|Figure 1: X-ray image of Pelvicachromis taeniatus|
"When it comes to sexual selection in the animal world, it is the usually the sex that puts more effort (and has more to lose) into the results that gets to be the choosy sex. Usually this means females get to be picky. After all, eggs are more expensive than sperm and females often end up contributing quite a bit with parental care. This choosiness means that males need to impress females through the evolution of secondary sexual traits. This could be, and usually is, just about anything: elaborate feathers, complicated dances or mating calls, gift giving, etc."
So what happens when you see a highly developed ornament on a female of a species? Usually such ornaments are seen as non-adaptive, they are simply the genetic correlations of male ornaments. But, if you go back to the who "gets to be the choosy sex" argument and apply it to females you may find that females are looking to attract males that give them something that they need in addition to a sperm contribution. Perhaps the male takes care of the female (or "invests in female quality"), or maybe he contributes a significant amount of parental investment (or he's a good baby-daddy). This idea of females evolving ornaments to attract males has gained some ground recently. And that topic is today's article.
There is a species of cichlid fish called Pelvicachromis taeniatus where both sexes have ornamentation. This fish is known to be biparental and socially monogamous. Both sexes develop large, colored pelvic fins that they present to their potential mates during courtship. The pelvic fin of female fish is exceedingly large and differs from the male fin in both color and shape (it is triangular rather than thread-shaped). During courtship, females spread out their violet, triangular pelvic fin and fan it at the males. This behavior enlarges "their violet ventral nuptial projection area," suggesting that the fin and behavior are used to attract males. This study took a look at the allometric relationship of the pelvic fin compared to other fins (anal, caudal, dorsal, and pectoral fin), and then tested male preferences for females showing larger or smaller pelvic fins to determine the effect of fin size on male mate choice.
I would be remiss and probably confuse the heck out of you if I didn't take a sentence, or a few, to explain allometry. The most basic of basic definitions is that allometry is the relationship between size and shape. You define the dimensions of a body part (or trait or character) in relation to the body size, with the scale relating trait size to body size. What you get is an allometric relationship, and these are typically classified in three ways:
(1) Isometry - the ratio of trait to body size is constant
(2) Negative Allometry - large individuals have small traits
(3) Positive Allometry - large individuals have relatively larger traits
This relationship is driven by evolutionary constraints, natural selection, and/or sexual selection. And when you look at those traits driven by sexual selection you usually see option (3) Positive Allometry. Although a positive allometry does not mean the trait was driven by sexual selection.
In the experiment, the researchers took X-ray images of the females so they could analyze trait and body size. Then they conducted mate choice experiments to see which female characters the males preferred. Typically this is achieved by putting a female fish in one tank and a male fish in an adjacent tank and observing the courtship or lack of. Then the trait of interest can be altered (fin clipping, etc) or a substitute used. In this case the researchers used computer animations of females with various pelvic fin sizes. The use of the computer allowed them to standardize the stimuli and eliminate many confounding variables.
The allometry study found that the female pelvic and caudal fins showed isometry in relation to body size, but the anal, dorsal, and pectoral fins showed negative allometry. The females showed a constant ratio of pelvic or caudal fin size to body size, but the other fins were relatively smaller in larger females. They also found that the size of the pelvic and caudal fin is more positively related to body size than the other fins, suggesting that these fins are under selective pressures that the other fins are not. Remember when I said that sexually selected traits usually show positive allometry? This study showed isometry and negative allometry, no positive. The authors hypothesize that when you incorporate viability selection you end up with a more complex relationship between body size and traits, that natural selection and sexual selection "could have synergistic effects on the evolution of traits, thus sexually selected traits may be scaled into isometry or even negative allometry." Perhaps the amount of variation in traits under directional sexual selection may be limited by natural selection, that natural selection is constraining the pelvic fin size of these fish. Basically, a gigantic fin attracts a mate but slows you down and makes it easier for predators to get you. So you find a middle ground, a fin that isn't too big and gets you eaten but isn't too small so that you never attract a mate. This scaling down for the predators is where the isometry and negative allometry are showing themselves.
The mate choice experiment showed that males like large pelvic fins. But why? Could be that the large fin is an indicator to female quality. These indicators may be direct (fertility, fecundity, or maternal investment) or indirect (genetic benefits, parasite resistance, pretty daughters). Translated - If she can carry around that big fin and survive then she must have good behaviors and genes. The researchers found pelvic fin size in females to be positively related to body condition. So it is likely that the indicators are reliable. I haven't discussed much about the violet color of the female pelvic fin, but the fins are colored similar to the ventral violet nuptial belly coloration. The size of this ventral coloration is associated with female quality, indicating the fecundity of the female, her readiness to spawn, and may reveal information about her maternal quality and offspring survival rates. Again, all indicators that tell the male that she's quite a catch (no pun intended).
Anyone who has spent time with a group of females (of any species) knows that there's some competition going on, and you can't rule that out in a study such as this. These cichlid fish show sequential aggressive behaviors towards other females. Before a fight escalates a female will present and fan out her pelvic fin to show off her body condition and body size in the hopes of intimidating and driving off the rival female.
When it comes to fish studies, especially sexual selection studies, there are many. This study is unique in that it is the first to show that male choice might scale the allometry of a female sexual trait. This is important in understanding the scaling relationship of female traits with body size and starting to tease out the selection pressures driving the evolution of these ornaments.
Read the study here:
Baldauf, Sebastian A., et al. (2010) Male mate choice scales female ornament allometry in a cichlid fish. BMC Evolutionary Biology: 10, 301. (DOI: 10.1186/1471-2148-10-301)