Much like humans, pigs feel heat based on temperature and humidity. Sows and boars both suffer from acute and persistent exposure to elevated ambient temperatures and humidity. As a result, infertility can be relatively short term or, sometimes, a permanent disability from which the animal will never recover. Heat stress to sows and boars occurs when ambient temperatures are outside of the animals' thermoneutral zone, which ranges between 45° and 70° F. In most cases, the effect of heat stress on reproduction has been related to ambient temperatures in excess of 80° F. In studies where temperatures were elevated experimentally for sows and gilts, anestrus increased and conception rates and embryo survival decreased.