Common vampire bat contact calls attract past food-sharing partners

Calling to the collective: contact calling rates within groups of disc-winged bats do not vary by kinship or association

Many group-living animals coordinate social behaviours using contact calls, which can be produced for all group members or targeted at specific individuals. In the disc-winged bat, Thyroptera tricolor, group members use 'inquiry' and 'response' calls to coordinate daily movements into new roosts (furled leaves). Rates of both calls show consistent among-individual variation, but causes of within-individual variation remain unknown. Here, we tested whether disc-winged bats produce more contact calls towards group members with higher kinship or association. In 446 experimental trials, we recorded 139 random within-group pairs of one flying bat (producing inquiry calls for roost searching) and one roosting bat (producing response calls for roost advertising). Using generalized linear mixed-effect models (GLMM), we assessed how response and inquiry calling rates varied by sender, receiver, genetic kinship and co-roosting association rate. Calling rates varied consistently across senders but not by receiver. Response calling was influenced by inquiry calling rates, but neither calling rate was higher when the interacting pair had higher kinship or association. Rather than dyadic calling rates indicating within-group relationships, our findings are consistent with the hypothesis that bats produce contact calls to maintain contact with any or all individuals within a group while collectively searching for a new roost site. This article is part of the theme issue 'The power of sound: unravelling how acoustic communication shapes group dynamics'.

Hierarchically embedded scales of movement shape the social networks of vampire bats

Social structure can emerge from hierarchically embedded scales of movement, where movement at one scale is constrained within a larger scale (e.g. among branches, trees, forests). In most studies of animal social networks, some scales of movement are not observed, and the relative importance of the observed scales of movement is unclear. Here, we asked: how does individual variation in movement, at multiple nested spatial scales, influence each individual's social connectedness? Using existing data from common vampire bats (Desmodus rotundus), we created an agent-based model of how three nested scales of movement-among roosts, clusters and grooming partners-each influence a bat's grooming network centrality. In each of 10 simulations, virtual bats lacking social and spatial preferences moved at each scale at empirically derived rates that were either fixed or individually variable and either independent or correlated across scales. We found that numbers of partners groomed per bat were driven more by within-roost movements than by roost switching, highlighting that co-roosting networks do not fully capture bat social structure. Simulations revealed how individual variation in movement at nested spatial scales can cause false discovery and misidentification of preferred social relationships. Our model provides several insights into how nonsocial factors shape social networks.

Neural activation associated with maternal and aversive interactions with pups in the Mongolian gerbil (Meriones unguiculatus)

According to approach-avoidance model, virgin female laboratory rats display maternal behaviour when the tendency to approach and interact with the pup is stronger than avoiding it. A positive neural mechanism that includes the medial preoptic area (mPOA)/bed nucleus of the stria terminalis (BNST) and a negative mechanism that involves the anterior hypothalamic nucleus (AHN)/ventromedial nucleus (VMN)/ periaqueductal grey (PAG) underlie to these behaviours. Unlike virgin rats, which avoid the pups, virgin females Mongolian gerbils (Meriones unguiculatus) can be immediately either maternal or aggressive with the pups. Furthermore, the Mongolian gerbil is monogamous and biparental species. Despite these difference, we hypothesised that maternal and aggressive interaction with the pups could activate mPOA/BNST and AHN/VMH/PAG, respectively, and that maternal response could be associated with high concentrations of estradiol (E₂). Twenty virgin maternal females and 20 aggressive toward the pups were selected. Ten maternal females interacted with the pups (MAT-pups) and 10 with candy (MAT-candy). Of the 20 aggressive females, 10 interacted with the pups (AGG-pups) and 10 with candy (AGG-candy). Immediately after the test, blood samples were taken to quantify E₂. The brains were dissected for c-Fos immunohistochemistry. MAT-pups females had significantly higher activation in mPOA/BNST than MAT-candy females, while AGG-pups showed significant activation in AHN/VMH/PAG compared with AGG-candy females. The maternal response was associated with high concentrations of E₂. These results suggested a positive and a negative mechanism in the regulation of maternal behaviour in the Mongolian gerbil, and that the immediate maternal response could be due to high E₂ concentrations.

The Potential to Encode Detailed Information About Parasites in the Acoustic Signals of Chinese Horseshoe Bats (Rhinolophus sinicus)

Condition-dependent acoustic signals that potentially reveal information about the signaler’s physical or physiological condition are common and important in the animal kingdom. Given the negative effects of parasites on the health and fitness of their hosts, it is reasonable to expect animal acoustic signals to reflect detailed information concerning parasite infection. However, despite previous studies having verified the potential of sexually selected vocalizations to provide information on parasitism based on the correlations between call acoustic properties and parasitism in some animal taxa, less is known about whether acoustic signals used in a non-sexual context also reflect parasite infection especially for highly vocal bats. We thus investigated the relationships between the acoustic properties of distress calls and echolocation pulses and the infestation intensity of gamasid mites and bat flies in Chinese horseshoe bats (Rhinolophus sinicus) to determine whether acoustic signals potentially contain information about parasite infection. We found that bats infected with more gamasid mites uttered significantly shorter echolocation pulses, suggesting that echolocation pulses may contain information on the intensity of mite infection. Additionally, bats infected with more gamasid mites emitted distress calls with narrower bandwidth, while bats with more bat flies emitted calls with longer pause duration. These results suggest that distress calls may not only reflect a signaler’s parasite infection intensity but also may provide information concerning infection with specific parasites. In short, our findings suggest that acoustic signals of bats potentially reflect detailed information about parasite infection.

Forced proximity promotes the formation of enduring cooperative relationships in vampire bats

Spatial assortment can be both a cause and a consequence of cooperation. Proximity promotes cooperation when individuals preferentially help nearby partners, and conversely, cooperation drives proximity when individuals move towards more cooperative partners. However, these two causal directions are difficult to distinguish with observational data. Here, we experimentally test if forcing randomly selected pairs of equally familiar female common vampire bats (Desmodus rotundus) into close spatial proximity promotes the formation of enduring cooperative relationships. Over 114 days, we sampled 682 h of interactions among 21 females captured from three distant sites to track daily allogrooming rates over time. We compared these rates before, during and after a one-week period, during which we caged random triads of previously unfamiliar and unrelated vampire bats in proximity. After the week of proximity when all bats could again freely associate, the allogrooming rates of pairs forced into proximity increased more than those of the 126 control pairs. This work is the first to experimentally demonstrate the causal effect of repeated interactions on cooperative investments in vampire bats. Future work should determine the relative importance of mere association versus interactions (e.g. reciprocal allogrooming) in shaping social preferences.

Gene losses in the common vampire bat illuminate molecular adaptations to blood feeding

Vampire bats are the only mammals that feed exclusively on blood. To uncover genomic changes associated with this dietary adaptation, we generated a haplotype-resolved genome of the common vampire bat and screened 27 bat species for genes that were specifically lost in the vampire bat lineage. We found previously unknown gene losses that relate to reduced insulin secretion (FFAR1 and SLC30A8), limited glycogen stores (PPP1R3E), and a unique gastric physiology (CTSE). Other gene losses likely reflect the biased nutrient composition (ERN2 and CTRL) and distinct pathogen diversity of blood (RNASE7) and predict the complete lack of cone-based vision in these strictly nocturnal bats (PDE6H and PDE6C). Notably, REP15 loss likely helped vampire bats adapt to high dietary iron levels by enhancing iron excretion, and the loss of CYP39A1 could have contributed to their exceptional cognitive abilities. These findings enhance our understanding of vampire bat biology and the genomic underpinnings of adaptations to blood feeding.

Reaching Out for Inaccessible Food Is a Potential Begging Signal in Cooperating Wild-Type Norway Rats, Rattus norvegicus

Begging is widespread in juvenile animals. It typically induces helpful behaviours in parents and brood care helpers. However, begging is sometimes also shown by adults towards unrelated social partners. Adult Norway rats (Rattus norvegicus) display a sequence of different behaviours in a reciprocal food provisioning task that have been interpreted as such signals of need. The first behaviour in this sequence represents reaching out for a food item the animal cannot obtain independently. This may reflect either an attempt to grasp the food object by itself, or a signal to the social partner communicating the need for help. To distinguish between these two possibilities, we tested in female wild-type Norway rats if the amount of reaching performed by a food-deprived rat changes with the presence/absence of food and a social partner. Focal rats displayed significantly more reaching behaviour, both in terms of number and total duration of events, when food and a potentially helpful partner were present compared to when either was missing. Our findings hence support the hypothesis that rats use reaching behaviour to signal need to social partners that can help them to obtain food.

Social foraging in vampire bats is predicted by long-term cooperative relationships

Stable social bonds in group-living animals can provide greater access to food. A striking example is that female vampire bats often regurgitate blood to socially bonded kin and nonkin that failed in their nightly hunt. Food-sharing relationships form via preferred associations and social grooming within roosts. However, it remains unclear whether these cooperative relationships extend beyond the roost. To evaluate if long-term cooperative relationships in vampire bats play a role in foraging, we tested if foraging encounters measured by proximity sensors could be explained by wild roosting proximity, kinship, or rates of co-feeding, social grooming, and food sharing during 21 months in captivity. We assessed evidence for 6 hypothetical scenarios of social foraging, ranging from individual to collective hunting. We found that closely bonded female vampire bats departed their roost separately, but often reunited far outside the roost. Repeating foraging encounters were predicted by within-roost association and histories of cooperation in captivity, even when accounting for kinship. Foraging bats demonstrated both affiliative and competitive interactions with different social calls linked to each interaction type. We suggest that social foraging could have implications for social evolution if "local" within-roost cooperation and "global" outside-roost competition enhances fitness interdependence between frequent roostmates.

Social dominance and cooperation in female vampire bats

When group-living animals develop individualized social relationships, they often regulate cooperation and conflict through a dominance hierarchy. Female common vampire bats have been an experimental system for studying cooperative relationships, yet surprisingly little is known about female conflict. Here, we recorded the outcomes of 1023 competitive interactions over food provided ad libitum in a captive colony of 33 vampire bats (24 adult females and their young). We found a weakly linear dominance hierarchy using three common metrics (Landau's h' measure of linearity, triangle transitivity and directional consistency). However, patterns of female dominance were less structured than in many other group-living mammals. Female social rank was not clearly predicted by body size, age, nor reproductive status, and competitive interactions were not correlated with kinship, grooming nor food sharing. We therefore found no evidence that females groomed or shared food up a hierarchy or that differences in rank explained asymmetries in grooming or food sharing. A possible explanation for such apparently egalitarian relationships among female vampire bats is the scale of competition. Female vampire bats that are frequent roostmates might not often directly compete for food in the wild.

Cooperation-based concept formation in male bottlenose dolphins

In Shark Bay, Western Australia, male bottlenose dolphins form a complex nested alliance hierarchy. At the first level, pairs or trios of unrelated males cooperate to herd individual females. Multiple first-order alliances cooperate in teams (second-order alliances) in the pursuit and defence of females, and multiple teams also work together (third-order alliances). Yet it remains unknown how dolphins classify these nested alliance relationships. We use 30 years of behavioural data combined with 40 contemporary sound playback experiments to 14 allied males, recording responses with drone-mounted video and a hydrophone array. We show that males form a first-person social concept of cooperative team membership at the second-order alliance level, independently of first-order alliance history and current relationship strength across all three alliance levels. Such associative concepts develop through experience and likely played an important role in the cooperative behaviour of early humans. These results provide evidence that cooperation-based concepts are not unique to humans, occurring in other animal societies with extensive cooperation between non-kin.

Tracking sickness effects on social encounters via continuous proximity sensing in wild vampire bats

Sickness behaviors can slow the spread of pathogens across a social network. We conducted a field experiment to investigate how sickness behavior affects individual connectedness over time using a dynamic social network created from high-resolution proximity data. After capturing adult female vampire bats (Desmodus rotundus) from a roost, we created “sick” bats by injecting a random half of bats with the immune-challenging substance, lipopolysaccharide, while the control group received saline injections. Over the next 3 days, we used proximity sensors to continuously track dyadic associations between 16 “sick” bats and 15 control bats under natural conditions. Compared to control bats, “sick” bats associated with fewer bats, spent less time near others, and were less socially connected to more well-connected individuals (sick bats had on average a lower degree, strength, and eigenvector centrality). High-resolution proximity data allow researchers to flexibly define network connections (association rates) based on how a particular pathogen is transmitted (e.g., contact duration of >1 vs. >60 min, contact proximity of <1 vs. <10 m). Therefore, we inspected how different ways of measuring association rates changed the observed effect of LPS. How researchers define association rates influences the magnitude and detectability of sickness effects on network centrality. When animals are sick, they often encounter fewer individuals. We tracked this unintentional “social distancing” effect hour-by-hour in a wild colony of vampire bats. Using bat-borne proximity sensors, we compared changes in the social network connectedness of immune-challenged “sick” bats versus “control” bats over time. “Sick” bats had fewer encounters with others and spent less time near others. Associations changed dramatically by time of day, and different measures of association influenced the sickness effect estimates.

Do bats use guano and urine stains to find new roosts? Tests with three group-living bats

Many animals use social cues to find refuges. Bats can find roosts using the echolocation and social calls of conspecifics, but they might also use scent cues, a possibility which is less studied. The entrances of bat roosts are often marked by guano and urine, providing possible scent cues. We conducted eight experiments to test whether bats use the scent of guano and urine to find potential roosts. In field experiments, we tested if Molossus molossus (velvety free-tailed bats) in Panama and Eptesicus fuscus (big brown bats) in Ohio would investigate artificial roost boxes that were scented with guano and urine more often than a paired unscented control. We did not detect any difference in flights near the scented versus unscented roosts, and we detected only one entrance into any artificial roost (scented). In six captive experiments, we tested for the attraction of Desmodus rotundus (common vampire bats) and Molossus molossus to areas scented with guano and urine, under several conditions. Results were mixed, but overall suggested that the scent of guano and urine does not act as a strong lure for the tested bat species. We suggest that further tests of olfaction-based roost choice in bats should manipulate existing scent cues on familiar roosts.

Immune-challenged vampire bats produce fewer contact calls

Vocalizations are an important means to facilitate social interactions, but vocal communication may be affected by infections. While such effects have been shown for mate-attraction calls, other vocalizations that facilitate social contact have received less attention. When isolated, vampire bats produce contact calls that attract highly associated groupmates. Here, we test the effect of an immune challenge on contact calling rates of individually isolated vampire bats. Sickness behaviour did not appear to change call structure, but it decreased the number of contact calls produced. This effect could decrease contact with groupmates and augment other established mechanisms by which sickness reduces social encounters (e.g. mortality, lethargy and social withdrawal or disinterest).

Social structure and relatedness in the fringe-lipped bat (Trachops cirrhosus)

General insights into the causes and effects of social structure can be gained from comparative analyses across socially and ecologically diverse taxa, such as bats, but long-term data are lacking for most species. In the neotropical fringe-lipped bat, Trachops cirrhosus, social transmission of foraging behaviour is clearly demonstrated in captivity, yet its social structure in the wild remains unclear. Here, we used microsatellite-based estimates of relatedness and records of 157 individually marked adults from 106 roost captures over 6 years, to infer whether male and female T. cirrhosus have preferred co-roosting associations and whether such associations were influenced by relatedness. Using a null model that controlled for year and roosting location, we found that both male and female T. cirrhosus have preferred roosting partners, but that only females demonstrate kin-biased association. Most roosting groups (67%) contained multiple females with one or two reproductive males. Relatedness patterns and recapture records corroborate genetic evidence for female philopatry and male dispersal. Our study adds to growing evidence that many bats demonstrate preferred roosting associations, which has the potential to influence social information transfer.

Cooperative bird discriminates between individuals based purely on their aerial alarm calls

From an evolutionary perspective, the ability to recognize individuals provides great selective advantages, such as avoiding inbreeding depression during breeding. Whilst the capacity to recognize individuals for these types of benefits is well established in social contexts, why this recognition might arise in a potentially deadly alarm-calling context following predator encounters is less obvious. For example, in most avian systems, alarm signals directed toward aerial predators represent higher predation risk and vulnerability than when individuals vocalize toward a terrestrial-based predator. Although selection should favor simple, more effective alarm calls to these dangerous aerial predators, the potential of these signals to nonetheless encode additional information such as caller identity has not received a great deal of attention. We tested for individual discrimination capacity in the aerial alarm vocalizations of the noisy miner (Manorina melanocephala), a highly social honeyeater that has been previously shown to be able to discriminate between the terrestrial alarm signals of individuals. Utilizing habituation–discrimination paradigm testing, we found conclusive evidence of individual discrimination in the aerial alarm calls of noisy miners, which was surprisingly of similar efficiency to their ability to discriminate between less urgent terrestrial alarm signals. Although the mechanism(s) driving this behavior is currently unclear, it most likely occurs as a result of selection favoring individualism among other social calls in the repertoire of this cooperative species. This raises the intriguing possibility that individualistic signatures in vocalizations of social animals might be more widespread than currently appreciated, opening new areas of bioacoustics research.

Do vampire bats groom others based on need?

Allogrooming provides a window into the social lives of many group-living mammals and birds. The fitness benefits of allogrooming are encouraged by proximate mechanisms that make it physiologically rewarding for both actors and receivers. However, receivers might not always benefit from allogrooming. Some allogrooming decisions might be the actor’s response to cues of the recipient’s need. Other decisions might only be caused by the actor’s motivational state. To test these ideas, we studied what triggers allogrooming in common vampire bats. In test 1, subjects that had experimentally disturbed and wetted fur were more likely to be allogroomed, even when controlling for increased self-grooming. In test 2, allogrooming rates were elevated not only by receiver self-grooming (a cue for receiver need) but also by the actor’s previous self-grooming. Both effects were significantly greater than the effect of self-grooming by third parties. Interestingly, we detected a negative interaction: the positive effect of receiver need on allogrooming was smaller when the actor was previously self-grooming. This is consistent with the hypothesis that there are “receiver-driven” allogrooming decisions, which are responses to recipient need, and “actor-driven” decisions, which are not. We predict that receiver-driven allogrooming will bestow greater benefits to recipients compared with actor-driven allogrooming.

A method for rapid testing of social learning in vampire bats

Designing experiments on social learning using an untested behaviour or species requires baseline knowledge of how the animals will perform. We conducted a pilot study of a procedure for rapidly testing social learning in the highly social common vampire bat (Desmodus rotundus) using a simple maze. To create demonstrators, we allowed captive bats to learn to exit a three-dimensional maze, which reunited them with their colony as a reward. We then filmed naive bats in the same maze, comparing their ability to exit the maze before, during and after the addition of a trained demonstrator. The presence of a demonstrator increased the exit rates of naive bats, presumably by attracting the attention of the naive bats to the maze exit. Four of the five naive bats that exited in the presence of a demonstrator retained the ability to exit without the demonstrator. No naive bat exited during trials without a potential demonstrator present. This experimental procedure appears to be a promising approach for efficient tests of social learning in vampire bats because maze difficulty can be manipulated to adjust learning rates and each trial requires only 15 min.

Social communication in bats

Bats represent one of the most diverse mammalian orders, not only in terms of species numbers, but also in their ecology and life histories. Many species are known to use ephemeral and/or unpredictable resources that require substantial investment to find and defend, and also engage in social interactions, thus requiring significant levels of social coordination. To accomplish these tasks, bats must be able to communicate; there is now substantial evidence that demonstrates the complexity of bat communication and the varied ways in which bats solve some of the problems associated with their unique life histories. However, while the study of communication in bats is rapidly growing, it still lags behind other taxa. Here we provide a comprehensive overview of communication in bats, from the reasons why they communicate to the diversity and application of different signal modalities. The most widespread form of communication is the transmission of a signaller's characteristics, such as species identity, sex, individual identity, group membership, social status and body condition, and because many species of bats can rely little on vision due to their nocturnal lifestyles, it is assumed that sound and olfaction are particularly important signalling modes. For example, research suggests that secretions from specialized glands, often in combination with urine and saliva, are responsible for species recognition in several species. These olfactory signals may also convey information about sex and colony membership. Olfaction may be used in combination with sound, particularly in species that emit constant frequency (CF) echolocation calls, to recognize conspecifics from heterospecifics, yet their simple structure and high frequency do not allow much information of individual identity to be conveyed over long distances. By contrast, social calls may encode a larger number of cues of individual identity, and their lower frequencies increase their range of detection. Social calls are also known to deter predators, repel competitors from foraging patches, attract group mates to roost sites, coordinate foraging activities, and are used during courtship. In addition to sound, visual displays such as wing flapping or hovering may be used during courtship, and swarming around roost sites may serve as a visual cue of roost location. However, visual communication in bats still remains a poorly studied signal modality. Finally, the most common form of tactile communication known in bats is social grooming, which may be used to signal reproductive condition, but also to facilitate and strengthen cooperative interactions. Overall, this review demonstrates the rapid advances made in the study of bat social communication during recent years, and also identifies topics that require further study, particularly those that may allow us to understand adaptation to rapidly changing environmental conditions.

Younger vampire bats (Desmodus rotundus) are more likely than adults to explore novel objects

The effects of age on neophobia and exploration are best described in birds and primates, and broader comparisons require reports from other taxa. Here we present data showing age-dependent exploration in a long-lived social species, the common vampire bat (Desmodus rotundus). A previous study found that vampire bats regurgitated food to partners trapped in a cage. Interestingly, while only a few adult bats visited the trapped bat, in every trial all or most of the eight young males in the colony would visit the trapped bat without feeding it. To test whether this behavioral difference resulted from age class differences in exploration, we compared responses of the bats to a trapped conspecific versus an inanimate novel object. Some adults and young showed interest in trapped conspecifics, but only the young males explored the novel objects. Additional novel object tests in a second captive colony showed that higher rates of novel object exploration were shown by young of both sexes. Our results corroborate past findings from other mammals and birds that age predicts exploration. If age-dependent exploration is indeed adaptive, then the role of age as a predictor of exploration tendency should depend on species-specific life history traits. Finally, because younger vampire bats also appear to have higher exposure to pathogens such as rabies virus, there may be implications for pathogen transmission if younger and more exploratory vampire bats are more likely to feed on novel hosts.

Ultimate and proximate mechanisms of reciprocal altruism in rats

The reciprocal exchange of goods and services among social partners is a conundrum in evolutionary biology because of its proneness to cheating, but also the behavioral and cognitive mechanisms involved in such mutual cooperation are hotly debated. Extreme viewpoints range from the assumption that, at the proximate level, observed cases of "direct reciprocity" can be merely explained by basic instrumental and Pavlovian association processes, to the other extreme implying that "cultural factors" must be involved, as is often attributed to reciprocal cooperation among humans. Here we argue that neither one nor the other extreme conception is likely to explain proximate mechanisms underlying reciprocal altruism in animals. In particular, we outline that Pavlovian association processes are not sufficient to explain the documented reciprocal cooperation among Norway rats, as has been recently argued.