Читать книгу SuperCooperators - Roger Highfield - Страница 17

It will have blood; they say, blood will have blood.

—Shakespeare, Macbeth

In the pitch darkness, the creatures take flight. They shun the moonlight, making the most of their sense of smell to track their victims, then land nearby to stalk them. After a quick loping run on all fours they latch on to their prey. Using a heat sensor on the nose, each one can tell where the blood courses closest to the surface of the victim’s skin. Often a meal begins with a quick bite to the neck. There they can hang for up to half an hour, using their long grooved tongues like straws to lap fresh warm blood. Over several nights they return to sup on the same wounds, and it is thought that they are able to recognize the breathing sounds of their victims in the same way as we use the sound of a voice to recognize each other.

What I find most extraordinary of all about vampire bats is what happens when they return to their roost, where hundreds, even thousands of them congregate, suspended upside down. If one member in the roost is unable to find prey during the night’s hunt, its peers will regurgitate some of their bloody fare and share it. The exchange of blood among the bats was first revealed in studies conducted in the early 1980s by Gerald Wilkinson of the University of Maryland. During fieldwork in Costa Rica, Wilkinson found that, on any given night, a few percent of adult bats and one-third of juveniles fail to dine. They rarely starve, however, since well-fed vampire bats disgorge a little precious blood to nourish their hungry peers. What was neat was that his experiments suggested that bats are more likely to share blood with a bat that has previously fed them (the bats spend time grooming each other, paying particular attention to fur around the stomach, enabling them to keep tally).

This is an example of what I call direct reciprocity. By this, I mean simply the principle of give-and-take. When I scratch your back, I expect you to scratch mine in return. The same goes for blood meals among bats. This form of reciprocity is recognized in popular sayings, such as “tit for tat” and the idiom “one good turn deserves another.” The Romans used the phrase quid pro quo—“something for something.” As the vampires suggest, this kind of cooperation dates back long before Romulus and Remus, long before the rise of modern humans.

For direct reciprocity to work, both sides have to be repeatedly in contact so that there is an opportunity to repay one act of kindness with another. They might live in the same road, or village. Perhaps they work together. Or they may encounter each other every Sunday in church. In the case of the bats, they hang about the same cave or hollow. In that way, they can form a “contract” based on helping each other.

The bats are one often cited example of direct reciprocity in nature. Another can be found on coral reefs, where fish of all kinds visit “cleaning stations” where they are scrubbed of parasites by smaller varieties of fish and by shrimps: the former get cleaned of pesky parasites and the latter get a free meal. When a wrasse tends a great grouper, the little cleaner sometimes swims into the gill chambers and mouth, demonstrating remarkable faith that it is not going to be eaten. When the grouper wants to depart, it tells its cleaner that it wants to go by closing its mouth a little and shaking its body. It does this even when it is in danger of being attacked. A safer way to proceed would be to gulp down the cleaner and leave immediately. The first strategy would be a form of cooperation, the second a form of defection.

The nuisance of parasites—ticks—has led to the emergence of another instance of this mechanism at work, in the form of reciprocal grooming, this time among impala, a kind of antelope found in Africa. And when it comes to our closest relatives, textbooks are crammed with examples. One was reported in 1977 by Craig Packer in the Gombe Stream Research Centre, Tanzania, where there has been a long-term study of olive baboons, so named because of their distinctive fur. Packer, now at the University of Minnesota, reported how one male will help another who had previously come to his aid in ganging up on more senior baboons, so that one of them can have sex with the senior’s female. Even though the helper will not have sex immediately after forming a coalition, he still cooperates because he expects the favor will be returned.

Sri Lankan macaques Macaca sinica will tend the wounds of a fellow male in order to secure the latter macaque’s support in future conflicts. Unsurprisingly, juvenile males are especially attentive to the injuries of hefty adults, who can provide more muscle in a future fracas. One study of macaques in Kalimantan Tengah, Indonesia, went so far as to suggest that males were more likely to mate with females that they have previously groomed, the grooming being a kind of payment for sex, a finding given the colorful interpretation that the “oldest profession”—prostitution—seems to date back long before humans.

Male chimpanzees share meat to bind social alliances, and there is some evidence that they increase the degree to which they cooperate in line with how much a partner has been helpful toward them. Reciprocity can be exchanged in all kinds of currencies, such as grooming, support in fights, babysitting, warning, teaching, sex, and of course food. Frans de Waal of Emory University, Atlanta, observed how a top male chimpanzee, Socko, had more chance of obtaining a treat from his fellow chimp May if he had groomed her earlier that day.

There are caveats, however. One is that different scientists use terms such as reciprocity in various ways. Another is that, when it comes to observing behaviors in the wild, it can take many lengthy and detailed studies to understand what is really going on. Tim Clutton-Brock, a professor of ecology and evolutionary biology at Cambridge University, says that it can be hard to sift concrete examples of reciprocity from the illusory ones that can be explained another way.

Let’s take Craig Packer’s inspirational olive baboon research, for example. Packer had originally thought that the males were trading favors in their pursuit of sex. His original argument went that the allies switch roles, so that each one benefits from the association. But follow-up studies suggested that the cooperating males actually compete with each other when it comes to snatching the prize. The only way they can have an opportunity to mate is to join forces and to cooperate, true enough. But once the existing consort is driven off, then it is every man for himself when it comes to getting the girl. Packer puts it like this: “In this scenario, cooperation is like a lottery, and you can’t win if you don’t buy a ticket. Because two against one gives very good odds of success, the price of the ticket is very low compared to the value of the prize. Participate in enough lotteries of this sort, and you will always come out ahead—and so will your partners.”