Wilkinson on vampire bats (small) - Sci Amer 1990

Wilkinson on vampire bats (small) - Sci Amer 1990 - Food...

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Unformatted text preview: Food Sharing in Vampire Bats Two nights Without a blood meal and a vampire bat starves to death—unless it can solicit food from a roostmate. A buddy system ensures that food distribution among the bats is equitable t night-“long after most visual predators have stopped prowl- ing-—vampire bats emerge from their roosts and take to the wing. flying low across the landscape in search of warm-blooded prey. Within an hour or two. having found appro- priate victims and fed on their blood. the bats return to the roost to sleep. feed their young and interact with nestmates. Until recently little was known about either the behavior or the life history of the common vampire bat. Desmodus rotundus. For many years biologists were more interested in the animal's physiology than in its social organization. which was thought to be relatively simple. A number of recent studies. however. reveal that vampire bats are remarkably social: females cluster together during the day but at night reassort themselves. creating a fluid social organization that is main- tained for many years. Moreover. it is now known that long-term associav tions among females enable bats to regurgitate blood to one another on a regular basis and so significantly in- crease their chances of survival. The reason for regurgitation behav- ior was revealed in studies carried out more than 15 years ago by Brian K. McNab of the University of Florida. who showed that a vampire bat will die if it fails to feed for two nights in a row. After 60 hours without food it GERALD S. WILKINSON has spent more than 10 years studying the social behav- ior of bats. having first become interest- ed in them while taking a field course in Costa Rica in 1978. Since then he has studied bats in South America. Africa. Australia. Southeast Asia and the US. Wilkinson received his 3.3. from the University of California. Davis. in 1977 and his Ph.D. in biology from the Univer- sity of California. San Diego. in 1984. He has been assistant professor of zoology at the University of Maryland at College Park since 1987. Gr by Gerald S. Wilkinson loses as much as 25 percent of its weight and can no longer maintain a critical body temperature. To fuel the body's metabolic engine and avoid death. individuals must consume 50— sometimes even loo—percent of their body weight in blood every night. Yet feeding is not always easy. espe- cially for young bats. who must learn to bite quickly and to do so without inflicting pain on their victims. I have seen horses toss their heads. swish their tails and rub against obstacles to rid themselves of hungry bats. Al- though the bats counter such defen- sive strategies by returning to the same animal (a known target) several nights in succession or by feeding sequentially from fresh wounds. from 7 to 30 percent of the bats in a cluster fail to obtain a blood meal on any given night. By soliciting food from a roostmate. a bat can fend off starva- tion—at least for one more night—and so have another chance to find a meal. in 1978 Uwe Schmidt. a zoologist at the University of Bonn. presented the first evidence that females regur- gitate blood to their pups. At that time Schmidt had kept bats for more than 10 years in a turret at Poppels- dorfer Schloss. an old castle that is now the main research building of the university's Zoological Institute. and had spent much of his career observ- ing their behavior. Schmidt discov— ered. for example. that shortly after birth. the pups are given regurgitated blood—in addition to milk—by their mothers; he also found that on some occasions a pup will take blood from an adult other than its mother. In one case he even observed an orphaned pup who was being suckled by an adoptive parent. Food sharing of this sort. in which individuals provision other members of a group. inextreme- ly rare in mammals; in addition to bats. only a few species—such as wild dogs. hyenas. chimpanzees and hu— man beings—are known to display such behavior. Food sharing appears to be altruis- tic: a donor bat gives up food—-which might otherwise be used to ensure either its own survival or the survival of its offspring—to a recipient bat. whose chances of survival are thereby increased at no apparent cost to itself. Yet true altruism has never been docu- mented in nonhuman animals. pre- sumably because such a one-way sys- tem is not evolutionarily stable. The reason is that donors. who lose re- sources. are eventually outcompeted by recipients. who have more resour- ces and so survive longer. produce more offspring and pass more of their genes on to the next generation. Care- ful studies of altruistic behavior by a number of investigators reveal that many acts of apparent altruism actu- ally take place either between relatives (and so are a form of kin selection) or between individuals who exchange re- sources on a more or less equal basis, in which case they can be considered to be acts of reciprocal altruism. or reciprocity. aving heard of Schmidt's work, i wanted to study vampire bats in their natural habitat to see whether blood regurgitation is an act of kin selection or reciprocity (or both). i set off for Costa Rica. and there. with the help of my assistants Robin Weiss. Michael L. Jones and Ter- ri Lamp. l studied a population of Desmodus rotundus for 26 months be- tween 1978 and 1983. l hoped by observing their regur- gitation behavior to see if the bats were feeding only their relatives. and COMMON VAMPIRE BAT. Desmodus ro- tundus. is found from Mexico south to Argentina and Chile. particularly in are- as where the land has been converted to pasture. Females. such as those shown here. gather together in caves and hol- low trees during the day. emerging only at night to search for warm-blooded prey. theretore engaged in km selection. or it they were reciprocth exchanging tood (with either related or nonrelated individualsl. and thus engaged in recl- prot'ity. In order to prm e reciprocity I needed to demonstrate that Joe crite- ria were helm: met: that females assoi ciate for long periods. so that each one has a large but unpredictable number of opportunities to engage in blood sharing; that the likelihood ol‘ an in- dividual regurgitating to a roostmate can be predicted on the basis of their past association; that the roles of do- nor and recipient frequently reverse; that the short-term benefits to the recipient are greater than the costs to the donor; and that donors are able to recognize and expel cheaters from the system. amptre bats. which are com- mon in tropical America wher- ever land has been converted to pasture and livestock are present. are ideal subjects for a study of this na- ture. For my research site I selected a cattle ranch in northwestern Costa Rica called Hacienda La Pacifica (which has since been turned into an ecologi- cal research station and is now called ('cntro L'cologit a La l’acil'ical. 'l'here l lountl that vampire bats. in the absence ol' (EHL‘H, spend the day in hollow trees. where temperatures are constant. the humidity is high and it is dark even during the day. Most of the trees at La l’acilica had a single open- ing at their base: by lying inside the opening and peering upward with the aid of binoculars and a diffuse light source, we could observe the bats for several hours at a time. By gradually increasing the amount of light (over a period of several months). we could habituate the bats to our presence and observe their interactions with one another. The single entrance at the base of the tree offered another ad- vantage: we could stretch a fine-mesh net in front of it and catch the bats when they emerged at night to hunt. In this way we were able to tag them and subsequently quantify individual pat- terns of behavior. We found that the bats emerge in search of prey every night at a time that varies with the phase of the moon: il it is too bright outside. the bats wait until the moon goes down. Unlike the other two species of blood- drinking bats (the white~winged vamA pire bat. Diaemus youngi. and the hairyvlegged vampire bat. IJiphylla ecaudata). which feed mostly on the blood of birds. Desmodus rotundus feeds primarin on mammalian blood. The hat seems to prel‘er horses to cows. and it locates them by a com- bination ol‘ smell. sound and echo- location. Having identified a victim. the bat usually lands on the animal’s tail or mane and hangs from it while search- ing for an appropriate spot to bite. Specialized heat-sensitive cells in the nose help the bat find a place where the victim's blood vessels are near the surface. When such a spot is found, the bat quickly excise-s a small patch of skin with its razor-sharp upper inci- sors and begins feeding. An anticoagu- lant in the bat’s saliva keeps the blood flowing during the 20 to 30 minutes needed to consume a meal. Then the bat. its stomach visibly swollen. re- turns to the roost. where it remains FACIAL FEATURES of the vampire bat reflect its blood-based diet. The enlarged ears help the animal search for prey and navigate by echolocation: the bat emits high-pitched sounds that reflect off objects in its path. and the echoes are picked up by the ears. Smell and heat receptors in the broad. fleshy nose enable the bat to home in on a suitable victim. After finding an appropriate spot to bite. the bat excises a small patch of flesh with its razor-sharp incisors. with the aid of an anticoagu- lant in its saliva. the bat then laps up the blood that flows from the victim's wound. until the next night. As the bats re- turned to their roosting trees just be- fore dawn. we netted them to deter- mine which individuals had success— fully obtained a blood meal. Our first goal on arriving at La Pa- cifica was to tag all the bats in our study area with lightweight bands of different colors; females were band- ed around the left wrist and males around the right. Each band also had a piece of reflective tape affixed to it. which enabled us to spot roosting in- dividuals even when light levels in the COMMON VAMPIRE BATS feed on a variety of mammalian prey but seem to prefer horses and mules to other species. They usually land on the animal's taii or mane. which gives them something to hang onto during the 20 to 30 minutes it takes to drink their fill. The animal is not always cooperative; it may shake its head and swish its tail in an attempt to dislodge the bats. A bat who fails to feed for two nights in a row will die from starvation unless it can solicit regurgitated blood from a roostmate. tree were very low. Once the banding process was completed. we could pick a bat at random and record its behav- ior every 10 seconds. 100 times in succession. By working in pairs, we tagged ($00 bats and accumulated more than 400 hours of such behav- ioral observations. Once a week we did a census of the animals in each tree to document patterns of association among bats occupying the same day- time roost. We also fitted a total of 37 bats with radio transmitters and so were able to determine the extent and degree of overlap of their forag- ing ranges. e found that the social or- ganization of vampire bats is dominated by groups of from eight to 12 adult females and an equal number of pups (one for each fe— male). Pups are born throughout the year at about 10—month intervals; fe- males stay with their mothers. where- as males leave between the ages of 12 and 18 months. when they become reproductively mature. in contrast to some other trepical bat species. in which males defend “harems.” vam- pire bat males defend territories. They form dominance hierarchies within hollow trees. fighting among them- selves for the alpha position near the top of the tree's hollow cavity (where females frequently cluster) and de- fend their territories vigorously— sometimes to the death—against in- truders. The intruders are males who normaily roost alone or in small groups during the day in trees that are rarely visited by females. As we tracked the bats and moni- tored their associations. we were sur- prised to find that their social organi- zation is stable and yet fluid. The bat population in our study area could be subdivided into three groups of about 12 adult females. each of which of- ten subdivided into smaller clusters. Although the three groups were iso- lated from one another. the composi- tion of the individual clusters that made up each group varied continual- ly. Each group had exclusive rights to a range of about six trees. and once or twice a week the females would shift roosts (sometimes carrying their pups with them to another tree). reassorting themselves in the process. Because female pups stay with their mothers past reproductive maturi- ty. several generations are typically found clustered together in one tree. Yet my biochemical analyses of blood sampies suggest that only about 50 percent of the offspring in a cluster share the same father. Presumably this is because females show no loyal— ty to a particular tree (whereas males do) and so are periodically exposed to new males. with whom they some- times mate. in addition. for reasons that are not well understood. females switch groups from time to time (pos- sibly because prey have become diffi- cult to locate); on the average a new female joins a group every two years. As a result. each group consists of several matrilines, within which re- latedness is high but between which it is low. Analysis of roosting associations reveals that adult females exhibit a preference for certain other females. which cannot be explained simply on the basis of some physical feature of the roosting site. Moreover. it seems that their preference is not always for relatives but may be for nonrelatives. a finding consistent with reciprocity theory. Having determined that the bats have an affinity for one another. we needed to answer the following question: Do the bats remain together for long periods and thus have the opportunity to develop and maintain a mutual support system? The answer appears to be yes. Rex- ford D. Lord. now at the lndiana Uni- versity of Pennsylvania, determined the maximum life expectancy of vam- pire bats by counting the annual growth rings of their teeth and found that females can live for as long as 18 years. And from banding studies un- dertaken at La Pacifica by Theodore H. Fleming of the University of Miami in the 1970‘s, we knew that at least two of the females in our study area had roosted together for more than 12 STUDYING VAMPIRE BATS in their natural habitat requires that many hours he spent in an uncomfortable position (left). To observe the hats. which roost in hollow trees (as well as in caves and other dark places). the author and his assistants years. In view of their longevity and the fact that each individual fails to feed periodically. we concluded that the bats meet the first criterion of reciprocity: not only do individuals have the ability to form long. stable relations with one another. but the opportunity to engage in food sharing is ever present among them. or next step was to determine whether blood is regurgitat- ed randomly within a group or whether the females regurgitatc only to close relatives or to prior roost— mates. as predicted by kin selection and reciprocity theory. respectively. To do so. we needed to estimate the frequency of blood sharing under nat- ural conditions. During the course of our five-year study we witnessed a total of 110 instances of blood sharing by regurgi- tation. Seventy percent of the regurgi- tations took place between a mother and her pup and can therefore be thought of as parental care. The re- maining 30 percent. however, involved adult females feeding young other than their own, adult females feeding other adult females and. on two occa- sions, adult males feeding offspring. To determine whether or not bats regurgitate selectively, we compared the degree of relatedness between a recipient and a donor as well as their roost-association index (the propor- tion of times two individuals were seen together in the same cluster) to see if—on either accountw-the regurgi- tation values were higher than if the re- cipient were randomly soliciting from all potential donors in the roost. We SCIENTIFIC AMERICAN February 1990 found as a result that both relatedness and prior association-are important predictors of an individual's response to a solicitation. Our results show that vampire bats do not share blood ran- dome but share preferentially with individuals who are frequent roost- mates and often. but not always. relat- ed. a finding that supports both reci- procity and kin-selection theories. The next step in our study was to test reciprocity experimentally. if re- ciprocal altruism does occur among vampire bats, then one might predict that individuals should aid only those in imminent danger of starvation and should preferentially repay those bats who had previously fed them. To test these predictions. we captured four adult females from our main study area at La Pacifica and four from a secondary study area at Parque Na- tional de Santa Rosa some 50 kilome- ters farther north. We knew from their tags that two of the La Pacifica bats were grandmother and granddaughter (related to one another by one fourth); the others were unrelated but had a high degree of roost association. We initiated our experiment by ha- bituating the hats to captivity and also to being fed nightly from plastic meas- uring bottles. which enabled us to re- cord the amount of blood each bat would ingest at mealtime. Once the bats were at ease in their cages. we selected one each night at random and put it in a separate cage. where it was deprived of food. The next morn- ing we returned the experimental bat to its original cage and observed its interactions with its cagemates. Our results indicate that blood shar- spent from two to six hours a day on their backs. peering upward with a diffuse light. Colored bands placed around the wrists of the bats (right) made it possible to identify indi- viduals and follow their interactions over a five-year period. 79 ing nearly always took place between bars from the same population. Only once did it occur between strangers. Moreover. we found that blood shar- ing was not random. even among bats who had a high degree of prior associ- ation. Instead it appeared that the un- related bats developed a buddy sys- tem. so that two individuals would regurgitate almost exclusively to each other—a strong indication that their roles reverse on a regular basis. Another criterion of reciprocity the- ory is that the cost of donating blood must be small relative to the benefit 0 iii *jii‘ii'l " - liila --ll .II. {ill 7 7 MALE 'l! l :i' it . E. . i II “I . HOLLOW TREE .|_ u u I . .I I c J u n ' l t I , _| a l l I ". i. l ROOST msnoe f l | I a o I I. FEMALE unit} CLUSTER ‘EE it provides to the recipient. in oth- er words. by regurgitating blood to a roostmate. the donor should be able to save its roostmate's life without substantially risking its own. To test whether or not this was true for vam- pire bats. we needed to measure the costs and benefits of blood sharing in two ways: directly by determining the amount of blood and the frequency of its ingestion required to prevent star- vation and indirectly by estimating the effect of blood sharing on long- term survival by means of computer simulations. From McNab's work as well as our owu. we knew that a bat must con- sume from 20 to 30 milliliters of blood every 60 hours to prevent star- vation. In addition. we knew that a bat on the brink of starvation can gain up to 12 hours of life and another chance to find food if it is given blood by a roostmate. A cooperative roostmate, who has recently fed and therefore has an elevated metabolic rate. loses less than 12 hours by donating a blood meal and so has 36 hours and two nights of hunting left before reaching the point of starvation. According to FEMALE NURSING MALE AND FEMALE vampire bats often roost in the same tree (a). Females cluster near the top of the cavity. some 12 or more feet from the ground. where they are guarded by a single domlnant male. No or three subordinate males occupy the same tree but roost closer to the ground. As many as 12 fe- males. each with a young pup (the pups differ in size because births Occur throughout the year). may gather in one tree (b). Although the composition of the roosting groups varies from day to day. some females associate for many years and regur- gitate blobd to one another. a behavior that is a form of recip- rocal altruism. A hungry bat solicits regurgitated blood from a roostmate first by grooming (cl. which consists of licking the potential donor under her wing. and then by licking the donor's lips (dl. If the donor is receptive. she responds by regurgitating blood (2). Only bats who are close relatives or who have had a long-term association give blood to each other. reciprocity theory, then. only bats with less than 24 hours of life remaining should be fed by their roosunates. Our experiment with captive bats, in which we withheld food for 24-hour periods, showed exactly that. For a reciprocal-exchange system to work. it is necessary for bats to recog- nize one another and be able to detect and exclude cheaters. Although we have yet to prove that cheaters either exist or are excluded from the system. we have reason to believe that the bats are capable of individual recognition. To begin with, we know from our ob- servations of captive hats that only individuals who have had a prior asso- ciation will regurgitate blood to one another. it therefore seems likely that they must recognize each other in some way. Circumstantial evidence strongly suggests that social grooming plays a role in roostmate recognition. The bats spend more than 5 percent of each day grooming and licking one another, and the behavior seems to be an Important prelude to regurgita‘ tion: hungry bats frequently groom potential donors (females who have recently fed). As with blood regurgita- tion. grooming occurs more frequent- ly among individuals who either are close relatives or have previously as- sociated with one another than it does among bats who are strangers. Additional evidence that bats rec- ognize each other has been provid- ed by Schmidt. By analyzing sono- grams he and his students have found that the bats emit individually distinct vocalizations. Such “contact calls" of- ten accompany grooming sessions and have the acoustic characteristics (variable frequency and low intensi- ty) that are necessary to encode indi- vidual identity. Auditory signals {and possibly olfactory cues) of this type presumably enable individuals to rec- ognize their long-term roostmates as well as cheaters who solicit blood but do not themselves respond to solicitation. t seems. then, that both reciprocity and kin selection promote blood sharing among roostmates and that regurgitation is beneficial—~at least in the short term—to a recipient. How might such energetically costly behavior affect overall survival rates within a population? To establish that reciprocity can persist in an evolution; ary sense. one must be able to demon- strate. at least theoretically. that bats who share food with their roostmates have a higher annual survival rate than those who do not. RECIPIENT TIME ‘ LOST PERCENTAGE OF PREFEEDING WEIGHT 8 C 90 RE STARVATION -- mm cameo w PM so _ , \ 700 12 24 36 43 60 TIME (HOURS) COST-BENEFIT ANALYSIS of blood sharing among vampire bats indicates that recipi- ents benefit more than donors lose. The author weighed adult females returning to the roost after feeding and then weighed them every hour for the next 24 hours. An individualwho had fed might return at 130 percent ofits prefeeding weight (half the weight of a blood meal is lost through urination within the first hour after feed- ing), whereas a bat who failed to feed on two successive nights might return at 80 percent of its earlier weight. By regurgitating live milliliters of condensed blood to a hungry roosunate. the donor bat might drop from 1 10 to 95 percent of its prefeed— ing weight but lose only six hours of the time it has remaining until starvation. The recipient bat. however. gains 18 hours and so benefits more than the donor loses. We knew from our netting studies that on the average about 30 percent of the immature bats (those younger than two years) fail to obtain a blood meal on a given night, whereas only 7 percent of reproductiver mature males and females fail to feed. We also knew from field observations that the failure to feed appears to be random: all individuals within an age group are affected equally. With that informa- tion and the help of a computer, we determined that annual mortality for adults in the absence of food sharing (given that 7 percent of the adults fail 7 to feed on a given night and that failure to feed two nights in a row will lead to death) should be about 82 percent. Because actual mortality among adult vampire bats is only 24 percent per year. we concluded that food sharing must be favored by natu- ral selection Vampire bats have evolved a system of food exchange whereby they share blood with mosh-hates in need. Al- though the behavior puts the donor at risk. the recipient is more likely to survive another night. Moreover. our studies have shown that individuals who exchange blood with their roost— mates gain an immediate advantage in terms of increasing their own survival and sometimes the survival of their relatives. Hence. both reciprocity and kin selection appear to be operative in this system. is food sharing a behavior unique to vampire bats? Not exactly. Females of several insectivorous bat species. in- cluding the bentnwinged bat. Minion- terus schreibersi, the Mexican free- tailed bat. Tadan'da mexicana, and the evening bat. Nyctt'ceius humeralis, nurse young other than their own. The bent-winged bat has not yet been studied in great detail. Gary P. McCracken and his students at the University of Tennessee at Knoxville have been studying the nurseries of free-tailed bats for the past nine years. Several million free-tails congregate in caves in the southwestern US every summer to give birth synchronously to their pups. The young are kept in creches. where the density of individu- als is as high as 40 pups per 16 square inches. The females roost elsewhere and visit their pups only twice a day to nurse them. As the females approach, the hungry pups swarm toward them: as many as four pups have been ob- served attempting to feed sequentially from one female. To an observer. it appears as if fe- males are feeding whichever pups reach them first. For that reason many investigators thought the fe- males were a communal resource; af- ter all. finding one's own pup amid SCIENTIFIC AMERICAN February 1990 81 MBGCAN FREE-TAIUZD BATS rear their pups in communal nurseries. which may contain a million or more newborn bats. Despite the density of pups—as many as 40 within the space of 16 square inches—mothers. such as the one at the upper left. locate their own pups 83 percent of the time and so rarely nurse unrelated pups. millions of others seemed impossible. Yet McCracken has shown. by compar- ing variations in blood enzymes be- tween lactating females and suckling pups. that mothers successfully find and nurse their own young 83 percent of the time. apparently with the help of olfactory and auditory cues. Any nonparental suckling that takes place appears to be milk stealing on the part of an aggressive pup. For species that form such enor- mous aggregations. such as the free- tailed bat. the benefits of creching (the pups stay warm. and the risk of predation to any one individual is re- duced) outweigh the cost of occasion- ally nursing the wrong pup. Among free-tailed bats nonparental nursing— when it occurs—seems to be neither a form of kin selection nor reciprocal altruism but simply the result of ran- dom error. For the past two summers my stu- dents and [ have been studying non- parental nursing in evening bats in northern Missouri. These bats. like the free—tailed hats. form nursing col- onies. Their colonies are relatively small. however. containing only from 82 QCH‘W'W-wr- ‘ururr-x 30 to 200 adults. and are usually found in attics rather than in caves. Unlike the free—tailed bat, which gives birth to one pup per summer. the evening bat gives birth usually to two and sometimes to three pups at a time. Our studies indicate that a mother nurses her pups faithfully during the week following their birth but that as the pups age they tend to feed occasionally from other females. When a pup is about three weeks old. it is generally nursing from a female other than its mother about 20 per- cent of the time. Is this a case of reciprocal altruism or kin selection? Because the females can be observed actively accepting or rejecting young pups who solicit milk from them. it seems they are dis— criminating between pups in some way. Preliminary evidence suggests that the females are selectively feed- ing relatives: analyses of b]ood«en- zyme markers indicate that most of- ten nonparental females ar‘e relat- edveat least distuntly--—-to the pups they nurse. In addition. data my grad. uate student _]. Andrew Scherrer has recently collected indicate that each Fol-grunn; ! mm pup has a unique call‘and that calls made by relatives resemble each oth- er. We suspect a nonparental female may recognize related pups by com— paring their calls with those oi‘ her own pup. esearch on food sharing in bats Rillustrates a common theme in evolutionary biology: that simi- lar behaviors seen in different species may result from entirely different evo- lutionary pressures. Although kin se- lection is widely regarded as a power- ful and pervasive evolutionary force. under certain circumstances—such as whenever animals live in small groups and the potential for frequent aid giv- ing among them is high—reciprocity is likely to be more beneficial than kin selection—provided that cheaters can be detected and excluded from the system. Further understanding of the for- ces responsible for social evolution in vertebrates requires that the mech- anisms underlying individual and kin recognition be identified. Our re- search on aid-giving behavior in bars demonstrates that the role both kin selection and reciprocity play in a society is dependent on the recog- nition capabilities of the animals in that society. Determining the extent to which in- dividuals recognize and interact pref- erentially with relatives should be greatly facilitated by modern molecu- lar tachniques (such as DNA finger- printing). which significantly enhance the ability of investigators to measure relatedness among animals in the field. Because bats possess a sophisti- cated auditory system that enables them to navigate and capture prey. I believe careful study of their vocaliza- tions in social situations may yield exciting information about the mech- anisms by which animals recognize their relatives and close associates. The results of such study should in turn elucidate much about vertebrate social behavior in general. FURTHER READING THE EVOLUTION or COOPERATION. Robert Axelrod. Basic Books. inc. 1984. Socw. EVOLUFION. Robert Trivcrs. Benjamin-Cummings Publishing ($0.. 1985. THE NATURAL llIsTORY or VAMPIRE BATS Edited by Arthur M. Greenhall and Uwc Schmidt. CRC Press, 1988. RECIPRUCAL ALTRUISM IN BATS AND 01'”- ER MAMMAIs. Gerald S. Wilkinson in Ethology and Sociobiology. Vol. 9. Nos. 2-4. pages 85400; July. 1988. ...
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Wilkinson on vampire bats (small) - Sci Amer 1990 - Food...

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