Does information of predators influence general wariness?

Identifying the most effective behavioural assays and predator cues for quantifying anti-predator responses in mammals: a systematic review

BACKGROUND

Mammals, globally, are facing population declines. Protecting and breeding threatened populations inside predator-free havens and translocating them back to the wild is commonly viewed as a solution. These approaches can expose predator-naïve animals to predators they have never encountered and as a result, many conservation projects have failed due to the predation of individuals that lacked appropriate anti-predator responses. Hence, robust ways to measure anti-predator responses are urgently needed to help identify naïve populations at risk, to select appropriate animals for translocation, and to monitor managed populations for changes in anti-predator traits. Here, we undertake a systematic review that collates existing behavioural assays of anti-predator responses and identifies assay types and predator cues that provoke the greatest behavioural responses.

METHODS

We retrieved articles from academic bibliographic databases and grey literature sources (such as government and conservation management reports), using a Boolean search string. Each article was screened against eligibility criteria determined using the PICO (Population-Intervention-Comparator-Outcome) framework. Using data extracted from each article, we mapped all known behavioural assays for quantifying anti-predator responses in mammals and examined the context in which each assay has been implemented (e.g., species tested, predator cue characteristics). Finally, with mixed effects modelling, we determined which of these assays and predator cue types elicit the greatest behavioural responses based on standardised difference in response between treatment and control groups.

REVIEW FINDINGS

We reviewed 5168 articles, 211 of which were eligible, constituting 1016 studies on 126 mammal species, a quarter of which are threatened by invasive species. We identified six major types of behavioural assays: behavioural focals, capture probability, feeding station, flight initiation distance, giving-up density, and stimulus presentations. Across studies, there were five primary behaviours measured: activity, escape, exploration, foraging, and vigilance. These behaviours yielded similar effect sizes across studies. With regard to study design, however, studies that used natural olfactory cues tended to report larger effect sizes than those that used artificial cues. Effect sizes were larger in studies that analysed sexes individually, rather than combining males and females. Studies that used 'blank' control treatments (the absence of a stimulus) rather than a treatment with a control stimulus had higher effect sizes. Although many studies involved repeat measures of known individuals, only 15.4% of these used their data to calculate measures of individual repeatability.

CONCLUSIONS

Our review highlights important aspects of experimental design and reporting that should be considered. Where possible, studies of anti-predator behaviour should use appropriate control treatments, analyse males and females separately, and choose organic predator cues. Studies should also look to report the individual repeatability of behavioural traits, and to correctly identify measures of uncertainty (error bars). The review highlights robust methodology, reveals promising techniques on which to focus future assay development, and collates relevant information for conservation managers.

Beyond simple habituation: Anthropogenic habitats influence the escape behaviour of spur-winged lapwings in response to both human and non-human threats

Habitat development may affect wildlife behaviour, favouring individuals or behaviours that cope better with perceived threats (predators). Bolder behaviours in human-dominated habitats (HDH; e.g. urban and rural settlements) may represent habituation specifically to humans, or a general reduction in predator-avoidance response. However, such carry-over effects across threat types (i.e. beyond humans) and phases of the escape sequence have not been well studied to date. Here we investigated escape behaviours of a locally common wader species, the spur-winged lapwing Vanellus spinosus. We assayed their flight initiation distance (FID) and subsequent escape behaviours in agricultural areas and in HDH. We found that lapwings in HDH were bolder, and that the difference was manifested in several phases of the predator-avoidance sequence (shorter FIDs, shorter distances fled, and a higher probability of escape by running vs. flying). When re-approached (by an observer) after landing, lapwings in HDH were also more repetitive in their FID than those in other habitats. To determine whether this apparent bolder behaviour in HDH areas is merely a consequence of habituation to humans or represents a broader behavioural change, we introduced an additional threat type-a remotely-operated taxidermic jackal ('Jack-Truck'). Finding bolder responses in the HDH to the human threat alone (and not to the Jack-Truck) could have supported the habituation hypothesis. In contrast, however, we found a bolder response in the HDH to both threat types, as well as a correlation between their FIDs across different sites. These bolder behaviours suggest that HDH impose a broader behavioural change on lapwings, rather than just simple habituation. Overall, our findings demonstrate how FID trials can reveal strong behavioural carry-over effects of HDH following human and non-human threats, including effects on the subsequent phases of escaping the predator. Further, FID assays may reveal consistent behavioural types when assessed under field conditions, and offer a direct way to differentiate among the various poorly understood and non-mutually exclusive mechanisms that lead to behavioural differences among organisms in HDH. The mechanistic perspective is essential for understanding how rapid urbanization impacts wildlife behaviour, populations, and the range of behaviours within them, even in species apparently resilient to such environmental changes.

What Does an Insect Hear? Reassessing the Role of Hearing in Predator Avoidance with Insights from Vertebrate Prey

Insects have a diversity of hearing organs known to function in a variety of contexts, including reproduction, locating food, and defense. While the role of hearing in predator avoidance has been extensively researched over the past several decades, this research has focused on the detection of one type of predator-echolocating bats. Here we reassess the role of hearing in antipredator defense by considering how insects use their ears to detect and avoid the wide range of predators that consume them. To identify the types of sounds that could be relevant to insect prey, we first review the topic of hearing-mediated predator avoidance in vertebrates. Sounds used by vertebrate prey to assess predation risk include incidental sound cues (e.g., flight sounds, rustling vegetation, and splashing) produced by an approaching predator or another escaping prey, as well as communication signals produced by a predator (e.g., echolocation calls, songs) or nonpredator (e.g., alarm calls). We then review what is known, and what is not known, about such sounds made by the main predators and parasitoids of insects (i.e., birds, bats, terrestrial vertebrates, and invertebrates) and how insects respond to them. Three key insights emerged from our review. First, there is a lack of information on how both vertebrate and insect prey use passive sound cues produced by predators to avoid being captured. Second, while there are numerous examples of vertebrate prey eavesdropping on the calls and songs of predators and nonpredators to assess risk, there are currently no such examples for eared insect prey. Third, the hearing sensitivity of many insects, including those with ears considered to be dedicated to detecting bats or mates, overlaps with both sound cues and signals generated by nonbat predators. Sounds of particular relevance to insect prey include the flight sounds and calls of insectivorous birds, the flight sounds of insect predators and parasitoids, and rustling vegetation sounds of birds and terrestrial predators. We conclude that research on the role of insect hearing in predator avoidance has been disproportionally focused on bat-detection, and that acoustically-mediated responses to other predators may have been overlooked because the responses of prey may be subtle (e.g., ceasing activity, increasing vigilance). We recommend that researchers expand their testing of hearing-mediated risk assessment in insects by considering the wide range of sounds generated by predators, and the varied responses exhibited by prey to these sounds.

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.

Risk allocation: acute and chronic predator exposure have contrasting effects on Song Sparrow (Melospiza melodia) singing behaviour

Increasing the danger posed by predators may cause prey animals to alter their behaviour. For example, they may be more vigilant and so feed more slowly. Breeding male Song Sparrows (Melospiza melodia (A. Wilson, 1810)) spend much time in conspicuous, loud song, which is an important behaviour for territorial defense and for mate attraction. We measured their singing behaviour in relation to both chronic (active Cooper’s Hawk (Accipiter cooperii (Bonaparte, 1828)) nest nearby) and acute (playback of hawk calls) predator exposure. We found that proximity to a Cooper’s Hawk nest had little or no influence. In contrast, the response to acute exposure was strong and immediate: Song Sparrows reduced the song rate and the proportion of time spent singing, lowered perch height, and increased concealment. The decline in the amount of song during the few minutes following playback attributable to the acute exposure was 34.6%. We analyze these results in light of theories about how animals adjust risk taking in response to predation danger. Given that the numbers of their predators have risen steadily for the past few decades and affect the level of singing, we consider the implications for trend estimates of songbird populations based on surveys using auditory methods.

Database of Bird Flight Initiation Distances to Assist in Estimating Effects from Human Disturbance and Delineating Buffer Areas

U.S. Fish and Wildlife Service biologists determine effects from disturbance to threatened and endangered bird species, and staffs of federal and state agencies estimate these effects when delineating protective buffers around habitat of bird species of concern on land management areas. These efforts can be informed by the distances at which human activities cause birds to react or move away. To that end, here we present a database of published alert distances (distances at which birds exposed to an approaching human activity exhibit alert behavior), flight initiation distances (distances at which birds exposed to an approaching human activity initiate escape behavior), and minimum approach distances (distances at which humans should be separated from wildlife). The database distinguishes between nesting and nonnesting situations. The nesting database includes 578 alert distances and 2,177 flight initiation distances from 45 studies representing 11 orders, 27 families, and 49 species of birds. The nonnesting database comprises 1,419 alert distances and 34,775 flight initiation distances from 50 studies representing 19 orders, 89 families, and 650 species.

Buses, cars, bicycles and walkers: the influence of the type of human transport on the flight responses of waterbirds

One way to manage disturbance to waterbirds in natural areas where humans require access is to promote the occurrence of stimuli for which birds tolerate closer approaches, and so cause fewer responses. We conducted 730 experimental approaches to 39 species of waterbird, using five stimulus types (single walker, three walkers, bicycle, car and bus) selected to mimic different human management options available for a controlled access, Ramsar-listed wetland. Across species, where differences existed (56% of 25 cases), motor vehicles always evoked shorter flight-initiation distances (FID) than humans on foot. The influence of stimulus type on FID varied across four species for which enough data were available for complete cross-stimulus analysis. All four varied FID in relation to stimuli, differing in 4 to 7 of 10 possible comparisons. Where differences occurred, the effect size was generally modest, suggesting that managing stimulus type (e.g. by requiring people to use vehicles) may have species-specific, modest benefits, at least for the waterbirds we studied. However, different stimulus types have different capacities to reduce the frequency of disturbance (i.e. by carrying more people) and vary in their capacity to travel around important habitat.

Vigilance of kit foxes at water sources: a test of competing hypotheses for a solitary carnivore subject to predation

Animals that are potential prey do not respond equally to direct and indirect cues related to risk of predation. Based on differential responses to cues, three hypotheses have been proposed to explain spatial variation in vigilance behavior. The predator-vigilance hypothesis proposes that prey increase vigilance where there is evidence of predators. The visibility-vigilance hypothesis suggests that prey increase vigilance where visibility is obstructed. Alternatively, the refuge-vigilance hypothesis proposes that prey may perceive areas with low visibility (greater cover) as refuges and decrease vigilance. We evaluated support for these hypotheses using the kit fox (Vulpes macrotis), a solitary carnivore subject to intraguild predation, as a model. From 2010 to 2012, we used infrared-triggered cameras to record video of kit fox behavior at water sources in Utah, USA. The refuge-vigilance hypothesis explained more variation in vigilance behavior of kit foxes than the other two hypotheses (AICc model weight=0.37). Kit foxes were less vigilant at water sources with low overhead cover (refuge) obstructing visibility. Based on our results, the predator-vigilance and visibility-vigilance hypotheses may not be applicable to all species of prey. Solitary prey, unlike gregarious prey, may use areas with concealing cover to maximize resource acquisition and minimize vigilance.

Behavioral responses of maned wolves (Chrysocyon brachyurus, Canidae) to different categories of environmental enrichment stimuli and their implications for successful reintroduction

The maned wolf (Chrysocyon brachyurus, Illiger, 1815, Canidae) is a threatened species that inhabits the cerrados of Brazil, Argentina, Peru, Bolivia, Paraguay, and Uruguay. Captive maned wolves could be potentially used in reintroduction programs for species conservation; however, it is necessary that their behavior and cognitive abilities are conserved. Environmental enrichment is a tool used to stimulate captive animals and maintain a natural behavioral repertoire. To compare the variation of captive maned wolves' behavioral responses to environmental enrichment, we studied three maned wolves held by Belo Horizonte Zoo, Brazil. Foraging, interspecific, and intraspecific stimuli were offered to the animals and their responses were compared with a baseline and postenrichment treatments. The test was used to help in choosing which one of the three individuals would participate in a reintroduction project. The results showed that stimuli type did influence the animal's responses, and that individually wolves responded differently to foraging, interspecific, and intraspecific enrichment items (P80.05 for some behaviors in each enrichment category). The individual's personality seemed to influence their behavioral responses, with animals showing bold and shy responses, and this trait should be considered during decision makings for reintroduction.

Flight initiation distance is differentially sensitive to the costs of staying and leaving food patches in a small-mammal prey

Escape theory predicts that a prey should flee from an approaching predator at a point in which the cost of staying equals the cost of escape. We manipulated the cost of fleeing upon approaching human predators by providing the small mammal Octodon degus (Molina, 1782) with varying amounts of supplementary food likely to disappear while the animals are not in the food patch (e.g., hidden in their burrows). Simultaneously, we manipulated the risk of remaining in the patch by providing supplementary food at varying distances from the nearest burrow. Degus fled at a shorter distance to approaching predators when foraging in patches closer to the nearest burrow and supplied with relatively high abundance of food, but only when these rodents were foraging socially. Also, degus fled at a greater distance to approaching predators when foraging in patches far from the nearest burrow. Thus, functions linked to the loss of feeding opportunities and the risk of predation interact to influence flight initiation distance after a simulated attack. This study represented one of the few demonstrations of an interactive effect between cost and risks on antipredator behavior in a small, social prey mammal.