The example of langur infanticide shows how individual interests can be favored by natural selection over group interests. How then can cooperative effort evolve within a species? After all, humans cooperate with one another all the time. In hunter-gatherer bands, for example, anthropologists have long noted the prevalence of food-sharing. The prey killed on hunts, in particular, tends to be widely shared among band members. Hunting returns are highly variable, and success depends more on luck than on skill. E.g., in some hunter-gatherer populations a man will return from a hunt empty-handed four out of ten times, on average. And on his successful forays, a hunter will often kill a large animal, which provides meat far in excess of his own needs and those of his family.

Interestingly, one finds a similar situation among vampire bats. Vampire bats are found from Mexico to Argentina, but everywhere they are found they feed exclusively on blood. Typically they prey upon domesticated livestock. Specialized heat-sensititve cells in the bat's nose allow the bat to find a location where the prey's blood vessels are near the surface of its skin. The bat hangs from its prey's mane (or sometimes other parts of its body), and bites it around the neck- shoulder. With its sharp teeth, the bat slices off a piece of skin on the prey, opening a wound from which the bat draws its meal. Its saliva contains an anticoagulant that keeps the wound open long enough for the bat to complete its meal -- about 20 to 30 minutes.

However, as with human hunters, vampire bats are not always successful in their pursuit of prey. Typically 7% of adult bats and up to 30% of juvenile bats are unsuccessful in obtaining a meal each evening; but a bat that succeeds in getting food gorges itself with 40 percent of its weight in blood or more. Even after gorging itself, a bat will die of starvation if it goes without food for 60 hours.

Vampire bats cope with variance in their food supply much like human hunter-gatherers do: they share food. Females roost together in groups of about 12, composed of several matrilines (grandmother, mother and her pups). Females within a matriline are related to one another (of course), but females in different matrilines are not related to one another. When a female returns to her roost having found no food, she will solicit food by licking a roostmate under her wing and on the lips. If the roostmate has found food, she will regurgitate part of her blood meal into the mouth of the female who solicited from her. Female vampire bats do not share food with strangers; they share with kin, and with unrelated female "buddies" with whom they have long-standing reciprocal relationships. These relationships last many years: vampire bats can live up to 18 years, and they roost with the same group of individuals for most of that time.

Lets suppose there were two groups of vampire bats. The bats in one group do not share food with their roostmates. They prosper or perish solely on the basis of their own feeding success. Contrast this group of bats with a group in which the members cooperate with one another. If a bat fails to feed it solicits food from a roostmate who was successful in finding food that evening. Which population do you think will suffer greater mortality? This program is designed to simulate these conditions. Each individual has circuits that generate a particular strategy for responding to requests for help. For example, an "Always Defect" (AD) individual is one who never donates blood to anyone who asks for it (but who *will* take blood that is offered to her). An "Always Cooperate" (AC) individual is one donates blood to anyone who asks her for it, regardless of whether the asker has ever helped her in the past. A Tit For Tat (TFT) individual will donate blood to an individual the first time that individual asks for it; but on subsequent requests, Tit For Tat will help only if that individual had helped her when she was in need. (AD, AC, and TFT are all designed to accept blood that is offered to them.)

Go to the strategies menu and select a population of all defectors (those who refuse to share), then run the simulation and see how well individuals in this population fare.

Now run the simulation again, but this time select a population of all cooperators using the strategies menu. Do you notice any difference in the survival of individuals in the two populations?

Using game theory, computer simulations of populations changing over time, and various mathematical models, evolutionary biologists have studied the conditions under which circuits that cause individuals to cooperate can spread via natural selection. Under what conditions will they cause one to cooperate, and with whom? What design features will they have: Do they allow one to recognize different individuals and remember how those individuals treated them in the past? What do they cause one to do when someone cheats -- fails to help someone who helped them in the past? Are different circuits selected for depending on the nature of the social environment? What will happen to a circuit that causes one to share food in an environment filled with individuals who never return the favor? And so on.

Hopefully when you ran the simulation you found that individuals in the group that cooperated did better than individuals in the group that did not cooperate.