetus than in the mother, such that the difference between fetal and maternal body temper- ature, on average 0.6°C before the thermal stress, fell signif- icantly by 0.54 􏰁 0.06°C (SE, n 􏰂 8) during heat exposure and by 0.21 􏰁 0.08°C (n 􏰂 7) during exercise. During 6 h of maternal exposure to cold, temperature fell significantly less in the fetus than in the ewe, and the difference between fetal and maternal body temperature rose to 1.16 􏰁 0.26°C (n 􏰂 9). Thermoregulatory strategies used by the pregnant ewe for thermoregulation during heat or cold exposure appear to protect the fetus from changes in its thermal environment.