Toads can't tell guys from gals.

Male western toads aren't totally undiscriminating, but they don't seem able to distinguish one sex from the other. They keep the species going with the philosophy: "if it moves--especially if it's big--grab it and mate."

Experiments offering males a choice of partners have revealed the details of a trial-and-error mating system, report Adolfo Marco from the University of Salamanca in Spain and his colleagues.

The toads have only a few days each year to mate, returning by the hundreds to the same spot every spring. According to some theorists, such a mating fest, where males literally scramble for partners, should encourage the evolution of specific ways to pick the best mates.

"We speculate, however, that under some conditions scramble competition may not favour effective sex and mate recognition," Marco's team says in the June Animal Behaviour. If making a pass at the wrong sex doesn't waste much time or energy, where's the pressure for improvement? Western toads, at least, seem to manage quite well.

The team studied the toads that crowd into a small section of Lost Lake in Oregon for four to six nights of mating. "They're constantly jumping on one another," says coauthor Joseph M. Kiesecker from the Yale School of Forestry and Environmental Studies in New Haven, Conn. As one of the males moves, the others think it's a female and they tackle it, he adds.

When a male gets grabbed by mistake, he protests. The researchers describe his characteristic series of peeps as "birdlike twittering calls." The mistaken suitor lets go quickly. Kiesecker has never seen one male clasp another for more than three seconds.

A female doesn't emit this release call when grabbed, so a male holds on for hours, sometimes days. The female drags him along until she releases her eggs.

When researchers offered the toads a choice, males picked large females over smaller ones. "But they can't make the distinction between males and females," Kiesecker reports.

That's not so strange, he says. The females of this species generally grow larger than males, and the biggest females, in theory, lay the biggest clutches of eggs. Getting fuddled on the male-female thing, however, causes only a few seconds of inconvenience.

"The more general question," says Paul Verrell from Washington State University in Pullman, "is, 'How choosy are males?"'

The standard answer has been that males of most species invest little in reproduction, so they are not too selective about their efforts. "We expect males to try to mate with the end of a beer bottle or the toe of your boot," Verrell says. During recent years, however, he has seen a shift in theory, recognizing a larger role for male selectiveness.