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Graeme D Ruxton

Publications and source records attributed to Graeme D Ruxton.

At least 19 recordsLinked to original sources
PubMed2006

Avian predators attack aposematic prey more forcefully when they are part of an aggregation.

Defended insects often advertise their unprofitability to potential predators using conspicuous aposematic coloration. Many aposematic insects are also gregarious, and it has been suggested that the aggregation of defended prey may have facilitated the evolution of aposematic coloration. Empirical studies have demonstrated that birds are more wary of aggregated aposematic prey, and learn to avoid them more quickly than solitary prey. However, many aposematic insects survive being attacked by birds, and the effect of aggregation on post-attack survival has not previously been investigated. Using domestic chicks as predators and artificially manipulated mealworms as prey, we provide empirical evidence that predators attack aggregated aposematic prey more forcefully than solitary prey, reducing the likelihood of prey surviving an attack. Hence, we suggest that previous works concluding that aggregation was an important pre-requisite for the evolution of aposematism may have overestimated the fitness benefits of aggregation, since aggregated prey may be attacked less but are also less likely to survive an attack.

Aggression↗
PubMed2006

Aposematic signals and the relationship between conspicuousness and distinctiveness.

It has long been recognized that prey that invest in toxic or other defenses often advertise these defenses by means of conspicuously coloured signals. One question that remains unanswered is why conspicuousness is such a universal trait of aposematic signals. Conspicuousness may allow more rapid avoidance learning by predators or improved retention of such learning. An alternative or complementary explanation is that defended species should adopt a conspicuous signal of their defence to make them visually distinct from inconspicuous undefended prey. Here, we use a neural network model of prey detection and attack decision making by a predator in combination with evolving, virtual prey to shed light on the relative importance of conspicuousness against the background and distinctiveness from other species as mechanisms underlying aposematic signalling. Our model suggests that prey conspicuousness may result from selection for distinctiveness, but that selection for distinctiveness does not result in maximization of conspicuousness. On the other hand, our model does not justify the exclusion of the possibility that conspicuousness as such may be a beneficial attribute of warning coloration. It is likely that the relative importance of the two selective forces (for conspicuousness and for distinctiveness) will differ on a case-by-case basis, however there is no empirical or logical reason for the current neglect of evolutionary pressure for distinctiveness. Thus, we suggest that description of aposematism as the teaming of a secondary defence with a conspicuous signal may be overly simplistic; we would rather that the signal were described as conspicuous and/or distinctive.

Animals↗
PubMed2006

Aggregation, defence and warning signals: the evolutionary relationship.

In a seminal contribution, Fisher argued how distastefulness could incrementally evolve in a prey species that was distributed in family groups. Many defended prey species occur in aggregations, but did aggregation facilitate the evolution of defence as Fisher proposed or did the possession of a defence allow individuals to enjoy the benefits of group living? Contemporary theory suggests that it can work both ways: pre-existing defences can make the evolution of gregariousness easier, but gregariousness can also aid the evolution of defence and warning signals. Unfortunately, the key phylogenetic analyses to elucidate the ordering of events have been hampered by the relative rarity of gregarious species, which in itself indicates that aggregation is not a pre-requisite for defence. Like the underlying theory, experimental studies have not given a definitive answer to the relative timing of the evolution of defence and aggregation, except to demonstrate that both orderings are possible. Conspicuous signals are unlikely to have evolved in the absence of a defence and aggregated undefended prey are likely to be vulnerable to predation in the absence of satiation effects. It therefore seems most likely that defence generally preceded the evolution of both aggregation and signalling, but alternative routes may well be possible.

Animals↗
PubMed2006

Structural correlates of increased adhesive efficiency with adult size in the toe pads of hylid tree frogs.

Tree frogs are able to climb smooth, vertical substrates using specialised toe pads which adhere via an area-based wet adhesive mechanism. Although the link between pads and arboreality in frogs is well-established, few studies have investigated the influence of morphology on adhesion. Trinidadian tree frogs from the genus Hyla are geometrically similar. There is a tendency towards comparatively reduced mass in larger species, but toe pad area increases as expected with isometry. As adhesion is area-dependent, forces are affected directly by the increase in mass relative to pad area, and there is a decrease in the ability of larger species to adhere to smooth rotation platforms. However, there is an increase in force per unit area that suggests larger species have more efficient toe pads. Toe pad structure is very similar though there are variations in the details of a number of features. Crucially, although differences in morphology appeared small they had demonstrable effects on adhesive efficiency of the pads. Epithelial cell area correlates positively with frog length and adhesive efficiency, related features of cell density and intercellular channel length correlate negatively. These findings are discussed in relation to the different forces involved in the tree frogs' wet adhesive system.

Adaptation, Physiological↗
PubMed2006

Artificial neural network properties associated with wiring patterns in the visual projections of vertebrates and arthropods.

We model the functioning of different wiring schemes in visual projections using artificial neural networks and so speculate on selective factors underlying taxonomic variation in neural architecture. We model the high connective overlap of vertebrates (where networks have a dense mesh of connections) and the less overlapping, more modular architecture of arthropods. We also consider natural variation in these basic wiring schemes. Generally, arthropod networks are as efficient or more efficient in functioning compared to vertebrate networks. They do not show the confusion effect (decreasing targeting accuracy with increasing input group size), and they train as well or better. Arthropod networks are, however, generally poorer at reconstructing novel inputs. The ability of vertebrate networks to effectively process novel stimuli could promote behavioral sophistication and drive the evolution of vertebrate wiring schemes. Vertebrate networks with less connective overlap have, surprisingly, similar or superior properties compared to those with high connective overlap. Thus, the partial connective overlap seen in real vertebrate visual projections may be an optimal, evolved solution. Arthropod networks with and without whole-cell neural connections within neural layers have similar properties. This indicates that neural connections mediated by offshoots of single cells (dendrites) may be fundamental to generating the confusion effect.

Animals↗
PubMed2006

Evolution of anti-predator traits in response to a flexible targeting strategy by predators.

Anti-predator benefits increase with vigilance rate and group size in many species of animal, while simultaneously resource intake rates usually decrease. This implies that there is an optimal group size and vigilance rate that will maximize individual fitness. While this basic theory of vigilance has been modelled and tested extensively, it has often been assumed that the predator represents a 'fixed-risk' such that groups of prey are essentially independent entities that exert little or no effect on one another either directly or indirectly. We argue that this is an over-simplification, and propose that the behaviour of one group of prey will likely affect the fitness of another local group of prey if the predator preferentially attacks the most vulnerable group-rather than attack both with constant rates. Using a numerical simulation model, we make the first examination of this game and allow the prey to dynamically evolve both optimal group size distributions between two habitats and vigilance rates in response to a predator with a preference for whichever group is the more vulnerable. We show that the density of prey in the population and the sensitivity of a predator to differences in prey vulnerability are likely to drive the dynamics of such a game. This novel approach to vigilance theory opens the door to several challenging lines of future research, both experimental and theoretical.

Animals↗
PubMed2006

How can automimicry persist when predators can preferentially consume undefended mimics?

It is common for species that possess toxins or other defences to advertise these defences to potential predators using aposematic ("warning") signals. There is increasing evidence that within such species, there are individuals that have reduced or non-existent levels of defence but still signal. This phenomenon (generally called automimicry) has been a challenge to evolutionary biologists because of the need to explain why undefended automimics do not gain such as a fitness advantage by saving the physiological costs of defence that they increase in prevalence within the population, hence making the aposematic signal unreliable. The leading theory is that aposematic signals do not stop all predatory attacks but rather encourage predators to attack cautiously until they have identified the defence level of a specific individual. They can then reject defended individuals and consume the undefended. This theory has recently received strong empirical support, demonstrating that high-accuracy discrimination appears possible. However, this raises a new evolutionary problem: if predators can perfectly discriminate the defended from the undefended and preferentially consume the latter, then how can automimicry persist? Here, we present four different mechanisms that can allow non-trivial levels of automimics to be retained within a population, even in the extreme case where predators can differentiate defended from undefended individuals with 100% accuracy. These involve opportunity costs to the predator of sampling carefully, temporal fluctuation in predation pressure, predation pressure being correlated with the prevalence of automimicry, or developmental or evolutionary constraints on the availability of defence. These mechanisms generate predictions as to the conditions where we would expect aposematically signalling populations to feature automimicry and those where we would not.

Animals↗
PubMed2006

Temporal and intrapopulation variation in prey choice of wintering geese determined by stable isotope analysis.

1. Individual variability in prey preferences can have marked effects on many demographic parameters from individual survival and fecundity to the vital rates of entire populations. A population level response is ultimately determined by individual prey choices; however, the effect of individual dietary choice is often overlooked. 2. We determined prey choice by individual consumers, light-bellied Brent geese Branta bernicla, during the overwintering period. Two hundred and eighty-one individuals were sampled at distinct temporal points over two winters. Stable isotopic ratios of carbon and nitrogen for blood cells and blood plasma, from each sampled individual were measured. Isotopic ratios for potential prey items were also measured. 3. Delta15N and delta13C for blood samples were both significantly different between sample months. Generally we found a decrease in both isotopic ratios during the course of the winter. All potential prey items were also isotopically distinct. Multisource mixing models (isosource) were used to determine the range of possible contribution to the diet of individuals. 4. During early winter, diet consisted almost exclusively of sea grass Zostera spp. The level of Zostera spp. in the diet dropped until mid-winter, and was supplemented by the utilization of green algae Ulva lactuca, and Enteromorpha spp., and terrestrial grasses. Terrestrial grass comprised an increasing proportion of the diet in late winter, representing virtually the exclusive food source by April. 5. By examining intrapopulation variability in resource utilization we highlight a number of ecologically important factors not addressed by previous population level studies.

Animals↗
PubMed2005

Antipredator vigilance in birds: modelling the 'edge' effect.

Many animals spend a large proportion of their time either foraging for food or watching out for predators (antipredator vigilance). There have been many theoretical and empirical studies investigating the trade-off between these two activities, especially in birds. Previous models of antipredator vigilance assume that all birds within the group spend the same amount of time feeding. However, many empirical studies have shown that individuals on the edge of flocks are more vigilant. Here we describe a vigilance model which investigates the effect of position on the birds' strategies by dividing the feeding area into an inner and outer region. The model examines how various parameters such as food availability and predation risk affect the strategies of individual birds according to whether they are in the inner or outer region. Our model predicts that birds in the outer group are always more vigilant than those in the inner region. Birds in the centre receive a higher payoff in each of the wide range of scenarios that we have considered, and so our model also indicates why dominant birds would choose to feed in the centre of the group; a prediction in accord with several empirical studies.

Algorithms↗
PubMed2005

Increasing search rate over time may cause a slower than expected increase in prey encounter rate with increasing prey density.

A previous experiment with birds searching for caterpillars in an aviary demonstrated a highly counterintuitive result, that the rate at which a forager encounters prey does not increase linearly with prey density. Here, I demonstrate that if search rate increases over time then this can produce exactly the observed type of behaviour. Further, I argue that declining perception of predation risk over time in the absence of reinforcement, coupled with a trade-off between anti-predator vigilance and searching ability (both widely reported in field and laboratory studies), could generate such a change in search rate over time. Hence, if my hypothesis is correct, the previous experimental results could have considerable generality, and invite reconsideration of our mathematical descriptions of predator-prey interactions.

Animals↗
PubMed2005

Why are small males aggressive?

Aggression is ubiquitous in the animal kingdom, whenever the interests of individuals conflict. In contests between animals, the larger opponent is often victorious. However, counter intuitively, an individual that has little chance of winning (generally smaller individuals) sometimes initiates contests. A number of hypotheses have been put forward to explain this behaviour, including the "desperado effect" according to which, the likely losers initiate aggression due to lack of alternative options. An alternative explanation suggested recently is that likely losers attack due to an error in perception: they mistakenly perceive their chances of winning as being greater than they are. We show that explaining the apparently maladaptive aggression initiated by the likely loser can be explained on purely economic grounds, without requiring either the desperado effect or perception errors. Using a game-theoretical model, we show that if smaller individuals can accurately assess their chance of winning, if this chance is less than, but close to, a half, and if resources are scarce (or the contested resource is of relatively low value), they are predicted to be as aggressive as their larger opponents. In addition, when resources are abundant, and small individuals have some chance of winning, they may be more aggressive than their larger opponents, as it may benefit larger individuals to avoid the costs of fighting and seek alternative uncontested resources.

Aggression↗
PubMed2005

Aposematism: what should our starting point be?

The evolution of aposematism is considered to be a major evolutionary problem because if new aposematic forms emerged in defended cryptic populations, they would face the dual problems of rarity and conspicuousness. We argue that this commonly assumed starting point might not have wide validity. We describe a novel evolutionary computer model in which prey evolve secondary defences and become conspicuous by moving widely over a visually heterogeneous habitat. Unless crypsis imposes high opportunity costs (for instance, preventing prey from efficient foraging, thermoregulation and communication), costly secondary defences are not predicted to evolve at all. However, when crypsis imposes opportunity costs, prey evolve secondary defences that facilitate raised behavioural conspicuousness as prey exploit opportunities within their environment. Optimal levels of secondary defence and of behavioural conspicuousness increase with population sizes and the costs imposed by crypsis. When prey are already conspicuous by virtue of their behaviours, the evolution of aposematic appearances (bright coloration, etc.) is much easier to explain because aposematic traits add little further costs of conspicuousness, but can bring large benefits.

Adaptation, Physiological↗
PubMed2005

The confusion effect in predatory neural networks.

A simple artificial neural network model of image reconstruction in sensory maps is presented to explain the difficulty predators experience in targeting prey in large groups (the confusion effect). Networks are trained to reconstruct multiple randomly conformed "retinal" images of prey groups in an internal spatial map of their immediate environment. They are then used to simulate prey targeting by predators on groups of specific conformation. Networks trained with the biologically plausible associative reward-penalty method produce a more realistic model of the confusion effect than those trained with the popular but biologically implausible backpropagation method. The associative reward-penalty model makes the novel prediction that the accuracy-group size relationship is U shaped, and this prediction is confirmed by empirical data gathered from interactive computer simulation experiments with humans as "predators." The model further predicts all factors known from previous empirical work (and most factors suspected) to alleviate the confusion effect: increased relative intensity of the target object, heterogeneity of group composition, and isolation of the target. Interestingly, group compaction per se is not predicted to worsen predator confusion. This study indicates that the relatively simple, nonattentional mechanism of information degradation in the sensory mapping process is potentially important in generating the confusion effect.

Animals↗
PubMed2005

Intimidating butterflies.

Many butterfly species feature two or four conspicuously coloured eyespots on their wings; the most commonly heard explanation for these markings is that they are used to intimidate predators. However, this explanation has stood, until recently, on the flimsiest of empirical foundations. Now, Adrian Vallin and colleagues have demonstrated empirically the effectiveness of these signals in dissuading avian predators from attacking the bearers. Their work suggests that predators can be intimidated by bold displaying by otherwise defenceless prey, and opens up an interesting challenge to evolutionary ecologists to explain how this apparently maladaptive behaviour is retained in predator populations.

Journal Article↗
PubMed2005

Warning displays in spiny animals: one (more) evolutionary route to aposematism.

To date, theoretical or laboratory simulations of aposematic evolution in prey animals have focused narrowly on internally stored chemical defense as the source of unprofitability and ignore aposematic advertisement of physical defenses such as spines (and defensive hairs, claws, etc.). This has occurred even though aposematism in spiny animals has been recognized since the 19th century. In this paper we present the first detailed theoretical consideration of aposematism in spiny animals, focusing on questions of initial evolution, costs of display, and coevolution of displays with defenses. Using an individual-based evolutionary model, we found that spines (or similar physical defenses) can easily evolve without aposematism, but when spines do evolve, aposematic displays can also easily evolve if they help to make the prey animal distinctive and if they draw attention to the physical threat. When aposematic displays evolve, they cause reduced investment in costly spines, so that, in addition to signaling unprofitability, aposematic display may enhance the cost-effectiveness of antipredator defenses (one exception to this conclusion is if the display is itself as costly as the defense). For animals with stinging spines, combining physical and chemical defense, the evolution of aposematic display may lead to reduced investment in the toxin compared to the spine. This occurs because spines act as both secondary (repellent) defenses and as primary defenses (their own visible, honest advertisement), whereas internally stored toxins only (generally) act as repellent secondary defenses. We argue that conspicuous aposematism in spines functions as an attention-getting mechanism, whereas conspicuous aposematic display in purely toxic animals may be explained by signal reliability arguments. Finally, one (more) route by which aposematism may initially evolve is by spiny rather than purely chemically defended species, spreading to species with other forms of secondary defense as the signal becomes common.

Animal Structures↗