Rachel Crisp is currently writing her masters thesis on dominance interactions in female vampire bats. Males have a clear dominance hierarchy in competition over roost territories, but do the female vampires have a dominance rank? If so, does it strongly correlate with cooperative interactions? Her work should come out next year!
Despite all the work on cooperation, not much is known or published about competitive interactions in female vampires. Interestingly, female vampire bats are larger than males, but there is no evidence for a strong female reproductive skew as in cooperative breeding societies. The little bit on competitive interactions that is available is in German (work in the 1970s supervised by Uwe Schmidt) or Korean (by Shi-Ryoung Park). Rachel Moon graciously translated this 1988 paper by Park from Korean to English.
For easy accessibility, here it is below.
Dominance relationships in a colony of vampire bat, Desmodus rotundus
Korea National Univ. of Education, Chongwon (Korea R.). Dept. of Biology
Translated by Rachel Moon, Harvard University
Originally published as Park, S. R. (1988). Dominance relationship in a colony of vampire bat, Desmodus rotundus. The Korean Journal of Zoology. 31 (4): 243-250.
Dominance relationship was investigated in a captive of Desmodus rotundus, a neotropical sangivorous bat, under seminaturalistic conditions. The hierarchy was determined from four different behaviors (flee, fly-out, avoid, wait) by the encounter of two adult bats on the feeding site. The aggressive action “flee after fighting” was relatively low (16%) compared to the other three observed behaviors. A hierarchy of the females was reflected sometimes in the feeding order. The harem male dominated the non-harem males and exhibited his territorial behavior. However, to his female partners he didn’t show aggression.
Except for a few species, most bat species (Chiroptera) live socially (Gopalakrishna, 1955; Eisentraut, 1957; Kulzer, 1958; Goodwin and Greenhall, 1961; Barbour and Davis, 1969). Bats exhibit complex social structures and diverse kinds of social behavior (Bradbury, 1977). However, the functional significance of bat social structure and social behavior is still not well studied; therefore, this study aims to understand the significance of complex social behavior of bats by studying dominance relationship in a group of South American neotropical vampire bats (Desmodus rotundus).
Vampire bats (Desmodus rotundus) have developed and adopted unique behaviors and physiological structures that enable them to feed on the blood of vertebrate animals. Vampire bats form long-term small colonies with highly developed social structures (Wimsatt, 1969; Schmidt et a., 1978; Wilkinson, 1985). Until now it has been nearly impossible to study dominance relationship structure of vampire bats in natural settings; therefore this study aims to investigate dominance relationship in a group of vampire bats in experimental settings that resemble naturalistic conditions.
MATERIALS AND METHODS
This study was conducted with 9 vampire bats (Desmodus rotundus; three males, six females). Only one male and one female were captured in the wild (Colombia, 1975), and others were born and raised in captivity. The bats were kept in a thermostatic chamber (with 12h/12h of light and dark) at a temperature of 25C and 70% humidity. They were provided cow or pig blood (with removed fibrin) on a bird water feeder. In order to distinguish each individual from another, I marked each bat by attaching aluminum loop of different colors on its forearm (Table 1).
The observation cage had a size of 250 x 180 x 100 cm, and its left side, right side, back side and ceiling were made with thin plastic surface with small holes so that the bats could hang easily. The front center part of the room was made with transparent glass (100 * 50 cm) so that the experimenter can observe easily.
In order to observe the bats’ behaviors, an infrared night vision scope (metascope 9902E) was used, during early hours (17:30-20:30) of the 12 dark hours. This study emphasized an understanding the quantitative aspects of behavior observed in a previous study of vampire bat social relationship (Schmidt and Manske, 1973). In other words, I recorded and analyzed frequency and duration of observed behaviors, distribution of behaviors, and subsequent behaviors.
The social behavior of bats takes place at a feeding site, where they often encounter one another. During the initial period of feeding time, aggressive physical fights were often observed. This behavior usually begins with two bats pushing and shoving each other. If the opponent does not back off immediately, this aggressive physical fight occasionally turns into a fierce combat. To first determine the feeding dominance relationship of bats in the colony, we investigated the feeding times of individual bats, as seen in Figure 1.
D (female) always feeds first; this female kept her place until she was full, even though other younger bats who were feeding at the same time often obstructed her by physical fight of pushing and shoving aside (Figure 2A). H (female) always showed up at the feeding site after the first feeding activity period, when other bats were mostly not feeding. During this period, H often encountered her 15-month-old daughter J at the feeding site. J would often push H to the side, although J had several unoccupied feeders nearby. Then J would wait right behind her mother H until H finishes feeding and returns (Figure 2B). As soon as H left the feeding site, J started to feed at the same spot H was feeding.
However, not all of the other colony members ate their meals following such straightforward feeding order. Depending on each feeding site situations (whether bats are at the feeding site or not, and if so which bats are present), the bats visited the feeding site at irregular intervals. Therefore, it was difficult to determine group dominance relationship solely based on feeding order. Because the bats frequently encountered different individuals at the feeding site, it was possible to observe several types of social behavior. I chose four different types of social behavior exhibited by two encountering bats as the main parameter to determine bats’ dominance hierarchy.
- Flee (after fight): After two adult bats meet and fight at feeding site, the winner bat continues feeding or starts to feed, whereas the loser bat flees away. For fighting behaviors, I observed shoving and pushing to the side, and attacking behavior.
- Fly-out: It is a behavior shown when the inferior bat immediately flies away without showing any aggressive behavior when two bats encounter at the feeding site.
- Avoid: When an inferior bat is feeding and another bat approaches, the inferior bat gives up its meal and moves to a different feeding site. The inferior bat feeds at a new feeding site or stands for awhile at a distance.
- Wait: The inferior bat waits at a nearby place, or goes back and forth hesitantly at a different place, until the superior bat finishes feeding and leaves.
Fleeing is exhibited after aggressive behavior, whereas fly-out, avoid, and wait behaviors are “direct-flight” behaviors performed by inferior bats themselves in order to avoid potential conflicts; therefore, these three types of behavior are characterized as unaggressive or submissive behavior.
A total of 294 encounters were observed at the feeding site, and the frequency of each behavior type is shown in Table 2. Aggressive behavior accounted for 15.6% of all observed behaviors, which shows that it happened relatively less frequently than unaggressive behavior (Figure 3). Aggressive behaviors between male bats at the feeding site rarely took place; the most common behavior between two encountering male bats was “fly-out” behavior, which accounted for half of all behaviors exhibited.
In order to determine dominance relationship, I used a point system in which the superior (winner) bat earns 1 point when superior bat and inferior (loser) bat are distinguished. The score difference between two bats is shown in Table 3 by comparing the scores of each bat. Using this result to build a sociogram of dominance hierarchy at the feeding site, the following phenomena in each group could be explained.
Dominance relationship among harem females (Figure 4)
When encounters took place between two females from five harem females, the most common social behaviors were “wait” and “avoid.” D was the most superior dominant bat among five harem females. B, who had the lowest dominance hierarchy ranking, exhibited “wait” and “avoid” behavior as a response to D’s behaviors. E had a middle dominance hierarchy ranking, which was higher than inferior bats C and B and lower than superior bats D and H. E responded by “fly-out” behavior all three times when she encountered H. Although I never observed direct encounters of H and D at a feeding site (their feeding period almost never overlapped), I concluded that D had a higher dominance hierarchy ranking than H depending on feeding ranking.
Dominance relationship among males (Figure 5)
When two out of three male bats (G, A, F) encounter at the feeding site, the inferior bat usually flees away. Thus a clear dominance hierarchy was revealed among these three males. The most superior bat was G; whenever A or F appeared at the feeding site, G forced them to leave the site. When G approached, the inferior bats rushed their feeding. The most inferior bat was F; whenever F was feeding, I often observed him looking around his surroundings and rushing his feeding. Similarly, A also exhibited typical feeding behavior of inferior bats, which was carefully approaching feeding sites, quickly feeding non-stop, and leaving in a hurry.
Out of 31 encounters between G and A at the feeding site, A showed “fly-out” response 26 times; of 42 encounters between G and F, F responded by flying out 32 times. Out of 12 encounters between A and F, F exhibited “fly-out” behavior in all encounters. Therefore it was determined that the most superior bat was G, the most inferior bat was F, and A was in the middle.
Dominance relationship among male bats and harem females (Figure 6)
At the feeding site, G (male) always showed passive behavior towards old harem females who have previously given birth to pups. When these females were feeding at the feeding site, G responded by exhibiting “wait” behavior 30-40 cm away from the site until the females finished feeding. I observed six encounters between G and B (female who hasn’t yet given birth), and four out of six times B showed submissive behavior (“fly-out”) towards G.
A (male) showed dominance towards B and C (females); however, the number of encounter between A and D or H (females) was too small to clearly determine their dominance hierarchy.
F (male) was certainly superior than B and C (females), who had low dominance hierarchy rankings within harem females. However, he ended up getting an inferior status after frequent pecking-order disputes with other harem females. His dominance hierarchy ranking was determined by his fights (“fleeing”) with other females, which were 19 times (73%) out of 26 encounters.
Vampire bats (Desmodus rotundus) form small groups when they go out to feed in the wild. Greenhall et al. (1969) observed behaviors such as combats, waiting, and co-feeding when vampire bats were feeding in groups in the wild. Furthermore, even in captivity the vampire bats often engage in pecking order disputes at feeding sites; therefore, the social structure based on dominance hierarchy seems to be a prominent characteristic of social behavior of vampire bats (Schmidt and Van de Flierdt, 1973). Vampire bats maintain close relationships through various social interactions at feeding sites, such as combats and submissive behaviors. According to the results of this study on a captive group of vampire bats, aggressive behavior (16%) took place less frequently than unaggressive behavior (submissive behavior), and most of the bats showed defensive or submissive behaviors (wait, fly-out, avoid) at the feeding site.
Park (1986) reported that aggressive behaviors are usually triggered by subadult bats, showing a strong tendency to claim high dominance status. Park observed that bats who are 14-16 months old often engaged more in dominance ranking fights compared to adult bats. The results of this study also revealed that group members abide by dominance hierarchy through a single consistent order when feeding and that aggressive behaviors between adult bats at feeding sites did not occur as often. Thus, it can be thought that vampire bats have especially well-developed mechanism of group formation, which serves to restrain mutual aggression by strengthening the cohesiveness among group members.
Vampire bats form a typical harem social structure (Load, 1976). Such social structure has been observed in other bat species such as Pipistrellus nanus (O’Shea, 1980), Phylolostomus hastatus (MaCreacken and Bradbury, 1981), Carollia perspicillata (Porter, 1979), and Artibeus jamaicensis (Morrison, 1978); the male bats in these species also form a single distinct dominance hierarchy, which is observed by ritualized combats. Park (1986) found out that alpha male (harem male) vampire bats stayed with females for a long period of time, whereas beta males stayed at a place where females use as their temporary shelter. On the contrary, gamma males stayed far away from female groups and did not exhibit any territorial behavior. Connecting to the results of this study, it seems that hierarchy system of male vampire bats is associated with territoriality. In the context of dominance behavior, Wicker and Uhrig (1969) observed Lavia frons engaging in a territorial fight with neighboring bats, and Bradbury and Emmons (1974) witnessed Saccopteryx leptura driving out outsider bats from its hunting territory. Several other bat species (Vespertilionidae: Dwyer, 1970; Brosset, 1976; Phyllostomatidae: Fenton and Kunz, 1977; Porter, 1979) are known to defend their harems and exhibit territorial behavior towards intruders and competitors.
Sex and age of bats are significant factors in formation of dominance hierarchy. Usually, older animals show dominance over younger animals, and males show dominance over females (Immelmann, 1983). This study reveals that alpha males (harem males) did not exhibit absolute dominance over females; on the other hand, harem males exhibited inferior behaviors toward their female partners. It is not easy to declare dominance relationship based on age only with the results of this study. Whether juvenile bats depend on dominance hierarchy rankings of their mother bats require further investigation.