Reciprocal Altruism: Vampire bats are one of the rare mammals that exhibit altruism, in this case sharing food at cost with recipient bats with no directly apparent benefits. Wilkinson studied food sharing in D. rotundus extensively to determine if altruistic behavior was only between relatives (and thus kin selection) or occurred between bats on a relatively equal basis (and thus reciprocity).
The
five criteria to demonstrate reciprocity vs kin selection
(Wilkinson 1990):
1: Females associate for long periods, so there are numerous chances
to be either the donor or recipient bat.
2: The likelihood of regurgitation to roostmates can be predicted
on the basis of past associations.
3: The roles of donor and recipient reverse often.
4: The short-term benefits to the recipient outweigh the costs
to the donor.
5: Donors can recognize and expel cheaters to this system.
The basics: Vampire bats will starve after 60 bloodless hours, losing as much as 25% of their bodyweight, making them unable to maintain a critical body temperature. They need to eat 50-100 % of their body weight in blood every night. Blood is preferentially donated to bats in critical need (those that would reach minimum weight within 24 hours) within a given roost; if a bat has more than 24 hours until starvation it will usually not be fed. Males in critical need, however, will still not be fed. Only age has been shown to influence feeding success, with bats under 2 years of age failing 33% of the time, while adult bats only fail 7% of the time. Juveniles fail more often because they are still learning how to inflict painless bites and often are shaken from their prey. A computer model shows that the annual mortality for adults, if 7% fail to feed per night and failure to feed 2 nights in a row leads to death, should be 82%. Actual adult mortality is only 24%, therefore foodsharing must be favored by natural selection (Wilkinson 1984, 1990).
Female bats begin sharing blood at 8 months of age. The frequency of donation is independent of age and reproductive condition. Adult females regurgitate to their young, other young, and each other. In the wild Wilkinson observed mothers feeding blood to their infants minutes after birth, perhaps to inoculate the digestive tract with the right bacteria. They supplement milk with blood to young older than 3 months and share blood after weaning (7 months). Such behavior in mammals is usually limited to those where a single female reproduces (kin selection), but vampires are an exception. (Wilkinson 85a, 88, 90).
Answers to the criteria:
1: The primary social unit is the female
group, composed of 8-12 adults with dependent young which move between
2-5 different day roosts. At least two females have been observed roosting
together over a 12 year time period. Therefore female bats meet the first
criteria, with many opportunities for foodsharing.
2: Of 110 observed regurgitations,
70% were between mother and dependent pups and was thus considered parental
care. The rest were between adult females feeding young other than
their own, feeding other adult females, and in two rare instances, adult
males feeding offspring. Degree of relatedness between sharing bats was
determined, and it was found that vampires share more with individuals
who are usually roostmates and often, though not always, relatives. This
finding supports both theories, though the effect of relatedness could
be secondary to that of association due to the matrilines present in the
groups. It has also been observed that only bats with prior associations
will regurgitate to each other.
3: It was found that unrelated bats
often formed a buddy system, with two individuals regurgitating mostly
to each other, indicating that roles often reverse.
4: Vampires lose weight exponentially
when they fast. Therefore the same weight of blood costs the donor less
time than the recipient bat gains. See graph at upper left.
5: For this system to work, bats must not be able to get away with taking but not reciprocating. It has not yet been proved that cheaters exist or are excluded, but bats are capable of individual recognition.
By donating, a bat indirectly benefits because it is more likely to obtain a regurgitation from bats to which it has previously donated.
Because vampires live in small groups and have high probabilities for giving and receiving aid, reciprocity is most likely more beneficial than kin selection, especially because groups of high association are not exclusively kin. (again, several matrilines are present).
Individual recognition: Schmidt found that mothers and young recognize each other through a series of calls. All bats have distinct contact calls they give when grooming and touching each other. (Schmidt 1972). Social grooming therefore aids in recognition of roostmates through these calls and through olfaction. Grooming behavior seems to be the precursor to foodsharing, with hungry bats grooming potential donors (see top picture) more often than expected. Grooming, like blood sharing, occurs more often between close relatives or bats with strong associations. Through grooming bats can also determine, based on stomach extension, how recently a potential donor has fed (Wilkinson 1986).
Regurgitation in captive male vampire bats: Until DeNault and Donald's 1995 study, reciprocal altruism had only been observed and discussed in female vampire bats, because it is the female social organization that cultivates it. Male social organization violates two of Trivers' criteria for the evolution of reciprocal altruism: low dispersal rates (which would maximize reciprocating opportunities) and weak dominance hierarchies (which would minimize aggressive, counter-altruistic behavior).
This study was conducted on a captive D. rotundus colony and reported that males had a "very low-intensity, non-linear dominance relationship." Non-aggressive male-male and male-female behavior was recorded (such as grooming), much more than previously reported, and reciprocal altruism between adult males of low relatedness was fairly common. True to Wilkinson's previous studies, there was no significant correlation between relatedness and foodsharing within the colony; bats operate more by associations.
It was suggested that this previously unheard-of male reciprocity was due to the fact that the male bats were "captive, socially acclimatized… in non-crowded conditions." Another possibility is because there were twice as many females in the study, there was not as much competition for mating rights.
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
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