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A major evolutionary force driving functionally referential alarm calls is the need for different strategies to escape various predator types in complex structured habitats. In contrast, a single escape strategy appears to be sufficient in less‐structured open habitats, and under such conditions urgency‐dependent alarm calls may be favored. Nevertheless, some species, such as meerkats (Suricata suricatta), have evolved functionally referential alarm calls despite living in open areas, using only bolt‐holes for retreat. To understand the evolution of different alarm call systems, we investigated the calls of sympatric Cape ground squirrels (Xerus inauris) and compared their antipredator and foraging behavior with that of meerkats. Cape ground squirrels emitted urgency‐dependent alarm calls and responded to playbacks depending on urgency, not predator type. Vigilance behavior and habitat use differed between the two species. Meerkats roam widely to find prey and for efficient foraging depend on coordinated predator vigilance and escape behavior. As herbivores with smaller territories, Cape ground squirrels depend less on coordinated antipredator behavior, and urgency‐dependent alarm calls encode all essential information. We conclude that habitat complexity does not explain the evolution of functionally referential alarm calls in all species, and other constraints, such as the need to coordinate group movements to maintain foraging efficiency, could be more relevant.

In the perpetual struggle between high-speed predators and their prey, individuals need to react in the blink of an eye to avoid capture. Alarm calls that warn of danger therefore need to do so sufficiently rapidly that listeners can escape in time. Paradoxically, many species produce more elements in their alarm calls when signalling about more immediate danger, thereby increasing the reliability of transmission of critical information but taking longer to convey the urgent message. We found that New Holland honeyeaters, Phylidonyris novaehollandiae , incorporated more elements in alarm calls given to more dangerous predators, but video analysis revealed that listeners responded in 100 ms, after only the first element. Consistent with this rapid response, the acoustic structure of the first element varied according to the danger, and playbacks confirmed that birds need hear only the first element to assess risk. However, birds hid for longer and were more likely to flee, after calls with more elements. The dual mechanisms of varying both element structure and number may provide a widespread solution to signalling rapidly and reliably about immediate danger.

Environmental noise from anthropogenic and other sources affects many aspects of animal ecology and behaviour, including acoustic communication. Acoustic masking is often assumed in field studies to be the cause of compromised communication in noise, but other mechanisms could have similar effects. We tested experimentally how background noise disrupted the response to conspecific alarm calls in wild superb fairy‐wrens, Malurus cyaneus, assessing the effects of acoustic masking, distraction and changes in vigilance. We first examined the birds' response to alarm‐call playbacks accompanied by different amplitudes of background noise that overlapped the calls in acoustic frequency. We then scored and videoed their response to alarm calls in two types of background noise, that did or did not overlap call frequency, but were broadcast at a constant amplitude. Birds were less likely to flee to alarm calls in higher amplitudes of overlapping noise, demonstrating that noise itself compromised communication independently of environmental correlates. Background noise affected the response only if it overlapped in frequency with the alarm calls, implying that the effect was not due to distraction. Further, birds were equally vigilant during background noise of overlapping or non‐overlapping frequency, indicating that the lack of response to alarm calls in overlapping noise was not due to enhanced vigilance and awareness that there was no predator. We conclude that alarm‐call reception was compromised by masking, a mechanism that is often assumed but rarely tested in an ecological context. Masking compromised reception of high‐frequency “aerial” alarm calls and so could reduce survival in background noise of similar frequency. While anthropogenic noise, which is often of lower frequency, is unlikely to affect communication with these calls, it could affect reception of acoustic cues of danger, or other conspecific or heterospecific alarm calls. A plain language summary is available for this article. Plain Language Summary

Background Animals often benefit from the alarm calls of other species to detect danger, but how such cues are integrated into vigilance strategies remains unclear. Giraffes ( Giraffa spp.) rely on early threat detection to avoid ambush and are known hosts of red-billed oxpeckers ( Buphagus erythrorhynchus ), which form mutualistic associations with large mammals by feeding on ectoparasites and emitting alarm calls in response to approaching threats. While these calls are thought to provide early-warning benefits, it remains unclear how giraffes interpret them, and whether their responses vary with prior exposure to predation risk. Results We conducted playback experiments across three giraffe populations differing in predator presence to test whether giraffes adjust vigilance in response to oxpecker alarm calls. Individuals in the predator-inhabited reserve maintained vigilance longer than those in predator-free areas, suggesting that prior exposure enhances responsiveness to alarm calls. Acoustic analyses revealed that oxpecker alarm calls are characterized by low harmonic-to-noise ratios, consistent with harsh, broadband signals that are known to enhance attention and urgency perception in alarm contexts. However, call structure alone did not explain vigilance responses; instead responses were modulated by ecological context, specifically whether giraffes lived in areas with or without lions. Conclusions Our findings suggest that oxpeckers serve a sentinel-like function and that giraffes use their alarm calls as early-warning signals, with stronger responses observed in populations exposed to predators. This supports the idea that eavesdropping on heterospecific alarm calls can provide context-dependent benefits, with predator-experienced giraffes showing greater sensitivity to oxpecker alarms. By linking behavioral flexibility with ecological context, this study offers a framework for understanding how mutualistic communication systems adapt to changing predation pressures.

Avian vocal mimicry has typically been examined through the lens of sexual selection acting on males. However, the females of many bird species are accomplished vocal mimics; a fact that cannot be accounted for by traditional male‐centric explanations for vocal mimicry. Female superb lyrebirds Menura novaehollandiae mimic primarily during nest defence, whilst male lyrebirds mimic predominantly during sexual advertisement. Here we examined the relationship between female age and vocal mimicry using a dataset of nesting female lyrebirds, several of which were recorded over multiple years. The vocal mimicry produced by females was diverse, and individuals varied greatly in how often they mimicked and what models they mimicked; however, neither the propensity to mimic nor the number of model sounds was explained by female age. Nevertheless, older females were more likely to mimic predators than younger females. There are two main implications of these findings. First, age is unlikely to explain intra‐population variation in female mimetic repertoires. Second, females might fine‐tune their mimetic repertoires as they age and mimic only models that are most effective during nest defence, such as predators. We discuss what these results mean for our understanding of vocal mimicry and vocal learning in songbirds of both sexes.

Increased predation risk is considered a cost of having conspicuous colours, affecting the anti-predator behaviour of colourful animals. However, this is difficult to test, as individual factors often covary with colour and behaviour. We used alarm call playback and behavioural observations to assess whether individual birds adjust their response to risk according to their plumage colour. Male superb fairy-wrens (Malurus cyaneus) change from a dull brown to conspicuous blue plumage each year, allowing the behaviour of different coloured birds to be compared while controlling for withinindividual effects. Because the timing of colour change varies among males, blue and brown birds can also be compared at the same time of year, controlling for seasonal effects on behaviour. While blue, fairy-wrens fled more often in response to alarm calls, and took longer to emerge from cover. Blue fairy-wrens also spent more time foraging in cover and being vigilant. Group members appeared to benefit from the presence of blue males, as they reduced their response to alarms, and allocated less time to sentinel behaviour when a blue male was close by. We suggest that fairywrens perceive themselves to be at a higher risk of predation while in conspicuous plumage and adjust their behaviour accordingly.

Species facing similar selection pressures should recognize heterospecific alarm signals. However, no study has so far examined heterospecific alarm-call recognition in response to parasitism by cuckoos. In this study, we tested whether two sympatric host species of the common cuckoo Cuculus canorus, Oriental reed warbler Acrocephalus orientalis (ORW, main host), and black-browed reed warbler Acrocephalus bistrigiceps (BRW, rare host), could recognize each other’s alarm calls in response to cuckoos. Dummies of common cuckoo (parasite) and Eurasian sparrowhawk Accipiter nisus (predator) were used to induce and record alarm calls of the two warbler species, respectively. In the conspecific alarm-call playback experiments, ORW responded more strongly to cuckoo alarm calls than to sparrowhawk alarm calls, while BRW responded less strongly to cuckoo alarm calls than to sparrowhawk alarm calls. In the heterospecific alarm-call playback experiments, both ORW and BRW responded less strongly to cuckoo alarm calls than sparrowhawk alarm calls. BRW seemed to learn the association between parasite-related alarm calls of the ORW and the cuckoo by observing the process of ORW attacking cuckoos. In contrast, alarm calls of BRW to cuckoos were rarely recorded in most cases. BRW with low parasite pressure still developed recognition of heterospecific parasite-related alarm call. Unintended receivers in the same community should recognize heterospecific alarm calls precisely to extract valuable information.

Several theories have been generated to understand the socio-cognitive mechanisms underlying the unique cooperative abilities of humans. The ‘interdependence hypothesis' posits first, that the cognitive dimension of human cooperation evolved in contexts when several individuals needed to act together to achieve a common goal, like when hunting large prey. Second, the more interdependent individuals are, the more likely they are to provide services to conspecifics in other contexts. Alternatively, the ‘social tolerance hypothesis' proposes that higher social tolerance allows conspecifics to cooperate more efficiently and with a wider range of partners. We conducted the first field experimental evaluation of both hypotheses in our closest living relatives by contrasting chimpanzees to the less interdependent but more tolerant bonobos. We compared each species' performance during a cooperative task: informing conspecifics about a danger. We presented Gaboon viper models to 82 individuals from five wild communities. Chimpanzees arriving late at the snake were significantly more likely to have heard a call and less likely to startle, indicating that chimpanzees were better informed about the presence of the threat than bonobos. This stems from clear species differences in how individuals adjusted their calling decisions to the level of information already available. Chimpanzees were more likely to call and produced more alarm calls when they had not yet heard a call, whereas bonobos did so when they already heard a call. Our results confirm the link between interdependence and cooperation performance. These species differences were most likely driven by differences in motivation rather than in cognitive capacities because both species tended to consider audience knowledge in their decision to call. Our results inform theories on the evolution of human cooperation by linking inter-group competition pressure and in-group cooperative motivation and/or capability.

Animals use vocal signals to provide information across a wide range of contexts. However, it is more complex to identify the information content when the same vocalizations are produced in different contexts, such as against predators and conspecific competitors. This raises the question whether information about the caller or context can be coded in relatively subtle variation within the call type and whether receivers respond differentially to such variation. Using playbacks of the general ‘chatter’ alarm call of Eurasian magpies ( Pica pica ), we tested whether or not territorial magpies respond differently to variation in call duration and rate. We show that magpies responded acoustically faster to chatters of longer duration, which may signal a greater motivation or urgency of territorial intruders. Alternatively, a delay in chatter response to shorter calls may reflect a period of hesitation and risk avoidance. Interestingly, magpies did not approach the loudspeaker more closely in response to longer calls and the total chatter response did not differ either. This suggests that after the short initial response, the persistence of the signal over time as well as visual information on the level or type of danger become essential for more differentiated response behaviours. Taken together, our results show that magpies perceive and respond differentially to variation in alarm call duration, suggesting that such variation encodes meaningful information.