Foraging mode switching : the importance of prey distribution and foraging currency

A.D.H. was supported by the European Research Council (Advanced Grant 250209 to Alasdair Houston), a College for Life Sciences Fellowship at the Wissenschaftskolleg zu Berlin, and a NERC Independent Research FellowshipNE/L011921/1.Foraging methods are highly variable, but can be grouped into two modes: searching and ambush. While research has focused on the functioning of each mode, the question of how animals choose which to use has been largely neglected. Here we consider a forager that exploits prey that are patchily distributed in space and time. This forager can either sit and wait for prey to appear or search for prey, which is more likely to result in encounters with prey but costs more energy and/or exposes the forager to greater predation risk. The currency that natural selection appears to have optimized will be determined by the additional costs of searching and whether there is a risk of starvation. We therefore compare the predictions of models based on currencies that consider only energy and predation risk to state-dependent models in which energy reserves are used to trade off predation rate, starvation rate and investment in growth. The choice of currency qualitatively affects how mode should change when prey abundance and prey patchiness increase. We show how differing prey distributions can explain variation in effects of experimentally increasing prey abundance. Our work has several implications for the study of foraging mode, population dynamics and the methods used to assess population size.Publisher PDFPeer reviewe

Current incentives for scientists lead to underpowered studies with erroneous conclusions

We can regard the wider incentive structures that operate across science – such as the priority given to novel findings – as an ecosystem within which scientists strive to maximise their fitness (i.e., publication record and career success). Here, we develop an optimality model that predicts the most rational research strategy, in terms of the proportion of research effort spent on seeking novel results rather than on confirmatory studies, and the amount of research effort per exploratory study. We show that, for parameter values derived from the scientific literature, researchers acting to maximise their fitness should spend most of their effort seeking novel results and conduct small studies that have only 10-40% statistical power. As a result, half of the studies they publish will report erroneous conclusions. Current incentive structures are in conflict with maximising the scientific value of research; we suggest ways that the scientific ecosystem could be improved

Evolution of a flexible rule for foraging that copes with environmental variation

PublishedThis is the final version of the article. It first appeared from Oxford University Press via http://www.currentzoology.org/paperdetail.asp?id=12437Models of adaptive behaviour typically assume that animals behave as though they have highly complex, detailed strategies for making decisions. In reality, selection favours the optimal balance between the costs and benefits of complexity. Here we investigate this trade-off for an animal that has to decide whether or not to forage for food - and so how much energy reserves to store - depending on the food availability in its environment. We evolve a decision rule that controls the target reserve level for different ranges of food availability, but where increasing complexity is costly in that metabolic rate increases with the sensitivity of the rule. The evolved rule tends to be much less complex than the optimal strategy but performs almost as well, while being less costly to implement. It achieves this by being highly sensitive to changing food availability at low food abun-dance - where it provides a close fit to the optimal strategy - but insensitive when food is plentiful. When food availability is high, the target reserve level that evolves is much higher than under the optimal strategy, which has implications for our under-standing of obesity. Our work highlights the important principle of generalisability of simple decision-making mechanisms, which enables animals to respond reasonably well to conditions not directly experienced by themselves or their ancestors.This work was supported by the European Research Council (Advanced Grant 250209 to A.I.H.) and a College for Life Sciences Fellowship at the Wissenschaftskolleg zu Berlin awarded to A.D.H

The evolution of mechanisms underlying behaviour

This is the final version of the article. It first appeared from Oxford University press via http://www.scopus.com/record/display.uri?eid=2-s2.0-84925394342&origin=inward&txGid=0-This work was funded by the European Research Council (Advanced Grant 250209 to Alasdair Houston)

Florivory as an opportunity benefit of aposematism

A.D.H. was supported by the European Research Council (Advanced Grant 250209 to A. Houston) and fellowships from the Wissenschaftskolleg zu Berlin and the Natural Environment Research Council (NE/L011921/1).Inconspicuous prey pay a cost of reduced feeding opportunities. Flowers are highly nutritious but are positioned where prey would be apparent to predators and often contain toxins to reduce consumption. However, many herbivores are specialized to subvert these defenses by retaining toxins for their own use. Here, we present a model of the growth and life history of a small herbivore that can feed on leaves or flowers during its development and can change its primary defense against visual predators between crypsis and warning coloration. When herbivores can retain plant toxins, their fitness is greatly increased when they are aposematic and can consume flowers. Thus, toxin sequestration leading to aposematism may enable a significant opportunity benefit for florivory. Florivory by cryptic herbivores is predicted when toxins are very potent but are at high concentration only in flowers and not in leaves. Herbivores should usually switch to eating flowers only when large and in most conditions should switch simultaneously from crypsis to aposematism. Our results suggest that florivory should be widespread in later instars of small aposematic herbivores and should be associated with ontogenic color change. Florivory is likely to play an underappreciated role in herbivorous insect life histories and host plant reproductive success.Publisher PDFPeer reviewe

The influence of the starvation-predation trade-off on the relationship between ambient temperature and body size among endotherms

Aim: The tendency for animals at higher latitudes to be larger (Bergmann's rule) is generally explained by recourse to latitudinal effects on ambient temperature and the food supply, but these receive only mixed support and do not explain observations of the inverse to Bergmann's rule. Our aim was to better understand how ecological variables might influence body size and thereby explain this mixed support. Location: World-wide. Methods: Previous explanations do not allow for the selective pressure exerted by the trade-off between predation and starvation, which we incorporate in a model of optimal body size and energy storage of a generalized homeotherm. In contrast to existing arguments, we concentrate on survival over winter when the food supply is poor and can be interrupted for short periods. Results: We use our model to assess the logical validity of the heat conservation hypothesis and show that it must allow for the roles of both food availability and predation risk. We find that whether the effect of temperature on body size is positive or negative depends on temperature range, predator density, and the likelihood of long interruptions to foraging. Furthermore, changing day length explains differing effects of altitude and latitude on body size, leading to opposite predictions for nocturnal and diurnal endotherms. Food availability and ambient temperature can have counteracting selective pressures on body mass, and can lead to a non-monotonic relationship between latitude and size, as observed in several studies. Main conclusions: Our work provides a theoretical framework for understanding the relationships between the costs and benefits of large body size and eco-geographical patterns among endotherms world-wide

The influence of the food–predation trade-off on the foraging behaviour of central-place foragers

PublishedThis is the author accepted manuscript. The final version is available from Springer via the DOI in this record.Social animals that bring resources to a central place have commonly been used to test the predictions of optimal foraging models. Such animals are amenable to test because they do not themselves reproduce, and so we might expect them to be selected to maximise some measure of input of food to the colony. Several currencies have been proposed to predict behaviour, such as net rate, efficiency, and the ratio of the mortality rate to energy gain rate. Observations on social animals, especially bees, show mixed support for each currency. Here, we examine how these currencies can be united by considering the expected lifetime input of energy to the colony in a representative study of patch residence time. This currency explains partial loads because it leads to the prediction that the energy that a forager delivers to the colony over its lifetime is maximised by returning to the colony after a critical amount has been collected, even if energy is gained at a constant rate. We show that the extent to which foraging carries a greater mortality risk than travelling controls whether this currency makes similar predictions to net rate or to efficiency. We assess the evidence that bee behaviour actually maximises this currency and argue that mortality risk at resource sites is likely to be a critical determinant of foraging strategies in central-place foragers

Adaptive learning can result in a failure to profit from good conditions: implications for understanding depression.

Published onlineThis is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/emph/eov009BACKGROUND AND OBJECTIVES: Depression is a major medical problem diagnosed in an increasing proportion of people and for which commonly prescribed psychoactive drugs are frequently ineffective. Development of treatment options may be facilitated by an evolutionary perspective; several adaptive reasons for proneness to depression have been proposed. A common feature of many explanations is that depressive behaviour is a way to avoid costly effort where benefits are small and/or unlikely. However, this viewpoint fails to explain why low mood persists when the situation improves. We investigate whether a behavioural rule that is adapted to a stochastically changing world can cause inactivity which appears similar to the effect of depression, in that it persists after the situation has improved. METHODOLOGY: We develop an adaptive learning model in which an individual has repeated choices of whether to invest costly effort that may result in a net benefit. Investing effort also provides information about the current conditions and rates of change of the conditions. RESULTS: An individual following the optimal behavioural strategy may sometimes remain inactive when conditions are favourable (i.e. when it would be better to invest effort) when it is poorly informed about the current environmental state. Initially benign conditions can predispose an individual to inactivity after a relatively brief period of negative experiences. CONCLUSIONS AND IMPLICATIONS: Our approach suggests that the antecedent factors causing depressed behaviour could go much further back in an individual s history than is currently appreciated. The insights from our approach have implications for the ongoing debate about best treatment options for patients with depressive symptoms.This work was supported by the European Research Council (Evomech Advanced Grant 250 209 to A.I.H.)

The impact of detoxification costs and predation risk on foraging : implications for mimicry dynamics

This work was supported by the European Research Council (Advanced Grant 250209 to Alasdair Houston), a Natural Environment Research Council Independent Research Fellowship (NE/L011921/1) awarded to A.D.H., a BBSRC-NERC project grant (BB/G00188X/1) awarded to J.S., C.R. and G.D.R. and a faculty fellowship awarded to C.G.H. (Medical Sciences, Newcastle University) with strategic support funding from the Wellcome Trust.Prey often evolve defences to deter predators, such as noxious chemicals including toxins. Toxic species often advertise their defence to potential predators by distinctive sensory signals. Predators learn to associate toxicity with the signals of these so-called aposematic prey, and may avoid them in future. In turn, this selects for mildly toxic prey to mimic the appearance of more toxic prey. Empirical evidence shows that mimicry could be either beneficial (‘Mullerian’) or detrimental (‘quasi-Batesian’) to the highly toxic prey, but the factors determining which are unknown. Here, we use state-dependent models to explore how tri-trophic interactions could influence the evolution of prey defences. We consider how predation risk affects predators’ optimal foraging strategies on aposematic prey, and explore the resultant impact this has on mimicry dynamics between unequally defended species. In addition, we also investigate how the potential energetic cost of metabolising a toxin can alter the benefits to eating toxic prey and thus impact on predators’ foraging decisions. Our model predicts that both how predators perceive their own predation risk, and the cost of detoxification, can have significant, sometimes counterintuitive, effects on the foraging decisions of predators. For example, in some conditions predators should: (i) avoid prey they know to be undefended, (ii) eat more mildly toxic prey as detoxification costs increase, (iii) increase their intake of highly toxic prey as the abundance of undefended prey increases. These effects mean that the relationship between a mimic and its model can qualitatively depend on the density of alternative prey and the cost of metabolising toxins. In addition, these effects are mediated by the predators’ own predation risk, which demonstrates that, higher trophic levels than previously considered can have fundamental impacts on interactions among aposematic prey species.Publisher PDFPeer reviewe