Costs and benefits of defences induced by predators differing in dangerousness

While theoretical studies predict that inducible defences should be fine-tuned according to the qualities of the predator, very few studies have investigated how dangerousness of predators, i.e. the rate at which predators kill prey individuals, affects the strength of phenotypic responses and resulting benefits and costs of induced defences. We performed a comprehensive study on fitness consequences of predator-induced responses by involving four predators (leech, water scorpion, dragonfly larva and newt), evaluating costs and benefits of responses, testing differences in dangerousness between predators and measuring responses in several life history traits of prey. We raised Rana dalmatina tadpoles in the presence of free-ranging predators, in the presence of caged predators, and exposed naive and experienced tadpoles to free-ranging predators. Tadpoles adjusted the intensities of their behavioural and morphological defences to predator dangerousness. Survival was lower in the nonlethal presence of the most dangerous predator, while we could not detect costs of induced defences at or after metamorphosis. When exposed to free-ranging predators, small, but not large, tadpoles benefited from exhibiting an induced phenotype in terms of elevated survival when compared to naive tadpoles, but we did not observe higher survival either in tadpoles exhibiting more extreme phenotypes or in tadpoles exposed to the type of predator they were raised with. These results indicate that while predator-induced defences can mirror dangerousness of predators, costs and benefits do not necessarily scale to the magnitude of plastic responses.     

水生生态环境中捕食信息素的生态学效应

捕食信息素是捕食者释放的,能够引发猎物反捕食反应的化学信号。在水生生态系统中,捕食信息素在捕食者和猎物之间信息传递及协同进化过程中发挥着重要的作用,其生态学效应在国际上受到广泛关注。捕食信息素的来源有多种形式,研究中常使用养殖过捕食者的水溶液作为捕食信息素的来源。捕食信息素的作用效果受到捕食者和猎物的种类、信息素的浓度、观察的指标等多方面因素的影响。捕食信息素可以对水生生物的行为、形态和生活史特征等方面造成影响。水生生物通过感知捕食信息素来提前预知潜在的被捕食风险,并作出适应性调整,以降低被捕食的风险。在某些情况下,捕食信息素可以与污染物产生交互作用,从而干扰污染物对水生生物的毒性。对水生环境中捕食信息素的研究现状做了综述,介绍了当前对捕食信息素来源和理化性质等本质问题的认识,总结捕食信息素对水生生物行为、形态和生活史特征的影响,以及捕食信息素对污染物毒性的干扰,并分析了这一研究领域尚存在的困难和今后的研究方向。加强对捕食信息素的研究,将为解析水生环境中捕食者和猎物的生态关系提供新依据。     

Inducible defences in Daphnia: responses to two closely related predator species

The predator-induced responses of two species of Australian Daphnia, with contrasting distributions and life history patterns, to the kairomones of two species of Anisops predators, were measured. Daphnia longicephala produced a large crest and attained a larger size when exposed to both predators. D. carinata sl matured earlier than D. longicephala and did not produce a crest. Surprisingly, kairomones of both predators inhibited the production of ephippia in D. carinata sl. Anisops stali, the larger of the two predator species, induced a significantly larger crest size in D. longicephala, and larger brood size in both species compared with the smaller A. gratus, indicating a quantitative but not qualitative effect of predator species on inducible defences. Received: 18 August 1999 / Accepted: 14 April 2000     

萼花臂尾轮虫对晶囊轮虫反捕食行为及形态响应的母体效应

为研究轮虫通过母体效应诱导能否产生行为响应, 以萼花臂尾轮虫(Brachionus calyciflorus)为例, 研究其反捕食漂浮行为响应的母体效应。通过控制轮虫母体在捕食者诱导液中的暴露时间及带卵状态, 收集母体产生的后代, 再将这些后代再次用捕食者诱导液处理, 观察后代的漂浮行为及形态特征。研究发现: 暴露于捕食者诱导液诱导较长时间的母体产生的后代个体, 当再次暴露于捕食者诱导液时, 其产生的行为响应强于没有母体暴露经历的后代; 母体暴露时间越长, 后代形态和行为响应均更加强烈。研究显示萼花臂尾轮虫可通过母体效应产生漂浮行为响应。     

Behavioural and life history effects of predator diet cues during ontogeny in damselfly larvae

A central issue in predator–prey interactions is how predator associated chemical cues affect the behaviour and life history of prey. In this study, we investigated how growth and behaviour during ontogeny of a damselfly larva (Coenagrion hastulatum) in high and low food environments was affected by the diet of a predator (Aeshna juncea). We reared larvae in three different predator treatments; no predator, predator feeding on conspecifics and predator feeding on heterospecifics. We found that, independent of food availability, larvae displayed the strongest anti-predator behaviours where predators consumed prey conspecifics. Interestingly, the effect of predator diet on prey activity was only present early in ontogeny, whereas late in ontogeny no difference in prey activity between treatments could be found. In contrast, the significant effect of predator diet on prey spatial distribution was unaffected by time. Larval size was affected by both food availability and predator diet. Larvae reared in the high food treatment grew larger than larvae in the low food treatment. Mean larval size was smallest in the treatment where predators consumed prey conspecifics, intermediate where predators consumed heterospecifics and largest in the treatment without predators. The difference in mean larval size between treatments is probably an effect of reduced larval feeding, due to behavioural responses to chemical cues associated with predator diet. Our study suggests that anti-predator responses can be specific for certain stages in ontogeny. This finding shows the importance of considering where in its ontogeny a study organism is before results are interpreted and generalisations are made. Furthermore, this finding accentuates the importance of long-term studies and may have implications for how results generated by short-term studies can be used.     

Temperature effects on chemical signalling in a predator–prey system

SUMMARY 1. We investigated the effect of temperature on chemical signalling in a predator–prey model system (planktivorous fish and Daphnia galeata ). Life-history changes in Daphnia in response to chemical cues (kairomones) derived from fish have become a paradigm for chemically induced anti-predator defences.
2. As temperature can affect both predator and prey, we carried out two experiments to disentangle these effects. In order to test for temperature effects on the predator, we kept prey at a single temperature and exposed them to kairomones from fish exposed to two different temperatures. Daphnia exhibited a higher intrinsic rate of population increase ( r ) when exposed to fish kairomones produced at high rather than low temperature. Assuming a positive correlation between r (because of an earlier maturation and/or increased clutch sizes) and kairomone concentration, our results suggest that kairomone production increases with rising temperature.
3. In the second experiment, to study the influence of temperature on the prey, Daphnia were kept at two different temperatures and exposed to fish kairomones produced at one constant temperature. We found no interaction between the effects of fish kairomone and temperature on Daphnia life history, suggesting that temperature does not directly alter life-history responses to fish kairomones.
4. Our results suggest that temperature influences Daphnia life history through its effects on fish kairomone concentration, but that temperature does not affect the strength of the response of Daphnia to the presence of fish.     

Chimpanzee and felid diet composition is influenced by prey brain size

Prey use a wide variety of anti-predator defence strategies, including morphological and chemical defences as well as behavioural traits (risk-modulated habitat use, changes in activity patterns, foraging decisions and group living). The critical test of how effective anti-predator strategies are is to relate them to relative indices of mortality across predators. Here, we compare biases in predator diet composition with prey characteristics and show that chimpanzee (Pan troglodytes) and felid show the strongest and the most consistent predator bias towards small-brained prey. We propose that large-brained prey are likely to be more effective at evading predators because they can effectively alter their behavioural responses to specific predator encounters. Thus, we provide evidence for the hypothesis that brain size evolution is potentially driven by selection for more sophisticated and behaviourally flexible anti-predator strategies.     

Predator regime influences innate anti-predator behaviour in the freshwater gastropod Lymnaea stagnalis

1. Predation incurs high fitness costs in aquatic organisms either through direct consumption or through avoidance responses that reduce time for activities such as feeding and reproduction. Hence, avoidance responses of aquatic organisms should vary to match closely the predation threat in their environment.
2. The freshwater gastropod Lymnaea stagnalis occurs in a variety of environments which vary in the presence or absence of predatory fish. We used naïve snails reared from six populations of this species experiencing different predator regimes (three co-occurring with molluscivorous fish and three without) to assess whether populations differed in the type and degree of their avoidance behaviours. Innate behavioural responses to four treatments (control, conspecific alarm cues, fish kairomones and fish kairomones paired with alarm cue) were compared in laboratory trials.
3. The primary anti-predator behaviour of L. stagnalis in response to fish kairomones was to crawl out of the water rather than seek refuge under water. This response was strongest when fish kairomones were paired with alarm cues, and varied depending on population origin; snails reared from populations co-occurring with predatory fish showed a stronger response than those raised from populations not experiencing such predators. In addition, populations co-occurring with predatory fish responded to the fish kairomones presented alone.
4. Our findings suggest that the degree of innate anti-predator behaviour shown by L. stagnalis , in terms of both the level of risk to which it responds and the degree of response, varies depending on the predator regime experienced by field populations. Together with previous work on cue association, this demonstrates that this gastropod is able to match its avoidance behaviour very closely to short and long term predation threats within its habitat.     

Waterborne metals impair inducible defences in Daphnia pulex: morphology, life-history traits and encounters with predators

1.  Inducible defences may be temporary and favoured where predation is intermittent and have been demonstrated in several invertebrates and vertebrates when prey detect chemical cues (kairomones) released by predators. Daphnia pulex (a water flea) exposed to Chaoborus (midge larvae) kairomones produce small neckteeth on the dorsal surface of the head as a defence against this gape-limited predator and survive better in the presence of Chaoborus . Recent studies have shown that waterborne copper (Cu) impairs the induction of neckteeth which could lead to lower survival.
2.  Here, we examined the effects of Cu on morphological changes and shifts in life-history traits in D. pulex exposed to kairomone from Chaoborus americanus . We exposed D. pulex mothers to chemical cues of C. americanus fed on either D. pulex neonates or on brine shrimp Artemia salina , the same Chaoborus cues combined with an environmentally relevant concentration of copper (10 μg L⁻¹), or dechlorinated tap water. We examined several morphological characteristics of neonates and life-historical characteristics of adults as well as assessing survival of neonates by staging encounters with predators.
3.  Neonates from mothers exposed to kairomone plus copper had fewer and shorter neckteeth than neonates from mothers exposed to kairomone alone. Moreover, neonates exposed to Cu had lower survival during encounters with predators than neonates exposed to kairomone without Cu.
4.  Adult female Daphnia exposed to kairomones released more neonates within the first 24 h of brood release and emptied their brood pouches quicker than mothers not exposed to kairomones, irrespective of the presence of Cu.
5.  Impairment by metals of morphological defences in zooplankton could lead to a decline in population density and alter community structure.     

Parasite-induced alteration of odour responses in an amphipod–acanthocephalan system

Odour-related behaviours in aquatic invertebrates are important and effective anti-predator behaviours. Parasites often alter invertebrate host behaviours to increase transmission to hosts. This study investigated the responses of the amphipod Hyalella azteca when presented with two predator chemical cues: (i) alarm pheromones produced by conspecifics and (ii) kairomones produced by a predatory Green Sunfish (Lepomis cyanellus). We compared the responses of amphipods uninfected and infected with the acanthocepalan parasite Leptorhynchiodes thecatus. Uninfected amphipods reduced activity and increased refuge use after detecting both the alarm pheromones and predator kairomones. Infected amphipods spent significantly more time being active and less time on the refuge than uninfected amphipods, and behaved as if they had not detected the chemical stimulus. Therefore, L. thecatus infections disrupt the amphipods’ anti-predator behaviours and likely make their hosts more susceptible to predation.     

Vertical distribution of Chlamydomonas changes in response to grazer and predator kairomones

Individuals in aquatic communities frequently assess their biotic environment through infochemicals. In particular, kairomones are commonly involved in interactions between predator and prey. However, the relationship between individuals and chemicals produced by other organisms that are not direct predators, but may indicate the presence of a predator, is not well characterized. We used experimental microcosms to test whether the unicellular green alga Chlamydomonas reinhardtii alters vertical migration patterns in response to kairomones produced by zooplankton ( Daphnia ) and planktivores (fish). Our results suggested that phototaxis in C. reinhardtii was strongly affected by the type of kairomone present, the concentration of the kairomone, and the duration of exposure to the kairomone. Kairomones generally increased phototaxis in C. reinhardtii . The adaptive significance of such behavioral changes in natural settings would depend largely on local community composition. The similarity in phototactic responses of C. reinhardtii to Daphnia and fish kairomone suggest that, in at least this species of phytoplankton, the underlying genetic elements responsible for kairomone detection may be responsive to a broad range of chemical stimuli, allowing this species to adjust its phototaxis in response to not only the presence of its grazers, but also to predators of its grazers.     

An inducible morphological defence is a passive by-product of behaviour in a marine snail

Many organisms have evolved inducible defences in response to spatial and temporal variability in predation risk. These defences are assumed to incur large costs to prey; however, few studies have investigated the mechanisms and costs underlying these adaptive responses. I examined the proximate cause of predator-induced shell thickening in a marine snail (Nucella lamellosa) and tested whether induced thickening leads to an increase in structural strength. Results indicate that although predators (crabs) induce thicker shells, the response is a passive by-product of reduced feeding and somatic growth rather than an active physiological response to predation risk. Physical tests indicate that although the shells of predator-induced snails are significantly stronger, the increase in performance is no different than that of snails with limited access to food. Increased shell strength is attributable to an increase in the energetically inexpensive microstructural layer rather than to material property changes in the shell. This mechanism suggests that predator-induced shell defences may be neither energetically nor developmentally costly. Positive correlations between antipredator behaviour and morphological defences may explain commonly observed associations between growth reduction and defence production in other systems and could have implications for the evolutionary potential of these plastic traits.     

Facing multiple enemies: parasitised hosts respond to predator kairomones

During their lifetime most organisms are exposed to various enemies influencing their victims in multiple direct and indirect ways. Most studies concentrate on the effects of one enemy at a time, thereby not taking into account that in nature organisms are often simultaneously exposed to more than one enemy. We conducted a life-history experiment to investigate the simultaneous effects of predators (fish, Leuciscus idus) and parasites (microparasite, Caullerya mesnili) on their victim (Daphnia galeata). D. galeata were exposed to predator kairomones, parasites or both. D. galeata are able to sense the presence of fish predators via chemical cues (= kairomones). Both fish predator kairomones and microparasite infections influence the life history of Daphnia. Some of the effects of fish predator kairomones are directly opposed to microparasite effects; fecundity, for example, is increased in the presence of fish kairomones and decreased in Daphnia parasitised with C. mesnili. We investigated the influence of both threats on age at maturity, body size at different adult instars, fecundity and survival of one D. galeata clone. In the presence of fish kairomones, all D. galeata matured significantly earlier and increased the number of eggs in the second brood significantly. Parasitised D. galeata matured significantly earlier than non-parasitised ones in the absence and presence of fish kairomones. An infection with the microparasite C. mesnili led to significantly lower clutch sizes at the second adult instar, to significantly smaller body sizes from adult instar three onwards and to significantly reduced survival. No significant interaction effect between the responses to fish presence and to parasite infection was found for any of the investigated life-history traits. The lack of interaction effects between the exposure to predator kairomones and parasite infection was most likely due to the different timing of the effects. Fish kairomones affected D. galeata early in its life history whereas C. mesnili increased in its effects over time. Our results show that parasitised D. galeata are able to exhibit life-history responses to fish predator presence early in their lives. Thus, D. galeata parasitised with C. mesnili have a similar chance as non-parasitised D. galeata to escape from fish predation via life-history changes. Since older parasitised D. galeata are smaller, they may have an even better chance to escape visual predators under actual predation.     

Reversibility of predator‐induced plasticity and its effect at a life‐history switch point

In natural systems, organisms are frequently exposed to spatial and temporal variation in predation risk. Prey organisms are known to develop a wide array of plastic defences to avoid being eaten. If inducible plastic defences are costly, prey living under fluctuating predation risk should be strongly selected to develop reversible plastic traits and adjust their defences to the current predation risk. Here, we studied the induction and reversibility of antipredator defences in common frog Rana temporaria tadpoles when confronted with a temporal switch in predation risk by dragonfly larvae. We examined the behaviour and morphology of tadpoles in experimental treatments where predators were added or withdrawn at mid larval development, and compared these to treatments with constant absence or presence of predators. As previous studies have overlooked the effects that developing reversible anti‐predator responses could have later in life (e.g. at life history switch points), we also estimated the impact that changes in antipredator responses had on the timing of and size at metamorphosis. In the presence of predators, tadpoles reduced their activity and developed wider bodies, and shorter and wider tails. When predators were removed tadpoles switched their behaviour within one hour to match that found in the constant environments. The morphology matched that in the constant environments in one week after treatment reversal. All these responses were highly symmetrical. Short time lags and symmetrical responses for the induction/reversal of defences suggest that a strategy with fast switches between phenotypes could be favoured in order to maximise growth opportunities even at the potential cost of phenotypic mismatches. We found no costs of developing reversible responses to predators in terms of life‐history traits, but a general cost of the induction of the defences for all the individuals experiencing predation risk during some part of the larval development (delayed metamorphosis). More studies examining the reversibility of plastic defences, including other type of costs (e.g. physiological), are needed to better understand the adaptive value of these flexible strategies.     

Carryover effects of predation risk on postembryonic life-history stages in a freshwater shrimp

For organisms with complex life histories it is well known that risk experienced early in life, as embryos or larvae, may have effects throughout the life cycle. Although carryover effects have been well documented in invertebrates with different levels of parental care, there are few examples of predator-induced responses in externally brooded embryos. Here, we studied the effects of nonlethal predation risk throughout the embryonic development of newly spawned eggs carried by female shrimp on the timing of egg hatching, hatchling morphology, larval development and juvenile morphology. We also determined maternal body mass at the end of the embryonic period. Exposure to predation risk cues during embryonic development led to larger larvae which also had longer rostra but reached the juvenile stage sooner, at a smaller size and with shorter rostra. There was no difference in hatching timing, but changes in larval morphology and developmental timing showed that the embryos had perceived waterborne substances indicative of predation risk. In addition to carryover effects on larval and juvenile stages, predation threat provoked a decrease of body mass in mothers exposed to predator cues while brooding. Our results suggest that risk-exposed embryos were able to recognize the same infochemicals as their mothers, manifesting a response in the free-living larval stage. Thus, future studies assessing anti-predator phenotypes should include embryonic development, which seems to determine the morphology and developmental time of subsequent life-history stages according to perceived environmental conditions.     

Oviposition habitat selection in response to risk of predation in temporary pools: mode of detection and consistency across experimental venue

Natural selection should favor females that avoid ovipositing where risk of predation is high for their progeny. Despite the large consequences of such oviposition behavior for individual fitness, population dynamics, and community structure, relatively few studies have tested for this behavior. Moreover, these studies have rarely assessed the mode of detection of predators, compared responses in prey species that vary in vulnerability to predators, or tested for the behavior in natural habitats. In an outdoor artificial pool experiment, we tested the oviposition responses of two dipteran species, Culiseta longiareolata (mosquito) and Chironomus riparius (midge), to the hemipteran predator, Notonecta maculata. Both dipteran species have similar life history characteristics, but Culiseta longiareolata larvae are highly vulnerable to predation by Notonecta, while Chironomus riparius larvae are not. As their vulnerabilities would suggest, Culiseta longiareolata, but not Chironomus riparius, strongly avoided ovipositing in pools containing Notonecta. An experiment in natural rock pools assessing oviposition by Culiseta longiareolata in response to Notonecta maculata yielded an oviposition pattern highly consistent with that of the artificial pool experiment. We also demonstrated that the cue for oviposition avoidance by Culiseta longiareolata was a predator-released chemical: Notonecta water (without Notonecta replenishment) repelled oviposition for 8 days. Oviposition avoidance and mode of detection of the predator have important implications for how to assess the true impact of predators and for the use of commercially produced kairomones for mosquito control.     

Testing the Threat‐Sensitive Hypothesis with Predator Familiarity and Dietary Specificity

In a system with multiple predators, the threat‐sensitive predator avoidance hypothesis predicts that prey respond differently to predators relative to the risks each poses (e.g., degree of dietary specialization). Aquatic animals often rely heavily on detecting predators via chemical cues (kairomones) and respond with a suite of behaviors including detection and avoidance. However, little is known about how animals respond to kairomones of specialist versus generalist predators. In laboratory experiments, we compared behavioral responses of a poorly studied aquatic salamander, the greater siren (Siren lacertina), to cues from specialist and generalist predator snakes to evaluate threat‐sensitive responses. Sirens exhibited a novel behavior (gill‐flushing) most often in the presence of specialist predator cues. Avoidance behavior (reversing direction following cue detection) was higher in response to specialist predator and novel animal control cues and lowest in response to generalist predator cues. An intermediate response to the animal control, an unfamiliar amphibian predator, indicated that sirens respond cautiously to a novel cue. The gradient of observed responses to different snake cues indicates that sirens may be evaluating predation potential of animals based on their foraging specificity and familiarity.     

Chemical Predator Inspection in a Characin Fish (Hemigrammus erythrozonus, Characidae, Ostariophysi): The Effects of Mixed Predator Diets

Many prey organisms will approach (inspect) potential predators, primarily to assess local risk of predation. It has been demonstrated that Ostariphysan prey fishes can detect conspecific alarm pheromones in the diet of potential predators and use this chemical information to reduce their risk of predation while still gaining significant benefits associated with predator inspection. We conducted the current study to examine the possible effects of mixed diets on the use of these chemical predator diet cues during inspection visits. Shoals of four glowlight tetras ( Hemigrammus erythrozonus ) were exposed to Jack Dempsey cichlids ( Cichlasoma octofaciatum ) which had been fed diets consisting of: 100% tetras (with alarm pheromone); 75% tetra, 25% swordtail ( Xiphophorus helleri , which lack a recognizable alarm pheromone); 25% tetra, 75% swordtail; or 100% swordtails. Tetras significantly increased their anti-predator behaviour in response to predators fed 100% tetra or the two mixed predator diets, but not when exposed to predators fed a 100% swordtail diet. Likewise, we observed significant differences in inspection behaviour. Tetras took longer to initiate an inspection, inspected in smaller groups and directed a greater proportion of inspection visits towards the tail region of the predator when it had been fed 100% tetra or either of the two mixed prey diets. We found no significant differences in either anti-predator or inspection behaviour among the three diet treatments containing tetras. These data strongly suggest that glowlight tetras are capable of detecting relatively small amounts of conspecific alarm pheromone in the diet of potential predators and that they modify their behaviour based on the presence or absence of these cues.     

Non-lethal effects of invertebrate predators on Daphnia: morphological and life-history consequences of water mite kairomone

1. Here, we report morphological and life-historical changes in the cladoceran Daphnia ambigua in response to chemical cues released by the predatory water mite Piona chilensis . Both species are common inhabitants of southern temperate lakes.
2. We found significant differences in adult body size at first, second and third reproduction. Also, individuals exposed to kairomones had longer tail spines at first reproduction, and the resultant offspring had smaller bodies and shorter tail spines.
3. Exposure to mite cues did not exert effects on brood size at first reproduction, but decreased offspring number in subsequent broods. Similarly, only the second and third reproduction events were delayed by kairomone exposure.
4. The intrinsic population growth rate of predator-induced animals was lower than that in controls, but simulations based on a parameterized matrix model showed that the fitness costs could be overcome if the reported phenotypic responses reduced predation rate moderately. The gain in protection from predators needed to cancel out the reduction in fitness associated with predator cues was directly related to juvenile survival and fertility, and inversely related to adult survival.
5. This is the first work reporting phenotypic plasticity in Cladocera in response to kairomones released by water mites, which are conspicuous predators in many austral fresh waters.     

Model vs. playback experiments: The impact of sensory mode on predator-specific escape responses in saki monkeys

Although experimentally simulating predator presence helps improve sample sizes in studies of free-ranging animals, few studies have examined whether auditory playbacks and visual models produce similar results. Additionally, it is unclear if anti-predator strategies are specific to predator hunting styles in understudied Neotropical pitheciid primates, limiting what we can generalize about this phenomenon across this taxonomic order. We conducted predator simulation experiments to assess whether wild Rylands' bald-faced saki monkeys (Pithecia rylandsi) recognize predators based solely on acoustic cues, exhibit predator-specific responses to different predator types, and vary responses to presentations in different sensory modes. In our playback experiments, sakis had weak responses to non-predator control vocalizations compared to jaguar growls and harpy eagle shrieks. In most predator playbacks, subjects' first glance corresponded to the direction from which simulated predators would typically attack (above vs. below). However, although sakis exhibited appropriate movement responses to harpy playbacks (i.e., descending canopy), they exhibited no clear movement patterns when presented with jaguar playbacks. In contrast, jaguar model experiments consistently elicited fast approaches, mobbing-style responses, and long alarm calling bouts. Thus, if we had relied on playbacks alone, we might have concluded that sakis have only generalized responses to terrestrial ambush predators. In fact, in all variables measured (e.g., latency, number of calls, and response duration), models of both predator species elicited stronger reactions than playbacks. Results indicate that bald-faced sakis can identify predators based solely on vocalizations, but do not exhibit predator-specific escape responses to terrestrial predators based on acoustic cues alone. The differential response to playbacks and models calls into question the reliability of using acoustic-only stimuli to assess the specificity of anti-predator behavior to predator hunting styles in some primate species.