Sophie Lowe

WHY DO BATS COOPERATE?

VAMPIRE BATS:

Vampire bats live in huge colonies with numbers ranging from 6 to 2000 with most colonies containing around 100 members. Colonies consists almost entirely of females and the degree of relatedness between them is between 0.08 and 0.11
* (Wilkinson, 1988). Bats can live for up to 20 years but 9 is the average age reached in the wild and the majority of this time is spent within the one colony. A long period of time is spent caring for young, and for this reason they only usually have one offspring per year, which they spend around nine months nursing. Bats are highly sociable creatures and spend a lot of time grooming each other and learning each other’s smells and sounds. It has been found that vampire bats have very large brains in relation to their size, with the largest neocortexes of all bats. This is thought to aid in recognition of roost mates and in remembering past events.

COOPERATIVE BEHAVIOUR:

Vampire bats feed on the blood of large birds, cattle, horses and pigs and require around 2 tablespoons of blood every day. However, it is very difficult for vampire bats to find and feed from a host and it is not uncommon for a bat to return from a nights hunting without having fed. When a bat is successful in obtaining a blood meal, they are often able to ingest much more blood than is required. Subsequently, they may ‘donate’ this extra blood to fellow roost members which have been unsuccessful and to which blood donation is vital to their survival (vampire bats may die within 48 hours if they do not feed). This kind of behaviour is known as reciprocal altruism, the bats donate blood to each other and it is donated to them in return (in most cases. It will be seen that this does not always occur).

This type of behaviour is relatively rare in the animal kingdom and when considering the ultimate aim of the gene is to propagate copies of itself by increasing the chances for its host to reproduce and to keep their offspring alive, the reason for this rarity may become apparent. Cooperation between non-related animals does not directly benefit reproductive success, and may even appear to reduce the chances of cooperative individuals reproducing or successfully rearing offspring. However, there are several possible explanations as to why this kind of behaviour has evolved in vampire bats and these reasons shall be explored.

1. Because they expect to be paid back:

Why do people do favours for other people? Because we are nice? This is unlikely! Usually it is because we expect something in return. This is the same for vampire bats and has been highlighted by the display of ‘tit for tat’ behaviour, (a phrase first used by Axelrod to describe the type of behaviour which is optimal during an iterated version of prisoners dilemma*). Vampire bats are able to recognise fellow roost mates and upon donation of blood to another bat, they will not donate again to the same bat unless the favour is returned to them. The phrase ‘tit for tat’ is used because an individual bat will copy the same behaviour shown to them by another bat on subsequent meetings. For example, if bat A donates blood to bat B, bat B will on another occasion donate blood to bat A. However, if bat B fails to donate blood to bat A, if it is again starving, bat A will remember that bat B has not donated blood to it since its own previous donation and will fail to help the starving bat. This grudging behaviour shows that bats will only donate blood if they get something in return. If they make the mistake of donating blood to a non-reciprocator, they have suffered a loss but the mistake will not be made again. The bats however are forgiving and will resume their reciprocal behaviour if the debt is repaid. Thus, not ‘cutting off their nose despite their face’, in other words, they are not unnecessarily eliminating possible donors.

2. To maintain population size:

The survival of vampire bats is not easy, on many occasions they are unsuccessful on their hunting missions and return to the roost with an empty stomach. It does not take very long for a starving bat to die, around 3 days in fact. With each passing day, the likelihood that it is able to find a meal decreases due to its increasing weakness. It is almost a certainty that if the bat does not receive a donation from one of its fellow roost mates, it will die. This kind of situation may occur on many occasions and to many bats. If cooperation amongst the colony did not exist, their numbers would probably decrease rapidly, and the colony may face extinction. Therefore, it is in the best interests of the bats to cooperate with each other and keep the colony alive (although the decision to cooperate may not be a conscious one as we will see later). With a larger colony, there is an increased chance of donations between bats, the gene pool will be maintained and the colony will be stable in numbers. This may be important for protection i.e. safety in numbers!

3. Mistaken identity:

Although the brain of vampire bats is highly developed to aid recognition of their fellow roost mates, it may be that the brain is not developed enough to distinguish non-related roost mates from their own offspring. The begging behaviour of nursing offspring is probably very similar to that of roost mates and it may be that the mothers maternal instincts are not restricted to her own children. In a normal situation this type of behaviour is likely to be to the detriment of animals showing this behaviour, however, if the whole species extended this behaviour to each other, the colonies would benefit greatly from this originally seeming ‘error’. However, we must remember that the gene is the unit of selection and the gene is ultimately selfish, therefore it must be to the benefit of the gene that this behaviour occurs and therefore of benefit to individuals showing cooperative behaviour.

4. Evolutionary aspects of bat anatomy and physiology have made it possible:

Basically, bats cooperate because they can! Successful cooperation is only possible if the bats are able to recognise their own roost-mates and also be able to bear a ‘grudge’ against those which have failed to repay it in the past. Therefore, the development of the vampire bat’s brain plays a major role in cooperation.The tit for tat strategy also requires the performers of the behaviour to have a high probability of meeting each other again, a criteria which is definitely met by vampire bats which live in large colonies and stay in the same group for most of their life. It may be that cooperation is beneficial to all species but their development does not allow it.

5. Because cooperation amongst vampire bats is an Evolutionarily Stable Strategy:

All of the previous reasons may be highly important in explaining the existence of cooperative behaviour amongst vampire bats. However, most of the previous possible reasons can be encompassed by the following; Cooperation amongst vampire bats is an evolutionarily stable strategy (to be more precise, the ‘tit for tat’ behaviour is a stable strategy).

An Evolutionarily Stable Strategy is: ‘A heritable strategy which , if adopted by (expressed in) most members of a population, cannot be supplanted in evolution by an alternative (mutant) strategy. The strategy may be complex and involve a variety of different sub-responses in accordance with environmental changes, not least other organisms’ behaviours’ (Abercrombie et al, 1992).

The cooperative strategy employed by vampire bats may appear to be a conscious decision to cooperate with colony members but this behaviour may be entirely unconscious and is likely to have developed as a product of natural selection. Imagine the scenario of a population of vampire bats in which blood donation does not occur. Some may die from starvation but none waste energy by donating blood. Some animals may have more food than needed and others may have none at all. The population may be at risk from extinction if the number of bats dying of starvation is high, especially as offspring require feeding by the mother for the first 9 months of life. Now imagine a mutant gene arose in the population which either made the host unable to distinguish between related and non-related conspecifics or told the bat to donate blood to any starving animal, the effect is irrelevant other than that the gene causes the bat to donate blood to those requiring it. Now suppose the bat did this for every starving bat, but no other bats contained this gene. The bat would expend a lot of energy helping non-related bats, which may be to the detriment of its own offspring and render the gene unlikely to be spread. Now suppose that 2 or more unrelated bats shared the same altruistic gene. On nights when one bat was starving and the other had a good feed and vice versa, they would help each other out. This would increase the likelihood of both their survival and therefore of their offspring too. This gene would do very well in the population and would rapidly spread to offspring and successive generations. This could be an explanation for the cooperative behaviour. However, the exact strategies employed by vampire bats are a little more complicated and can be explained by the following scenario;

If there arose another mutant gene, one which accepted donations but did not donate to other bats at all, a ‘cheating gene’, it would do very well in a population of bats which always donated blood. The gene would proliferate to the detriment of the donors until they reached extinction. This would also lead to the decrease and possible extinction of the bats which never donated as there were now no donors to help them out and a state identical to the original situation would be reached. A population consisting of purely donors and cheaters is not stable. However, when a third gene comes into play, a gene which remembers past events but which is inherently ‘nice’ and ‘forgiving’, a stable state may be reached in which there are a relatively low number of donors and cheaters. This new gene will donate blood to those who need it but remembers individuals which fail to repay the favour and will refuse to feed them in the future unless the debt is repaid. The bats bearing this gene may be said to bear a grudge against those which ‘cheat’ it.

The game of prisoner dilemma can be used to evaluate the possible ‘decisions’ made by vampire bats as whether to cooperate or defect. As previously mentioned, a tit for tat strategy is an optimal behaviour when employed by the majority of the colony and cannot be bettered by any other strategy (that is known of to date). The various possible rewards and punishments received by an individual (bat B) are summarised in the following table;

If the bats were only to meet once, the only sensible option for them both would be to defect. However, in a colony of bats, there is a high probability they will meet again on an indefinite number of occasions, and in this situation, for each bat to cooperate with each other would give a much higher average payoff than if they were to ‘defect’ some of the time and ‘cooperate’ at other times. For both bats to ‘defect’ all of the time would give a much lower average payoff. It has been shown that, given the choice to defect or cooperate, the strategy which gives the highest average payoff to an individual employing the strategy is ‘tit for tat’ which has 4 simple rules:

1. Never be the first to defect
2. Retaliate only after your partner has defected
3. Be prepared to forgive after carrying out just one act of retaliation
4. Adopt this strategy only if the probability of meeting the same player again exceeds 2/3

The criteria for a prisoners dilemma and a ‘tit for tat’ strategy is met by vampire bats; the temptation to defect is greater than the reward for mutual cooperation which is greater than punishment for mutual defection which, in turn, is greater than a suckers payoff. The bats belonging to the same colony have a high probability of meeting each other again and they are capable of recognition and remembering past events.

CONCLUSION:

• Main reason being ESS
• Selfish gene
• Other reasons may contribute to it being an ESS as mentioned
• Summary of reasons

 

REFERENCES:

Abercrombie, M., Hickman, M., Johnson, M.L., Thain, M. (1992) The penguin dictionary of biology Penguin books; London
Dawkins, R. (1989) The selfish gene Oxford University Press; Oxford UK
staff.science.nus.edu/~parwani/sim/simproject1/cooperation.doc
www.brembs.net/ipd/vampire.html www.cquest.toronto.edu/zoo/bio150y/pdgame/titfortat.html
www.nitle.org/etcon/examples/sj_tit-for-tat.html
www.heretical.com/gameplay/evolcoop.html
www.abc.net.au/science/slab/tittat/story.htm
www.thewildones.org/Animals/vampire
www.tower.org/menagerie/safari/vampirebat.html