can surely count on convergent evolution of woodpecking. Not surprisingly, woodpeckers are very successful birds. There are nearly 200 species, many of them common. They come in all sizes, from tiny birds the size of kinglets up to crow-sized species. They are widespread over most of the world, with a few exceptions that I shall mention later. They do not have to migrate in winter. Some species have even exploited their woodpecking skills to live in treeless places, excavate nest holes in the ground, and feed on ants. While the earliest known fossil woodpeckers date only from the Pliocene (about seven million years ago), molecular evidence indicates that woodpeckers evolved about fifty million years ago.
How hard is it to evolve to become a woodpecker? Two considerations seem to suggest, 'Not very hard'. Woodpeckers are not an extremely distinctive old group without close relatives, like egg-laying mammals. Instead ornithologists have agreed for a long time that their closest relatives are the honey-guides of Africa, the toucans and barbets of tropical America, and the barbets of the tropical Old World, to which woodpeckers are fairly similar except in their special adaptations for woodpecking. Woodpeckers have numerous such adaptations, but none is remotely as extraordinary as building radios, and all are readily seen as extensions of adaptations possessed by other birds. The adaptations fall into four groups.
First and most obvious are the adaptations for drilling in live wood. These include a strong, straight, chisel- like bill with a hard, horny covering at the tip; nostrils protected with feathers to keep out sawdust; a thick skull; strong head and neck muscles; a broad base of the bill, and a hinge between that base and the front of the skull, to help spread the shock of pounding; and possibly a brain/skull design like a bicycle helmet, to protect the brain from shock. I hese features for drilling in live wood can be traced to features of other birds much more easily than our radios can be traced to any primitive radios of chimpanzees. Many other birds, such as parrots, peck or bite holes in dead Wood. Some barbets can actually excavate in live wood, but they are much more, clumsier, and less neat than woodpeckers and peck from the side rather than straight. Within the woodpecker family there is a gradation of n' ing ability—from wrynecks, which cannot excavate at all, to the many Woodpeckers that drill in softer wood, to hardwood specialists like sapsuckers and the pileated woodpecker.
Another set of adaptations are those for perching vertically on bark, such as a stiff tail to press against bark as a brace, strong muscles for manipulating the tail, short legs, long curyed toes, and a pattern of moulting the tail feathers that saves the central pair of tail feathers (crucial in bracing) as the last to be moulted. The evolution of these adaptations can be traced even more easily than can the adaptations for woodpecking. Even within the woodpecker family, wrynecks and piculets do not have stiff tails for use as braces. Many birds outside the woodpecker family, including creepers and pygmy parrots, do have stiff tails that they evolved to prop themselves on bark. The third adaptation is an extremely long and extensible tongue, fully as long as our own tongue in some woodpeckers. Once a woodpecker has broken into the tunnel system of wood-dwelling insects at one point, the bird uses its tongue to lick out many branches of the system without having to drill a new hole for each branch. Some woodpeckers have barbs at the tip of the tongue to spear insects, while others have big salivary glands to catch insects by making the tongue sticky. Woodpeckers' tongues have many animal precedents, including the similarly long insect-catching tongues of frogs, anteaters, and aardvarks and the brush-like tongues of nectar- drinking lories.
Finally, woodpeckers have tough skins to withstand insect bites plus the stresses from pounding and from strong muscles. Anyone who has skinned and stuffed birds knows that some birds have much tougher skins than others. Taxidermists groan when given a pigeon, whose paper-thin skin tears almost as soon as you look at it, but smile when given a woodpecker, hawk, or parrot. While woodpeckers have many adaptations for woodpecking, most of those adaptations have also evolved convergently in other birds or animals, and the unique skull adaptations can at least be traced to precursors. You might therefore expect the whole package of woodpecking to have evolved repeatedly, with the result that there would now be many groups of large animals capable of excavating into live wood for food or nest sites. Some animal groups defined initially by distinctive ways of feeding have proved to be polyphyletic, meaning that the group is actually an unnatural one, consisting of several groups that evolved similar adaptations from different ancestors. For instance, vultures are now known, and bats and seals are suspected, to be polyphyletic. But all the classical evidence, and now the newer molecular evidence, have uncovered no hint of polyphyly for woodpeckers. Modern woodpecker are all more closely related to each other than to any non-woodpecker. Woodpecking thus appears to have evolved only once.
Picologists, the scientists who specialize in studying woodpeckers, take that conclusion for granted. On reflection, though, it is startling to the rest of us non-picologists who had convinced ourselves that woodpecking would evolve repeatedly. Could it be that other pseudo-woodpeckers did evolve, but that our surviving woodpeckers were so superior that they exterminated their unrelated competitors? For example, separate groups of mammalian carnivores evolved in South America, Australia, and the Old World. But the Old World carnivores (our cats and dogs and weasels) proved so superior that they exterminated South America's carnivorous mammals millions of years ago and are now in the process of exterminating Australia's carnivorous marsupials. Was there a similar shootout in the woodpecker niche?
Fortunately, we can test that theory. True woodpeckers do not fly far over water, with the result that they never colonized remote oceanic land masses like Australia/New Guinea (formerly joined in a single land mass), New Zealand, and Madagascar. Similarly, placental terrestrial mammals other than bats and rodents were never able to reach Australia/ New Guinea, where instead marsupials evolved good functional equivalents of moles, mice, cats, wolves, and anteaters. Evidently it was not so hard to fill those mammalian niches by convergent evolution. Let's see what happened to the woodpecker niche in Australia/New Guinea. We find there a diverse array of birds that evolved convergently to feed on or under bark, including pygmy parrots, birds of paradise, honey-eaters, Australian creepers, Australian nuthatches, ploughbills, ifritas, and flycatchers. Some of those birds have powerful bills used to dig into dead wood. Some of them have evolved elements of the woodpecker anatomical syndrome, such as stiff tails and tough skins. The species that has come the closest to filling the woodpecker niche is not a bird at all but a mammal, the striped possum, which taps on dead wood to detect insect tunnels, rips open the wood with its incisor teeth, then inserts its long tongue or very long fourth finger to pull out the insects.
However, none of these would-be woodpeckers has actually made it into the woodpecking niche. None can excavate live wood. Many are visibly inefficient; I recall seeing a black-throated honeyeater trying to hop up a tree trunk and repeatedly falling off. The ploughbill and striped possum seem to be the would-be's most effective at digging in dead Wood, but both are quite uncommon and evidently cannot make a good wving by their efforts. New Zealand's and Madagascar's pseudo-Woodpeckers are no better. In a stunning instance of convergent Solution, Madagascar's best would-be is also a mammal, a primate called the aye-aye, that operates like a striped possum except for having a very long third instead of a fourth finger. But just as in Australia/New Guinea, none of the would-be's in New Zealand or Madagascar can excavate in live wood.
Thus, in the absence of woodpeckers, many try, and none succeeds. The woodpecker niche is flagrantly vacant on those masses not reached by woodpeckers. If woodpeckers had not evolved that one time in the Americas or Old World, a terrific niche would be flagrantly vacant over the whole Earth, just as it has remained vacant in Australia/New Guinea, New Zealand, and Madagascar.
I have dwelt on woodpeckers at length to illustrate that convergence is not universal, and that not all opportunities are seized. I could have illustrated the same point with other, equally flagrant examples. The most ubiquitous opportunity available to animals is to consume plants, much of whose mass consists of cellulose. Yet no higher animal has managed to evolve a cellulose-digesting enzyme. Those animal herbivores that digest cellulose instead have to rely on microbes housed within their intestines. Among such herbivores, none comes close to achieving the efficiency of ruminants, the cud-chewing mammals exemplified by cows. To take another example that I discussed in Chapter Ten, growing your own food would seem to offer obvious advantages for animals, but the only animals to master the trick before the dawn of human agriculture 10,000 years ago were leaf-cutter ants and their relatives plus a few other insects, which cultivate fungi or domesticate aphid 'cows'.
Thus, it has proved extraordinarily difficult to evolve even such obviously valuable adaptations as woodpecking, digesting cellulose efficiently, or growing one's own food. Radios do much less for one's food needs and would seem far less likely to evolve. Are our radios a fluke, unlikely to have been duplicated on any other planet?
Consider what biology might have taught us about the inevitability of radio evolution on Earth. If radio- building were like woodpecking, some species might have evolved cerUm elements of the package or evolved them in inefficient form, although only one species managed to evolve the complete package. For instance, we might
