It was a cold december day, but things were heating up in the shallows of Lagunitas Creek. A male coho salmon, a massive hook-jawed creature, had found a female. They hovered above the streambed in the ancient ritual of spawning, bodies parallel, the male’s mouth gaping. Below them was the redd, a scrape in the gravel where the female’s eggs were being laid.
Just downstream a smaller salmon watched the spawning pair. Every few minutes it would dash up to the spawners, and the big male would lunge at the interloper. The third salmon was a jack, an undersized but sexually mature male, and it was attempting to fertilize some of those eggs.
Watching the threesome made me wonder about the advantages to a male fish of being small and sneaky, as opposed to big, powerful, and endowed with a kype, that trademark hooked jaw. (Salmon nomenclature is a wonderful thing. Hatchlings start out as alevins, then become parr, then smolts. An Atlantic salmon that survives spawning—unlike coho and other Pacific species, it isn’t a terminal event for them—is called a kelt). The jack’s strategy seemed to counter the way sexual selection is supposed to work.
In The Descent of Man, Charles Darwin attempted to explain why so many creatures had traits with no clear adaptive value, traits that might even reduce their chances of survival: the tail of the peacock is a classic example. He attributed these to either male competition (size, strength, and weapons that give males more reproductive opportunities) or female choice (plumage or other attributes that potential mates find attractive). The notion of sexual selection has been refined some since Darwin’s time, but most evolutionary biologists still feel it accounts for a lot of sexually specific features and behavior, from the song of the mockingbird to the antlers of the moose.
So the big hook-jawed male coho makes evolutionary sense. But where does the jack fit in? Is he just an under-endowed male, the product of poor nutrition or other vicissitudes, using speed and stealth to make the best of a bad situation?
Not necessarily. It turns out that the phenomenon of parasitic spawning, as ichthyologists call it, is widespread among fish. Documented in 13 families, it’s most common among salmonids like the coho, wrasses (tropical reef fish), and cichlids (freshwater fish of African and South American lakes and rivers). Bluegill sunfish practice it, as do the famous humming toadfish of Sausalito. There’s a long list of names for them: sneakers, streakers, scroungers, cuckolders, satellite males, and my favorite, machos furtivos. Jacks are as much the product of a selection process as hooknoses—being a jack is not a fluke, but an evolutionarily stable strategy.
Typical male coho (and almost all females) return to their natal streams to spawn after a year and a half at sea. Jacks, however, spend only six months in the ocean. There’s no second chance for them; they die at the end of their journey, whether they’ve spawned or not. And some don’t get past the larger males. Observing coho in Lagunitas Creek, Peter Moyle, professor of fish biology at UC Davis, watched as a hooknose male “grabbed a jack between its jaws and lifted it out of the water with a shaking motion.” One study of chinook salmon found that attacks by big males accounted for almost half of jack mortality on the spawning grounds.
There’s evidence that jackhood is genetically determined, maybe with some contribution from environmental factors. And hit-and-run spawning seems to work well enough to keep a stable proportion of those genes in the population. Jacks in another study obtained 82 percent of their matings through sneaking, while hooknoses got 91 percent of theirs through fighting. Since intermediate-sized fish would have less success as either fighters or sneakers, the evolutionary process has been selecting for the two distinct kinds of males. Interestingly, Moyle implies that jacks are less likely to home in on the streams where they were hatched, and that this reduces genetic isolation among different coho runs.
With bluegill sunfish, introduced in California as sport fish, there are other complexities. Bluegills, unlike salmon, don’t die after spawning. And there are three kinds of bluegill males: parentals, who hold a territory centered on the nest where females lay their eggs; female mimics, who use their resemblance to females to get close enough for fertilization; and sneakers, small males like the coho jacks.
But the prize for complication goes not to a fish but to a reptile, the side-blotched lizard, common in western scrublands. Male lizards are color-coded, with orange, blue, or yellow throats, and each color morph has a different reproductive strategy. Orange-throats defend large territories that are home to several females. Blue-throats have smaller territories and pair with a single female. Yellow-throats are sneakers. Depending on the circumstances, each type can outcompete one of the others, in a kind of rock-scissors-paper game, and the numbers of each cycle over time.
The persistence of jack coho, yellow-throated side-blotched lizards, and other machos furtivos suggest that sexual selection doesn’t always favor the Schwarzeneggers of the animal world. To paraphrase Ecclesiastes, the race is sometimes to the swift (and sneaky).
