Examining the phenotypic plasticity in ejaculates of the Arctic charr by experimentally inducing successive changes in social status.

Abstract

According to theory, a male’s reproductive role should predict investment in ejaculate quality, i.e. subordinate males should invest in ejaculate quality to compensate for mating in unfavourable roles, and vice versa for dominant males. The Arctic charr is known to possess fluctuating characteristics of its sperm triggered by rapid changes in social status. The relationship between social status and phenotypic plasticity in Arctic charr ejaculates was examined through a caging experiment. Changes in status of size matched males were experimentally induced through two successive social encounters. Results show that males inhabiting subordinate roles throughout the experiment were able to maintain sperm swimming speed, as did males going from a dominant to a subordinate state. Further, males attaining and defending dominant roles did not decrease investment in their ejaculates as one would expect. Surprisingly, subordinate males attaining dominance showed increased sperm swimming speed in early post activation. The investments in short term fitness benefits become evident in this experiment, partly, as males becoming subordinate will not increase initial sperm swimming speed because it is possibly at an optimum already, but in particular, as males attaining dominance increase initial sperm swimming speed. As the rapidly altered competitive ability of Arctic charr ejaculates may increase individual fitness, individual positioning in a social hierarchy is an important aspect of sperm competition.