The phenomenon of estrus occurs due to the specific
influence of ovarian steroid hormones on behavioral
centers in the mammalian brain [8]. Initiation of standing
estrus has been associated with peak peripheral
estradiol concentrations [9]. Furthermore, estradiol
concentration at insemination may influence fertilization
(through sperm transport) by altering the uterine
environment [10]. After initiation of standing estrus,
the uterine environment is modified to enhance sperm
motility and longevity [11–13]. Consequently, proestrous
estradiol concentrations from either endogenous
or exogenous sources may play an important role in
sperm transport and the sustained viability of sperm
until ovulation and fertilization.