Tactic-specific differences in seminal fluid influence sperm performance

Abstract

Seminal fluid often makes up a large part of an ejaculate, yet most empirical and theoretical studies on sperm competition have focused on how sperm characteristics (number and quality) affect fertilization success. However, seminal fluid influences own sperm performance and may potentially influence the outcome of sperm competition, by also affecting that of rivals. As a consequence males may be expected to allocate their investment in both sperm and seminal fluid in relation to the potential level of competition. Grass goby (Zosterisessor ophiocephalus) is an external fertilizer with guard-sneaker mating tactics, where sperm competition risk varies according to the tactic adopted. Here, we experimentally manipulated grass goby ejaculates by separately combining sperm and seminal fluid from territorial and sneaker males. While sperm of sneaker and territorial males did not differ in their performance when they interacted with their own seminal fluid only, sperm of sneakers increased their velocity and fertilization rate in the presence of territorial males' seminal fluid. By contrast, sneaker males' seminal fluid had a detrimental effect on the performance of territorial males' sperm. Sperm velocity was unaffected by the seminal fluid of males employing the same tactic, suggesting that seminal fluid's effect on rival-tactic sperm is not based on a self/non-self recognition mechanism. Our findings show that cross interactions of sperm and seminal fluid may influence the fertilization success of competing ejaculates with males investing in both sperm and seminal fluid in response to sperm competition risk.

Figure 1.

Velocity of territorial and sneaker males' sperm after incubation in different seminal fluids (mean±s.e.m.). (a) Sperm tested in the presence of own fluid or with the fluid of a male performing the same or a rival tactic (n = 20 territorial, 20 sneaker; white bars, own fluid; grey bars, other sneaker fluid; black bars, other territorial fluid). (b) Sperm tested in the presence of only own fluid or with the addition of rival-tactic male fluid (n = 24 territorial, 24 sneaker; sneaker's fluid pre-diluted 1 : 10; white bars, own fluid; grey bars, own + rival-tactic male fluid). *Adjusted p < 0.05; **adjusted p ≤ 0.01; ***adjusted p ≤ 0.001.

Figure 2.

Fertilization percentage (mean±s.e.m.) of territorial (n = 10) and sneaker (n = 9) males' sperm after incubation in own seminal fluid and in a mix of own + fluid of a rival-tactic male. The seminal fluid of sneaker males was diluted prior to mixing (1 : 10). *Adjusted p < 0.05. Statistics on arcsine square root-transformed data. White bars, own fluid; grey bars, own + rival-tactic male fluid.