Barter deals among vampire bats

Press, 30 April 1984, Page 20

Barter deals among vampire bats

From ‘The Economist’ London

A starving vampire bat will be given regurgitated blood by a fellow bat — if it has been similarly generous to the donor on a previous occasion. A vervet monkey can expect help in a battle — if it has previously groomed the monkey that comes to its aid. These two recent discoveries show that it is not only human beings that pay for services. Animals helping each other is not itself news. A wildebeest defending its calf against a lion will risk its life; a bee will commit almost certain suicide by stinging to defend its nest. But these examples are explained by the fact that the animals are related: they share the same genes. (The biologist, J. B. S. Haldane, once remarked that he would sacrifice his life for three brothers — or nine cousins — because he would preserve more copies of his genes.) What about examples of mutual support between unrelated individuals? One theory says these are cases of reciprocal altruism: you scratch my back, I’ll scratch yours. The vampire bats studied by Dr Gerald Wilkinson of the University of California at San Diego fit this

neatly. Vampire bats fly out of their roosts each night in search of a horse or cow from which to drink blood. One in five of them fails to find a meal (more among young bats); if they go two or three nights without blood, they die. Fortunately, back in the roost, successful bats will often regurgitate some blood to help those in need, especially when the latter have less than 24 hours to live. Why? Helper and helped are not always related. Nor is the donor brimming over with blood; it usually has had an average meal, no more. Reciprocal altruism is the best explanation: Dr Wilkinson found that a bat gave blood to one that had helped it in the past. The vervet-monkey example is still more clear-cut. Vervet mon- i keys live in large troops in the 1 African savannah. i There is frequent bickering among troop members, and a ' monkey which gets another to help ' it often wins a fight. Monkeys also help each other to groom their fur in search of parasites —to reach 1 the parts one animal cannot. j Can one type of help buy the i other? Dr Robert Seyfarth and Dr ; Dorothy Cheney of the University < of California at Los Angeles have i done a series of experiments to i

find out. When in a fight, a monkev gives a characteristic call; this is what alerts another to come to its aid. The scientists played tape recordings of these calls and observed the reaction of individual monkeys, knowing in each case whether the monkev in question was related to the' caller (first cousin or closer) and whether it had helped to groom it in the past two hours. Often the two monkeys were related and, in these cases, the response was much the same whether the two had indulged in mutual grooming or not. But when they were unrelated, there was a consistent difference. If the caller had not groomed the monkey under observation, the latter looked up from his feeding only very briefly. If the caller had groomed the other recently, the monkey looked in the direction of the concealed tape recorder for several seconds. That is hardly rushing to the caller’s aid, but then there was no real fight going on. It certainly looks as if the caller could reckon it had bought help in fights by grooming its fellow. The first demonstration of barter in animals? — Copyright, “The Economist”.