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Image copyright Carin Bondar 'The Nature of Human Nature'
It’s a tough world out there. For the average Homo sapiens, having security in your relationship is a critical part to feeling successful. If someone out there loves you enough to be faithful to you and to forsake the advances of all others, it re-enforces our self confidence and thereby makes us happier (and more productive) beings. In the real world however, not all partnerships involve parties that adhere quite as strictly to the faithfulness that I mentioned above…what happens if you are feeling less than secure about the fidelity of your partner? In the 15th century humans invented a nifty contraption that provided a guarantee to insecure males that their partners would not accept genetic donations in his absence: the chastity belt. Made out of tough material (usually a combination of steel and leather), the belt covered her sensitive areas and prevented any kind of sexual interaction. In my opinion it seems a rather drastic measure to take, but as I mentioned above it’s a tough world out there. It turns out that the ‘chastity belt’ method of preventing your mate from copulating with any others isn’t limited to our species.
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A ‘mating plug’ is defined as a structure that is utilized to block the female genital reproductive tract. Males of many species (including insects, crustaceans, reptiles and even mammals) utilize such plugs to prevent further mating by a female once they have deposited their DNA into her reproductive tract. In species where there is intense sperm competition, biologists have described some sophisticated mating plugs. For example, the spermatophore of ground beetles (Oedothorax retusus) contains sticky substances that allow it to act as a mating plug by physically blocking the females’ reproductive opening. In addition to its physical blockage, the seminal fluid of ground beetles induces refractory behavior in mated females (i.e. it effects a change in her behavior that makes her avoid further copulation events with other males)1. Dwarf spider (Leptocarabus procerulus) males utilize glandular secretions that harden once they are deposited inside a female2. There is an extremely high level of sperm competition in dwarf spiders, as the sperm from a single mating event can remain fertile in a females’ sperm storage organ for several months. The glandular-produced plugs utilized by the dwarf spiders are highly effective at preventing further mating of females: an experimental study showed that a large plug (produced by an uninterrupted copulatory event) prevented remating of the female 93% of the time2.
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If you think that the ‘chastity belt’ strategies utilized by male ground beetles and dwarf spiders is shocking, read on. In arachnid species where sexual cannibalism is demonstrated (these are species where the female kills and ingests the male subsequent to copulation), males go to an even greater extent to ensure paternity. This makes sense. If he’s going to die anyway, he might as well do whatever it takes to make sure that his DNA is the prize winning seed for the next generation…even if this means breaking off his copulatory device inside the female genital opening (the human equivalent of a male breaking off his penis inside a females’ vagina in order to prevent future copulations). Males of the orb-web spider Argiope lobata and the white widow spider Latrodectus pallidus have been shown to do just that3,4. Plugging up the female with his copulatory apparatus has the effect of blocking the opening from use by other males subsequent to the sperm transfer, and it has been experimentally shown that such plugging is effective at reducing the paternity share of males that attempt to mate once a plug is in place3. Further investigation on sexual behavior in this species demonstrated that males that were cannibalized on their first copulation attempt had a much higher probability of damaging their pedipalps (sexual sperm transfer appendages) than males that escaped (74% vs 15%)3. Since this self-induced damage has negative consequences for future reproductive bouts (no kidding!), it makes the most sense biologically for a male to undertake it if his chances at securing paternity are high.
Although males of several species in the animal kingdom have developed some foolproof ways in which to ensure the fidelity of their sexual partners (i.e. in order to guarantee paternity of the resultant offspring and maximize fitness), I am much relieved that in our species such methods are almost entirely lacking. For the most part the chastity belt is a thing of the past, having been replaced by a simple trust in one’s partner or the taking of ceremonial vows in order to maintain a sexually monogamous partnership. I think that both males and females of our species would agree that such methods are a much more acceptable compromise than either losing your copulatory organ or having your opening plugged up indefinitely. It’s a tough world out there.
1Takami, Y., Sasabe, M., Nagata,N., and Sota, T. 2008. Dual function of seminal substances for mate guarding in a ground beetle. Behavioral Ecology 19: 1173-1178.
2Uhl, G. and Busch, M. 2009. Securing paternity: mating plugs in the dwarf spider Oedothorax retusus (Araneae: Erigoniae). Biological Journal of the Linnean Society 96: 574-583.
3Nessler, S.H., Uhl, G. and Schneider, J.M. 2009. Sexual cannibalism facilitates genital damage in Argiope lobata (Araneae: Araneidae). Behavioral Ecology and Sociobiology 63: 355-362.
4Segoli, M., Lubin, Y., and Harari, A.R. 2008. Frequency and consequences of damage to male copulatory organs in a widow spider. The Journal of Arachnology 36: 533-537.