Reciprocation in vampire bats

     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 in 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 
group that cooperated did better than individuals in the group that did 
not cooperate.