For decades, scientists have detailed the intricacies of male fruit fly sexual behavior and neurobiology. Thanks to this work, we know about the various molecules, behaviors, and neurons involved in the male’s pursuit of the female fly. But when it comes to the neurobiology of female sexual behavior, the opposite is true: few studies have looked into the mechanisms that allow females to choose a mate. This, some researchers say, has to do with how obvious the males' behaviors are — they're easier to study and are more eye-catching — but others suggest it has more to do with the lens through which scientists look at male and female behaviors.
Fortunately, three studies published today in Neuron and Current Biology are challenging the male bias that has marked fruit fly sex research by describing, in detail, the neurobiology of female sexual receptivity. And, unsurprisingly, their findings reveal that it’s more complicated than some scientists previously assumed.
"the female must be doing a lot, but we just couldn’t see it."
"When you read accounts of fly courtship, it’s all male-centric; it’s all based on what you can observe about the male," says Joel Levine, a biologist and fruit fly expert at the University of Toronto at Mississauga. "But a lot of us have felt all along the way that the problem has been our own; that the female must be doing a lot, but we just couldn’t see it."
In the first study, published today in Current Biology, scientists describe a novel female sexual receptivity behavior called "pausing." This behavior’s name is pretty spot on: when the female decides to interact with a courting male, she stops her movements. Researchers also linked the pausing behavior to a gene called Abdominal-B, which is well-known as a gene involved in fly development. When the researchers silenced the gene during development, or when they artificially inactivated neurons that are linked to the gene, female flies were unable to pause to interact with males."The gene needs to function normally during the development of the nervous system to create the high probability of female mating," says Jennifer Bussell, a molecular biologist at The Rockefeller University in New York City and lead author of the study. "So we think these neurons are part of the real meat of female sexual receptivity." Levine, who did not participate in any of the three studies, describes Bussell’s results as "cool" because "they introduced something new, and then defined it."
"these neurons are part of the real meat of female sexual receptivity."
In another study, published in Neuron, researchers traced how the "sex peptide" — a small protein in the male’s ejaculate — acts on the female’s brain to prevent her from mating for about 10 days after her initial copulation. "We identified cells in the central nervous system that communicate with the sex peptide receptors located on the uterus," says Mark Palfreyman, a molecular biologist at the Research Institute of Molecular Pathology in Austria and co-first author of the study. When these neurons were inactivated, the female flies disregarded the signals from the sex peptides and mated with other males much sooner than expected.
In a third study, also published in Neuron, researchers identified two small groups of brain cells involved in organizing female behavior and integrating the different sensory cues from the male — his pheromones and his courtship song — in the female brain. Researchers were able to demonstrate the role these neurons play by artificially inactivating them, which caused females to delay mating, as well as by activating them, which spurred the females to mate more readily with courting males. "The basis for how the female connects song quality to a decision to mate or not mate has been a mystery," says Bruce Baker, a neuroscientist at the Howard Hughes Medical Institute and a co-author of the study. But "our work suggests that the neurons we identified likely contribute to the female's judgment of whether a courting male is an attractive suitor that she should mate with."
"in the animal world, females have been seen as passive."
"Since the early 1900s, just watching the flies’ behavior, it was easy to see that the male chases the female around, extends a wing, and sings," Bussell says. But the female has always been seen as having a "sort of passive acceptance response," she says — "and I do think that, in general, in the animal world, females have been seen as passive." That’s why Bussell thinks scientists need to take a closer look at the female fly’s behavior to understand and identify the neurons that control the steps leading to copulation.
Combined, the studies help demonstrate that scientists who believed there was more to female receptivity in fruit flies were correct, Levine says — a feeling echoed by Palfreyman, who explained that "it took a lot of time really, and a lot of people staring at flies, for researchers to start saying ‘hey the female is doing something quite interesting and we should look at this.’"
scientists prefer studying male genitalia
This isn’t the first time researchers have spoken out about science’s tendency to prioritize male sexuality, and males in general. In May, for instance, researchers published a study demonstrating that scientists display a marked preference toward studying male genitalia, as opposed to female genitalia, and that this tendency has actually worsened since 2000. This bias, the researchers wrote, "reflects enduring assumptions about the dominant role of males in sex, and invariant female genitalia," despite mounting evidence of "how rapidly female genital traits can evolve."
"There’s this cliche that if men menstruated, we would probably have a different approach to the science of PMS and, you know, that’s probably true," Levine says. But it’s also possible to look at the publication of these three studies positively, he says. "It’s a metaphor for what’s happening in human society as well." More labs have started to look at female reproduction and sexual behavior, and they’re doing so "in a big way." This is an important development, he says, because if we want to get a handle on the "broader biological issues, like how species evolved and what makes for healthy offspring, we need to have a more balanced perspective."