A key component of classical sexual selection theory is the idea that males maximize their evolutionary fitness—the number of children they ultimately have—by mating with lots of females, while females maximize their fitness by selecting only one or a few high-quality partners. It’s pretty clear that this model works well for some species (like ducks), but also that there are many it doesn’t fit so well. Now it looks like one of the “classic” experimental examples of sexual selection may actually fall into the latter category.
Sexual selection was first proposed by Charles Darwin, in his 1871 follow-up to The Origin of Species, The Descent of Man, and Selection in Relation to Sex; but one of the earliest experimental tests of the model wasn’t published until 1948 [PDF]. The biologist A.J. Bateman allowed small groups of fruit flies—good old Drosophila melanogaster—containing equal numbers of males and females to mate at random, then reared the resulting eggs and reconstructed the parentage of the offspring to determine (1) the number of offspring each of the male and female parent flies had produced and (2) how many parters each parent fly had had.
How did Bateman reconstruct parentage decades before the advent of modern genetic testing? He used mutations with known, visible phenotypic effects as “markers”:
The fertility of individual flies of both sexes was measured by means of dominant marker genes. Several flies of each sex were mated together in one bottle, each fly carrying a different dominant marker gene. In this way, assuming the complete viability of all the marker genes, half the progeny of each fly could be identified.
That’s a pretty clever design given the technological limitations of the time. But it also turns out to be the fatal flaw in Bateman’s experiment.
There’s some more new evidence for one of the theories as to how gene variants that make men more likely to be gay could persist in human populations in the face of their obvious selective disadvantages: the same genes could, when carried by women, lead to greater fertility.
The authors interviewed women who were the biological mothers or aunts of gay men, and compared them to women who were mothers or aunts of straight men. They gave each participant a questionaire covering the key question—how many children they’d had. It also covered a sort of focused medical history, covering a slew of conditions that might have affected their fertility—anything from chlamydia infections to ovarian cysts to complicated pregnancies—and asked about their sexual behavior and history. Finally, the team gave the women in their sample a standardized personality test.
Even this relatively small sample showed the previously documented effect of shared genetics with gay men—women who had gay sons or nephews had more children than those who didn’t. Mothers and aunts of gay men also reported lower rates of medical conditions that could reduce their ability to have children. They said they’d had more partners than mothers and aunts of straight men (but this difference wasn’t statistically significant) and were also less concerned about family issues, and more likely to have been divorced. Finally, the personality test revealed that mothers and aunts of gay men were more extraverted.
That’s a big pile of factors tested, which makes me wonder about multiple testing issues with a small sample size. The study’s authors build a somewhat complicated narrative out of it all: They speculate that the same genes that make men gay make women less likely to have fertility-reducing conditions, but also more extraverted and more “relaxed” about building a family—which apparently also helps them have more children. So, okay, I guess that’s plausible given the results.
Here’s what the study doesn’t do, however: it doesn’t identify any specific genes involved in making gay men gay. It can’t actually test the hypothesis that there’s a genetic basis to same-sex attraction at all, much less the hypothesis that genes promoting same-sex attraction in men are located on the maternally-inherited X-chromosome. For those questions, you really need full pedigree data—or, better yet, lots and lots of genetic data; interviewing only female relatives isn’t remotely enough.
The text of the article doesn’t necessarily make that point as clearly as it could. The authors spend a great deal of time talking about the X-chromosome hypothesis, and though they make the requisite disclaimer in the Conclusions section—
With this type of limited data, we cannot directly derive a causal connection between the hypothetical sexually antagonistic autosomal or X-chromosome-linked genetic factors and health, behavior, and personality.
—that disclaimer elides the point that their data set can’t really test anything to do with genetics indirectly either.
The authors repeatedly describe their sample as a “pilot study,” however, so maybe something bigger, and more rigorous, is in the works.◼
Camperio Ciani, A., Fontanesi, L., Iemmola, F., Giannella, E., Ferron, C., & Lombardi, L. (2012). Factors associated with higher fecundity in female maternal relatives of homosexual men. The Journal of Sexual Medicine DOI: 10.1111/j.1743-6109.2012.02785.x
The central idea of sexual selection theory is pretty simple: Females, who invest relatively more in making and raising offspring, have an incentive to be choosy about mating. Males, on the other hand, may be able to get away with no more investment that a squirt of semen—so they have an incentive to mate with any female who’ll have them. How widely that model applies in the animal kingdom is very much an open question, but it does make some specific predictions that can be tested in an evolutionary context.
One of those predictions is that, when relatively more resources are at stake in the process of making babies, sexual selection should be stronger. Austin Hughes, a biologist at the University of South Carolina, recently set out to test for that pattern in waterfowl [$a].
Ducks and their relatives already look like a good fit for classic sexual selection. In many duck species mating is coercive, so females have evolved maze-like reproductive tracts to slow down unwelcome penises—and males have, in turn, evolved corkscrewing penises to navigate those mazes. And in many species, the sexes have strikingly different coloration—generally thought to mean that males are vying for female attention with brightly colored plumage, while females are more concerned with staying hidden while sitting on a nest.
However, there are also plenty of waterfowl species where males and females are almost indistinguishable—think of swans or geese, especially. If sexual plumage differences are related to the strength of sexual selection, maybe that reflects differences in the sexual “stakes” at play in each species. Hughes tested this hypothesis by comparing closely related pairs of waterfowl species or subspecies.
As an index of the reproductive effort made by females of each species, he used the mass of the average clutch of eggs laid, as a fraction of the mass of the average female. He then tested whether the species in each pair whose females made the larger “investment” in reproducing was also the species in the pair with more pronounced sexual differences in plumage coloration. And this was, indeed, what he found.
So that looks like a neat confirmation for one predicted effect of sexual selection. A worthwhile follow-up might be to add male parental care—which may be, admittedly, harder to measure—into the mix. If males help feed and protect the brood (which is often the case for waterfowl), that should offset the cost of reproduction from a female’s perspective, which might also reduce the strength of sexual selection.◼
Hughes, A. (2012). Female reproductive effort and sexual selection on males of waterfowl. Evolutionary Biology DOI: 10.1007/s11692-012-9188-1
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
One of the key evolutionary puzzles of same-sex sexuality, as it manifests in modern, Western human societies, is that those of us attracted to members of our own biological sex don’t make a lot of babies. I’ve already spent a lot of pixels on the question of how genes for same-sex attraction might persist in human populations in the face of that selective cost—but a paper just published in PLoS ONE adds some evidence in favor of one popular hypothesis: that gene variants that make men more likely to be gay could also make their straight relatives more fertile.
The new paper presents data from Samoa, where the traditional culture has long had a place for men who are attracted to other men, in the role of fa’afafine—literally, men who “live in the manner of women.” Samoan boys who show interest in feminine activities are recognized by their families as members of this “third gender,” which is more like the modern Western conception of transgender identity than what we call “gay.” Fa’afafine often present and dress like straight women, and as adults, they generally have relationships with straight-identified men. But fa’afafine aren’t exactly “transgendered” as we understand that concept in the West—they don’t have the sense that their bodies don’t match their gender identity.
The fact that Samoan culture accommodates and accepts same-sex sexuality makes it an especially interesting context for testing hypotheses about the evolution of queer sexuality, including the idea that relatives of fa’afafine might be more fertile than people with no fa’afafine in the family. The study’s coauthors surveyed Samoan fa’afafine and straight men, asking how many children their grandmothers, aunts, and uncles had had. And they found that grandmothers of fa’afafine—both maternal and paternal grandmothers—had more children than grandmothers of the straight-identified men they interviewed.◼
VanderLaan, D., Forrester, D., Petterson, L., & Vasey, P. (2012). Offspring production among the extended relatives of Samoan men and fa’afafine. PLoS ONE, 7 (4) DOI: 10.1371/journal.pone.0036088
I’ve just set myself up a Google Scholar alert for papers on the evolution of same-sex mating behavior. The plan is, I’ll post some brief notes on anything interesting that shows up in my inbox. First up: bisexual budgies!
Male budgerigars—or parakeets, to those of us in the States—live in female-dominated social groups when they’re not caged in a petstore. In these groups, apparently, it’s quite common for pairs of males to engage in behaviors that look a lot like what males do when courting female budiges. It’s been hypothesized that this same-sex courting is practice for the real, reproductive deal. If that were the case you’d expect that male budgies who put in more time practicing with other males would have better luck with females later on.
However, when Puya Abbassi and Nancy Tyler Burley of the University of California Irivine compared the frequency with which individual male budgies engage in same-sex courting to their later success with females, they found a negative relationship—males that had more same-sex interactions were less likely to find female mates [$a]. The authors propose that the same-sex interactions are actually males assessing each others’ social status. That would square with Abbassi and Burley’s observations if low-status males, who are less likely to get lucky in the mating game, spend a lot more time sorting out relative rankings amongst themselves—and this is what the authors suggest may be going on.◼
Abbassi, P., & Burley, N. (2012). Nice guys finish last: same-sex sexual behavior and pairing success in male budgerigars Behavioral Ecology DOI: 10.1093/beheco/ars030