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Initiating and developing social relationships with strangers can provide fitness benefits, but it is an inherently risky process. To mitigate potential risks and develop trust, strangers may ‘test the waters’ by gradually escalating the type of social investment from low-cost to high-cost. Opportunities to capture the moment animals first encounter one another in the wild are rare, and detailed quantitative assessments of when and how animals initiate relationships are limited. We introduced four unfamiliar groups of feral monk parakeets together into a single 22-bird group and observed the sequence of social behaviours that occurred as relationships developed over 22 days. We tested the effect of relationship status (stranger versus familiar) on the probability of dyads following predicted sequences and whether strangers who progressed their relationships maintained higher rates of no-contact proximity compared with dyads that did not. We found that stranger dyads, but not familiar dyads, were more likely to (i) approach each other without contact before making contact and (ii) follow predicted sequences of affiliative behaviours. Strangers that progressed to contact also had higher rates of associations than did birds that never made contact. These findings provide support for ‘Testing the Waters’ during new relationship formation in a socially and cognitively complex species. 

Regurgitated food sharing in vampire bats is a cooperative behavior that has garnered scientific interest as an example of reciprocal helping among kin and non-kin. The amount of food given is estimated via the duration of mouth-licking. However, a growing body of evidence across other animal taxa, especially social insects, shows that mouth-to-mouth material transfer can serve many functions besides food sharing. In this review, we asked whether and to what extent mouth-licking in the common vampire bat (Desmodus rotundus) could be explained by functions other than regurgitated food sharing. We first review the evidence, including new analyses of published data, that food sharing occurs during mouth-licking bouts in vampire bats. We then review interpretations of mouth-licking in other mammal species and assess the likelihood that various hypothetical functions suggested in other species could occur in vampire bats. We conclude that the primary function of prolonged bouts of mouth-licking in vampire bats is sharing of ingested blood, but that microbial sharing is another likely benefit, and that short bouts of mouth-licking also function as social signals of begging or offering of food. Future work on this behavior should keep alternative explanations in mind when interpreting observations. 

Abstract In many group‐living animals, survival and reproductive success depend on the formation of long‐term social bonds, yet it remains largely unclear why particular pairs of groupmates form social bonds and not others. Can social bond formation be reliably predicted from each individual's immediately observable traits and behaviors at first encounter? Or is social bond formation hard to predict due to the impacts of shifting social preferences on social network dynamics? To begin to address these questions, we asked how well long‐term cooperative relationships among vampire bats were predicted by how they interacted during their first encounter as introduced strangers. In Study 1, we found that the first 6 h of observed interactions among unfamiliar bats co‐housed in small cages did not clearly predict the formation of allogrooming or food‐sharing relationships over the next 10 months. In Study 2, we found that biologger‐tracked first contacts during the first 4–24 h together in a flight cage did not strongly predict allogrooming rates over the next 4 months. These results corroborate past evidence that social bonding in vampire bats is not reducible to the individual traits or behaviors observed at first encounter. Put simply, first impressions are overshadowed by future social interactions. 

Social structure can emerge fromhierarchically 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. 

Blood-feeding (sanguivory) has evolved more than two dozen times among birds, fishes, insects, arachnids, molluscs, crustaceans, and annelids; however, among mammals, it is restricted to the vampire bats. Here, the authors revisit the question of how it evolved in that group. Evidence to date suggests that the ancestors of phyllostomids were insectivorous, and that carnivory, omnivory, and nectarivory evolved among phyllostomids after vampire bats diverged. Frugivory likely also evolved after vampire bats diverged, but the phylogeny is ambiguous on that point. However, vampire bats lack any genetic evidence of a frugivorous past, and the behavioural progression from frugivory to sanguivory is difficult to envision. Thus, the most parsimonious scenario is that sanguivory evolved in an insectivorous ancestor to vampire bats via ectoparasite-eating, wound-feeding, or some combination of the two—all feeding habits found among blood-feeding birds today. Comparing vampire bats with other sanguivores, the authors find several remarkable examples of convergence. Further, it was found that blood-feeding has been ca. 50 times more likely to evolve in a vertebrate lineage than in an invertebrate one. The authors hypothesize that this difference exists because vertebrates are more likely than invertebrates to have the biochemical necessities required to assimilate the components of vertebrate blood. 

Abstract Reciprocity and pseudo‐reciprocity are two important models for the evolution of cooperation and often considered alternative hypotheses. Reciprocity is typically defined as a scenario where help givencauseshelp received: cooperation is stabilized because each actor's cooperative investments are conditional on the cooperative returns from the receiver. Pseudo‐reciprocity is a scenario where helpenablesbyproduct returns: cooperation is inherently stable because the actor's cooperative investments yield byproduct returns from the receiver's self‐serving behavior. These models are strict alternatives only if reciprocity is defined by the restrictive assumption of zerofitness interdependence, meaning that the helper has no “stake” in the receiver's fitness. Reciprocity and interdependence are, however, not mutually exclusive when helping can increase both reciprocal help and byproduct returns. For instance, helping partners survive can simultaneously increase their willingness to reciprocate, their ability to reciprocate, and byproduct benefits of their existence. Interdependence can “pave the road” to reciprocal helping, and partners who reciprocate help can also become interdependent. However, larger cooperative investments can increase the need for responsiveness to partner returns. Therefore, most long‐term cooperative relationships involve both responsiveness and interdependence. Categorizing these relationships as “reciprocity” can be viewed as ignoring interdependence, but calling them ‘pseudo‐reciprocity’ is confusing because stability also comes from the cooperative investments being conditional on returns. Rather than conceptualizing cooperation intodiscrete categories, it is more insightful to imagine a coordinate system with responsiveness and interdependence ascontinuous dimensions. One can ask: To what degree is helping behavior responsive to the partner's behavior? And to what degree does the helper inherently benefit from the receiver's survival or reproduction? The amounts of responsiveness and interdependence will often be hard to estimate, but both are unlikely to be zero. Identifying their relative importance, and how that changes over time, would greatly clarify the nature of cooperative relationships. 

Abstract To forage efficiently, animals should selectively attend to and remember the cues of food that best predict future meals. One hypothesis is that animals with different foraging strategies should vary in their reliance on spatial versus feature cues. Specifically, animals that store food in dispersed caches or that feed on spatially stable food, such as fruits or flowers, should be relatively biased towards learning a meal’s location, whereas predators that hunt mobile prey should instead be relatively biased towards learning feature cues such as odor or sound. Several authors have predicted that nectar-feeding and fruit-feeding bats would rely relatively more on spatial cues, whereas closely related predatory bats would rely more on feature cues, yet no experiment has compared these two foraging strategies under the same conditions. To test this hypothesis, we compared learning in the frugivorous bat, Artibeus jamaicensis, and the predatory bat, Lophostoma silvicolum, which hunts katydids using acoustic cues. We trained bats to find food paired with a unique and novel odor, sound, and location. To assess which cues each bat had learned, we then dissociated these cues to create conflicting information. Rather than finding that the frugivore and predator clearly differ in their relative reliance on spatial versus feature cues, we found that both species used spatial cues over sounds or odors in subsequent foraging decisions. We interpret these results alongside past findings on how foraging animals use spatial cues versus feature cues, and explore why spatial cues may be fundamentally more rich, salient, or memorable.