Spiders exchange gifts for sex
Female spiders like being courted with gifts from their male counterparts. New research shows that the females store more sperm from males if they bring a gift prior to mating.
It is not only human males who benefit from flattering that special lady with gifts.
In the world of nursery spiders it is normal for males to initiate mating by giving the female a gift – an insect prey wrapped in silk.
Now a new study shows that this courting behaviour actually pays off for the male nursery spider:
”Our studies have shown that the females store almost twice as much sperm from males who have given them a gift, compared to those who have not,” says Professor Trine Bilde, who heads the ‘Genetics, Ecology and Evolution’ unit at Aarhus University’s Department of Bioscience.
The new study is published in the journal Proceedings of the Royal Society B.
Male spider mates through ’boxing gloves’
Our studies have shown that the females store almost twice as much sperm from males who have given them a gift, compared to those who have not.
Unlike most other spiders, the nursery web spider (Pisaura mirabilis) does not catch its prey in webs. It goes hunting for its prey.
Like all other spiders, however, the nursery spider has a thread, which it often uses to wrap up insect prey as a gift for the female.
While the female is feasting on the contents of the gift, the male may get the chance to crawl beneath her belly and transfer his sperm into her antheridium.
Bilde explains that during mating, the male keeps one of his legs on the gift, so he always knows what the female is doing – and whether she will try to run away with the gift.
The male then transfers his sperm through two small organs that look like boxing gloves and are located on his head.
”The female can store the sperm for a very long time and can activate and use it whenever she wants. Her eggs only become fertilised if and when she wants them to be fertilised,” says the researcher.
Dissecting the antheridia
Most female spiders have mated with several males, but the Danish researchers wished to test the hypothesis that the female spider is capable of ensuring that she can store the greatest amounts of sperm when it comes from gift-giving spiders.
To study this, they presented female spiders with a group of males that brought a gift and another group that did not bring a gift.
After mating, the females were dissected in order to determine how much sperm they had stored from the individual males.
”We ensured that the mating sessions were of equal duration, so as to avoid differences in sperm transfer between the two groups of male spiders,” says Bilde.
“Our experiment focused on finding out whether the females store more sperm from gift-giving males, and our results showed that they actually do.”
The experiments were carried out by PhD fellow Maria Albo, who is tutored by Bilde.
Gift-giving males make better fathers
So why would a female spider rather store sperm from a gift-giving male?
The theory, according to Bilde, is that the female regards this gift-giving as a sign of quality in the male – and thus also his quality as the father of her offspring.
”If the male is capable of catching a prey, wrap it in and serve it to the female, then this shows that he is a good hunter. If these traits are inheritable, this is something the female would like to see passed on to her offspring.”
- "Sperm storage mediated by cryptic female choice for nuptial gifts", Proceedings of the Royal Society B, 2013, DOI:10.1098/rspb.2013.1735