Abstract
Female mate choice after copulation or spawning is cryptic when a female differentially influences the fertilization success of sperm from different males. We tested whether ovarian fluid could act as a potential mechanism of cryptic female choice (CFC) in chinook salmon (Oncorhynchus tshawytscha) by comparing how ovarian fluid from each of 7 females affected the sperm behavior of 11 different males. Using computer-assisted sperm analysis, we measured sperm velocity, motility, longevity, and linearity from the ejaculates of each male activated in the ovarian fluid from each female. Mean sperm swimming speed, path trajectory, and longevity differed significantly among males, and within males depended on the female's ovarian fluid in which it was activated. Most important, the pattern of within-male variation in these traits also varied significantly among males in response to different females' ovarian fluids. As sperm velocity is known to be a prime determinant of fertilization success in externally fertilizing fishes. This finding suggests that variation in the composition of female ovarian fluid may be a mechanism for CFC, whereby females differentially enhance the swimming speed of sperm from different males. Thus, female ovarian fluid can alter relative male fertilization success when there is intense sperm competition, as there is in this and other group spawning fish species.