Cooperative Behavior

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There are numerous ways common vampire bats cooperate with each other. Only one instance of cooperation has been termed reciprocal altruism. This instance is blood sharing or food sharing (Wilkinson 1990). Although this behavior has been the primary focus of most studies, there are at least three other cases of cooperation among common vampire bats.
The female common vampire bat participates in many cooperative, social activities (Wilkinson 1986). Females have been the focus for study when observing food sharing (DeNault & MacFarlane 1995). The females in the harem have strong bonds that they reinforce using social interactions in the roost (Wilkinson 1990). The males in the harem have bonds, but not as strong since resident males emigrate rather often (Wilkinson 2001). It is not only male bats in the harem that display cooperative behavior, but also bachelor males (Delpietro & Russo 2002). Bachelor males cooperate with bats in the harem under certain circumstances (Delpietro & Russo 2002).

Bachelor males and bats in the harem will congregate during times when the temperature is really low. The huddling of the bats is a mechanism for thermoregulation, called social thermoregulation. The common vampire bat does not hibernate during the winter, thus it has to find avenues around the cold temperatures it faces during the winter. When the temperature drops below eleven degrees Celsius, the bachelor males are allowed into the harem so that all the bats can aggregate together and keep warm. This event is amazing, considering harem males and bachelor males are very aggressive toward each other under any other circumstances (Delpietro & Russo 2002).

Female vampire bats in a roost cooperate exclusively with each other (Wilkinson 1990). The females exhibit a type of allomothering by feeding each other’s young (Wilkinson 1990). A lactating female in a roost will feed pups that have lost their mothers as well as those whose mother is still alive (Wilkinson 1985). This action is probably a mechanism that has evolved to keep young from starving, since they are fed by regurgitation, thus may not get enough to eat (Delpietro and Russo 2002). Also, this behavior seems to be a way for females to help each other with the burden of raising young (Delpietro and Russo 2002).
Social grooming is another activity that common vampire bats partake in (Wilkinson 2001). These bats spend a great deal of time cleaning each other. The grooming of one bat by another is called directional grooming (Wilkinson 1986). Mutual grooming occurs when two bats groom each other simultaneously (Wilkinson 1986). This practice is believed to serve more than the obvious cleaning purpose. The common vampire bat is believed to use social grooming as a way to create and strengthen bonds between bats that share blood (Wilkinson 1986). Grooming rate is dependent on both relatedness and association (Wilkinson 1986). Mothers groom their offspring more than they groom any other bat (Wilkinson 1986). The grooming by the mother promotes recognition between her and her pup (Wilkinson 1986). The strong bonds influenced by social grooming directly correlates to which bats share blood with each other (Wilkinson 1988). Common vampire bats that groom each other also share food with each other (Wilkinson 1988). Therefore it appears that social grooming may be a way for bats to assess if a bat begging for food is starving or well fed (Wilkinson 1986). The bats examine the shape of the bat they are grooming (Wilkinson 1986). If a bat has eaten, it will be noticeably swollen from the excessive amount of blood it has eaten (Altringham 1998). The grooming bat assesses the swollenness of the other bat while it is grooming it; hence it knows if the bat is really in need of food (Wilkinson 1986).

Food sharing is the donating of blood to another bat (Wilkinson 1990). Food sharing entails the regurgitation of blood by one bat to another bat (Lord 1993). Seven percent of bats will be unsuccessful in feeding on a given night; consequently they solicit blood from a roost-mate (Wilkinson 1990; Wilkinson 2001). Foraging requires precise techniques, such as not inflicting pain when biting an animal, which takes a long time to perfect (Wilkinson 1990). Thus, starvation is a large problem for vampire bats, thus this behavior is a means for vampire bats to fight off starvation (DeNault & McFarlane 1995). The common vampire bat cannot go more than two nights without food before it will die (Wilkinson 1990). After sixty hours the bat loses 25 percent of its body weight, and dies because it can no longer maintain the proper body temperature (Wilkinson 1990).

Although females have been the focus of most studies, resident males also participate in food sharing (DeNault & McFarlane 1995). Female bats share with their offspring the most, however they also share with roost-mates that are unrelated (Wilkinson 1990). The females in a roost are not all closely related, as evidenced by the average genetic relatedness coefficient of .11 (Matsuda 1989). Hence, this behavior is reciprocal altruism (Matsuda 1989). For reciprocal altruism in males that share food, the dominant behavior has to be repressed (DeNault & McFarlane 1995). Common vampire bats share with specific bats that they have relationships with (Wilkinson 1990; Wilkinson 2001). They will not share with a bat unless they have roosted together for more than sixty percent of the time (Wilkinson 2001). Also, the role of donor and recipient changes depending on which bat is starving, which creates reciprocity (Wilkinson 2001). One bat may donate blood one night and two nights later receive blood from the bat it donated blood to two nights prior. This behavior is altruistic because the amount of time in between feedings that the donating bat loses by donating blood is less than the amount of time gained before the receiving bat goes into starvation (Wilkinson 1990). Up to twelve hours is added to the time before the recipient bat will go into starvation (Wilkinson 1990).
Natural selection favors food sharing in vampire bats. The mortality rate of common vampire bats that did not share food was computed using a computer simulation. It was found that if bats did not share food, their mortality rate would be 82 percent. However, the mortality rate for common vampires is 24 percent, thus showing that food sharing is more beneficial (Wilkinson 1990).

Common Vampire Bat Homepage--General Overview--Mating System--

Feeding Behavior--Cooperative Behavior--References