This picture taken by Dr. Gil Rosenthal's research team shows two male swordtail fish aggressively displaying their dorsal fins.


Country crooner Charlie Robison is known for many a classic song line well beyond Texas Red Dirt Country music circles -- none possibly more fitting on the eve of Valentine's Day than, "You're not the best, but you're the best that I can do."

All is indeed fair in love, lyrics and mating, according to Texas A&M University biologist Dr. Gil Rosenthal, who has studied the latter behavior in swordtails, a small fish species native to streams in central Mexico for nearly two decades.

Rosenthal's assessment of love in all its mucky glory, at least when it comes to swordtails, is that Robison's mantra is right on the money, especially for females, whose ideal man has traits that in actuality are seldom found in any single male swordtail.

What a Girl Wants

So what is their idea of the perfect mate? Rosenthal says females have eyes for large males with small dorsal fins -- a desire that poses quite the predicament, given that, naturally, the larger the male, the larger the dorsal fin.

"So, female swordtails essentially want what they can't have," says Rosenthal, an associate professor in the Department of Biology.

Although the male's extravagant dorsal fin is raised in full display during courtship, this behavior is not merely a technique to attract females. A raised dorsal fin also is a signal of aggression toward other males who might be considering stealing his sweetheart. As a rule of thumb -- er, tail -- the bigger the dorsal fin, the bigger and badder the fish.

The downside, however, is that large dorsal fins not only scare off smaller males, they also scare off females. Hence the research-supported, love-connection conundrum: While females still want large mates, they are turned off by huge dorsal fins.

"If the only thing that's available at the time is small males with small fins or big males with big fins, then that's going to maintain the variation," Rosenthal says. "Being a small male with a small fin doesn't make you any worse off than being a big male with a big fin. So, there's variation, but the females are always going to be frustrated.

"What the females really want is really an impossible combination of things. They want a big male with a small dorsal fin -- in other words, strong, yet the sensitive type."

Boys Will Be Boys

As luck would have it, even in nature, some females manage to find the perfect guy after all. Rosenthal points to organic runoff from deforestation period in Mexico 15 years ago that has increased humic acid levels in streams. Although nontoxic, these high concentrations of humic acid interfered with the ability of two species of swordtails, the Xiphophorus birchmanni and the Xiphophorus malinche, to tell each other apart, resulting in interbreeding. Rosenthal says this inconsistent mixing of genomes leads to a weakening of the correlation among common traits that every now and then may produce the highly sought large male with a small dorsal fin.

Evolution aside, it pays to have a shorter-term back-up plan, which in the smaller male swordtail fish's case is his trademark blade-shaped tail. Rosenthal notes the prevailing theory has always been that the larger the tail, the more beautiful and therefore sexually appealing the guy -- at the potential expense of the rest of his body. According to studies conducted by Rosenthal's research group and others, males without plentiful food sources will forgo the nourishment of the rest of their body in order to invest everything into growing their swords -- in essence, a sneaky trick designed to make them appear larger and prettier for the girls.

Rosenthal's analysis appears to indicate that female swordtails have gotten wise to this ploy. To determine the importance of the sword in mating habits, Rosenthal showed the females videos of computer-generated males, both with and without their swords. When animated males were shown to females without their sword, they were digitally stretched to the size they would have been with it. Rosenthal and his team found that females could care less about the sword and just wanted, bar none, a big, strong guy. Furthermore, in some species, the sword is actually a turn-off in and of itself: Females find males with the ornament less attractive.

Rosenthal says the main take-away, at least for this stream of star-crossed lovers, is that what males have to offer almost never matches up to female needs and desires when it comes to choosing a partner.

Why Can't This Be Love

Love-language lessons notwithstanding, Rosenthal's laboratory is now embarking on a new quest to genotype individual male and female swordtail DNA sequences. His team is collaborating with a group of researchers at Princeton University who are mapping half a million markers onto a genome of a swordtail to help determine which parts of the genome are associated with genetic variation in certain traits at the single-gene level.

One of Rosenthal's main goals is to understand which part of the genome controls male traits and female preferences and, specifically, why it is that female swordtails are so adamant about wanting what they seemingly can't have.

"It's going to give us some insight into how it is that females want the impossible," Rosenthal explains. "Can we genetically separate their preferences for body sizes or fins?"

To learn more about Rosenthal's research, visit http://swordtail.tamu.edu/.


Contact: Chris Jarvis, (979) 845-7246 or cjarvis@science.tamu.edu or Dr. Gil Rosenthal, (979) 845-3614 or grosenthal@bio.tamu.edu

Jarvis Chris

  • Dr. Gil Rosenthal

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