I have to admit, I’ve never liked spiders. I don’t know what it is about them that freaks me out so much, but there’s definitely a few things that any true arachnophobe would have to say about them. First of all, they have too many legs. Or at least a couple too many. Then they lack pleasant proportions between body parts. Seriously, how can your legs be ten or more times longer than your body and not look gross? Also, they have a very unappealing colour and texture. Not to mention the fact that they are perfectly silent but horribly fast and, what’s worse, they resemble the evil monsters that I used to see in my nightmares as a kid.
This said, who would have thought that today, as I opened my Internet browser, I was going to discover one more disgusting reason not to like spiders? When I stumbled across a paper called “How effective and persistent are fragments of male genitalia as mating plugs [in St Andrew’s Cross spider]” I knew that was it. I had come across something truly awful about my worst enemy. I felt a shiver down my spine, went on to read the rest of the study and discovered the deep dark secrets of the intimate lives of these creatures.
This is how the story goes. A brave group of researchers from Macquarie University in Australia closely observed the mating process of dozens of Argiope Keyselingi, also known as St Andrew’s Cross spiders. After multiple sexual organ examinations before and after copulation, they discovered that the males from this species of spiders always try to break off a fragment of their genitalia inside the female body to prevent her from remating. How effective this nifty trick is depends on how long the fragment can stay in place before falling off the female’s genital tract. This relies on how much control the female has over the copulation duration time and how big and aggressive she is compared to the male. It turns out females of the St Andrew’s Cross spiders can be very bossy and dominating during sex and they can sometime do pretty nasty things to their partners. Such as attacking them. And occasionally eating them. Yes, to make the whole process even more brutal and ruthless, as if breakage of genitalia wasn’t enough, these spiders engage in cannibalistic acts.
What is interesting about the St Andrew’s Cross spiders is that the females are always trying to eat the male during copulation, but they are only successful around 50% of the time, depending on the partner’s size and strength. Scientists have also noticed that not every male succeeds in lodging a mating plug in the female body. So, what they hypothesised is that smaller males who are more likely to get eaten by the females are more efficient at securely lodging the mating plug. In other words, it’s a trade-off between their own life and the mating plugs that increase their chances of paternity. It’s a cruel fate, the one that these spiders have to face.
However, mating plugs are not unique to these spiders. Different kinds of mating plugs have been observed in a variety of animal species including nematodes, insects, reptiles and mammals. The plug itself varies depending on the animal: it may be coagulated sperm, like in the case of pigs and some primates, or accessory gland products, like in bumblebees and some types of butterflies and moths, or even the whole dead animal itself, like in the case of the orb-weaving spider (with spiders, perversity seems to be the rule).
Each plug can serve a variety of purposes. The most obvious one, of course, is to block the female genital opening, thus preventing her to mate with other males and reducing sexual competition. When the plug is big enough to be seen by other males, it can also discourage them from approaching the female in the first place. The plug may also contain chemicals that influence the female behaviour, making her less likely to engage with other males.
My favourite sexual plug depositing animal (up to this morning, I never thought I would have written such a sentence) has got to be the spider Leucauge marina. In this romantic species, the female has to cooperate in order for the plug to form successfully. This is dependent on male courtship performance. So the nicer he is, the more likely the female is to not mate with other spiders afterwards. It might not be as pretty as a ring, but this is as sweet as spiders get.