Testes,which are mainly responsible for producing sperm and secreting male hormones,are the functional center of the male repro-ductive system in mammals.During embryonic development in mammals with descended testes,the testicles descend from the uro-genital ridge to the scrotum,located outside the body cavity (Hutson et al.,2015).The cooling hypothesis is proposed to explain testicular descent and the evolution of the scrotum;it argues that the testes descend into the scrotum because the scrotum is at a temperature 2-4℃ lower than that of the core body and therefore more beneficial for normal spermatogenesis (Moore,1926).Spermatogenesis,the most basic biological process of male reproduction,is temperature sensitive (Thonneau et al.,1998).Failure of the testis to descend into the scrotum causes cryptorchidism,which can hamper sper-matogenesis and ultimately result in infertility (Kim et al.,2013;Durairajanayagam et al.,2015) and other severe secondary testicular diseases,such as cancers (Chung and Brock,2011;Cobellis et al.,2014).