The living rainbow: “Masculine” is actually “territorial” in electric fish

ResearchBlogging.orgOne 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

Selfishness aids cooperation

In evolutionary ecology, cooperation is a perpetual puzzle. It makes sense for organisms to help each other if they can reliably expect to be repaid in kind some time in the future, but cooperative societies are vulnerable to invasion by folks who selfishly accept help without returning the favor. The classical theoretical result is that, once selfishness evolves in a cooperative society, selfish individuals are able to out-compete cooperative ones, until cooperation (and, potentially, the society) dies away altogether.

One possible way to prevent this outcome is for cooperative individuals to punish selfishness, for instance by refusing to help those who don’t reciprocate. In this week’s PNAS, a new paper suggests that another way to stabilize cooperation is to have selfish individuals punish selfishness themselves [subscription needed].

As the authors put it,

This behavior might seem hypocritical in moral terms, but it makes sense as an evolutionary strategy. It can even be looked upon as a division of labor, or mutualism, whereby cheating during first-order interactions becomes a “payment” for altruism (punishment) in second-order interactions.

In other words, these “selfish punishers” may not return cooperation in kind, but they pay for it by punishing other selfish individuals. Ayn Rand would probably love this stuff, but it puts me in mind not of unfettered individualism but a feudal society – a mass of cooperators working for the benefit of the “greater good,” with a handful of punishers taking the benefits of that work and keeping everyone else in line.

Eldakar OT and DS Wilson. Selfishness as second-order altruism. PNAS 105(19): 6982-6.

Hummingbirds chirp with their tails

Online at Proc. R. Soc. B: high-speed video and experimental results suggest that male Anna’s Hummingbirds use their tail feathers to produce a chirping sound during courtship displays. This sort of thing is known from other bird species (notably two genera of Manakins), but there was some dispute about how Anna’s Hummingbird chirps. The high-speed video is on You Tube, though it basically only shows a male hummingbird flaring its tail at the bottom of a display dive. The experimental evidence apparently consists of air jet and wind tunnel tests on feathers collected (ouch) from wild birds.

Clark, C.J., and T.J. Feo. 2008. The Anna’s hummingbird chirps with its tail: a new mechanism of sonation in birds. Proceedings of the Royal Society Series B.