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Creationists keep coming up with the same old arguments. One favorite: “If humans evolved from apes, why are there still apes?” A fine example of what Judge Jones called “breathtaking inanity.” 

Apart from the fact that the surviving great ape species are not on the direct line of human ancestry, there are still apes because their apish morphology and behavior has, up to now, passed the tests of natural selection. There are still reptiles, too, and fish and worms and bacteria. There is much to be learned from comparative study of all these creatures, down to the genomic level, as representatives of stages in an evolutionary process—but not as identifiable ancestors and descendants. 

Just as it’s possible to compare steps in the development of an organ—eyes, brains, wings—it’s also instructive to look for evolutionary trajectories in the behavior of related species. Take the phenomenon of polyandry in birds. In many (if not most) avian species, males are larger than females, have brighter plumage, are more aggressive in defending territory, and take the initiative in courtship. Some seek multiple mates and leave care of the eggs and nestlings to the female. 

There are, however, a handful of bird species in which that pattern is reversed. Female phalaropes, pelagic shorebirds that nest in the Arctic and the interior West, are larger and more brightly colored than males, and practice serial polyandry, mating with one male at a time then moving on. 

How did phalaropes get that way? The reproductive lives of other, more “normal” shorebirds provides a clue. Female sanderlings lay two clutches of eggs every breeding season; her mate incubates the first while she takes responsibility for the second. It’s an efficient way of making the most of the brief Arctic summer. But note that the same male, in theory, sires both broods. 

But what if a female was able to free herself of child care altogether? Consider the spotted sandpiper, a widespread bird that nests along freshwater ponds and streams—they’re common in the Sierra—and winters on the seacoast, including San Francisco Bay, foraging on riprap and other rocky substrates. Males and females look identical in plumage, although females are larger. (They’re not spotted at this time of year, only in the breeding season.) They can be separated from other small sandpipers by the full-body teetering motion they make, a trait shared with such unrelated birds as dippers, waterthrushes, and wagtails. Its functional significance, if any, is unknown. 

Female spotted sandpipers migrate earlier than males to claim breeding territories. That process often involves physical altercations with other females, sometimes leading to injury. They peck at each other’s heads or use their legs, wings, and bill in a kind of avian judo to flip their opponents. 

With the arrival of the males, each female enters courtship mode. She performs aerial flight displays, goes through the ritualized motions of nest construction, and even sings, a repeated “weet-weet”—nothing for any songbird to worry about. After a pair has bonded, the female lays a batch of eggs and begins to incubate them. It looks like a classic monogamous relationship. But it won’t last. 

As new males appear, a female’s eye wanders. She may stay on territory or go cruising for a new prospect. Some females desert their first mates and set up housekeeping at a new location with a second male (serial polyandry). Others lure second—sometimes even third—males back to the home turf (simultaneous polyandry.) In either case, the original male is left to fend for the first offspring while the female incubates her new eggs. 

All this is accompanied by hormonal changes in the sandpipers’ bodies. Males, as is typical among vertebrates, start the season with higher testosterone levels than females. Their testosterone concentrations then drop when the female they’re paired with lays her eggs. At the same time, a male’s level of prolactin, another hormone involved with parental behavior, rises until it exceeds typical female levels. Although low relative to males, female testosterone levels rise sevenfold in the early part of the breeding season when the birds are battling for territories and wooing mates. 

This is a fairly complex syndrome involving differences between the sexes in size, aggressiveness, and hormonal patterns. The only thing missing is plumage dimorphism, as in phalaropes. In that lineage, having brighter feathers must have given some females a competitive advantage in the mating game; they were able to pass the trait along to their daughters and it spread through the population. At some point in evolutionary time, that might also happen to the spotted sandpiper.