The California mouse and the deer mouse are very similar, except California mice find one partner and mate with them for life, while deer mice take on as many mates as possible. That lifestyle difference has a major biological impact.
Despite living in similar environments and facing comparable external threats to their health, the immune system of the deer mouse is much, much stronger than that of the California mouse. According to UC Berkeley researchers, this disparity can be traced to the promiscuous lifestyle of the deer mice. Because they have sex with a wide variety of partners, they are exposed to more diverse bacteria than their California cousins are.
For proof, look no further than the deer mouse vagina. (OK, there was probably a better way of phrasing that.) There's twice as much bacterial diversity in the reproductive systems of deer mice, and that means they need the stronger immune systems to ward off the potentially infectious bacteria. What's more, there's similar genetic diversity in the DNA of the deer mouse immune system. The deer mice are much more likely to possess multiple forms of the gene MHC-DQa, which gives their immune systems much greater ability to recognize and neutralize unusual infections. In a statement, researcher member Matthew McManes commented on the findings:
"The promiscuous mice, by virtue of their sexual system, are in contact with more individuals and are exposed to a lot more bacteria. They need a more robust immune system to fend off all of the bugs that they're exposed to. The things an animal does, the way it behaves, and who it interacts with, are important to natural selection. These factors can cause immunogenes to evolve at a much faster rate, or slower in the case of monogamous mice. That connection is important and probably under-recognized."
While the actual change in the underlying genetics is intriguing, the fundamental finding — that promiscuity exposes an individual to more sexually transmitted bacteria than monogamy would — might seem a little obvious. However, it's the kind of intuitive result that is a huge pain in the ass to verify. That's because so few species are monogamous in the first place — only about 5% of all animal species go in for that whole monogamy thing — and there are very, very few instances where two similar species, one promiscuous and the other monogamous, live in close proximity to each other.
That's why these mice are so useful to the researchers. As a followup, McManes is hoping to examine tuco tucos, a gopher-like species found in South America, which sometimes lives in large social groups and sometimes lives alone. That's a different kind of variation in social structure, but it goes back to how differences in sociability can impact genetics — or, perhaps, be shaped by a species's underlying genetics. As McManes explains, we're likely to start seeing much closer links between behaviors and genetics, thanks to recent technological breakthroughs:
"Now that we have these new sequencing technologies, people are going to be really interested in looking at the mechanisms that underlie these behaviors. How might genes control what we do, and how we behave? We're going to see an explosion in these studies where people start to understand the very basic genetic mechanism for all sorts of behaviors that we know are out there."