In most animal species, there is one basic form of female and one basic form of male. Not so among the salmon fishes.

Evolution has given the coho salmon two kinds of males that differ markedly in form and behavior. One weighs eight to 10 times as much as the other and takes half again as long to reach maturity.

The two use completely different strategies for gaining access to the species' one kind of female. The big red-colored males, called hooknoses because of the shape of their snouts, fight one another for dominance in the classical hierarchy. The top-ranked male gets the female. The little silver-colored males, called jacks, have more ordinary snouts and gain their mating opportunities by hiding among the rocks until they can sneak into the females' nests while the big males are busy fighting.

These differences have been known for some years by fish biologists, who have also seen the same differences among the chinook, or king, salmon. Scientists have generally assumed that the little males were pathological in developing sexual maturity while still so small. The jacks' practice of sneaking their mating opportunities was presumed simply to be making the best of a bad situation.

Now a Canadian biologist has found evidence that the two kinds of males are about equally successful in reproducing. This, along with another study finding that the differences between the two kinds of males is hereditary, indicates that evolution has produced two different but equally stable strategies for the survival of males within a single species.

The research was done by Mart R. Gross, of Simon Fraser University in Burnaby, British Columbia, and published in a recent issue of the British journal Nature.

Gross said his finding contradicts the widely held opinion of biologists that the pressures of natural selection on any given species should favor only one optimal strategy for survival, a strategy that dictates the ideal physical form and behavior. The findings also lend support to the theories of sociobiology, which hold that social behavior can be under genetic control that evolves like any other genetically governed feature.

"Age of maturity in salmon," Gross wrote, "has probably not evolved as a single optimum but rather as a 'mixed evolutionarily stable strategy' in which precocious maturity is an evolutionarily viable alternative life history strategy."

Gross began his study by capturing males as they migrated upstream from the ocean toward the spawning grounds where the females had already arrived and scraped out bowl-shaped nests on the stream bottoms. The males were measured, tagged so that they could be recognized individually by observers on the shore, and released.

During the spawning season the two forms of males migrate upstream at the same time. Both forms were born in the same stream but one year apart. Then both migrated out into the open ocean. The larger hooknoses, often weighing around 25 pounds, return as mature three-year-olds. The smaller jacks, typically only three to four pounds, return as mature two-year-olds. The females are all three years old.

Observers then followed each fish upstream and recorded its behavior. In particular, they noted whether the fish fought with other males to be first at the nests where the females waited or whether it sneaked to the nests by hiding among rocks whenever it needed to avoid a fight.

The final tallies showed that the larger males were most likely to succeed by fighting and that the chances of the smaller males were better if they sneaked along. Big males who tried to sneak were usually too big to hide from their rivals. Little males who tried to fight almost always lost.

Gross said that when all factors were taken into account, including differences in survival to maturity, breeding lifespan, and mating success, the small males, the jacks, were 95 percent as successful as the large males, the hooknoses, in passing on genes that would perpetuate two rather different versions of what it means to be a male coho salmon.