Predation pressure shapes brain anatomy in the wild

There is remarkable diversity in brain anatomy among vertebrates and evidence is accumulating that predatory interactions are crucially important for this diversity. To test this hypothesis, we collected female guppies (Poecilia reticulata) from 16 wild populations and related their brain anatomy to several aspects of predation pressure in this ecosystem, such as the biomass of the four major predators of guppies (one prawn and three fish species), and predator diversity (number of predatory fish species in each site). We found that populations from localities with higher prawn biomass had relatively larger telencephalon size as well as larger brains. Optic tectum size was positively associated with one of the fish predator’s biomass and with overall predator diversity. However, both olfactory bulb and hypothalamus size were negatively associated with the biomass of another of the fish predators. Hence, while fish predator occurrence is associated with variation in brain anatomy, prawn occurrence is associated with variation in brain size. Our results suggest that cognitive challenges posed by local differences in predator communities may lead to changes in prey brain anatomy in the wild.     

Predator–prey interactions,flight initiation distance and brain size

Prey avoid being eaten by assessing the risk posed by approaching predators and responding accordingly. Such an assessment may result in prey–predator communication and signalling, which entail further monitoring of the predator by prey. An early antipredator response may provide potential prey with a selective advantage, although this benefit comes at the cost of disturbance in terms of lost foraging opportunities and increased energy expenditure. Therefore, it may pay prey to assess approaching predators and determine the likelihood of attack before fleeing. Given that many approaching potential predators are detected visually, we hypothesized that species with relatively large eyes would be able to detect an approaching predator from afar. Furthermore, we hypothesized that monitoring of predators by potential prey relies on evaluation through information processing by the brain. Therefore, species with relatively larger brains for their body size should be better able to monitor the intentions of a predator, delay flight for longer and hence have shorter flight initiation distances than species with smaller brains. Indeed, flight initiation distances increased with relative eye size and decreased with relative brain size in a comparative study of 107 species of birds. In addition, flight initiation distance increased independently with size of the cerebellum, which plays a key role in motor control. These results are consistent with cognitive monitoring as an antipredator behaviour that does not result in the fastest possible, but rather the least expensive escape flights. Therefore, antipredator behaviour may have coevolved with the size of sense organs, brains and compartments of the brain involved in responses to risk of predation.     

Transplant experiments demonstrate that larger brains are favoured in high-competition environments in Trinidadian killifish

The extent to which the evolution of a larger brain is adaptive remains controversial. Trinidadian killifish (Anablepsoides hartii) are found in sites that differ in predation intensity; fish that experience decreased predation and increased intraspecific competition exhibit larger brains. We evaluated the connection between brain size and fitness (survival and growth) when killifish are found in their native habitats and when fish are transplanted from sites with predators to high-competition sites that lack predators. Selection for a larger brain was absent within locally adapted populations. Conversely, there was a strong positive relationship between brain size and growth in transplanted but not resident fish in high-competition environments. We also observed significantly larger brain sizes in the transplanted fish that were recaptured at the end of the experiment versus those that were not. Our results provide experimental support that larger brains increase fitness and are favoured in high-competition environments.     

Predator induced life-history shifts in a freshwater cladoceran

Summary Life-history theory predicts that maturity and resource allocation patterns are highly sensitive to selective predation. Under reduced adult survival, selection will favour genotypes capable of reproducing earlier, at a smaller size and with a higher reproductive effort. When exposed to water that previously held fish, (size selective predators which prefer larger Daphnia), individuals of Daphnia hyalina reproduced earlier, at a smaller size and had a higher reproductive investment. Hence the prey was able to switch its life history pattern in order to become less susceptible to predation by a specific predator. The cue that evokes the prey response is a chemical released by the predator.     

Foraging mode affects the evolution of egg size in generalist predators embedded in complex food webs

Ecological networks incorporate myriad biotic interactions that determine the selection pressures experienced by the embedded populations. We argue that within food webs, the negative scaling of abundance with body mass and foraging theory predict that the selective advantages of larger egg size should be smaller for sit‐and‐wait than active‐hunting generalist predators, leading to the evolution of a difference in egg size between them. Because body mass usually scales negatively with predator abundance and constrains predation rate, slightly increasing egg mass should simultaneously allow offspring to feed on more prey and escape from more predators. However, the benefits of larger offspring would be relatively smaller for sit‐and‐wait predators because (i) due to their lower mobility, encounters with other predators are less common, and (ii) they usually employ a set of alternative hunting strategies that help to subdue relatively larger prey. On the other hand, for active predators, which need to confront prey as they find them, body‐size differences may be more important in subduing prey. This difference in benefits should lead to the evolution of larger egg sizes in active‐hunting relative to sit‐and‐wait predators. This prediction was confirmed by a phylogenetically controlled analysis of 268 spider species, supporting the view that the structure of ecological networks may serve to predict relevant selective pressures acting on key life history traits.     

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.     

Brain size affects the behavioural response to predators in female guppies (Poecilia reticulata)

Large brains are thought to result from selection for cognitive benefits, but how enhanced cognition leads to increased fitness remains poorly understood. One explanation is that increased cognitive ability results in improved monitoring and assessment of predator threats. Here, we use male and female guppies (Poecilia reticulata), artificially selected for large and small brain size, to provide an experimental evaluation of this hypothesis. We examined their behavioural response as singletons, pairs or shoals of four towards a model predator. Large-brained females, but not males, spent less time performing predator inspections, an inherently risky behaviour. Video analysis revealed that large-brained females were further away from the model predator when in pairs but that they habituated quickly towards the model when in shoals of four. Males stayed further away from the predator model than females but again we found no brain size effect in males. We conclude that differences in brain size affect the female predator response. Large-brained females might be able to assess risk better or need less sensory information to reach an accurate conclusion. Our results provide experimental support for the general idea that predation pressure is likely to be important for the evolution of brain size in prey species.     

Stomach fullness modulates prey size choice in the frillfin goby,Bathygobius soporator

Behaviours related to foraging and feeding in predator–prey systems are fundamental to our understanding of food webs. From the perspective of a predator, the selection of prey size depends upon a number of factors including prey vulnerability, prey size, and the predator's motivation to eat. Thus, feeding motivation and prey visual cues are supposed to influence predator decisions and it is predicted that prey selection by visual cues is modulated by the predator's stomach fullness prior to attacking a prey. This study was conducted using an animal model from the rocky shores ecosystem, a predatory fish, the frillfin goby Bathygobius soporator, and a benthic prey, the mottled shore crab Pachygrapsus transversus. Our results demonstrate that frillfin gobies are capable of visually evaluating prey size and that the size evaluation process is modulated by the level of stomach fullness. Predators with an empty stomach (0% fullness) attacked prey that was larger than the predicted optimal size. Partially satiated predators (50% stomach fullness) selected prey close to the optimal size, while fully satiated predators (100% stomach fullness) showed no preference for size. This finding indicates an integrative response of the predator that depends on the input of both internal and external sensory information when choosing prey. Predator perceptions of visual cues (prey size) and stomach fullness modulate foraging decisions. As a result, a flexible feeding behaviour emerges, evidencing a clearly adaptive response in line with optimal foraging theory predictions.     

Food selection by a sit-and-wait predator,the monkfish,Lophius upsicephalus,off Namibia (South West Africa)

Synopsis We investigated prey selection by the monkfish, Lophius upsicephalus. This sit-and-wait predator preferentially selected prey types that would be expected to react to the lure of the illicium. Prey size selection was dependent on predator size, with larger predators feeding on a wider range of prey sizes.     

Fish and dragonfly nymph predators induce opposite shifts in color and morphology of tadpoles

Many prey species, including amphibian larvae, can adaptively alter coloration and morphology to become more or less conspicuous to predators. Despite abundant research on predator-induced plasticity in tadpoles, the combination of color and morphological responses to predators remains largely unexplored. We measured predator-induced morphological and color plasticity in tadpoles. We reared tadpoles of the neotropical treefrog Dendropsophus ebraccatus with dragonfly nymph or fish predators, or in a predator-free control. After 10 days, we digitally photographed tadpoles and measured eight morphometric variables and five tail color variables. Tadpoles reared with nymphs developed the largest and reddest tails, but incurred a developmental cost, being the smallest overall. Cues from fish induced an opposite tail phenotype in tadpoles, causing shallow achromatic tails. Control tadpoles developed intermediate tail phenotypes. This provides the first experimental evidence that tadpoles can shift both color and morphology in opposite, predator-specific directions in response to a fish and an odonate predator. Despite mean differences, however, there was substantial variation in the degree of phenotype induction across treatments. Tail redness was correlated with tail spot size, but not perfectly, indicating that color and morphology may be partially decoupled in D. ebraccatus . Balancing selection from multiple conflicting predators may result in genetic variation for developmental plasticity.     

Size-dependent prey selection in piscivorous pikeperch Sander lucioperca and Volga pikeperch Sander volgensis shaped by bimodal prey size distribution

Prey size and species selection of pikeperch Sander lucioperca and Volga pikeperch Sander volgensis were investigated in relation to predator size in the shallow Lake Balaton, Hungary. Although their gape sizes were similar, S. lucioperca shifted to piscivory earlier and consumed fewer, but larger, prey than S. volgensis. Prey species preference of the two piscivores also differed. A bimodal prey size distribution resulted in a reclining sigmoid curve for the life span predator size to prey size relationship with inflexion points between 266 and 284 mm predator standard length (L(S) ) in S. lucioperca. In S. volgensis, as well as in S. lucioperca L(S) ≤ 350 mm, prey size increased monotonically with predator L(S) , following a power trend for all prey size variables. Prey depth to predator L(S) relationship varied significantly with prey species and prey number in both piscivores, and prey depth tended to be smaller in predators consuming more than one prey. Both predator species characteristically selected less active, benthic prey fishes in spite of their spiny fin rays, and small- and mid-sized predators selected for small prey. Relatively large prey were also eaten, however, especially by the smallest and largest S. lucioperca.     

Coordinated evolution of brain size,structure, and eye size in Trinidadian killifish

Brain size, brain architecture, and eye size vary extensively in vertebrates. However, the extent to which the evolution of these components is intricately connected remains unclear. Trinidadian killifish, Anablepsoides hartii, are found in sites that differ in the presence and absence of large predatory fish. Decreased rates of predation are associated with evolutionary shifts in brain size; males from sites without predators have evolved a relatively larger brain and eye size than males from sites with predators. Here, we evaluated the extent to which the evolution of brain size, brain structure, and eye size covary in male killifish. We utilized wild‐caught and common garden‐reared specimens to determine whether specific components of the brain have evolved in response to differences in predation and to determine if there is covariation between the evolution of brain size, brain structure, and eye size. We observed consistent shifts in brain architecture in second generation common garden reared, but not wild caught preserved fish. Male killifish from sites that lack predators exhibited a significantly larger telencephalon, optic tectum, cerebellum, and dorsal medulla when compared with fish from sites with predators. We also found positive connections between the evolution of brain structure and eye size but not between overall brain size and eye size. These results provide evidence for evolutionary covariation between the components of the brain and eye size. Such results suggest that selection, directly or indirectly, acts upon specific regions of the brain, rather than overall brain size, to enhance visual capabilities.     

The role of size and colour pattern in protection of developmental stages of the red firebug (Pyrrhocoris apterus) against avian predators

We investigated how predator/prey body‐size ratio and prey colour pattern affected efficacy of prey warning signals. We used great and blue tits (Parus major and Cyanistes caeruleus), comprising closely related and ecologically similar bird species differing in body size, as experimental predators. Two larval instars and adults of the unpalatable red firebug (Pyrrhocoris apterus), differing in body size and/or coloration, were used as prey. We showed that prey body size did not influence whether a predator attacked the prey or not during the first encounter. However, smaller prey were attacked, killed, and eaten more frequently in repetitive encounters. We assumed that body size influences the predator through the amount of repellent chemicals better than through the amount of optical warning signal. The larger predator attacked, killed and ate all forms of firebug more often than the smaller one. The difference between both predators was more pronounced in less protected forms of firebug (chemically as well as optically). Colour pattern also substantially affected the willingness of predators to attack the prey. Larval red–black coloration did not provide a full‐value warning signal, although a similarly conspicuous red‐black coloration of the adults reliably protected them. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 890–898.     

Foraging habitat determines predator–prey size relationships in marine fishes

Predator–prey size (PPS) relationships are determined by predator behaviour, with the likelihood of prey being eaten dependent on their size relative to that of the consumer. Published PPS relationships for 30 pelagic or benthic marine fish species were analysed using quantile regression to determine how median, lower and upper prey sizes varied with predator size and habitat. Habitat effects on predator foraging activity/mode, morphology, growth and natural mortality are quantified and the effects on PPS relationships explored. Pelagic species are more active, more likely to move by caudal fin propulsion and grow more rapidly but have higher mortality rates than benthic species, where the need for greater manoeuvrability when foraging in more physically complex habitats favours ambush predators using pectoral fin propulsion. Prey size increased with predator size in most species, but pelagic species ate relatively smaller prey than benthic predators. As pelagic predators grew, lower prey size limits changed little, and prey size range increased but median relative prey size declined, whereas the lower limit increased and median relative prey size was constant or increased in benthic species.     

Crab shell-crushing predation and gastropod architectural defense

The shell-breaking behavior of the crabs Ozius verreauxii Saussure 1853 and Eriphia squamata, Stimpson 1859 from the Bay of Panama is described. The master claws of both these crabs are well designed for breaking shells. Small shells, relative to the size of a crab predator, are crushed by progressively breaking off larger segments of a shell's apex, while larger shells are peeled by inserting a large dactyl molar into the aperture of a shell and progressively chipping away the lip of the shell.

Heavy gastropod shells are shown to be less vulnerable to crab predators than lighter shells, and narrow shell apertures and axial shell sculpture are demonstrated to be architectural features that deter crab predation. The incidence of architectural features which deter crab predation appears to be higher for smaller gastropod species than for larger gastropods which are too large for most crab predators. Large fish predators prey upon both gastropods and shell-crushing crabs. To avoid fish predators, both these prey groups seek refuge under rocks when covered by the tide. Fish predation thus appears to enforce a close sympatry between smaller gastropods and their crab predators.     

Partitioning the non-consumptive effects of predators on prey with complex life histories

Non-consumptive effects (NCEs) of predators on prey can be as strong as consumptive effects (CEs) and may be driven by numerous mechanisms, including predator characteristics. Previous work has highlighted the importance of predator characteristics in predicting NCEs, but has not addressed how complex life histories of prey could mediate predator NCEs. We conducted a meta-analysis to compare the effects of predator gape limitation (gape limited or not) and hunting mode (active or sit-and-pursue) on the activity, larval period, and size at metamorphosis of larval aquatic amphibians and invertebrates. Larval prey tended to reduce their activity and require more time to reach metamorphosis in the presence of all predator functional groups, but the responses did not differ from zero. Prey metamorphosed at smaller size in response to non-gape-limited, active predators, but counter to expectations, prey metamorphosed larger when confronted by non-gape-limited, sit-and-pursue predators. These results indicate NCEs on larval prey life history can be strongly influenced by predator functional characteristics. More broadly, our results suggest that understanding predator NCEs would benefit from greater consideration of how prey life history attributes mediate population and community-level outcomes.     

Prey selection by young trout fry (Salmo trutta)

Factors influencing the choice of invertebrate food by early feeding trout are sought, special attention being given to the dimensions of the prey. The stomach contents of fish of similar age feeding on lake plankton and drifting stream fauna are compared. Little consistent preference was displayed for the smaller planktonic organisms but prey size was of importance in the distribution of stream benthos among predators of different lengths. The width of the prey was of greater significance to selection than the mean volume or the area of the largest surface of the food items. When the fish were in obvious competition for food the correlation between predator length and all prey dimensions improved.     

The Ability of Three Species of Tadpoles to Differentiate among Potential Fish Predators

The ability of prey to respond to novel predator cues may depend on the generality or specificity of the response to predator cues. We used laboratory behavioral experiments to examine the ability of tadpoles of three species of anurans (American toad, Bufo americanus ; bullfrog, Rana catesbeiana ; and green frog, R. clamitans ) to respond to the presence of two native potential predators (bluegill, Lepomis macrochirus ; and largemouth bass, Micropterus salmoides ) and one non-native potential predator (goldfish, Carassius auratus ). We also examined the effect of tadpole size on the behavioral responses of American toads and green frogs to predator cues. All three species of tadpoles responded to the presence of predator cues, although the specific responses varied among species. American toads and green frogs reduced activity in the presence of at least some fish cues, but bullfrog tadpoles did not change their activity. Bullfrogs decreased use of vegetation in the presence of some predator cues, whereas American toads and green frogs did not. American toads only responded to the presence of bluegill cues but not the other fish predator cues, whereas bullfrogs and green frogs responded more generally to the fish predators. In both American toads and green frogs, tadpole size affected behavior. For American toads, activity increased, as did the use of the vegetated side of the aquarium, in larger tadpoles. Not only did size affect American toad behavior, but it also influenced the responses of the tadpoles to predator cues. For green frogs, activity decreased in larger tadpoles. Our results suggest that behavioral responses of tadpoles to predator cues can be influenced by both the identity of the predator and the prey, as well as the size of the potential prey.     

The evolution of prey body size reaction norms in diverse communities

1. Heterogeneous predation risks can select for predator-specific plastic defences in prey populations. However, diverse predation threats can generate diffuse selection, which, in turn, can lead to the evolution of more generalized reaction norms. Unreliable predator cues also can select for more generalized plasticity in prey. 2. Here, I evaluated the extent to which variation in risk from a focal predator vs. variation in risk from predator diversity and composition were associated with variation in body mass reaction norms in 18 prey populations. Toward this end, I assayed the body mass reaction norms in a common garden experiment for spotted salamander larvae Ambystoma maculatum in response to marbled salamander predators Ambystoma opacum, local predator richness and the densities of two auxiliary predator species. 3. When raised under controlled conditions, prey larvae generally were smaller when exposed to A. opacum kairomones. Among populations, the mean and slope of body mass variation was unrelated to A. opacum's local density. 4. Predator richness and several key environmental factors were not associated with reaction norm variation. Instead, the density of an auxiliary newt predator species was correlated with reduced mass reaction norm slopes. Results suggest that diffuse selection from auxiliary predators can modify the evolution of life-history plasticity.     

哺乳动物进化过程中体重增大的原因浅析

作者认为,哺乳动物进化过程中体型增大有4个方面的原因,即：（1）较大的种类在与利用相同资源的较小种类的竞争中具有优势;（2）为了避免激烈的种间竞争,一些种类专门利用某些只适合于体型较大者利用的资源;（3）某些食草动物依靠增大体型来增强对付天敌的能力;（4）捕食者—猎物之间的协同进化导致两者体型增大。这些机制反映了竞争作用,捕食作用以及资源利用的特化对哺乳动物进化过程的影响。文章还分析了贝格曼定律不能说明恒温动物体型增大机制的原因,并用化石证据对贝格曼定律的种种误解给予阐释。此外,对体型特大化的哺乳动物在第四纪冰期灭绝的原因给出了一个纯生态学意义的解释。