Being monogamous has its advantages: The couple has help rearing offspring and exclusive access to their mate's resources. Oddly enough, however, the practice may not be much help against sexually transmitted infections (STIs) — evolutionarily speaking, anyway.
According to a new mathematical study, the presence of sexually transmitted pathogens is often more likely to dissuade the evolution of monogamy than encourage it. Sure, individuals who go condom-free with a long-term partner are safer from diseases such as Chlamydia and gonorrhea than those who ditch the protection with a series of one-night stands. But over generations, STIs encourage monogamy only if they are moderately transmissible and fatal.
"Certainly, if everyone in the population just decided at a certain time they were all going to pair up and stick to this strategy, you could have a pathogen eradicated," said study researcher David McLeod, a doctoral student at Queen's University in Ontario. "What happens in nature is most of these behaviors are rare initially, so whether or not they succeed or fail is determined [at a time] when they are rare." [Hidden STD Epidemic: 110 Million Infections in the US (Infographic)]
Despite years of research on the evolution of pairing up, no one had ever tried a mathematically based analysis of how STIs might influence monogamy's evolution, McLeod told Live Science. Sexually transmitted infections can cause sterility or even death, which translates to fewer opportunities to pass on genes.
To find out if the presence of sexually transmitted pathogens might nudge species toward a monogamous lifestyle, McLeod and his colleagues created a mathematical model of a promiscuous population.
"We assume that in this baseline population, individuals are fully promiscuous, so each sexual interaction is a one-off," McLeod said.
Once the population reached a stable size, the researchers added a twist: Mutant individuals that preferred to form pair bonds, 'til death do them part. These monogamous mutants were released into the general population to thrive or die, depending on how well their mating strategy worked. [The Animal Kingdom's 9 Most Devoted Dads]
Finally, the model included a pathogen, transmissible by sex. Individuals who caught the pathogen were programmed either to become sterile, to die earlier, or both. In some conditions, the pathogen spread rapidly; in others, it was difficult to catch.
Transmission and virulence
Next, the researchers let their imaginary species run wild, mating and producing offspring. They found that two factors influenced whether the monogamous mutants won out or the promiscuous population carried the day.
The first was logical: Transmissibility. The STI had to be somewhat transmissible, but couldn't run rampant through the population. If the disease wasn't easy to catch, there simply wasn't much incentive to settle down with one partner, because promiscuity didn't carry much risk. On the other hand, if the disease spread easily, most individuals in the population caught it. In that case, chances of picking a mate without the disease were slim — so the individuals decided they might as well sow some wild oats instead.
"Only at intermediate transmission rates should you expect pair formation," McLeod said.
The second finding was a bit more surprising. Evolutionary biologists had suspected that sterility was a fate worse than death for individuals trying to pass on their genes; thus, researchers expected that an STI that caused sterility would promote monogamy as an avoidance tactic even more so than an STI that caused death.
Not so, McLeod and his colleagues found. In fact, individuals were more likely to enter into monogamy when the risk of infection was death than when it was sterility.
The reason? Imagine an invisible STI that causes sterility. You might pair up with a partner with this disease without knowing it, only to find out that the two of you can't produce offspring. You might not catch the STI yourself, but your genes are still out of the gene pool. In this situation, being monogamous disadvantages even healthy individuals.
Now imagine an STI that causes death. You might hook up with a partner carrying this STI, but the disease will soon carry your mate away. The partnership may have produced few to no offspring, but you are now free. If you didn't catch the disease yourself, you can go find another partner and try again. A monogamous relationship might protect you if you happen to choose an uninfected partner, but even if you're unlucky, you're only out of the game for a short period.
These findings don't take into account the other factors, such as resource sharing and offspring raising, which contribute to the evolution of monogamy, McLeod said. And they apply to individuals that mate for life. The findings could explain certain real-world scenarios, he said, such as ladybug populations afflicted by highly virulent STIs. These ladybugs don't adopt monogamy despite the risk of catching the disease, because so many have the disease already, McLeod said.
The bottom line, he said, is that when monogamy is rare, it's difficult to get the practice to take hold — at least from a disease perspective.
"It's difficult to get the strategy off the ground," McLeod said.
The researchers published their findings today (Oct. 14) in the journal Proceedings of the Royal Society B: Biological Sciences.