For this study, researchers experimented on a microscopic roundworm (C. elegans) that has been used in labs for decades to understand the nervous system, with lessons applicable across the animal kingdom. They come in two genders: male and "females" (which are technically hermaphrodites since, in some cases, they can self-fertilize).
Scientists had previously discovered that the males and "females" end up making different decisions about feeding vs. finding a mate. The hermaphrodites prioritize finding food. But the males will "spontaneously leave a food source" to look for a mate in a lab setting, even "suicidally," ending up dead on a petri dish, said Douglas Portman, a University of Rochester associate professor and lead author of the new study.
Portman and others wanted to figure out just why the males did that. Could it be their genetic makeup, by virtue of being males, has programmed them to behave this way? Researchers zeroed in on the roundworm's sense of smell, and specifically, one particular gene that is related to receptors sensitive to the smell of food. The "females" produced more of these receptors, whereas the males just had less.
Researchers genetically modified a batch of males to produce more of the smelling-receptors, and compared them to normal males. "Females" were placed in the middle of a petri dish, and males were next to a food source on the edge of a petri dish, with another ring of food serving as a barrier between the two sexes. As expected, the normal males left their food source, went around the barrier and mated. But the genetically-modified males were much less successful at mating -- possibly because they were too busy eating.
Before you go thinking, "Ugh. Typical. Men," remember, it's not the male roundworms' fault! They aren't making a conscious decision to go mate instead of eat. "Part of the reason the males leave food to mate is that they don't smell it as well," Portman said.
Now what does this mean for other animals, including humans? Well, people are way more complex creatures than roundworms. "Social and cultural factors clearly have a very strong -- and maybe even dominant -- contribution to sex differences in human behavior," Portman said.
But, we still don't know a lot about how the male vs. female human brain functions. Portman said there has long been "dogma" that behavioral differences has only to do with hormones, but "the brain also has access to that genetic information, of whether the brain is male or female," which may result in "subtle tweaks" that have big impacts on behavior.
"The extent to which there are innate sex differences in human behavior is still actively debated. And it really comes to the nature-versus-nurture question..," Portman said. "But there is a growing appreciation that there are biologically-based differences in the nervous systems themselves, and we understand very little where they come from.”
The findings, which bolster the idea of "neural plasticity" rather than animals being hard-wired to do something that can't be altered, could have bigger implications when it comes to disease susceptibility for different sexes, Portman said.
"The bigger picture here is we're interested in the more general problem of how the brain works, how animals make decisions and how neural circuits are modulated to change the way animals make decisions," Portman said. "Surprisingly little is understood, really at the level of how the nervous system integrates all that dynamic information to allow it to make a decision."