Global warming : consequences for sperm quality and fertility of dairy and beef bulls

Genital heat stress is a substantial risk factor for male infertility. At present, it is known that bulls exposed to natural heat stress experience a decrease in sperm quality, even in regions with temperate climates. Slight increases in scrotal temperature can disrupt spermatogenesis, resulting in a complex stress response. Negative effects of heat exposure were observed in semen collected between 14 and 42 days after artificial scrotal insulation, indicating that heat stress is more prejudicial for developing germ cells than for mature sperm cells. Bulls exposed to heat stress are more likely to suffer reduction of testicular weight, alteration of testosterone levels and increase of respiratory rate, rectal temperature, and water consumption. Moreover, hyperthermia has been found to disrupt spermatogenesis, reduce the fertilizing ability, sperm motility, normal morphology, viability, and to alter both the concentration and composition of lipids in bovine sperm. Furthermore, ROS production significantly increases in sperm, leading to lipid peroxidation (LPO), DNA fragmentation, and major sperm defects. Rising temperatures as a consequence of climate change not only have adverse effects on male reproduction, but also on animal physiology, welfare, and health. For decades, the continuing rise in global temperatures has been a concern worldwide and it is expected that temperatures will continue to increase. Global temperature has risen 0.9ºC in the last century. This temperature increase has accelerated for the past 35 years; the warmest five years on record have taken place since 2010, where 2016 was the warmest year and July 2019 the hottest month on record since modern recordkeeping began in 1880. Moreover, the number of days with a temperature humidity index (THI) above 68, which is considered the comfort threshold for cows, has increased by 4.1% from 1973 to 2008 in certain parts of Central Europe with 80 to 86 hot heat stress-causing days per year. Therefore, the main objective of this thesis was to elucidate the effects of rising temperatures on bovine sperm cells and the subsequent generations. Although the detrimental effects of heat stress on bovine reproduction have been studied before, the impact of heat stress on in vivo bull fertility and the molecular mechanisms of sperm damage on bulls exposed to heat stress have not been entirely characterized. First, we aimed to evaluate the sperm quality of the heat-stressed sperm cells, the effect on thei