During summer and early fall, many sow farms experience a variety of reproductive problems, such as, anestrus, extended weaning-to-estrus intervals, poor conception rates, higher embryo mortality rates and low farrowing rates. This phenomenon is commonly referred to as "seasonal infertility" and can also be present in boar studs, where the result is lower semen output and lower semen quality. Most researchers have attributed this seasonal infertility to heat stress and the fact that pigs are inherently seasonal breeders. Before pigs were domesticated, both sows and boars were seasonal breeders. In North America, pigs entered an anestrous season in the summer and early fall, which avoided the birthing of offspring in winter, a time when survival would not be optimal.