One of the most interesting ideas in Joan Roughgarden's book Evolution's Rainbow is that across the animal kingdom, many behaviors that we associate with gender—aggressiveness in males, nurturing of young by females—do not line up with biological sex as cleanly as we might think. One good example I've discussed before is white-throated sparrows, a species in which either the male or the female in a mated pair can take the aggressive role of defending the pair's nesting territory.
That principle is echoed in a paper recently published in the journal Behavioral Ecology and Sociology. This time the subject is not birds, but electric fish. Electric fish generate, and can sense, weak electric fields, which they use to navigate their environment—and for social signalling.
Here's video of a male and female of the species Brachyhypopomus pinnacaudatus interacting, via the website of Philip Stoddard, the senior author on the new study. The fishes' electric fields are made audible in the soundtrack, as sort of scratchy noises.
Male and female electric fish typically generate detectably different electric signals. However, Stoddard's team have found some evidence that "masculine" electrical signals may be more generally associated with aggressive social interaction for eletric fish of both sexes—in more crowded conditions, female electric fish start to signal more like males.
The team recorded electric signals from the electric fish Brachyhypopomus gauderio in both a natural population in Guatemala, where population density varied over several sampling periods, and in the lab, under experimentally varied population densities. In both the field and the lab, female fish generated signals with greater amplitude—a "masculine" signal trait—when the population density was higher. Females also generated signals of longer duration—another "masculine" trait—when the ratio of females to males in the population was greater.
Blood tests on female fish in the field study found increased levels of testosterone—which has previously been connected to more "masculine" electrical signals—associated with a higher female-to-male ratio. However, this wasn't replicated in the lab study.
So it looks like the female fish in this study use the same kind of signalling for aggressive social interaction that males do. That suggests the general differences between male and female signals are more due to differences in how often each sex interacts aggressively than because of physiological differences between the sexes per se.◼
Gavasa, S., Silva, A., Gonzalez, E., Molina, J., & Stoddard, P. (2012). Social competition masculinizes the communication signals of female electric fish. Behavioral Ecology and Sociobiology DOI: 10.1007/s00265-012-1356-x