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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 \u201ccleaning stations\u201d 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.<\/p>\n\n\n\n
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: \u201cIn this scenario, cooperation is like a lottery, and you can\u2019t win if you don\u2019t 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\u2014and so will your partners.\u201d\u00a0<\/p>\n\n\n\n