Rapid adjustments of sperm characteristics in relation to social status

Abstract

Sperm competition models predict that males typically mating in disfavoured roles should be selected to compensate for their disadvantage by investing more into sperm. We studied the effect of rapid changes in social status on ejaculate investments during experimental trials with an externally fertilizing teleost--the Arctic charr (Salvelinus alpinus). We document that males becoming dominant produce less sperm with lower velocity, but have higher sex steroid concentrations than subordinate males. These differences in sperm characteristics seem mainly to result from a decreased investment in sperm among fish that become dominant compared to pre-trial levels. Moreover, these adjustments of sperm production and sperm velocity seem not to be traded against sperm longevity. Our results support theoretical models of sperm competition, as males forced to mate in disfavoured roles seem to invest more into ejaculate quality than males in favoured roles. Additionally, we are the first to report that males, in a species with status-dependent shifts in reproductive tactics, have evolved rapid tactic specific adjustments of sperm production and sperm velocity corresponding to what could be predicted from their reproductive roles.

Gamete plasticity in a broadcast spawning marine invertebrate

“…Recently, manipulative experiments on internal fertilizers suggest that males may indeed adjust their sperm quality in response to the perceived risk of sperm competition (29)(30)(31)(32). However, although our understanding of the role of sperm competition in shaping selection on sperm size and number in internal fertilizers has increased, it is difficult to relate these findings back to the ancestral condition of broadcast spawning.…”

“…In external fertilizers with (relatively) fast blocks to polyspermy, faster sperm are predicted to have an advantage at finding unfertilized eggs (26,28,54,55); however, high-velocity sperm may also be more likely to induce polyspermy (42). Although we were unable to directly measure sperm velocity, velocity is expected to trade off against longevity (6,26,55; but see 29,56). Therefore, given that sperm from high-density animals lived longer, it is possible that the high-density sperm had a reduced velocity, which could lower collision rates and serve a similar function to reducing sperm numbers under high competition [as predicted by Bode and Marshall (3)].…”

“…Much of this variability may arise from the fact that sperm size may covary with velocity and longevity, and the benefits of these traits may be highly context dependent. The relationship between sperm competition and sperm longevity may be positive (21), negative (27,28), or absent (29), and fertilization success may increase for sperm that are faster (26) or longer lived (27). Hence, it remains unclear whether males should adaptively adjust sperm traits in response to changes in the risk of sperm competition.…”

Ejaculate Allocation and Sperm Competition in Alternative Reproductive Tactics of Salmon and Trout: Implications for Aquaculture

“…Interestingly, brown trout sea-running males were found to preferentially fertilize eggs of intermediate sizes, on the contrary to precocially maturing parr, which fertilized all eggs sizes indiscriminately; this mechanism was proposed as an expression of cryptic male choice (Vladić 2006). Recently, phenotypic plasticity in sperm production as a response to sperm competition risk was emphasized (Rudolfsen et al 2006;Cornwallis & Birkhead 2007;Pizzari et al 2007;Ota et al 2010). Importantly for conditional strategies to evolve, environmental cue affecting gonad maturation must be reliable, whereas finesses of the alternative maturation life histories are not necessarily equal (Tomkins & Hazel 2008).…”

Social Status and Availability of Females Determine Patterns of Sperm Allocation in the Fowl

“…Social status mediates access to females, placing dominant and subordinate males in favored and disfavored roles, respectively (LeBoeuf 1974;Cheng and Burns 1988;Rudolfsen et al 2006). Theoretically, favored and disfavored mating roles are expected to lead to the evolution of alternative ejaculate strategies and it is predicted that males in disfavored roles will allocate more sperm to initial copulations due to a lower probability of obtaining extra copulations and a higher risk of facing sperm competition (Parker 1990a(Parker , 1998Ball and Parker 2000;Reinhold et al 2002).…”