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| Pictures courtesy of lastrefuge.co.uk, The Order Chiroptera |
Reciprocation in Vampire Bats
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| Pictures courtesy of lastrefuge.co.uk, The Order Chiroptera |
FOR THIS ASSIGNMENT YOU MUST DOWNLOAD AND RUN A DOS PROGRAM. HENCE, YOU CAN'T DO THIS LAB ON A MAC, OR IN WINDOWS VISTA.
Natural selection can lead to behavior that promotes individual interests over group interests. For example, on the Indian sub-continent the leaf-eating Langur monkey exhibits an adaptive selfish behavior. When a new male becomes a troop's dominant male, he will kill all the infants in the troop. Evolution selects this behavior because it allows the new dominant male to better disseminate his genome.
However, evolution can also select for cooperative behaviors within a species. One example of such a cooperative behavior is food sharing. For example, humans in hunter-gatherer bands share food. Meat from hunting, in particular, tends to be widely shared among band members. Successful hunting involves luck as well as skill. In some hunter-gatherer populations a man will return from a hunt empty-handed forty percent of the time. Moreover, successful hunts often yield more meat than the hunter and his family can utilize. Thus, cooperation helps everyone by pooling resources that might go unused and decreasing each family's meatless days.
Food sharing also occurs in the other animal species. The purpose of this exercise is to study the evolutionary pressures that support food sharing, i.e., reciprocity, in vampire bats. Vampire bats (Desmodus rotundus) have a body size of about three inches and a wingspan of approximately eight inches. Vampire bats are found from Mexico to Argentina (see map below). These bats feed exclusively on blood gathered from large birds, cattle, horses, pigs, and other domesticated livestock. However, vampire bats do not suck the blood of their prey. Typically, the bat locates a sleeping animal, then uses specialized heat-sensitive cells in its nose to find a location where the prey's blood vessels are near the surface of its skin. Once the bat locates a spot, it hangs from its prey's mane or other body part and bites the prey's skin. The bat's sharp teeth slice off a piece of skin from which the bat draws its meal by lapping the blood oozing from the wound. Vampire bat saliva contains an excellent anticoagulant to keep the wound open and a anesthetic to numb the wound long enough for the bat to complete its meal -- about 20 to 30 minutes.
As with human hunters, vampire bats are not always successful in their pursuit of prey. Lack of hunting success is even more dire in the case of vampire bats, who will die of starvation if they go without food for 60 hours. Typically 7% of adult bats and up to 30% of juvenile bats are unsuccessful in obtaining a meal each evening. As with human hunters a successful bat has food to spare, usually gorging itself with 40 percent or more of its weight in blood. Vampire bats cope with variance in their food supply in much manner as human hunter-gatherers: 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 soliciting female. 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.
The program and accompanying questions are designed to simulate the survival rates of three different groups of vampire bats. The program is designed to simulate the interactions of bats where each individual has genetically determined 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 that 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.) Since each strategy is genetically determined, the offspring of each type of bat will have the circuit/strategy of its parent. You will want to see which strategy works out best for the bats in the long run.
To download the simulation program (1) create a file on your desktop, (2) right click here and select the save link as option on your menu, select your folder and select the save button. Once you have downloaded the file go to your folder and click on the file. The winzip program will ask you where you want unzip the file, (1) select browse, (2) select your folder, (3) select unzip. Once the files have been unzipped, go to your folder and double click on BATS.EXE. This should open the program. Click on the opening window to access the interface.
Below is a picture of the program interface. The top left box contains your stop, start, reset, and quit buttons. Below that is the Strategy Pull down Menu. This pull down menu allows you to choose the mix of cooperators, defectors, and tit for tat strategists in your group of 12 bats. There are three numbers; the first is the number of cooperators, the second is the number of defectors, the third is the number of tit for tat stratagists. For example, 0,12,0 selects zero cooperators, twelve defectors, and zero tit for tat strategists. Below the strategy pull down menu is the pull down menu that allows you to choose the length of time between reproductive cycles for the bats. So, 20 selects twenty days in between reproduction cycles. The final box on the left gives you the history of the interactions and the make up of the group. On the right hand side of the program interface, one finds a large grid telling you that status, overall stretegy, and bat-to-bat strategy for every bat at a given time. The Food Status Box on the top right gives you a running representation the each bat's current status. Along the top of the grid are the bats in the matriline listed left to right as 1-12. Below each number is the food status bar that rises and falls to indicate the individual bat's status, i.e., how close or far away the bat is from starving to death at that moment. If the line goes below the starvation threshold, then that bat dies. Immediately below the food status bars is the cooperation grid. This grid tells you what overall strategy each bat is using in its interactions, and what that sort of interactions the bat has with each member of the group. The values in the top box of each column indicate the overall strategy that the bat uses in its interactions with other bats. Its values can be: coop (cooperation), def (defectors), and TFT (tit-for-tat). Below the overall strategy box, each box in the column tells you what sort of interactions (cooperate or defect) the bats has with each of the other bats on any given day. The number on the side of the grid indicates the bat. So, for column 1 (leftmost) each box indicates the interaction type that the bat will adopt towards other individual bats. "+" indicates cooperation, while "-" indicates defection/non-cooperation. For instance, if the box numbered 4 in column one has a plus, that means that bat one will cooperate wiht bat 4 that day. The plus and minus values will change for bats using TFT as the simulation progresses.
When a bat dies off it will be replaced at the end of a reproductive cycle with a new bat. That new bat's overall strategy will be determined by the strategies of it's parents. As the simulation progresses through reproductive cycles bats die and are replaced in accordance with the survival rate and make-up of the group. The simulation show that the matriline has reached a nash equilibrium, when one all of the bats adopt one strategy or the distribution of strategies remains stable from generation to generation. That is, when the mix of bats and strategies represents the optimal interaction strategies between members of a matriline.
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As you go through the questions below, make a note of your answers. You'll have to enter your answers on the beachboard bat quiz to get your extra credit.
1.) To familiarize yourself with the program, (1) go to the strategies menu and select a population of all defectors (those who refuse to share 0,12,0), (2) select a reproduction cycle of 100 on the reproduction cycle menu below, and (3) hit the go button (top left) to run the simulation. The program will simulate the lives of your 12 bats for the first 100 days (1 reproduction cycle). It will then prompt you to start the next cycle. Look at the cooperation grid and the history box. What happened?
2.) Now run the simulation again, but this time (1) select a population of 4,4,4 (even numbers of each strategy) using the strategies menu, (2) select a reproduction cycle of 100 on the reproduction cycle menu below, and (3) hit the go button (top left) to run the simulation. The program will simulate the lives of your 12 bats for the first 100 days (1 reproduction cycle). It will then prompt you to start the next cycle. Go through as many reproduction cycles as necessary (counting them) until the group consists exclusively of bats one type of strategy. Which strategy won? How many reproduction cycles did it take?
3.) Now run the simulation again. This time (1) select a population of 4,4,4 using the strategies menu, (2) select a reproduction cycle of 10 on the reproduction cycle menu below, and (3) hit the go button (top left) to run the simulation. The program will simulate the lives of your 12 bats for the first 10 days (1 reproduction cycle). It will then prompt you to start the next cycle. Go through as many reproduction cycles as necessary (counting them) until the group consists exclusively of bats one type of strategy. Which strategy won? How many reproduction cycles did it take?
4.) Now run the simulation again. This time (1) select a population of 6,6,0 (6 cooperators, 6 defectors, 0 tit for tat) using the strategies menu, (2) select a reproduction cycle of 100 on the reproduction cycle menu below, and (3) hit the go button (top left) to run the simulation. The program will simulate the lives of your 12 bats for the first 100 days (1 reproduction cycle). It will then prompt you to start the next cycle. Go through as many reproduction cycles as necessary (counting them) until the group consists exclusively of bats one type of strategy. Which strategy won? How many reproduction cycles did it take?
5.) Run the simulation again. This time (1) select a population of 0,6,6 (0 cooperators, 6 defectors, 6 tit for tat) using the strategies menu, (2) select a reproduction cycle of 100 on the reproduction cycle menu below, and (3) hit the go button (top left) to run the simulation. The program will simulate the lives of your 12 bats for the first 100 days (1 reproduction cycle). It will then prompt you to start the next cycle. Go through as many reproduction cycles as necessary (counting them) until the group consists exclusively of bats one type of strategy. Which strategy won? How many reproduction cycles did it take?
6.) Run the simulation again. This time (1) select a population of 6,0,6 (6 cooperators, 0 defectors, 6 tit for tat) using the strategies menu, (2) select a reproduction cycle of 100 on the reproduction cycle menu below, and (3) hit the go button (top left) to run the simulation. The program will simulate the lives of your 12 bats for the first 100 days (1 reproduction cycle). It will then prompt you to start the next cycle. Go through as many reproduction cycles as necessary (counting them) until the group consists exclusively of bats one type of strategy. Which strategy won? How many reproduction cycles did it take?
7.) Which strategy or strategies showed an evolutionary advantage? Which seemed to work best? Which strategy seemed to work the worst? Why do suppose the best strategy worked the best?
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. You'll read more about this in the Ridley readings.
