SummaryThe competition for power in a complex social world is hypothesized to be a driving force in the evolution of intelligence [1]. More specifically, power may be obtained not only by brute force but also by social strategies resembling human politics [2]. Most empirical evidence comes from primate studies that report unprovoked aggression by dominants to maintain power by spreading fear [3] and third-party interventions in conflicts [4–6]. Coalitionary support has also been described in other animals [7, 8] and is often linked to social bonding [9, 10]. As coalitions can lead to a gain in power [5, 11] and fitness benefits [12], individuals may try to prevent coalitionary support or indirectly prevent others from forming social bonds that might lead to coalitions. Although there is some empirical evidence that coalitionary support can be manipulated [13], little is known about the indirect strategy. We show here that wild ravens (Corvus corax) regularly intervene in affiliative interactions of others even though such interventions are potentially risky and without immediate benefits. Moreover, the identities of both interveners and intervened pairs are not randomly distributed. Ravens with existing ties initiate most interventions, and ravens that are creating new ties are most likely to be the targets of interventions. These patterns are consistent with the idea that interventions function to prevent others from forming alliances and consequently becoming future competitors. We thus show previously undescribed social maneuvers in the struggle for power. These maneuvers are likely to be of importance in other social species as well.

Discriminating between different individuals is considered as prerequisite for any forms of social knowledge. In birds, discriminating between conspecifics based on individual characteristics has been tested mainly in the auditory domain with territorial calls and songs for neighbour and kin discrimination but little is known about discriminating between signallers in food calls. Ravens utilize a large set of calls and show individually distinctive call repertoires. Moreover, they show advanced social tactics during foraging, suggesting that they are capable of dealing with conspecifics on an individual basis. When confronted with food that is difficult to access, ravens produce particular calls (‘haa’, yells); these calls attract other ravens and, thus, have been hypothesized to serve as ‘functionally referential signals’. We here examined whether ravens are able to differentiate between individuals on the basis of these food calls. We first analysed individual differences in call parameters, using 424 food calls recorded from 18 individually marked wild ravens in the Austrian Alps. We then tested 18 captive ravens for recognition of individual differences in food calls with playbacks, using a habituation-dishabituation design. We found evidence that food calls show individual call characteristics in fundamental frequency and intensity-related measurements providing ravens with the opportunity to respond according to these individually distinct features. Furthermore, ravens discriminated between unfamiliar ravens in the habituation-dishabituation experiment, indicating that they may discern individual differences. Our results suggest that raven food calls are individually distinct and that the birds may be capable of differentiating between food-calling individuals.

In group-living animals, it is adaptive to recognize conspecifics on the basis of familiarity or group membership as it allows association with preferred social partners and avoidance of competitors. However, animals do not only associate with conspecifics but also with heterospecifics, for example in mixed-species flocks. Consequently, between-species recognition, based either on familiarity or even individual recognition, is likely to be beneficial. The extent to which animals can distinguish between familiar and unfamiliar heterospecifics is currently unclear. In the present study, we investigated the ability of eight carrion crows to differentiate between the voices and calls of familiar and unfamiliar humans and jackdaws. The crows responded significantly more often to unfamiliar than familiar human playbacks and, conversely, responded more to familiar than unfamiliar jackdaw calls. Our results provide the first evidence that birds can discriminate between familiar and unfamiliar heterospecific individuals using auditory stimuli.

Upon discovering food, common ravens, Corvus corax, produce far-reaching ‘haa’ calls or yells, which are individually distinct and signal food availability to conspecifics. Here, we investigated whether ravens respond differently to ‘haa’ calls of known and unknown individuals. In a paired playback design, we tested responses to ‘haa’ call sequences in a group containing individually marked free-ranging ravens. We simultaneously played call sequences of a male and a female raven in two different locations and varied familiarity (known or unknown to the local group). Ravens responded strongest to dyads containing familiar females, performing more scan flights above and by perching in trees near the respective speaker. Acoustic analysis of the calls used as stimuli showed no sex-, age- or familiarity-specific acoustic cues, but highly significant classification results at the individual level. Taken together, our findings indicate that ravens respond to individual characteristics in ‘haa’ calls, and choose whom to approach for feeding, i.e. join social allies and avoid dominant conspecifics. This is the first study to investigate responses to ‘haa’ calls under natural conditions in a wild population containing individually marked ravens.

Parental decisions in animals are often context‐dependent and shaped by fitness trade‐offs between parents and offspring. For example, the selection of breeding habitats can considerably impact the fitness of both offspring and parents, and therefore, parents should carefully weigh the costs and benefits of available options for their current and future reproductive success. Here, we show that resource‐use preferences are shaped by a trade‐off between parental effort and offspring safety in a tadpole‐transporting frog. In a large‐scale in situ experiment, we investigated decision strategies across an entire population of poison frogs that distribute their tadpoles across multiple water bodies. Pool use followed a dynamic and sequential selection process, and transportation became more efficient over time. Our results point to a complex suite of environmental variables that are considered during offspring deposition, which necessitates a highly dynamic and flexible decision‐making process in tadpole‐transporting frogs.

Acoustic ranging allows identifying the distance of a sound source and mediates inter-individual spacing and aggression in territorial species. Birds and mammals are known to use more complex cues than only sound pressure level (SPL), which can be influenced by the signaller and signal transmission in non-predictable ways and thus is not reliable by itself. For frogs, only SPL is currently known to mediate inter-individual distances, but we hypothesise that the strong territoriality of Dendrobatids could make the use of complex cues for ranging highly beneficial for this family. Therefore, we tested the ranging abilities of territorial males of Allobates femoralis (Dendrobatidae, Aromobatinae) in playback trials, using amplitude-normalized signals that were naturally degraded over distance, and synthetic signals that were masked with different levels of noise. Frogs responded significantly less to signals recorded from larger distances, regardless of SPL and signal-to-noise ratio (SNR), but showed no differential response to natural minimum and maximum SNRs across the typical communication range in wild populations. This indicates that frogs used signal amplitude and SNR only as ancillary cues when assessing the distance of sound sources and relied instead mainly on more complex cues, such as spectral degradation or reverberation. We suggest that this ability mediates territorial spacing and mate choice in A. femoralis. Good ranging abilities might also play a role in the remarkable orientation performance of this species, probably by enabling the establishment of a mental acoustic map of the habitat.Significance statementAcoustic ranging allows the distance of vocalizing competitors and mates to be identified. While birds and mammals are known to use complex cues such as temporal degradation, frequency-dependent attenuation and reverberation for ranging, previous research indicated that frogs rely only on signal amplitude (sound pressure level) to assess the distance of other callers. The present study shows for the first time that also poison frogs can make use of more complex cues, an ability which is likely to be highly beneficial in their territorial social organization and probably can also be used for orientation.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-017-2340-2) contains supplementary material, which is available to authorized users.

A fundamental attribute of social intelligence is the ability to monitor third-party relationships, which has been repeatedly demonstrated in primates, and recently also in captive ravens. It is yet unknown how ravens make use of this ability when dealing with different types of social relationships simultaneously during complex real-life situations. Free-ranging non-breeder ravens live in societies characterized by high fission–fusion dynamics and structured by age, pair-bond status and kinship. Here, we show that free-ranging ravens modify communication during conflicts according to audience composition. When being attacked by dominant conspecifics, victims of aggression signal their distress via defensive calls. Victims increased call rates when their kin were in the bystander audience, but reduced call rates when the bystanders were bonding partners of their aggressors. Hence, ravens use social knowledge flexibly and probably based on their own need (i.e. alert nearby allies and avoid alerting nearby rivals).

BackgroundAcoustic properties of vocalizations can vary with the internal state of the caller, and may serve as reliable indicators for a caller’s emotional state, for example to prevent conflicts. Thus, individuals may associate distinct characteristics in acoustic signals of conspecifics with specific social contexts, and adjust their behaviour accordingly to prevent escalation of conflicts. Common ravens (Corvus corax) crowd-forage with individuals of different age classes, sex, and rank, assemble at feeding sites, and engage in agonistic interactions of varying intensity. Attacked individuals frequently utter defensive calls in order to appease the aggressor. Here, we investigated if acoustic properties of defensive calls change with varying levels of aggression, and if bystanders respond to these changes.ResultsIndividuals were more likely to utter defensive calls when the attack involved contact aggression, and when the attacker was higher in rank than the victim. Defensive calls produced during intense conflicts were longer and uttered at higher rates, and showed higher fundamental frequency- and amplitude-related measures than calls uttered during low-intensity aggression, indicating arousal-based changes in defensive calls. Playback experiments showed that ravens were more likely to react in response to defensive calls with higher fundamental frequency by orientating towards the speakers as compared to original calls and calls manipulated in duration.ConclusionsArousal-based changes are encoded in acoustic parameters of defensive calls in attacked ravens, and bystanders in the audience pay attention to the degree of arousal in attacked conspecifics. Our findings imply that common ravens can regulate conflicts with conspecifics by means of vocalizations, and are able to gather social knowledge from conspecific calls.Electronic supplementary materialThe online version of this article (10.1186/s12983-017-0244-7) contains supplementary material, which is available to authorized users.