Function of copulatory plugs in house mice: mating behavior and paternity outcomes of rival males

Abstract

Polyandry is widespread across animal taxa, and subjects males to intense post-copulatory sexual selection which favors adaptations that enhance a male's paternity success, either by decreasing the risk of sperm competition and/or by increasing the competitiveness of the ejaculate. Copulatory plugs deposited by males are thought to have evolved in the context of sperm competition. However, experimental studies that assess the function of copulatory plugs remain scarce. Moreover, most studies have used unnatural manipulations, such as ablating plug-producing male glands or interrupting copulations. Here, we investigated whether repeated ejaculation affects plug size in a mammalian model species, the house mouse. When males experience short periods of sexual rest we found that plug size decreased over repeated ejaculations so that time since last ejaculation can be applied as an approximation for plug size. We induced natural variation in plug size arising from variation in male sexual restedness, and investigated the behavior and paternity success of rival males. Male behavior in the offensive mating role (second) was influenced, albeit not significantly, by the sexual restedness of the first male-to-mate, and therefore the size of his plug. However, second males sired a significantly greater proportion of embryos when competing against a male that had recently mated compared to a male that had not. This supports a potential role of the plug in promoting a male's competitive fertilization success when remating occurs, which could be mediated both by delaying female remating and by ensuring efficient sperm transport through the female reproductive tract.

The copulatory plug delays ejaculation by rival males and affects sperm competition outcome in house mice

“…Female remating was not affected by the plug removal. Overall, female remating rate was high (83%), similar to previous laboratory studies on wild‐derived house mice (Rolland et al ., ; Sutter & Lindholm, ; Sutter et al ., ; but see Ramm & Stockley, ). When we investigated the effects on copulatory behaviour in more detail, we found significant associations between certain aspects of copulatory behaviour and plug removal.…”

“…Plug removal trials may thus often have mimicked situations where smaller plugs are deposited (e.g. when males are not fully sexually rested; Sutter et al ., ). Thus, a contrast between the presence and complete absence of a copulatory plug might result in more pronounced differences in the copulatory behaviour of rival males than those observed here.…”

“…Besides timing, the number of ejaculations influences paternity in mice (Sutter & Lindholm, ; Sutter et al ., ) and more generally in rodents (Stockley & Preston, ). Our analyses showed that when accounting for the ejaculation interval, the number of ejaculations showed an additional positive effect on P 1 (Table ).…”

“…Some studies have directly observed female remating after the deposition of a copulatory plug, showing that copulatory plugs do not prevent remating (Ramm & Stockley, ; Sutter & Lindholm, ; Sutter et al ., ). Nonetheless, large copulatory plugs could be beneficial in the context of sperm competition by delaying ejaculation of rival males to a suboptimal time relative to ovulation (Parker, ; Sutter et al ., ). In house mice, first males sire the majority of offspring even if plugs are removed (Levine, ; Firman & Simmons, ), probably because they ejaculate at an optimal time relative to the release of ova (Gomendio et al ., ).…”

The molecular basis and reproductive function(s) of copulatory plugs

“…If first‐to‐mate males had their plugs experimentally removed following mating, they lost paternity to second‐to‐mate males, compared to control matings where first male plugs were left intact (Sutter and Lindholm, ). Furthermore, if first‐to‐mate males had recently mated, their copulatory plugs were smaller and they too lost paternity to second‐to‐mate males, compared to matings where first males were sexually rested (Sutter et al, ).…”