Social Organization: Vampire bats social organization can be classified as a multi male/multi female group, with the primary social unit being the female group consisting of 8-12 adult females and their dependent offspring. Each group of females is loyal to a set of 2-5 diurnal roosts and splinters off into 2-4 female (with young) subgroups whose composition fluctuates. (Wilkinson 85a, 88, 90) Thus vampires are also a fusion-fission group.

Female groups consist of several matrilines due to the immigration of, on average, one new female per group per 2 years (though admission into the group is not guaranteed), therefore cannot be classified as straight kin groups. Matrilines are established by female recruitment into the natal group and male dispersal. (Wilkinson 85a,b, 88, 90)

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Males hang out in one of three methods. If they are strong enough they fight to hold the alpha spot, which is right next to where the females roost, in the top of the tree. They are not actually in the group, however - there is no physical contact with the females, who are often aggressive towards the top male. (Wilkinson 85b, 88, Park 91) If the tree holds many females, there may be a beta and even gamma male residing in the same tree, the gamma male being at the lowest elevation and thus farthest from the females.
These males defend these small territories with the hopes of gaining mating access. (Wilkinson 84, 85a,b, 88, 90). Weaker males occupy trees rarely frequented by females (or not at all), either solitarily or in small bachelor groups of 2-3 males (Wilkinson 85b, Park 1991). These are the visiting males that sometimes sneak copulations while the alpha is out hunting.

Aggression: as Turner's description of aggressive behavior fits our model of an agonistic encounter:

~Assesment/threat: the attacker maintains a lower body posture (more stable), and bristles its neck hairs: a bluff to make itself appear bigger and therefore have more Recourse Holding Power.

~Testing/ritualized fighting: The two bats push at each other side to side, though if the fight intensifies they will knock a wing to the ground or drum them both rapidly. This is an example of displacing aggression, to show what an aggressor is capable of in an attempt to scare off the other.

~Intense fight: Turner calls this the "real attack," with the attacker using a frontal approach to lunge at its opponent, drumming its wings, scratching, and vocalizing loudly. Biting is described as rare and only done by the subordinate animal if cornered, however Wilkinson found otherwise: see below.

male:male:
Fighting for access to females. More specifically, fighting over territory that is close to females, not the females themselves. Fights are often vicious and can result in death. Wilkinson observed a bloody fight between a visiting male and a resident alpha, which ended with the intruder replacing the resident as top male and the loser moving to a tree not populated by females. Almost all adult males caught had wounds or scars from being bitten, which contradicts Turner's observation that intraspecific biting was rare (Wilkinson 1988, Turner 1975).

Male:female:
Aggression toward the female during mating.

Female:male:
Aggression toward the male to keep him out of the female social group or to avoid mating.

Aggression was also noted at times on prey by Turner, Greenhall, and Wilkinson, between females, between unmarked bats, or between males and yearlings. The vampire in "possession" of the animal would fly at the other, vocalizing loudly and aggressively, like the vocalizations when defending a roost territory. Wilkinson also noted aggression between females within a colony but could not yet specify a reason.

In contrast to the brutal fights of the common vampire, D. youngi exhibits only ritualized, "stylized" fighting to settle disputes (Schutt et al. 1999)

Females maintain their bonds through roosting together, grooming, and foodsharing. Wilkinson (1985a) evaluated 8 hypotheses as to why female bonds are so strong - why they roost together:
1: To share an appropriate microclimate
2: Predator avoidance
3: Avoid ectoparasite infestations
4: Minimize distance to prey
5: Fend off aggressive males
6: Feed from the same bite simultaneously
7: Reduce ectoparasites through grooming
8: Share food through regurgitation
He found that of these variables, only the last one, that females roost together to facilitate reciprocity, sufficiently effects long-term association.

Older females formed more associations, and though each females did not form associations with all other females, all females formed at least some associations, averaging 5.7 nonrandom associations per female.
Grooming: (Wilkinson 1986)

Wilkinson observed and compared three types (self, directional/allo, and mutual) to see whether allogrooming, like food sharing, is more dependent on relatedness or association in who grooms whom. It was not quite the kin-selection vs reciprocity argument because there is no evidence there is a cost (to reproductive success or survival) to the donor or direct benefit to the recipient. An indirect benefit, like in foodsharing, was apparent; grooming influences a partner’s future behavior through regurgitation.

Self: Bats as young as one month spend most of their waking day self-grooming, which is their primary form of ectoparasite removal. It occurs about 10 times as often as either directional or mutual grooming. Wilkinson found more parasitic flies on bats from trees where self-grooming was most frequent.

Directional: Defined as one bat grooming another. This was more common among juveniles and females that were either relatives or frequent roostmates than would be expected by chance. When mother-young pairs were excluded from analysis (in that case, grooming is considered to be a form of parental care), the effect of association in grooming became more important, while relatedness decreased in significance. Amount of directional grooming was independent of ectoparasite levels and roost location but not sex, as males were only infrequently observed engaged in allogrooming. Park (1991) observed that as captive males grew up they more often formed social contacts with non-alpha males than within the group, until they were completely excluded and dispersed.

Mutual: Two bats simultaneously grooming each other; an offshoot of directional grooming. Time spent roosting together is as significant as relatedness in determining if two bats will groom socially.

Because directional grooming appears to have no significant impact on ectoparasite levels, it must serve a social purpose. Bats that receive have higher rates of directional grooming (towards the donor bat) in the 2 minutes before regurgitation but not in the 2 minutes after, suggesting that grooming is an important social precursor to securing one. Not only does grooming facilitate individual recognition (through olfaction, which is said to rival that of fruit bats and humans, and contact calls as described by Schmidt (1972), it allows bats to gauge how recently a potential donor and recipient have fed, through stomach distention. Most grooming before a donation was directed under the wings, near the stomach.

By allowing individual recognition, grooming fulfills one of the conditions on reciprocity to be evolutionarily stable: that donors can recognize and therefore not aid cheaters. Wilkinson observed frequent female fights within the group studied but was unable to determine if the attacked bat was a possible cheater.

Because bats begin self-grooming at one month yet there are still high levels of mother-offspring directional grooming (considered a form of parental care), there must be an explanation other than parasite removal. Most likely, mothers groom their young to aid in recognition. Their olfaction is good enough for the potential to identify through saliva marking (Vampires have no scent glands).

Calls: Schmidt described social calls within vampire bats for the first time in 1972, albeit only studying two captive juveniles (a male and female) and their mothers. Mother and offspring must be able to communicate with each other, especially to maintain contact when the mother cannot directly watch her offspring (e.g., as they fly together in search of prey) and to distinguish each other from the rest of the colony. It maintains a strong bond between the two.

He described three types of juvenile calls: separation calls, recognition calls, and contact calls. Upon hearing separation calls, the mother finds and takes her offspring under her wing and licks it. Recognition calls are given after separation calls, when the offspring recognizes its mother's calls but cannot be reunited with her immediately. Contact calls are very soft, variable "peeps" and are given when there is body-contact with the mother, a response to the mother's grooming.

Mother vampire bats rarely give calls audible to the human ear but Schmidt described two in his 1972 study: Stimmfühlungslauten and Kontaktlauten (contact calls). Stimmfühlungslauten (literally, "true-feeling calls") are the mother's response to the juvenile's separation calls (which lead to the juvenile's recognition calls, upon hearing the mother). Contact calls are given when reunited with her offspring and are very variable, effecting individual recognition.

Please note this is an undergraduate website created for Animal Behavior 323 at Davidson, College, NC, by Julie Perry. Questions, comments, suggestions, corrections: please email Juperry@davidson.edu

© Copyright 2002 Department of Biology, Davidson College, Davidson, NC 28035 - - - - - - - - - - - - - - - - - - - - - - Last updated: 19 April 2002