Cooperation experiments often ask cognitive questions. Do the actors realize they need a partner? Do they know the partner’s role? Are they prepared to share the spoils? If one individual were to hog all the benefits, this obviously would imperil future cooperation. So we assume that animals watch not only what they get but also what they get compared to what their partner gets. Inequity is something to worry about.
This insight inspired an immensely popular experiment that Sarah Brosnan and I conducted with pairs of brown capuchins. After they performed a task, we rewarded both monkeys with cucumber slices and grapes after determining that they all favored the latter over the former. The monkeys had no trouble with the task if they received identical rewards, even if they both got cucumber. But they were vehemently opposed to unequal outcomes, if one got grapes and the other got cucumber. The cucumber monkey would contentedly munch on her first slice, but after noticing that her companion was getting grapes, she would throw a tantrum. She’d ditch her measly veggies and shake the testing chamber with such agitation that it threatened to break apart.46
Refusing perfectly fine food because someone else is better off resembles the way humans react in economic games. Economists call this response “irrational,” since getting something is by definition better than getting nothing. No monkey, they say, should ever refuse food that she’d normally eat, and no human should reject a small offer. One dollar is still better than no dollar. Sarah and I are unconvinced that this kind of reaction is irrational, though, since it seeks to equalize outcomes, which is the only way to keep cooperation flowing. Apes may even go further than monkeys in this respect. Sarah found that chimpanzees sometimes protest inequity that goes the other way. They object not only to getting less than the other but also to getting more. Grape receivers may reject their own advantage! This obviously brings us close to the human sense of fairness.47
An odd couple of hunters: a coral trout and a giant moray eel prowl together around the reef.
Without going into further details, something encouraging happened in these studies. They were soon extended to other species, including outside the primates. It is always a sign of a field’s maturity when it expands. Researchers who applied inequity tests to dogs and corvids found reactions similar to those of the monkeys.48 Apparently, no species can escape the logic of cooperation, whether it involves the selection of good partners or the balance between effort and payoff.
The generality of these principles is best illustrated by the work on fish by Redouan Bshary, a Swiss ethologist and ichthyologist. For years Bshary has been enchanting us with observations of the interplay and mutualism between small cleaner wrasses and their hosts, the large fish from which the cleaners nibble away ectoparasites. Each cleaner fish owns a “station” on a reef with a clientele, which come and spread their pectoral fins and adopt postures that offer the cleaner a chance to do its job. In perfect mutualism, the cleaner removes parasites from the client’s body surface, gills, and even the inside of its mouth. Sometimes the cleaner is so busy that clients have to wait in queue. Bshary’s research consists of observations on the reef but also experiments in the laboratory. His papers read much like a manual for good business practice. For example, cleaners treat roaming fish better than residents. If a roamer and a resident arrive at the same time, the cleaner will service the roamer first. Residents can be kept waiting since they have nowhere else to go. The whole process is one of supply and demand. Cleaners occasionally cheat by taking little bites of healthy skin out of their client. Clients don’t like this and jolt or swim away. The only clients that cleaners never cheat are predators, which possess a radical counterstrategy: to swallow them. The cleaners seem to have an excellent understanding of the costs and benefits of their actions.49
In a set of studies in the Red Sea, Bshary observed coordinated hunting between the leopard coral trout—a beautiful reddish-brown grouper that can grow to three feet in length—and the giant moray eel. These two species make a perfect match. The moray eel can enter crevices in the coral reef, whereas the trout hunts in the open waters around it. Prey can escape from the trout by hiding in a crevice and from the eel by entering open water, but it cannot get away from the two of them together. In one of Bshary’s videos, we see a coral trout and a moray eel swimming side by side like friends on a stroll. They seek each other’s company, with the trout sometimes actively recruiting an eel through a curious head shake close to the eel’s head. The latter responds to the invitation by leaving its crevice and joining the trout. Given that the two species don’t share the prey with each other but swallow it whole, their behavior seems a form of cooperation in which each achieves a reward without sacrificing anything for the other. They are out for their own gain, which they attain more easily together than alone.50
The observed role division comes naturally to two predators with different hunting styles. What is truly spectacular is that the entire pattern—two actors who seemingly know what they are going to do and how it will benefit them—is not one we usually associate with fish. We have lots of cognitively high-level explanations for our own behavior and find it hard to believe that the same might apply to animals with much smaller brains. But lest one think that the fish are showing a simplified form of cooperation, Bshary’s recent work challenges this notion. Coral trout were presented with a fake moray eel (a plastic model capable of performing a few actions, such as coming out of a tube) that was able to help them catch fish. The setup followed the same logic as the pulling tests in which chimpanzees recruit help when needed, but not if they can complete the task alone. The trout acted in every way similar to the apes and were equally adept at deciding on their need for a partner.51