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Deimatic behaviours, also referred to as startle behaviours, are used against predators and rivals. Although many are spectacular, their proximate and ultimate causes remain unclear. In this review we aim to synthesise what is known about deimatic behaviour and identify knowledge gaps. We propose a working hypothesis for deimatic behaviour, and discuss the available evidence for the evolution, ontogeny, causation, and survival value of deimatic behaviour using Tinbergen's Four Questions as a framework. Our overarching aim is to direct future research by suggesting ways to address the most pressing questions in this field. peerReviewed

This work was funded by BBSRC grants awarded to J.M.H. and O.P. (BB/N006569/1), C.R. and J.S (BB/N00602X/1), P.G.L (BB/N005945/1), and I.C.C. (BB/N007239/1). An apparent and common feature of aposematic patterns is that they contain a high level of achromatic (luminance) contrast, for example, many warning signals combine black spots and stripes with a lighter colour such as yellow. However, the potential importance of achromatic contrast, as distinct from colour contrast, in reducing predation has been largely overlooked. Here, using domestic chicks as a model predator, we manipulated the degree of achromatic contrast in warning patterns to test if high luminance contrast in aposematic signals is important for deterring naïve predators. We found that the chicks were less likely to approach and eat prey with high contrast compared to low contrast patterns. These findings suggest that aposematic prey patterns with a high luminance contrast can benefit from increased survival through eliciting unlearned biases in naïve avian predators. Our work also highlights the importance of considering luminance contrast in future work investigating why aposematic patterns take the particular forms that they do. Publisher PDF

Animal warning signals show remarkable diversity, yet subjectively appear to share visual features that make defended prey stand out and look different from more cryptic palatable species. Here we develop and apply a computational model that emulates avian visual processing of pattern and colour to Lepidopteran wing patterns to show that warning signals have specific neural signatures that set them apart not only from the patterns of undefended species but also from natural scenes. For the first time, we offer an objective and quantitative neural-level definition of warning signals based on how the pattern generates neural activity in the brain of the receiver. This opens new perspectives for understanding and testing how warning signals function and evolve, and, more generally, how sensory systems constrain general principles for signal design.

Whether and to what extent animals experience emotions is crucial for understanding their decisions and behaviour, and underpins a range of scientific fields, including animal behaviour, neuroscience, evolutionary biology and animal welfare science. However, research has predominantly focused on alleviating negative emotions in animals, with the expression of positive emotions left largely unexplored. Therefore, little is known about positive emotions in animals and how their expression is mediated. We used tail handling to induce a negative mood in laboratory mice and found that while being more anxious and depressed increased their expression of a discrete negative emotion (disappointment), meaning that they were less resilient to negative events, their capacity to express a discrete positive emotion (elation) was unaffected relative to control mice. Therefore, we show not only that mice have discrete positive emotions, but that they do so regardless of their current mood state. Our findings are the first to suggest that the expression of discrete positive and negative emotions in animals is not equally affected by long-term mood state. Our results also demonstrate that repeated negative events can have a cumulative effect to reduce resilience in laboratory animals, which has significant implications for animal welfare.

Rhesus macaques ( Macaca mulatta) used in behavioural neuroscience are often required to complete cognitively complex tasks, for which a high level of motivation is essential. To induce motivation, researchers may implement fluid-restriction protocols, whereby freely available water is limited, such that fluid can be used as a reward in the laboratory. A variety of different rewards and schedules are used, but there exists a lack of data assessing their effectiveness. In this study, we aimed to quantify fluid preference in rhesus macaques and to use these preferences to compare the motivational quality of different reward schedules: the monkey's previous reward (i.e. the fluid used to reward them in past studies), their new preferred reward, a variable schedule of previous and preferred reward, and a choice between the previous and preferred rewards. We found that it may be possible to reduce the level of restriction if an adequately motivating preferred reward is identified, but that this is dependent on the animal. Each monkey responded differently to both the fluid-preference assessments and to the different reward schedules. As such, monkeys should not be subject to ‘blanket’ protocols but should be assessed individually to maintain adequate scientific data collection at the least severe level of fluid restriction.

Land-based source pollutants (LBSP) actively threaten coral reef ecosystems globally. To achieve the greatest conservation outcome at the lowest cost, managers could benefit from appropriate tools that evaluate the benefits (in terms of LBSP reduction) and costs of implementing alternative land management strategies. Here we use a spatially explicit predictive model (InVEST-SDR) that quantifies change in sediment reaching the coast for evaluating the costs and benefits of alternative threat-abatement scenarios. We specifically use the model to examine trade-offs among possible agricultural road repair management actions (water bars to divert runoff and gravel to protect the road surface) across the landscape in West Maui, Hawaii, USA. We investigated changes in sediment delivery to coasts and costs incurred from management decision-making that is (1) cooperative or independent among landowners, and focused on (2) minimizing costs, reducing sediment, or both. The results illuminate which management scenarios most effectively minimize sediment while also minimizing the cost of mitigation efforts. We find targeting specific “hotspots” within all individual parcels is more cost-effective than targeting all road segments. The best outcomes are achieved when landowners cooperate and target cost-effective road repairs, however, a cooperative strategy can be counter-productive in some instances when cost-effectiveness is ignored. Simple models, such as the one developed here, have the potential to help managers make better choices about how to use limited resources.

The question, "Why should prey advertise their presence to predators using warning coloration?" has been asked for over 150 years. It is now widely acknowledged that defended prey use conspicuous or distinctive colors to advertise their toxicity to would-be predators: a defensive strategy known as aposematism. One of the main approaches to understanding the ecology and evolution of aposematism and mimicry (where species share the same color pattern) has been to study how naive predators learn to associate prey’s visual signals with the noxious effects of their toxins. However, learning to associate a warning signal with a defense is only one aspect of what predators need to do to enable them to make adaptive foraging decisions when faced with aposematic prey and their mimics. The aim of our review is to promote the view that predators do not simply learn to avoid aposematic prey, but rather make adaptive decisions about both when to gather information about defended prey and when to include them in their diets. In doing so, we reveal what surprisingly little we know about what predators learn about aposematic prey and how they use that information when foraging. We highlight how a better understanding of predator cognition could advance theoretical and empirical work in the field.

Rhesus macaques (

Remote sensing and geographic analysis of woody vegetation provide means of evaluating the distribution of natural resources, patterns of biodiversity and ecosystem structure, and socio-economic drivers of resource utilization. While these methods bring geographic datasets with global coverage into our day-to-day analytic spheres, many of the studies that rely on these strategies do not capitalize on the extensive collection of existing field data. We present the methods and maps associated with the first spatially-explicit models of global tree density, which relied on over 420,000 forest inventory field plots from around the world. This research is the result of a collaborative effort engaging over 20 scientists and institutions, and capitalizes on an array of analytical strategies. Our spatial data products offer precise estimates of the number of trees at global and biome scales, but should not be used for local-level estimation. At larger scales, these datasets can contribute valuable insight into resource management, ecological modelling efforts, and the quantification of ecosystem services.

Aposematic prey advertise their toxicity to predators using conspicuous warning signals, which predators learn to use to reduce their intake of toxic prey. Like other types of prey, aposematic prey often differ in body size, both within and between species. Increasing body size can increase signal size, which make larger aposematic prey more detectable but also gives them a more effective and salient deterrent. However, increasing body size also increases the nutritional value of prey, and larger aposematic prey may make a more profitable meal to predators that are trading off the costs of eating toxins with the benefits of ingesting nutrients. We tested if body size, independent of signal size, affected predation of toxic prey as predators learn to reduce their attacks on them. European starlings (Sturnus vulgaris) learned to discriminate between defended (quinine-injected) and undefended (water-injected) mealworm prey (Tenebrio molitor) using visual signals. During this process, we found that birds attacked and ate more defended prey the larger they were. Body size does affect the probability that toxic prey are attacked and eaten, which has implications for the evolutionary dynamics of aposematism and mimicry (where species share the same warning pattern).

Visually hunting predators forage on prey possessing a range of different defence strategies. The two most commonly studied defence strategies are camouflage (in which palatable and undefended prey have colour patterns to minimize detection) and aposematism (in which toxic prey advertise their defences using conspicuous warning signals). Typically, these two defence strategies are studied in isolation, but when multiple prey species are eaten by the same predators, changes in the defence strategy of one species could affect the selection pressures acting on the defensive strategy of another. In this experiment, we tested this idea by investigating whether predators increased their foraging behaviour on prey that they knew to be toxic when undefended prey were better concealed. European starlings, Sturnus vulgaris , were allowed to forage freely for undefended (water-injected) and defended (quinine-injected and coated) mealworms, Tenebrio molitor , in bowls containing woodchip (prey type was signalled by bowl colour). Toxic prey were always covered with 30 ml of woodchip, while undefended prey were covered with either 30 ml or 150 ml of woodchip (all birds received both woodchip depths). Increasing the amount of woodchip covering the undefended prey made them harder to find, but did not affect birds' propensity to search for toxic prey. However, birds did attack and eat more toxic prey, but only when they received the 150 ml treatment before the 30 ml treatment. Our experiment shows that better concealed palatable prey can increase predation of toxic prey, with implications for the study of prey defences.

Across a range of disciplines, researchers are becoming increasingly interested in studying the variation in cognitive abilities found within populations. Behavioral ecology is no exception: the pursuit to understand the evolution of cognition has lead to a rapidly expanding literature that uses various tasks to measure individuals’ cognitive abilities. While this is an exciting time, we are concerned that without being clearer as to the cognitive abilities under test it will be difficult to design appropriate experiments and the interpretation of the data may be unsound. The aim of this review is 3-fold: 1) to highlight problems with designing tasks for measuring individual variation in cognitive abilities and interpreting their outcomes; 2) to increase awareness that noncognitive factors can cause variation in performance among individuals; and 3) to question the theoretical basis for thinking that performance in any cognitive task should necessarily correlate with a measure of fitness. Our take-home message is that variability in performance in cognitive tasks does not necessarily demonstrate individual variation in cognitive ability, and that we need to both design more stringent cognitive tests and be more cautious in their interpretation.

Summary Evolutionary biologists have long been fascinated by camouflage patterns that help animals reduce their chances of being detected by predators [ 1 , 2 , 3 , 4 ]. However, patterns that hide prey when they remain stationary, such as those that match their backgrounds [ 5 , 6 ], are rendered ineffective once prey are moving [ 7 , 8 , 9 , 10 ]. The question remains: can a moving animal ever be patterned in a way that helps reduce detection by predators? One long-standing idea is that high-contrast patterns with repeated elements, such as stripes, which are highly visible when prey are stationary, can actually conceal prey when they move fast enough [ 11 , 12 , 13 , 14 ]. This is predicted by the “flicker fusion effect,” which occurs when prey move with sufficient speed that their pattern appears to blur, making them appear more featureless and become less conspicuous against the background [ 2 , 8 ]. However, although this idea suggests a way to camouflage moving prey, it has not been empirically tested, and it is not clear that it would work at speeds that are biologically relevant to a predator [ 13 ]. Combining psychophysics and behavioral approaches, we show that speed and pattern interact to determine the detectability of prey to the praying mantis (Sphodromantis lineola) and, crucially, that prey with high-contrast stripes become less visible than prey with background-matching patterns when moving with sufficient speed. We show that stripes can reduce the detection of moving prey by exploiting the spatiotemporal limitations of predator perception, and that the camouflaging effect of a pattern depends upon the speed of prey movement.

Predators that have learned to associate warning coloration with toxicity often continue to include aposematic prey in their diet in order to gain the nutrients and energy that they contain. As body size is widely reported to correlate with energetic content, we predicted that prey size would affect predators' decisions to eat aposematic prey. We used a well-established system of wild-caught European starlings, Sturnus vulgaris, foraging on mealworms, Tenebrio molitor, to test how the size of undefended (water-injected) and defended (quinine-injected) prey, on different coloured backgrounds, affected birds’ decisions to eat defended prey. We found that birds ate fewer defended prey, and less quinine, when undefended prey were large compared with when they were small, but that the size of the defended prey had no effect on the numbers eaten. Consequently, we found no evidence that the mass of the defended prey or the overall mass of prey ingested affected the amount of toxin that a predator was willing to ingest, and instead the mass of undefended prey eaten was more important. This is a surprising finding, challenging the assumptions of state-dependent models of aposematism and mimicry, and highlighting the need to understand better the mechanisms of predator decision making. In addition, the birds did not learn to discriminate visually between defended and undefended prey based on size, but only on the basis of colour. This suggests that colour signals may be more salient to predators than size differences, allowing Batesian mimics to benefit from aposematic models even when they differ in size., Highlights • The size of toxic prey did not affect the amount of toxin ingested by birds. • Total prey mass eaten did not affect the amount of toxin ingested by birds. • The amount of toxin ingested by birds depended on the mass of nontoxic prey. • Colour signals may be more salient to predators than size differences. • Findings have implications for the selection pressures acting on aposematic prey.

Aposematic prey warn predators of their toxicity using conspicuous signals. However, predators regularly include aposematic prey in their diets, particularly when they are in a poor energetic state and in need of nutrients. We investigated whether or not an environmental factor, ambient temperature, could change the energetic state of predators and lead to an increased intake of prey that they know to contain toxins. We found that European starlings, Sturnus vulgaris, increased their consumption of mealworm, Tenebrio molitor, prey containing quinine (a mild toxin) when the ambient temperature was reduced below their thermoneutral zone from 20 °C to 6 °C. The birds differed in their sensitivity to changes in ambient temperature, with heavier birds increasing the number of toxic prey they ate more rapidly with decreasing temperature compared to birds with lower body mass. This could have been the result of their requiring more nutrients at lower temperatures or being better able to detoxify quinine. Taken together, our results suggest that conspicuous coloration may be more costly at lower temperatures, and that aposematic prey may need to invest more in chemical defences as temperatures decline. Our study also provides novel insights into what factors affect birds' decisions to eat toxic prey, and demonstrates that selection pressures acting on prey defences can vary with changing temperature across days, seasons, climes, and potentially in response to climate change., Highlights • We investigated the effect of temperature on birds' decisions to eat toxic prey. • As it got cooler, birds were more likely to eat prey containing toxins. • Heavier birds were more sensitive to changes in temperature. • Selection pressures on prey defences will change over days, seasons and climes.

For over 150 years, researchers have investigated the anti-predator function of animal patterns. However, this work has mainly focused on when prey remain still, and has only recently started to incorporate motion into the study of defensive coloration. As motion breaks camouflage, a new challenge is to understand how prey avoid predators while moving around their environment, and if a moving prey can ever be camouflaged. We propose that there is a solution to this, in that a ‘flicker fusion effect’ can change the appearance of the prey in the eyes of their predators to reduce the chances of initial detection. This effect occurs when a high contrast pattern blurs at speed, changing the appearance of the prey, which may help them better match their background. Despite being widely discussed in the literature, the flicker fusion effect is poorly described, there is no clear theoretical framework for testing how it might reduce predation, and the terminology describing it is, at best, rather confusing. Our review addresses these three key issues to enable researchers to formulate precise predictions about when the flicker fusion effect occurs, and to test how it can reduce predation.

The warning signals of toxic insects are often 'multimodal', combining bright coloration with sounds or odours (or both). Pyrazine (a common insect warning odour) can elicit an intrinsic avoidance in domestic chicks Gallus gallus domesticus, both against novel coloured food, and also against food colours that are specifically associated with aposematism, namely yellow and red. In three experiments, we investigated the role of novelty in this innate bias against yellow coloured food in the presence of pyrazine. Naive chicks were familiarized either to pyrazine odour or to coloured food before being tested for a bias against yellow (warningly coloured) food as opposed to green (nonwarningly coloured) food. In experiment 1, pyrazine novelty was shown to be vital for eliciting a bias against yellow food. However, experiment 2 suggested that colour novelty was not important: chicks familiarized with coloured crumbs still avoided yellow crumbs when pyrazine was presented. In a third experiment that gave chicks an even greater degree of pre-exposure to coloured crumbs, the bias against yellow food eventually waned, although pyrazine continued to elicit an aversion to yellow even after birds had had experience of up to 24 palatable yellow crumbs. Pyrazine novelty has been an important pressure in the evolution of multimodal warning signals, and can continue to promote the avoidance of warningly coloured food, even when it is relatively familiar. The implications for warning signals are discussed. Copyright 1999 The Association for the Study of Animal Behaviour.

Hooded crows (Corvus cornix) nest on power poles throughout the north of Scotland, and the interruptions to electricity supply caused by the nests cost the electricity provider in excess of (UK) £250,000 annually. In the Orkney Isles, where pole nesting is relatively common, most nests are actively removed before they can cause a fault. However, rebuilding often occurs. Although the electrical company routinely fits Firefly FF-type diverters (P & R Tech Inc., Beaverton, OR, USA) after nest removals to deter the crows from rebuilding, there has been no field test of the effectiveness of the Fireflies as a deterrent. In our study, carried out in Orkney in the Spring/Summer of 2009 and 2010, Fireflies were fitted at half of the sites from which nests were removed and not fitted at the other half of the sites. We found that crows were equally likely to rebuild at sites fitted with Fireflies as they were to rebuild at sites without Fireflies. However, rebuilding was less likely to occur the later in the season that nests were removed, and nests in the middle phase of construction were the most likely to be rebuilt. Therefore, making an appropriate decision as to when to remove a crow nest seems to be a more effective method for deterring nest rebuilding than is the fitting of Firefly diverters. © 2012 The Wildlife Society.

Animals often eat foods containing toxins to benefit from the nutrients that they contain. Understanding how animals balance the costs of eating toxins with the benefits of gaining nutrients is important for understanding the evolution of antipredator defenses, particularly aposematism and mimicry. In this study, we tested whether predators could learn to use color signals to make strategic decisions about when to include prey that varied in their toxin content in their diets. We gave European starlings (Sturnus vulgaris) daily sessions of sequentially presented mealworms (Tenebrio molitor). There were 3 types of mealworm which were made discriminable using color signals: undefended mealworms injected with water, mildly defended mealworms injected with 1% quinine solution, and moderately defended mealworms injected with 3% quinine solution. Birds learned to eat more undefended than defended prey and more mildly than moderately defended prey. Crucially, when we manipulated the birds’ energetic states using food restriction, we found that they increased the number of defended prey that they ate but maintained their relative preferences. Birds made state-dependent decisions based upon their knowledge of the amount of toxin prey contained and their current energetic need. Our results provide novel insights into the evolution of aposematic signals and also demonstrate that we may need to develop new models of the evolution of mimicry based on the state-dependent behavior of predators. Our data also have broader implications for the study of nutrient–toxin trade-offs across a range of different ecological scenarios.

Many prey species have evolved bitter-tasting toxins that effectively protect them from potential predators. While predators can learn to associate the taste of defended prey with the noxious effects of the toxins, it is unclear whether bitter tastes also function as deterrents. We explicitly tested the effectiveness of a nontoxic distasteful chemical as an antipredator defence. We gave four groups of European starlings, Sturnus vulgaris, a sequential colour discrimination task, where one colour signalled undefended mealworms, Tenebrio molitor, and the other signalled defended mealworms that had either been injected with or coated with either a high or a low concentration of Bitrex solution (a nontoxic bitter-tasting solution). Birds ate all of the prey presented in this way, but performed disgust responses after eating prey coated in Bitrex solution. Birds were then given a series of trials in which they received defended and undefended prey items simultaneously. Birds that received prey injected with Bitrex attacked similar numbers of defended and undefended prey, whereas birds that received prey coated in Bitrex ate significantly fewer defended than undefended prey. Birds given prey coated in a high concentration of Bitrex showed a stronger preference for undefended prey than birds given prey coated in a low concentration of Bitrex. Our experiment demonstrates that a nontoxic distasteful chemical can protect insect prey from predation, but only under very specific conditions.

Many defended prey advertise their defences to potential predators by being conspicuously coloured and/or having distinct bold markings. These colourful warning signals benefit prey since they are easy to learn to avoid, and seem particularly effective as deterrents. However, the evolution of conspicuous coloration is problematic, because if a distinct conspicuous morph arises in a cryptic species, it would be more easily detected and the risk of being attacked would be high. However, this argument assumes that attack probability will be the only factor determining prey survival, but in fact many prey have external defences that predators can detect, allowing prey to survive attacks. This experiment tests whether a rare conspicuously coloured defended morph can have a selective advantage over a cryptic defended morph when defence chemicals can be detected before ingestion by a naive predator. Using chicks, Gallus gallus domesticus, foraging on coloured chick crumbs, we found that naive birds can learn to avoid defended prey when they are conspicuous but not when they are cryptic. Crucially, when prey ingestion, and not attack probability, was used as our measure of mortality we found that there can be a selective advantage to being conspicuous in a cryptic population. This advantage can occur even in the first avoidance-learning trial, providing an initial selective advantage for rare conspicuous defended morphs. However, this does not occur in all conspicuously coloured prey populations, and it is evident that colour biases are important and their role in the initial evolution of conspicuous colour signals should be considered.

Aposematic insects conspicuously advertise their unprofitability to potential predators. However, when these prey initially evolved, they were likely to have been rare and presumably at a greater risk of being detected and killed by naive predators. Both kin and individual selection theories have been used in attempts to explain this apparent paradox, with much of the empirical research supporting kin selection--based theories. Here, we experimentally test how chemical defence levels in prey and avian color biases influence the probability of a rare conspicuous morph having an initial survival advantage. We used newly hatched domestic chicks (Gallus gallus domesticus) foraging on green and purple prey, on a green or purple background, to model the evolutionary scenario of a rare conspicuous morph arising in a population of already defended cryptic prey. Defended prey were produced by spraying them with quinine solution, which the birds readily detect and can learn to avoid. Although attack rates were initially similar for both defended prey types, the chicks only learned to avoid defended prey when they were conspicuous, not when they were cryptic. In addition, defended conspicuous prey were more likely to be rejected on attack than defended cryptic prey, even when first encountered by a predator. These data suggest that there could be a selective advantage for a rare conspicuous morph to arise in a population of cryptic defended prey due to increased avoidance learning and taste-rejection in naive predators. Our findings also suggest that being a non-preferred color and/or highly defended will increase the probability of this evolutionary scenario. Copyright 2008, Oxford University Press.

Avian predators learn to avoid defended insects on the basis of their conspicuous warning coloration. In many aposematic species, the level of chemical defence varies, with some individuals (automodels) being more defended than others (automimics). Production of defence chemicals is often costly, and automimics enjoy the benefits of the warning signal without paying the cost of chemical production. This is a problem for the evolutionary stability of aposematism, since automimics appear to be at a selective advantage. If predators sample aposematic prey and selectively reject automodels, aposematism could become evolutionarily stable. However, it is then unclear how automimicry persists in aposematic species. Using chicks as predators and coloured crumbs for prey, we investigated how birds selectively rejected aposematic prey at different automimic frequencies. We found that birds tasted and rejected automodels at a constant level regardless of automimic frequency, but the rejection rates of automimics decreased with increasing automimic frequency. Birds only discriminated between automodels and automimics at frequencies above 25%; below this automimics would be at a selective advantage over automodels since they do not pay the costs associated with toxicity. Selective rejection of aposematic prey by predators can therefore explain both why aposematism is evolutionarily stable, and how automimicry can persist in aposematic populations. We also found that birds relearned that red crumbs were palatable more quickly with increasing automimic frequency. Therefore, the benefit of automimicry may depend upon birds' previous experiences with unpalatable prey as well as the actual frequency of automimics in the population.

Aposematic prey advertise their defences, such as toxins or stings, to visually hunting predators using conspicuous warning coloration. Both the conspicuousness and the chemical content of prey determine the speed of avoidance learning by naive predators, and it has long been assumed that predator education is the main selective pressure in the evolution of aposematism. However, recent theoretical models have considered how educated predators could also exert significant selection pressures on aposematic prey by increasing their attack rates on defended prey in times of food shortage. Currently, there are no clear experimental data to support these models. In this study, we show that European starlings (Sturnus vulgaris) increase their attack rates on chemically defended insect larvae when their body masses and fat stores are experimentally reduced. In addition, the increase in attack rate is not simply due to indiscriminate attacks made when energy reserves are low but is based on knowledge about the prey’s defences. Taken together, these results suggest that educated adult predators will strategically trade off the energetic benefits of prey against their toxic costs according to their energetic needs. This result challenges classic theoretical models of the evolution of aposematism based purely on predator learning and forgetting rates and demonstrates the need to consider energy-toxin trade-offs in foraging decisions on defended prey. We discuss the implication of these results for the evolution of chemical defences and warning signals. Key words: aposematism, discrimination, energetic state, predation, quinine, receiver psychology. [Behav Ecol]

In recent years, there have been over 50 comparative analyses carried out in which social or ecological variables have been used to explain variation in whole brain size, or a part thereof, in a range of vertebrate species. Here, we review this body of work, pointing out that there are a number of substantial problems with some of the assumptions that underpin the hypotheses (e.g. what brain size means), with the data collection and with the ways in which the data are combined in the analyses. These problems are particularly apparent in those analyses in which attempts are made to correlate complex behaviour with parts of the brain that carry out multiple functions. We conclude that now is the time to substantiate these results with data from experimental manipulations.

Chemically defended insects sometimes advertise their unpalatability to foraging avian predators by using warning displays that predators learn to avoid. One important aspect of many warning displays is the secretion of some or all of the chemical defence upon attack. In this experiment, we used European starlings, Sturnus vulgaris, foraging on mealworms, Tenebrio moloitor, to investigate how secreting defence chemicals influences the way in which predators assess palatability and learn to avoid unpalatable prey. We gave two groups of starlings a sequential colour discrimination task, where one colour signalled palatable mealworms and the other signalled mealworms that had been either injected with or coated with quinine solution (designed to simulate prey that stored and secreted their chemical defences, respectively). Birds in both groups readily acquired the colour discrimination, but differed in how they avoided the unpalatable prey. Birds presented with mealworms with secreted defences learned to avoid unpalatable prey on the basis of their colour signals more quickly than birds presented with mealworms with stored defences. However, they also attacked more unpalatable individuals at asymptote, perhaps because they learned to wipe the quinine from the surface of the mealworms. This suggests that there may be an optimum strategy for secreting chemicals that could explain why many unpalatable insects secrete only a proportion of their defence chemicals.

Müllerian mimicry, where 2 unpalatable species share a warning pattern, is classically believed to be a form of mutualism, where the species involved share the cost of predator education. Birds learn to avoid a color signal faster when individual prey possesses 1 of 2 bitter-tasting chemicals rather than all having the same chemical, suggesting that Müllerian mimics that possess different defense chemicals are better protected than those that possess the same defense chemical. Using domestic chicks as predators and flavored, colored crumbs for prey, we investigated whether birds learn to avoid 2 visually distinct crumb types faster when each crumb type possesses a different defense chemical than when both crumb types share the same defense chemical. We found that birds learned to avoid 2 visually distinct color signals at a similar rate, irrespective of whether each color signal represented a different defense chemical or whether both color signals represented the same defense chemical. This experiment, therefore, indicates that in terms of predator avoidance learning, possessing 2 defense chemicals is more advantageous when prey look the same than when they look different. This suggests that Müllerian mimics with different defense chemicals not only are better protected than Müllerian mimics that share a single chemical but also benefit more from their mimetic resemblance. Copyright 2006.

Aposematic insect species are thought to gain protection from avian predators by advertising their unpalatability with bright or conspicuous coloration. However, individuals within a population may differ in their level of chemical defence. Since the speed and strength of avoidance learning are often assumed to be determined by the degree of defence, the presence of more palatable individuals in a population is thought to decrease the value of the warning coloration. Using chicks, Gallus gallus domesticus, as model predators and coloured crumbs flavoured with different concentrations of quinine sulphate solution for unpalatable prey, we tested these learning assumptions using a colour discrimination task. The mean level of prey defence was positively correlated with the speed of avoidance learning, and negatively correlated with the ease with which learned aversions were reversed. However, it was unclear how prey palatability influenced the strength of avoidance learning, since the number of unpalatable crumbs attacked at asymptote was not correlated with the mean level of prey defence, although the reluctance to attack an unpalatable crumb within a trial was positively correlated with prey defence. We also found that the proportion of the attacked unpalatable crumbs that were subsequently rejected was positively correlated with the mean level of prey defence. Our results show the importance of the level of prey defence in determining both the attack and ingestion rates of avian predators.

Stimuli and events that animals have to learn about in both their natural environments and in modern environments, such as our homes and farms, are often “multisensory,” i.e., they usually occur in more than one sensory modality. There are studies in humans and animals showing that stimuli that are multisensory are learned more quickly than stimuli presented in just a single sensory modality. My aim is to highlight how animals can combine information from several sensory modalities simultaneously, and show how using multisensory stimuli in training can enhance an animal's ability to learn new behaviors.

Unpalatable insects often advertise their defences to avian predators by conspicuous colours, such as red and yellow. Therefore, perhaps not surprisingly, birds tend to have unlearned biases against warningly coloured food. These biases are particularly evident when other components of insect warning displays, such as novel sounds and odours, are also present. We tested whether bitter taste, often associated with the defensive chemicals used by aposematic insects, can elicit or enhance specific colour aversions to red and yellow food in young domestic chicks, Gallus gallus domesticus. In our first experiment, subjects were given familiar brown chick crumbs sprayed with either a 0, 1, or 4% quinine solution. In each palatability treatment, chicks were then offered a choice of palatable crumbs that were coloured either red and green or yellow and green. Chicks attacked (pecked or ate) fewer red and yellow crumbs and more green crumbs with increasing quinine concentrations. Chicks also ate fewer red and yellow crumbs with increasing unpalatability, although there was no effect on the numbers of green crumbs eaten. In a second experiment, we gave chicks a colourless drop of either 0 or 0.3% quinine solution and found that this also produced similar attack biases against red and yellow as is in the first experiment. Taken together, these results show that birds use unpalatable taste to adapt their visual foraging decisions, which has consequences for the evolution and stability of mimicry systems, and also has implications for the pairing of colour and taste in psychological experiments.

The evolution of animal communication is a complex issue and one that attracts much research and debate. 'Receiver psychology' has been highlighted as a potential selective force, and we review how avian psychological processes and biases can influence the evolution and design of signals as well as the progress that has been made in testing these ideas in behavioural studies. Interestingly, although birds are a focal group for experimental psychologists and behavioural ecologists alike, the integration of theoretical ideas from psychology into studies of communication has been relatively slow. However, recent operant experiments are starting to address how birds perceive and respond to complex natural signals in an attempt to answer evolutionary problems in communication. This review outlines how a psychological approach to understanding communication is useful, and we hope that it stimulates further research addressing the role of psychological mechanisms in signal evolution.

Toxic prey that signal their defences to predators using conspicuous warning signals are called ‘aposematic’. Predators learn about the toxic content of aposematic prey and reduce their attacks on them. However, through regulating their toxin intake, predators will include aposematic prey in their diets when the benefits of gaining the nutrients they contain outweigh the costs of ingesting the prey's toxins. Predators face a problem when managing their toxin intake: prey sharing the same warning signal often vary in their toxicities. Given that predators should avoid uncertainty when managing their toxin intake, we tested whether European starlings ( Sturnus vulgaris ) preferred to eat fixed-defence prey (where all prey contained a 2% quinine solution) to mixed-defence prey (where half the prey contained a 4% quinine solution and the other half contained only water). Our results support the idea that predators should be more ‘risk-averse’ when foraging on variably defended prey and suggest that variation in toxicity levels could be a form of defence.

The evolution and maintenance of toxicity in a prey population is a challenge to evolutionary biologists if the investment in toxin does not benefit the individual. Recent experiments suggest that taste-rejection behaviour enables predators to selectively ingest less toxic individuals, which could stabilize investment in defences. However, we currently do not know if taste rejection of defended prey is accurate across different contexts, and that prey always benefit according to their investment. Using avian predators, we show that the rejection probability does not solely depend on the investment in defence by an individual, but also on the investment by other individuals in the same population. Therefore, taste rejection by predators could lead to destabilization in the investment in defences, and allow variability in prey defences to exist.

Many animals produce and respond to signals made up of multiple components. For example, many avian sexual displays are highly extravagant combinations of visual and acoustic elements, and are described as being 'multicomponent'. One possible reason for the evolution of such complex signals is that they provide more reliable information for receivers. However, receivers also influence signal evolution in another important way, by how they perceive and process signals: signallers will be selected to produce signals that are more easily received. The potential role of receiver psychology in the evolution of multicomponent signals has not previously been considered; in this review I present psychological results that support the notion that two components are better received than one alone. Detection can be improved by producing two components together, thus reducing the reaction time, increasing the probability of detection and lowering the intensity at which detection occurs. Discriminability of multicomponent stimuli is also made easier through better recognition, faster discrimination learning and multidimensional generalization. In addition, multicomponent stimuli also improve associative learning. I show that multicomponency does indeed improve signal reception in receivers, although the benefits of producing components in two sensory modalities (bimodal multicomponent signals) may be larger and more robust than producing them in just one (unimodal multicomponent signals). This highlights the need for consideration of receiver psychology in the evolution of multicomponent signals, and suggests that where signal components do not appear to be informative, they may instead be performing an important psychological function. Copyright 1999 The Association for the Study of Animal Behaviour.

Multimodal warning displays combine visual signals with components produced in other sensory modalities, for instance, aposematically coloured insects often produce a pungent odour or harsh sound when they are attacked. Recent research has focussed upon a particular odour, pyrazine, which is commonly associated with warning coloration. Our experiments have shown that pyrazine elicits hidden unlearned biases against particular visual aspects of food in foraging domestic chicks. Here we asses the current state of our knowledge about these biases, reviewing our results using pyrazine and other odours, and also presenting new data showing that sound can produce similar effects. We will discuss potential psychological mechanisms by which these foraging biases are achieved in avian predators, and potential pathways for their evolution.

Avian predators learn to avoid defended insects on the basis of their conspicuous warning coloration. In many aposematic species, the level of chemical defence varies, with some individuals being more defended than others. Sequestration and production of defence chemicals is often costly and therefore less defended individuals enjoy the benefits of the warning signal without paying the full costs of chemical production. This is a fundamental theoretical problem for the evolutionary stability of aposematism, since less defended individuals appear to be at a selective advantage. However, if predators sample aposematic prey and selectively reject individuals on the basis of their chemical investment, aposematism could become evolutionarily stable. Previous research aimed at testing whether birds can use taste to discriminate between palatable and unpalatable prey has been confounded by other experimental factors. Here, we show that birds can taste and reject prey entirely on the basis of an individual's level of chemical defence and more importantly, they can make decisions on whether or not to consume a defended individual based upon their level of chemical investment. We discuss these results in relation to the evolution of aposematism, mimicry and defence chemistry.

Rhesus macaques are an important model in behavioral neuroscience due to their advanced cognitive abilities. To motivate animals to engage in complex tasks, fluid rewards, in conjunction with fluid control protocols, are often used. The impact of these protocols on animal welfare is controversial. We compared two fluid control protocols against a protocol providing free access to water and evaluated the impacts on physiological states of hydration, behavioral measures of welfare, and scientific output. Blood physiology did not significantly differ between any of the protocols, and urine measures were indicative of well functioning, healthy kidneys. Changes in behaviors were limited, the main one being an increase in motivation to drink on the stricter fluid control protocol, and improved task performance early in the week. Overall, fluid control protocols had little measurable impact on the welfare of rhesus macaques while ensuring that scientific data of high quality could be obtained.

Camouflage is one of the most widespread forms of anti-predator defence and prevents prey individuals from being detected or correctly recognized by would-be predators. Over the past decade, there has been a resurgence of interest in both the evolution of prey camouflage patterns, and in understanding animal cognition in a more ecological context. However, these fields rarely collide, and the role of cognition in the evolution of camouflage is poorly understood. Here, we review what we currently know about the role of both predator and prey cognition in the evolution of prey camouflage, outline why cognition may be an important selective pressure driving the evolution of camouflage and consider how studying the cognitive processes of animals may prove to be a useful tool to study the evolution of camouflage, and vice versa. In doing so, we highlight that we still have a lot to learn about the role of cognition in the evolution of camouflage and identify a number of avenues for future research.

Nature 525, 201–205 (2015); doi:10.1038/nature14967 In the first boldface paragraph of this Article, the global number of trees should be approximately ‘1.30 trillion’ (rather than ‘1.39 trillion’) for tropical and subtropical forests and ‘0.66 trillion’ (rather than ‘0.61 trillion’) for temperate regions.