Research from a team of scientists at Aarhus University in Denmark has shown that Pisaura mirabilis females will store more of the gametes of sperm from gift-bearing males.  The team counted sperm retained in newly mated P. mirabilis females and the number of eggs that hatched. Not all male partners provided gifts, and the team found that females kept about 40 percent less sperm from non-gifting males compared with those that were gift-bearing.  Egg-hatching rates from matings with non-gift bearing males were also reduced. It appears females manipulated the sperm retention to distort the fertilization success among males.

This sperm manipulation is called cryptic female choice. It essentially gives females more of an upperhand in reproduction. Cryptic female choice is difficult to demonstrate in most cases; however the Aarhus team were able to effectively monitor it by keeping influences on sperm transfer constant, such as copulation time. The only other alternative would be that gift-bearing males produce more sperm than non-gift bearing males, but there is no evidence to support this.

Females usually expend more resources than males during sexual reproduction in order to produce offspring. This costly reproduction has meant females tend to pick their mates. Some females mate with several males and store the sperm in a bag-like organ called the spermatheca; they can then later choose whose sperm they wish to use.

Cryptic female choice is only seen in species where females have multiple mates, so that they can prioritise the sperm of one male over another.  Bringing a gift to the P. mirabilis females makes for a tempting offer. The food gift is wrapped with silk into a sphere by the male. After offering the gift to the female, when she reaches for it he manoeuvres himself into position and transfers his sperm into her. This is done quickly as females can make off with the gift without mating.  

The time spent copulating is down to the female, and depends on whether there is a gift and the size of that gift. As females consume the gift during mating, a bigger gift takes longer to eat and so results in a longer copulation time. The longer the copulation time, the more sperm the male transfers into the female.

Nuptial gifts of food may also make the male’s sperm appear more valuable as they indicate a healthy and well-fed male. Starved males had more difficulty in wrapping the prey in silk and regularly ate the prey themselves. Males that brought nuptial gifts were more likely to be better hunters who could afford to give away food.

There is no conclusive evidence of a direct correlation between nuptial gifts and the male’s hunting ability however. More studies are required to show whether hunting abilities are heritable too.