Marzluff set out to test this recognition with rubber face masks like those we put on at Halloween. After all, crows may recognize certain people by their bodies, hair, or clothes, but with masks you can move a human “face” around from one body to the next, isolating its specific role. His angry birds experiment involved capturing crows while wearing a particular mask, then have coworkers walk around with either this mask or a neutral one. The crows easily remembered the mask of the capturer, far from fondly. Funny enough, the neutral mask was Vice President Dick Cheney’s face, which elicited more negative reactions from the students on campus than from the crows. Not only did birds that had never been captured recognize the “predator” mask, but years later they still harassed its wearers. They must have picked up on the hateful response of their fellows resulting in massive distrust of specific humans. As Marzluff explains, “It would be a rare hawk that would be nice to a crow, but with humans you have to classify us as individuals. Clearly, they’re able to do that.”9
While corvids always impress us, sheep seem to go a step further in that they remember one another’s faces. British scientists led by Keith Kendrick taught sheep the difference between twenty-five pairs of their own species’ faces by rewarding a choice for one face and not for the other. To us, all these faces look eerily alike, but the sheep learned and retained the twenty-five differences for up to two years. In doing so, they used the same brain regions and neural circuits as humans, with some neurons responding specifically to faces and not to other stimuli. These special neurons were activated if the sheep saw pictures of companions that they remembered—they actually called out to these pictures as if the individuals were present. Publishing their study under the subtitle “sheep are not so stupid after all”—a title to which I object, since I don’t believe in stupid animals—the investigators put the face-recognition ability of sheep on a par with that of primates and speculated that a flock, which to us looks like an anonymous mass, is in fact quite differentiated. This also means that mixing flocks, as is sometimes done, may cause more distress than we realize.
Having made primate chauvinists look sheepish, science piled it on with wasps. The northern paperwasp, common in the American Midwest, has a highly structured society with a hierarchy among its founding queens, who are dominant over all workers. Given the intense competition, each wasp needs to know her place. The alpha queen lays most eggs, followed by the beta queen, and so on. Members of the small colony are aggressive to outsiders as well as to females whose facial markings have been altered by experimenters. They recognize one another by strikingly different patterns of yellow and black on every female’s face. The American scientists Michael Sheehan and Elizabeth Tibbetts tested individual recognition and found it to be as specialized as that of primates and sheep. The wasps distinguish their own species’ mugs far better than other visual stimuli, and they also outperform a closely related wasp that lives in colonies founded by a single queen. These wasps hardly have a hierarchy and have far more homogeneous faces. They don’t need individual recognition.10
If face recognition has evolved in such disparate pockets of the animal kingdom, one wonders how these capacities connect. Wasps do not have the big brains of primates and sheep—they have minuscule sets of neural ganglia—hence they must be doing it in a different manner. Biologists never tire of stressing the distinction between mechanism and function: it is very common for animals to achieve the same end (function) by different means (mechanism). Yet with respect to cognition, this distinction is sometimes forgotten when the mental achievements of large-brained animals are questioned by pointing at “lower” animals doing something similar. Skeptics delight in asking “If wasps can do it, what’s the big deal?” This race to the bottom has given us trained pigeons hopping onto little boxes to disparage K?hler’s experiments on apes and the holding up of intelligence outside the primate order to cast doubt on mental continuity between humans and other Hominoids.11 The underlying thought is that of a linear cognitive scale, and the argument that since we rarely assume complex cognition in “lower” animals, there is no reason to do so in “higher” ones.12 As if there were only one way to achieve a given outcome!
Evolutionary science distinguishes between homology (the traits of two species derive from a common ancestor) and analogy (similar traits evolved independently in two species). The human hand is homologous with the bat’s wing since both derive from the vertebrate forelimb, as is recognizable by the shared arm bones and five phalanges. The wings of insects, on the other hand, are analogous to those of bats. As products of convergent evolution, they serve the same function but have a different origin.
This is not the case. Nature abounds with illustrations to the contrary. One that I know firsthand is a pair-bonding Amazonian cichlid, the discus fish, that has achieved the equivalent of mammalian nursing. Once the fry have absorbed the egg yolk, they gather along the flanks of Mom and Dad to nibble mucus off their bodies. The breeding pair secretes extra mucus for this purpose. The young enjoy both nutrition and protection for about a month until they are “weaned” by parents who now turn away each time they approach.13 No one would use these fish to make a point about the complexity or simplicity of mammalian nursing for the obvious reason that the mechanisms are radically different. All that they share is the function of feeding and raising the young. Mechanism and function are the eternal yin and yang of biology: they interact and intertwine, yet there is no greater sin than confusing the two.
To understand how evolution works its magic across the evolutionary tree, we often invoke the twin concepts of homology and analogy. Homology refers to shared traits derived from a common ancestor. Thus, the human hand is homologous with the wing of a bat, since both derive from an ancestral forelimb and carry the exact same number of bones to prove it. Analogies, on the other hand, arise when distant animals independently evolve in the same direction, known as convergent evolution. The parental care of the discus fish is analogous to mammalian nursing but certainly not homologous, since fish and mammals do not share an ancestor that did the same. Another example is how dolphins, ichthyosaurs (extinct marine reptiles), and fish all have strikingly similar shapes owing to an environment in which a streamlined body with fins serves speed and maneuverability. Since dolphins, ichthyosaurs, and fish did not share an aquatic ancestor, their shapes are analogous. We can apply the same line of thought to behavior. The sensitivity to faces in wasps and primates came about independently, as a striking analogy, based on the need to recognize individual group mates.
Convergent evolution is incredibly powerful. It has equipped both bats and whales with echolocation, both insects and birds with wings, and both primates and opossums with opposable thumbs. It has also produced spectacularly similar species in distant geographic regions, such as the armored bodies of armadillos and pangolins, the prickly defense of hedgehogs and porcupines, and the predatory weaponry of the Tasmanian tiger and the coyote. There is even a primate, the aye-aye of Madagascar, that looks like E.T. with an extremely elongated middle finger (to tap for hollow spots and extract grubs from wood), a trait that it shares with a small marsupial, the long-fingered triok of New Guinea. These species are genetically miles apart, yet they have evolved the same functional solution. We should not be surprised therefore to find similar cognitive and behavioral traits in species that are eons and continents apart. Cognitive rippling is common precisely because it isn’t bound by the evolutionary tree: the same capacity may pop up almost anywhere it is needed. Instead of taking this as an argument against cognitive evolution, as some have done, it perfectly fits the way evolution works through either common descent or adaptation to similar circumstances.
A prime example of convergent evolution is the use of tools.