In a recently published study, Purdue University biologist Rick Howard shows that female toads seek the deeper calls of older, larger males, and that males may disguise their voices to increase their chances of luring a mate.
Surprisingly, Howard found in a separate study that, for the offspring, there may be no genetic advantage whether mom falls for the imitation call or finds the real thing. The findings pose new questions about the role played by genetics in natural selection.
"Natural selection might suggest that the females' preference for a certain type of male has a genetic or evolutionary factor behind it," Howard says. "But in this case, and similar cases throughout the animal kingdom, evidence for such a genetic link remains elusive."
Howard began studying the mating behavior of American toads 12 years ago to see if there are any laws of nature that could be applied to other species, including humans.
"I started out working with birds, but found I was spending most of my time trying to catch them, and very little time actually watching what they were doing," he says.
Toads are much easier to catch and tag for identification purposes, Howard says. And, because they are not wary of humans, the scientists are able to get within a few feet of the animals to record their calls and behavior.
After finding that large male toads had more mating success than small males, Howard and his colleagues began studies to see how females identify the largest males. The group began analyzing the animals' calls, because vocal cues often are used as signals to attract mates. To eavesdrop on the courting males, the researchers carry cassette tape players with microphones mounted on fishing rods as they wander through small ponds and marshes.
"In most species the pitch of a male's call should be a relatively straightforward signal, because larger males produce deeper calls," Howard says. "But this species, at least initially, was thought to be an exception to that rule."
In a study published in the February edition of the scientific journal Copeia , Howard presents evidence that larger toads do indeed produce deeper calls, and that what males do in isolation is very different from what they do if another calling male is nearby.
When alone, males toads of all sizes will call at a higher frequency and exhibit more variety in the types of pitches they use, Howard says. If other males are in the area, males lower the pitch of their calls, thereby making themselves seem larger.
"It's a bedlam out there in the chorus, because everyone's screaming their heads off," Howard says. "Somehow in this madness, a female has to go around and assess males by listening to the pitch of their calls. Meanwhile, they're confronted with the fact that these males are playing vocal games."
Howard says similar pitch changing by males has been seen in only two other frog species.
"In one of those two species, the males will fight over females and territories, so lowering his pitch may be a way of communicating 'I'm bigger than I look and you'd better leave me alone,'" Howard says. "But American toads don't fight, so the rationale behind their vocal games isn't clear."
What's also not clear is why females prefer the lower-pitched calls in the first place, he adds. "In many species, the males contribute resources that the female can use -- such as protection or assistance in caring for the young," he says. "In the case of the American toad, all the female gets from the male is sperm, or genetic material for her offspring."
That fact, which appears repeatedly in different types of organisms, helped spark Howard's scientific interest in the amphibians' love connections.
"One thing that's rather bizarre in the animal kingdom is that in species like this, where all a female gets from her mate is sperm, the males are often much more elaborate than the females, or they develop some specific trait that helps their mating success," he says.
The male peacock, for example, contributes only genetic material to the young, yet it is endowed with lavish tail feathers and ornate plumage.
"Certainly the kinds of elaborations we see in males are far in excess of what's necessary for the female to find a mate of her species," Howard says. "The females would have to be nearly sensory deprived to confuse a male peacock with something else."
One theory to explain this phenomenon, called the "good genes" theory, suggests that females seek out genetically superior mates, trying to gain the best possible genetic makeup for their offspring, Howard says.
"There is a raging controversy in the field of reproductive behavior right now as to whether there is really a genetic argument underlying this whole phenomenon that we're seeing in many types of organisms," he says. "For that to hold true, the males must exhibit some trait to indicate their genetic superiority, and that trait must be readily recognized by the females."
In an earlier study published in the August 1994 issue of Evolution , Howard found that, in the case of the American toad, the low-pitched bass offered no genetic advantage over a tenor.
Starting with 19 females with eggs, Howard and his colleagues crossed each female with three males representing three body-size categories. Five hundred hatchlings from each cross were reared to metamorphosis in seminatural ponds in the field.
"One of the nice things about frogs and toads is that females produce large numbers of eggs, so you have large samples to work with," he says. "Also, because the eggs are fertilized externally, portions of eggs from each females can be fertilized by different males."
As the tadpoles developed into their adult stage, they were measured for weight, metamorphosis age, and total number of survivors. Though genetic variation could be seen between the three groups from each female, the study showed no correlation between the offspring's fitness and the sire's body size or age.
"The question as to why females prefer to mate with larger males remains an open question," Howard says.
Howard and his group are pursuing similar studies with tiger salamanders -- a species in which females prefer males with longer tails -- to see if offspring from the long-tailed sires benefit from this trait.
The researchers will expand studies with toads to look for performance differences in offspring that may be linked to the size or age of the sire. Howard also is analyzing the calling behavior in toads to see what kinds of vocal games the males are playing with each other. In addition, he is studying how female toads respond to calls of various lengths and pitches.
Howard says that studies such as his may add evidence to one side or another of the good gene theory. In addition, the studies may bring to light basic biological drives that come into play in sexual selection for all species.
"The kinds of questions that are being asked in these studies are questions you could ask of any species on the face of the Earth," he says. "There's a lot of research done on humans, for example, addressing the same kinds of questions. But we're perhaps the hardest organism to analyze because of our cultural practices, and the fact that we have conscious reasoning that makes answering these questions very difficult."
Source: Rick Howard, (765) 494-8136; Internet, firstname.lastname@example.org
Writer: Susan Gaidos, (765) 494-2081; Internet, email@example.com
Purdue News Service: (765) 494-2096; e-mail, firstname.lastname@example.org
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