The vampire bat (Demodus rotundus) is a small bat, no bigger than a plum, that “removes a small patch of flesh [from its prey] with its razor-sharp incisors and laps up the blood flowing from the wound. A vampire’s saliva has anticoagulant to keep the blood from clotting. After one bat has drunk its fell, another continues at the same spot.”
Hunting for food is hard and up to one-third of bats miss a meal on any given night.
Bats die after 60 hours without food, after freezing from dropping 25% of its weight.
Life as a vampire is hard. Bats are warm-blooded and, without feathers or fur, lose lots of heat. Their requirements for energy are huge. A vampire bat consumes 50 to 100 percent of its weight in each meal. Yet up to one-third of the bats may not obtain a meal on any given night.
Going without a meal is dangerous. A vampire dies after sixty hours without food because by then its weight has dropped 25 percent, and it can no longer maintain its critical body temperature. To survive, vampire bats have developed an elaborate buddy system for sharing meals. The sharing takes place between mother and pup, as well as between adults.
The dominant male bat fathers half the group’s young by staying and guarding the inner top of the nest.
Subordinate males hang out by the entrance.
Photo credit: Alex Hyde/NPL (https://www.nature.com/articles/d41586-019-03005-5)
One study of vampires on a ranch in Costa Rica focused on a population divided into three groups of a dozen females. The members of a group often stay together for a long time, twelve years in some cases, and get to know one another very well. The group of a dozen adult bats is a family unit from a vampire’s standpoint.
Most of the group consists of females, each of whom usually cares for one pup. A female pup stays in the group as she matures, whereas a male pup leaves. The females in a group span several generations. Group membership is not entirely static, however. A new female joins the group every two years, so at any time the females in the group belong to several lineages, called matrilines.
The bats live in the hollows of trees. Imagine a hollow tree with an opening at its base and a long vertical chamber reaching up into the tree trunk. The females congregate at the top of the chamber. About three males hang out, so to speak, in the tree hollow.
One male assumes a position near the top of the chamber, nearest to the females, and defends this location against aggressive encounters from other males. This dominant male fathers about half of the group’s young. Subordinate males take up stations near the base of the tree by the entrance. Other males are out of luck, roosting alone or in small male-only groups rarely visited by females.
70% of food transfers are mother to pup.
The other 30% are female to someone else’s pup, female to female, and male to pup.
Photo credit (C) Gerry Carter
The food is transferred by one bat regurgitating into the mouth of another. Most (70 percent) of the food transfers are from a mother to her pup. This food-sharing supplements the mother’s lactation [milk production]. The other 30 percent involves adult females feeding young other than their own, adult females feeding other adult females, and on rare occasions, adult males feeding offspring.
Increased cooperation in bats includes same-sex grooming and food sharing.
Some adult females have a “special friendship” with females who are not their kin (males also have same-sex relations; see Roughgarden 141). This bond is brought about in part by social grooming. The bats expend 5 percent of each day grooming and licking one another. Some of this grooming is between special friends, and the remaining among kin. A hungry bat grooms one who has recently fed to invite a donation of food. To solicit food, a hungry bat licks a donor on her wing and then licks her lips. The donor may then offer food.
The mutual assistance is significant. If they didn’t help each other, the annual mortality of vampires would be about 80 percent, based on the chance of missing a meal two nights ina row. Instead, the annual mortality rate is around 25 percent because food-sharing tides bats through their bad nights.
References
G. Wilkonson, 1990, Food sharing in vampire bats, Scientific American (February), 76-82.
Roughgarden uses “cooperation” here in the wider sense to include both helping and not hurting. See R. Trivers, 1984, Social Evolution, Benjamin-Cummings.
Roughgarden, J. (2013) Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press, Berkeley. p. 61-2.