Do yellow-bellied marmots respond to predator vocalizations?

Behavioural changes in aposematic Heliconius melpomene butterflies in response to their predatory bird calls

Prey-predator interactions have resulted in the evolution of many anti-predatory traits. One of them is the ability for prey to listen to predators and avoid them. Although prey anti-predatory behavioural responses to predator auditory cues are well described in a wide range of taxa, studies on whether butterflies change their behaviours in response to their predatory calls are lacking. Heliconius butterflies are unpalatable and form Müllerian mimicry rings as morphological defence strategies against their avian predators. Like many other butterflies in the Nymphalidae family, some Heliconius butterflies possess auditory organs, which are hypothesized to assist with predator detection. Here we test whether Heliconius melpomene change their behaviour in response to their predatory bird calls by observing the behaviour of male and female H. m. plessini exposed to calls of Heliconius avian predators: rufous-tailed jacamar, migratory Eastern kingbird, and resident tropical kingbird. We also exposed them to the calls of the toco toucan, a frugivorous bird as a control bird call, and an amplified greenhouse background noise as a noise control. We found that individuals changed their behaviour in response to jacamar calls only. Males increased their walking and fluttering behaviour, while females did not change their behaviour during the playback of the jacamar call. Intersexual behaviours like courtship, copulation, and abdomen lifting did not change in response to bird calls. Our findings suggest that despite having primary predatory defences like toxicity and being in a mimicry ring, H. m. plessini butterflies changed their behaviour in response to predator calls. Furthermore, this response was predator specific, as H. m. plesseni did not respond to either the Eastern kingbird or the tropical kingbird calls. This suggests that Heliconius butterflies may be able to differentiate predatory calls, and potentially the birds associated with those calls.

Sensory integration of danger and safety cues may explain the fear of a quiet coyote

Sensory integration theory predicts natural selection should favour adaptive responses of animals to multiple forms of information, yet empirical tests of this prediction are rare, particularly in free-living mammals. Studying indirect predator cues offers a salient opportunity to inquire about multimodal risk assessment and its potentially interactive effects on prey responses. Here we exposed California ground squirrels from two study sites (that differ in human and domestic dog activity) to acoustic and/or olfactory predator cues to reveal divergent patterns of signal dominance. Olfactory information most strongly predicted space use within the testing arena. That is, individuals, especially those at the human-impacted site, avoided coyote urine, a danger cue that may communicate the proximity of a coyote. By contrast, subjects allocated less time to risk-sensitive behaviours when exposed to acoustic cues. Specifically, although individuals were consistent in their behavioural responses across trials, 'quiet coyotes' (urine without calls) significantly increased the behavioural reactivity of prey, likely because coyotes rarely vocalize when hunting. More broadly, our findings highlight the need to consider the evolution of integrated fear responses and contribute to an emerging understanding of how animals integrate multiple forms of information to trade off between danger and safety cues in a changing world.

White rhinos and other herbivores decrease visitations and increase vigilance in response to human vocalizations

Harnessing the fear animals have of humans has the potential to aid in the conservation of wildlife. Most vertebrates perceive humans as “super predators.” While predator cues are an important nonlethal management tool, the use of human cues for management has rarely been implemented or experimentally tested. Extensive poaching is threatening the persistence of white rhinos (Ceratotherium simum simum), and there is a need to deter them from areas with elevated poaching risks. To investigate the feasibility of harnessing the fear white rhinos have of humans to aid in their conservation, we conducted playback experiments at rhino middens. We broadcasted repeated human (treatment) and bird (control) vocalizations, and measured changes in visitations and antipredator responses. We found that overall rhino visitations did not change in response to controls but decreased by 46% in response to human vocalizations. This pattern appears to be driven by the response of females, who decreased their visitations by 70% in response to human vocalizations, while visitations by males remained unchanged. This difference is likely related to males defending small exclusive territories. Providing evidence that changes in female visitation rates were a function of the perceived fear of white rhinos, we found that both sexes exhibited more vigilance in response to human vocalizations (males 69.5%, females 96%) compared to controls. We also saw a 63% reduction of other herbivores at treatment sites. Our findings provide evidence that the fear of humans can be used to alter the movements and behavior of female white rhinos, critical for population recovery, as well as other large herbivores.

Intersexual differences in the number of genes differentially expressed in wild mammals in response to predation risk

Predation is a psychological stressor in prey animals. Besides direct killing and consumption by predators, the perception of predation risk indirectly influence prey population behavior, dynamics and physiology. Few studies identified the transcriptomic response associated with predator presence/abundance in natural populations and uncontrolled settings. However, to our knowledge, intersexual differences in the number of genes whose expression change in response to high predation risk have not been previously reported in wild mammals. Here, by using publicly available gene expression data in wild yellow-bellied marmots (Marmota flaviventer), we found that the number of differentially expressed genes in response to predator stress is higher in female marmots (n = 516) than males (n = 387). Only a small percentage of these differentially expressed genes (n = 36) are shared between the sexes, and that the most of the differentially expressed genes are expressed in a sex-specific manner in response to predation stress. Overall, our results provide new insight into sex-specific variation in gene expression changes in wild mammals under high predation risk.

Habituation or sensitization? Long-term responses of yellow-bellied marmots to human disturbance

Continuous exposure to humans causes wildlife to either habituate or sensitize. Although increased tolerance may play an important role in coexistence with humans, the mechanisms and fitness outcomes of long-term changes of tolerance are not fully understood because only a few studies have assessed individual- and population-level responses over many years. We developed a novel predictive framework to study habituation and sensitization to humans and applied it to yellow-bellied marmots (Marmota flaviventer) in areas of high and low human disturbance. We focused on two antipredator behaviors—time allocation to vigilance during foraging to quantify baseline vigilance levels and flight initiation distance (FID)—to quantify subsequent responsiveness to threat. We used the rate of body mass gain during the active season as a fitness outcome. Assessing 15 years’ population and individual-level responses to human disturbances, marmots in highly disturbed colonies allocated more time to vigilance, but this did not change over time. FID decreased on average when they were approached more and also tended to decrease in highly disturbed colonies and over 15 years. Yet, there was individuality in FIDs; marmots that fled at greater distances became sensitized with repeated approaches. Additionally, the marmots in highly disturbed colonies gained less body mass over time compared to conspecifics in less disturbed colonies. These results suggested that, although marmots habituated to humans, long-term human disturbance has negative fitness consequences. Our framework should help wildlife managers evaluate the comprehensive impact of human activities on wildlife.

Conservation implications of using an imitation carnivore to assess rarely used refuges as critical habitat features in an alpine ungulate

Understanding relationships between animals and their habitat is a central goal in ecology with important implications for conservation. Misidentified habitat requirements can have serious repercussions because land protection or reintroductions might occur in less than optimal habitat. Studies of resource selection have greatly facilitated an understanding of ecological relationships but can be improved when vital yet infrequently utilized habitat features are more fully described. A critical element for many prey species is escape terrain or some other form of refuge to avoid predation. Mountain goats (Oreamnos americanus) are well known for their use of cliffs to avoid predation, but a survey of the literature revealed at least twelve different approximations of goat escape terrain, ranging from > 25° to > 50° slopes. Here, we seek to (1) enhance estimates of mountain goat escape terrain and antipredator behavior, and (2) highlight the limitations of the assumption that the time an animal spends in an area is proportional to importance. To improve estimates of goat escape terrain, we conducted field work across two years (2014–15) in Glacier National Park, Montana USA and manipulated apparent predation risk by exposing mountain goats to a threatening simulated grizzly bear (Ursus arctos) treatment and a non-threatening ungulate (control) treatment. Mountain goats moved in response to the simulated bear but not in response to the simulated ungulate, with shorter latencies to move for subjects in larger groups and at shorter distances to the simulated threat. Through a used-unused resource selection function we tested 22 landscape variables to describe the use of escape terrain. Proximity to slopes greater than 60° best explained the locations to which mountain goats fled after exposure to the simulated bear, and the average slope of these escape locations was 56.5° (±14.1 S.D.). Our results suggest that mountain goat escape terrain be considered at slopes of 60° as a minimum because our simulated threat did not include pursuit of goats and, thus, slopes of 60° are likely underestimates of actual escape terrain. Additionally, because direct interactions between carnivores and goats seldom occur, serious escape terrain is infrequently used. Past estimates of escape may have miscalculated the slopes which goats select for in response to predation risk. Based on experimental approaches in the wild, we suggest that anti-predator behavior should be included in studies of resource selection when the goal is to consider habitat as a predictor for conservation success. Finally, we discuss evidence suggesting a past mountain goat introduction failed due to lack of adequate escape terrain and subsequent recolonization of a predator.

Predator-induced fear causes PTSD-like changes in the brains and behaviour of wild animals

Predator-induced fear is both, one of the most common stressors employed in animal model studies of post-traumatic stress disorder (PTSD), and a major focus of research in ecology. There has been a growing discourse between these disciplines but no direct empirical linkage. We endeavoured to provide this empirical linkage by conducting experiments drawing upon the strengths of both disciplines. Exposure to a natural cue of predator danger (predator vocalizations), had enduring effects of at least 7 days duration involving both, a heightened sensitivity to predator danger (indicative of an enduring memory of fear), and elevated neuronal activation in both the amygdala and hippocampus – in wild birds (black-capped chickadees, Poecile atricapillus), exposed to natural environmental and social experiences in the 7 days following predator exposure. Our results demonstrate enduring effects on the brain and behaviour, meeting the criteria to be considered an animal model of PTSD – in a wild animal, which are of a nature and degree which can be anticipated could affect fecundity and survival in free-living wildlife. We suggest our findings support both the proposition that PTSD is not unnatural, and that long-lasting effects of predator-induced fear, with likely effects on fecundity and survival, are the norm in nature.

Assessing blue wildebeests' vigilance, grouping and foraging responses to perceived predation risk using playback experiments

Two aspects of reactive antipredator behaviour are still unclear for ungulates. First, when there is a direct predation threat, how do prey balance antipredator and social vigilance to learn a predator's location and assess the risk? Second, how do an individual's group and environment affect its responses? We tested the responses of adult females in 101 groups of wildebeest to playbacks of lion roars or car noises in Etosha National Park, Namibia. We analysed how the times they spent in different types categories of vigilance, and their within-group density, were affected by the playbacks and how a range of social and environmental variables affected those responses. Females increased their antipredator vigilance but not their social vigilance, after lion roars but not car noises, suggesting that they mostly relied on their own vigilance rather than social information to try to find the source of the lion roars. Females' antipredator vigilance increased more when they were further from cover and with other prey species, suggesting that both circumstances increased their perception of risk. They 'bunched' more after lion roars than car noises and their bite rates decreased as they bunched. Animals' use of social information about threats is likely to be context-dependent.

Vocal recognition of a nest-predator in black grouse

Corvids count among the important predators of bird nests. They are vocal animals and one can expect that birds threatened by their predation, such as black grouse, are sensitive to and recognize their calls. Within the framework of field studies, we noticed that adult black grouse were alerted by raven calls during periods outside the breeding season. Since black grouse are large, extremely precocial birds, this reaction can hardly be explained by sensitization specifically to the threat of nest predation by ravens. This surprising observation prompted us to study the phenomenon more systematically. According to our knowledge, the response of birds to corvid vocalization has been studied in altricial birds only. We tested whether the black grouse distinguishes and responds specifically to playback calls of the common raven. Black grouse recognized raven calls and were alerted, displaying typical neck stretching, followed by head scanning, and eventual escape. Surprisingly, males tended to react faster and exhibited a longer duration of vigilance behavior compared to females. Although raven calls are recognized by adult black grouse out of the nesting period, they are not directly endangered by the raven. We speculate that the responsiveness of adult grouse to raven calls might be explained as a learned response in juveniles from nesting hens that is then preserved in adults, or by a known association between the raven and the red fox. In that case, calls of the raven would be rather interpreted as a warning signal of probable proximity of the red fox.

Nest predation risk modifies nestlings' immune function depending on the level of threat

Predation risk is thought to modify the physiology of prey mainly through the stress response. However, little is known about its potential effects on the immunity of animals, particularly in young individuals, despite the importance of overcoming wounding and pathogen aggression following a predator attack. We investigated the effect of four progressive levels of nest predation risk on several components of the immune system in common blackbird (Turdus merula) nestlings by presenting them with four different calls during 1 h: non-predator calls, predator calls, parental alarm calls and conspecific distress calls to induce a null, moderate, high and extreme level of risk, respectively. Nest predation risk induced an increase in ovotransferrin, immunoglobulin and the number of lymphocytes and eosinophils. Thus, the perception of a potential predator per se could stimulate the mobilization of a nestling's immune function and enable the organism to rapidly respond to the immune stimuli imposed by a predator attack. Interestingly, only high and extreme levels of risk caused immunological changes, suggesting that different immunological parameters are modulated according to the perceived level of threat. We also found a mediator role of parasites (i.e. Leucocytozoon) and the current health status of the individual, as only nestlings not parasitized or in good body condition were able to modify their immune system. This study highlights a previously unknown link between predation risk and immunity, emphasizing the complex relationship among different selective pressures (predation, parasitism) in developing organisms and accentuating the importance of studying predation from a physiological point of view.

Receivers matter: the meaning of alarm calls and competition for nest sites in a bird community

Animal communities may constitute information networks where individuals gain information on predation risk by eavesdropping on alarm calls of other species. However, communities include species in different trophic levels, and it is not yet known how the trophic level of the receiver influences the informative value of a call. Furthermore, no empirical study has yet tested how increased competition may influence the value of alarm calls for distinct receivers. Here, we identify the importance of alarm calls emitted by a small owl, the little owl (Athene noctua), on the structure of a cavity-nesting bird community including mesopredators and primary prey under variable levels of competition for nest holes. Competitors sharing top predators with the callers and prey of the callers interpreted alarm and non-alarm calls differently. Competitors chose preferentially alarm and non-alarm patches over control patches to breed, while prey selected alarm patches. In contrast, competition for nest sites affected habitat selection of prey species more than that of competitors of the callers. This study provides support for a changing value of alarm calls and competition for nest sites for distinct receivers related to niche overlapping among callers and eavesdroppers, therefore, calling attention to possible cascading effects by the use of information in natural communities.

Female in-nest chatter song increases predation

Female song is an ancestral trait in songbirds, yet extant females generally sing less than males. Here, we examine sex differences in the predation cost of singing behaviour. The superb fairy-wren (Malurus cyaneus) is a Southern Hemisphere songbird; males and females provision the brood and produce solo song year-round. Both sexes had higher song rate during the fertile period and lower song rate during incubation and chick feeding. Females were more likely than males to sing close to or inside the nest. For this reason, female but not male song rate predicted egg and nestling predation. This study identifies a high fitness cost of song when a parent bird attends offspring inside a nest and explains gender differences in singing when there are gender differences in parental care.

Fear of large carnivores causes a trophic cascade

The fear large carnivores inspire, independent of their direct killing of prey, may itself cause cascading effects down food webs potentially critical for conserving ecosystem function, particularly by affecting large herbivores and mesocarnivores. However, the evidence of this has been repeatedly challenged because it remains experimentally untested. Here we show that experimentally manipulating fear itself in free-living mesocarnivore (raccoon) populations using month-long playbacks of large carnivore vocalizations caused just such cascading effects, reducing mesocarnivore foraging to the benefit of the mesocarnivore's prey, which in turn affected a competitor and prey of the mesocarnivore's prey. We further report that by experimentally restoring the fear of large carnivores in our study system, where most large carnivores have been extirpated, we succeeded in reversing this mesocarnivore's impacts. We suggest that our results reinforce the need to conserve large carnivores given the significant “ecosystem service” the fear of them provides.

Top predators may indirectly influence ecological processes through fear-induced behavioural changes in their prey. By experimentally manipulating this ‘landscape of fear', Suraci et al. show that fear of large carnivores in a mesopredator can cause cascading effects down the food web that benefit its prey.

Anti-predator behaviour of kudu and impala in response to mimicked African wild dog presence: do age and sex matter?

Predators not only prey upon certain prey species, but also on certain age–sex classes within species. Predation risk and an individual’s response to this risk might therefore vary with an individual’s characteristics. We examined the proportion of time different age–sex classes of kudu (Tragelaphus strepsiceros) and impala (Aepyceros melampus) spent high quality vigilant (costly vigilance that detracts from all other activities) in response to mimicked predation risk by African wild dogs (Lycaon pictus). For both species predation risk was the main factor determining the investment in high quality vigilance behaviour. Age–sex class-specific responses were not related to age–sex class specific lethality risk presented by African wild dogs. For impala, regardless of predation risk, age seemed to have some effect on the investment in high quality vigilance with sub-adult impala spending more time high quality vigilant than adult impala, which is possibly why African wild dogs predominantly preyed upon adult impala.

The sensory ecology of nonconsumptive predator effects

Nonconsumptive effects (NCEs) have been shown to occur in numerous systems and are regarded as important mechanisms by which predation structures natural communities. Sensory ecology-that is, the processes governing the production, propagation, and masking of cues by ambient noise-provides insights into the strength of NCEs as functions of the environment and modes of information transfer. We discuss how properties of predators are used by prey to encode threat, how the environment affects cue propagation, and the role of single sensory processes versus multimodal sensory processes. We discuss why the present body of literature documents the potential for strong NCEs but does not allow us to easily determine how this potential is expressed in nature or what factors or environments produce strong versus weak NCEs. Many of these difficulties stem from a body of literature in which certain sensory environments and modalities may be disproportionately represented and in which experimental methodologies are designed to show the existence of NCEs. We present a general framework for examining NCEs to identify the factors controlling the number of prey that respond to predator cues and discuss how the properties of predators, prey, and the environment may determine prey perceptive range and the duration and frequency of cue production. We suggest how understanding these relationships provides a schema for determining where, when, why, and how NCEs are important in producing direct and cascading effects in natural communities.

Short-term behavioural responses of impalas in simulated antipredator and social contexts

Prey animals often have to trade off foraging against vigilance. However, vigilance is costly and individuals are expected to adjust their vigilance and its cost in relation to social cues and their predation risk. To test this, we conducted playback experiments in the field to study how lions’ (Panthera leo) roars and male impalas’ (Aepyceros melampus) territorial vocalizations affected the vigilance and foraging behaviours as well as movements of female impalas. Our results show that impalas adjusted their activities in different ways depending on the vocalizations broadcast. After lions’ roars were played, female impalas increased their vigilance activity (in particular increasing their high-cost vigilance – vigilance without chewing), decreased their bite rates and increased their movements, whereas male impalas’ vocalizations caused females to decrease their vigilance (decreasing their low-cost vigilance – vigilance while chewing) and increase their movements without affecting their bite rates. Therefore, it appears that predators’ vocalizations stimulate anti-predator behaviours such as vigilance and movement at the expense of foraging, whereas males’ vocalizations increase individuals’ displacements at the expense of vigilance. Overall, this study shows that both predator and social cues have direct effects on the behaviour of gregarious prey and need to be considered in future studies.

The sound of danger: threat sensitivity to predator vocalizations, alarm calls, and novelty in gulls

The threat sensitivity hypothesis predicts that organisms will evaluate the relative danger of and respond differentially to varying degrees of predation threat. Doing so allows potential prey to balance the costs and benefits of anti-predator behaviors. Threat sensitivity has undergone limited testing in the auditory modality, and the relative threat level of auditory cues from different sources is difficult to infer across populations when variables such as background risk and experience are not properly controlled. We experimentally exposed a single population of two sympatric gull species to auditory stimuli representing a range of potential threats in order to compare the relative threat of heterospecific alarm calls, conspecific alarms calls, predator vocalizations, and novel auditory cues. Gulls were able to discriminate among a diverse set of threat indicators and respond in a graded manner commensurate with the level of threat. Vocalizations of two potential predators, the human voice and bald eagle call, differed in their threat level compared to each other and to alarm calls. Conspecific alarm calls were more threatening than heterospecfic alarm calls to the larger great black-backed gull, but the smaller herring gull weighed both equally. A novel cue elicited a response intermediate between known threats and a known non-threat in herring gulls, but not great black-backed gulls. Our results show that the relative threat level of auditory cues from different sources is highly species-dependent, and that caution should be exercised when comparing graded and threshold threat sensitive responses.

Host response to cuckoo song is predicted by the future risk of brood parasitism

Introduction

Risk assessment occurs over different temporal and spatial scales and is selected for when individuals show an adaptive response to a threat. Here, we test if birds respond to the threat of brood parasitism using the acoustical cues of brood parasites in the absence of visual stimuli. We broadcast the playback of song of three brood parasites (Chalcites cuckoo species) and a sympatric non-parasite (striated thornbill, Acanthiza lineata) in the territories of superb fairy-wrens (Malurus cyaneus) during the peak breeding period and opportunistic breeding period. The three cuckoo species differ in brood parasite prevalence and the probability of detection by the host, which we used to rank the risk of parasitism (high risk, moderate risk, low risk).

Results

Host birds showed the strongest response to the threat of cuckoo parasitism in accordance with the risk of parasitism. Resident wrens had many alarm calls and close and rapid approach to the playback speaker that was broadcasting song of the high risk brood parasite (Horsfield’s bronze-cuckoo, C. basalis) across the year (peak and opportunistic breeding period), some response to the moderate risk brood parasite (shining bronze-cuckoo, C. lucidus) during the peak breeding period, and the weakest response to the low risk brood parasite (little bronze-cuckoo, C. minutillus). Playback of the familiar control stimulus in wren territories evoked the least response.

Conclusion

Host response to the threat of cuckoo parasitism was assessed using vocal cues of the cuckoo and was predicted by the risk of future parasitism.

Yellow-bellied marmots: insights from an emergent view of sociality

Ecological factors explain variation in sociality both within and between species of marmots-large alpine ground squirrels. Fifty years of study, by me and my colleagues, of the yellow-bellied marmots (Marmota flaviventris) at the Rocky Mountain Biological Laboratory, near Crested Butte, CO, USA, has created opportunities to see how sociality changes with population and group size. Over the past decades, we have witnessed a natural experiment whereby the population tripled in size. If we view sociality as an emergent process, then demography acts as a constraint on interactions between individuals, and the threefold increase in population size should have consequences for group structure. We have used social network statistics to study the causes and consequences of social interactions by capitalizing on this demographic variation. Such an emergent view is ideally studied in an integrative Tinbergian way that focuses on both mechanism and function. We have determined that some social attributes are heritable, that social cohesion is established through age and kin structure, that well-embedded females (but not males) are less likely to disperse, and that there are fitness consequences of social attributes. Together, this integrative relationship-centred view expands on the traditional ecological model of sociality and offers a framework that can be applied to other systems.

Age and sex influence marmot antipredator behavior during periods of heightened risk

Animals adjust their antipredator behavior according to environmental variation in risk, and to account for their ability to respond to threats. Intrinsic factors that influence an animal's ability to respond to predators (e.g., age, body condition) should explain variation in antipredator behavior. For example, a juvenile might allocate more time to vigilance than an adult because mortality as a result of predation is often high for this age class; however, the relationship between age/vulnerability and antipredator behavior is not always clear or as predicted. We explored the influence of intrinsic factors on yellow-bellied marmot (Marmota flaviventris) antipredator behavior using data pooled from 4 years of experiments. We hypothesized that inherently vulnerable animals (e.g., young, males, and individuals in poor condition) would exhibit more antipredator behavior prior to and immediately following conspecific alarm calls. As expected, males and yearlings suppressed foraging more than females and adults following alarm call playbacks. In contrast to predictions, animals in better condition respond more than animals in below average condition. Interestingly, these intrinsic properties did not influence baseline time budgets; animals of all ages, sexes, and condition levels devoted comparable amounts of time to foraging prior to alarm calls. Our results support the hypothesis that inherent differences in vulnerability influence antipredator behavior; furthermore, it appears that a crucial, but poorly acknowledged, interaction exists between risk and state-dependence. Elevated risk may be required to reveal the workings of state-dependent behavior, and studies of antipredator behavior in a single context may draw incomplete conclusions about age- or sex-specific strategies.

Behavioral evidence for community-wide species discrimination from echolocation calls in bats

Recognizing species identity is crucial for many aspects of animal life and is often mediated by acoustic signals. Although most animals are able to distinguish acoustic signals of their own species from other sympatrically occurring species, it is yet unknown whether animals can distinguish among acoustic signals of different closely related sympatric species. In this context, echolocating bats are a particularly interesting model system: their echolocation system evolved primarily for spatial orientation and foraging, but recent studies indicate that echolocation also has an important communicative function. Yet, the role of echolocation calls for species discrimination and thus potentially for interspecific communication has not been investigated. Using a behavioral discrimination assay, we found that two species of wild horseshoe bats could discriminate calls of their own species from those of three sympatric congeneric species. We further show that the bats were able to discriminate between echolocation calls of different congeneric species from the local community. In both cases, discrimination ability was high despite strong overlap of species' call frequency bands. This study provides the first experimental evidence for species discrimination based on echolocation calls. On a more general level, it shows for the first time that animals can distinguish among acoustic signals of different closely related and ecologically similar species from their local community.

Two sympatric species of passerine birds imitate the same raptor calls in alarm contexts

While some avian mimics appear to select sounds randomly, other species preferentially imitate sounds such as predator calls that are associated with danger. Previous work has shown that the Greater Racket-tailed Drongo (Dicrurus paradiseus) incorporates predator calls and heterospecific alarm calls into its own species-typical alarm vocalizations. Here, we show that another passerine species, the Sri Lanka Magpie (Urocissa ornata), which inhabits the same Sri Lankan rainforest, imitates three of the same predator calls that drongos do. For two of these call types, there is evidence that magpies also use them in alarm contexts. Our results support the hypothesis that imitated predator calls can serve as signals of alarm to multiple species.

Frequency information in the vibration-cued escape hatching of red-eyed treefrogs

Incidental acoustic and vibrational cues generated by predators are a potential source of information for prey assessing risk. Substrate vibrations should be excited by most predators, and frequency, amplitude or temporal properties could allow prey to distinguish predator from benign-source vibrations. Red-eyed treefrog embryos detect egg predators using vibrations excited during attacks, hatching rapidly and prematurely to escape. We recorded vibrations in egg clutches during attacks by five species of predators and three common types of benign physical disturbance. We analyzed their frequency distributions to assess if and how frequency properties could be used to discriminate between vibration sources and used vibration playbacks to examine the effects of frequency properties on the escape hatching response. Vibrations produced by predators and benign disturbances generally have broad and overlapping frequency distributions, and all frequencies excited by attacks are also excited by benign disturbances. Decision rules based on the frequency distribution of vibrations alone would therefore result in either high levels of hatching in response to benign vibrations (false alarms) or common failures to hatch in response to predators (missed cues). Nevertheless, embryos hatch in response to predator and not benign disturbances in nature, and our playback results show that vibration frequency information is an important component of their hatching decision. Embryos combine frequency with temporal information to refine their hatching response. Moreover, comparing frequency spectra of predator and benign vibrations suggests that the presence of energy in frequencies outside the range characteristic of attacks might serve as an indicator of benign disturbance.