I’m sorry to break it to you, but all of those beautiful stories about swans and other birds being completely monogamous are rubbish. The more genetic testing we do, the more often we find that species once thought to be monogamous are actually cheating on each other fairly regularly. Behavioral ecologists have also noted that individuals of “monogamous” species will occasionally leave their present mate to go find a new one, a phenomenon we refer to as “divorce”.
That’s right, if you’re sitting in a room of behavioral ecologists you might hear us talking about how often ducks, fish, primates, lizards, rodents, etc get divorced. It’s a strange little world we live in.
Anyway, I found a neat paper today on divorce rates in the more-often-than-not monogamous trematode parasite, Schistoma mansoni. Before I tell you more about the study, here is some background on one of the most important parasites in the world from a human health and socio-economic perspective.
Schistosomiasis is a disease that effects approximately 200 million people and results in 20 million deaths a year in tropical and neotropical regions. Infection with a schistosome parasite produces a number of unpleasant symptoms, including bloody urine, abdominal swelling, fever, enlargement of the liver, and high blood pressure in the portal vein, pulmonary artery or pulmonary vein. The three main schistosome species responsible for the cases of schistosomiasis in humans are Schistosoma haematobium, S. mansoni, and S. japonicum. I’m going to focus on S. mansoni for the rest of the article.
Schistosome parasites require 2 hosts, one of which is a snail and the other a vertebrate. The parasites asexually reproduce in the snail, creating offspring that mature into free-swimming larvae which leave the snail in search of humans. These larvae recognize human skin secretions (mainly by honing in on the amino acid arginine) and swim in the direction of the secretions until they come into contact with human skin.
Once on the skin, the parasites will look for a good place (like a hair follicle) to enter the human body. In as brief a time as 10-30 seconds, the parasites can go from being visible on the surface of the skin to disappearing into the human body.
In the body S. mansoni uses the circulatory system for public transportation. On the journey, the parasites make a brief pit stop at the lungs and then travel to left side of the heart. The parasites then hang out in the heart, waiting for potential partners to arrive on the scene. When two parasites pair up, they sexually mature and journey together to the mesenteric veins.
Male schistosomes have a large groove in their body into which a female schistosome can slip. When a female makes this groove her home, she begins diverting all of her energy into producing eggs while the male continuously pumps food and sperm her way. Female S. mansoni can produce 300 eggs per day. These eggs are the cause of much of the pain and suffering associated with schistosomiasis.
S. mansoni offspring need to make their way into the intestine so that they can hitch a ride with feces to their way out of the body. If the feces end up in water, then the offspring swim out and search for a snail so that the life cycle can begin again.
Unfortunately for us, a lot of the parasites don’t find their way to the intestine and instead end up lodged in various parts of the human body (these parasites apparently have as good a sense of direction as I do). The immune system finds these trapped parasites and forms a protective shell around it called a granuloma. If enough granulomas form in a small area then blood flow is impaired and a number of unpleasant problems follow.
This parasite has been causing humans problems for thousands of years. Numerous historical texts describing diseases plaguing people at that time are almost certainly describing schistosomiasis. In fact, eggs from S. haematobium have been identified in Egyptian mummies dating back to 1200 BC.
The really sad thing about this parasite is that its transmission could easily be slowed by changes in cultural practices or by enhanced sanitation techniques. Much of the world is either unaware of this, unable to afford to make the changes or are set in their ways and unwilling to change.
Divorce in S. mansoni
S. mansoni are fairly monogamous, but mate switching has been observed from time to time. In numerous species (including humans), sex ratio appears to be an important factor in whether or not couples divorce. This means, for example, that divorce may occur more often in populations where there are a lot more females than males around. The researchers in this study decided to examine how divorce rate changes as the proportion of males to females was varied.
As mentioned earlier, schistosomes reproduce asexually when they are residing in snail hosts. These clones were collected and genetic markers were identified so that the researchers could distinguish between clones. In lay terms, this just means that the researchers found a characteristic in each group of offspring that allowed them to be uniquely identified.
The researchers first infected a mouse with males (let’s call them 1M) and females (call them 1F) of known genotypes and gave them time to pair off. After a sufficient amount of time had passed, the researchers added either more females or more males (2F and 2M, respectively) of a different genotype to see if this had any effect on the “monogamous” pairs that had previously formed. If at the end of the experiment 1F females were only paired with 1M males, then the couples had remained monogamous. If 1F females were found with 2M males or if 1M males were found with 2F Females, then divorce and partner switching had occurred.
After retrieving the couples and identifying the clones in each couple, the researchers discovered that sex ratio was an important factor governing divorce rates in S. mansoni. The more males that were present in the population the more likely it was that couples were divorcing and finding new partners.
The researchers were unable to determine whether the divorces were due to females choosing to leave their male partners (the “female choice hypothesis”) and/or if the strongest schistosome males were capable of prying a female out of another male’s groove (the “forced divorce hypothesis”). Either way, the more males that were present the less stable were the schistosome “marriages”.
I thought this was a really cool use of genetic techniques to figure out what’s going on in a world that can’t be observed directly. It was also neat that they were able to show a factor we know to be important in the mating behaviors of many vertebrates (i.e., sex ratio) is also important in trematode parasites.
Robert’s and Janovy’s Foundations of Parasitology (8th edition)