Science / Medicine : Males in Nature Find Flashy Adornments Improve Sex Lives : Reproduction: The ability to attract females outweighs the disadvantages of attention-getting traits, researchers find. UC Santa Barbara study is a prototype for work of evolutionary biologists.
Male animals have evolved a wide variety of bizarre and elaborate characteristics, such as the lion’s mane, the peacock’s tail and the bright coloration characteristic of many male fish.
In many cases, these adornments actually decrease the male’s ability to survive by drawing it to the attention of predators or by impairing its ability to fight or escape. So why the window dressing?
Sex--or ultimately, reproduction.
Researchers are finding that the disadvantage of obvious adornments is far outweighed by the males’ increased ability to attract mates and thus propagate their genes.
Most recently, UC Santa Barbara researchers have demonstrated that females of a certain species of fish seem to become more attracted to the male of the species when males are fitted with plastic swords on the ends of their tails. The findings suggest what may have led to the evolution of a related fish species called the swordtail.
Biologists have long been puzzled by the question of how these traits arise genetically. Do males evolve traits that females subsequently come to prefer, do the traits and the preferences arise at the same time, or do the females have a preexisting preference that males evolve in response to?
It is unlikely that males would develop adornments first because there would exist no evolutionary pressure to select for the traits. Most researchers have argued that the traits and preferences tend to arise at the same time because that seems most logical, and they have developed elaborate theories about how the two are linked.
But new results presented last week in the journal Science by biologist Alexandra L. Basolo of UC Santa Barbara provide strong evidence that, at least in one case, the female’s preference arose first and that males adapted to it. The study is a prototype of new studies being conducted by evolutionary biologists to shed light on sexual preferences in a variety of species.
“It represents a new way of going about these studies,” said biologist Michael Ryan of the University of Texas at Austin.
“We’re one step closer to understanding what causes these sexual preferences to develop,” added biologist Douglas Futuyma of the State University of New York at Stony Brook.
Basolo has been studying two closely related species of fish: the green swordtail, in which males have a sword-shaped extension on their tails, and the platy, which has a normal tail. The swordtail and the platy are believed to derive from a common ancestor, and biologists believe that the platy is more similar to that ancestor--that it is more primitive.
In earlier studies, Basolo had shown that female swordtails prefer males with longer tails. These studies were conducted by using a three-chambered aquarium, placing the female in the center and the males on each side.
An observer then monitored courtship behavior, determining which of the males the female spent more time close to. Basolo observed that the females spent more time near the male with the longer tail. When she cut off part of that male’s tail so that it was shorter than the other’s, the female then began courting the previously spurned male.
Basolo then began wondering how the closely related female platy might react to tails. To find out, she sewed a plastic sword onto the tail of a male platy and determined the female’s preference between it and a regular male.
She found that the female spent about twice as much time with the sword-tailed male as with the other--about the same ratio as shown in the swordtail species. When she sewed the tail on the other male, it became the preferred partner.
Because the platy is much more like the ancestor of the two species, she said, “it is clear that the female’s preference arose before the appearance of the sword” and that preference may have figured in the evolution of the swordtail.
The challenge now is to determine what causes the female’s preference. One strong possibility is that the tail simply makes the male look bigger; previous studies have shown that females of the two species prefer bigger males. But it is also possible that the preference arose because the tail resembles a favored food object or perhaps because the females see horizontal objects better than vertical, she said.
Only one similar study is known. Ryan is studying two closely related species of frog. One makes a mating call composed of a whining noise followed by a deep-throated “chuck.” The second makes only the whining noise.
Females in the first species prefer males whose chuck has a low vibrational frequency because those males tend to be bigger. The ears of the females are shaped so that they are especially sensitive to the lowest notes. He has also found that females of the second species have a similarly shaped ear, even though males do not make the chucking noise. He will report in a forthcoming paper that, if he gives males of the second species an artificial chuck with a tape recorder, females strongly prefer them.
Now, he said, biologists are looking for other closely related species to see if they too have such preexisting preferences. If the preferences are common, researchers will then have to develop new theories about how sexual traits develop.
