Optimal Foraging Models Research Articles

Vulture culture: dietary specialization of an obligate scavenger.

Individual dietary variation has important ecological and evolutionary consequences. However, it has been overlooked in many taxa that are thought to have homogeneous diets. This is the case of vultures, considered merely as 'carrion eaters'. Given their high degree of sociality, vultures are an excellent model to investigate how inter-individual transmissible behaviours drive individual dietary variation. Here, we combine GPS-tracking and accelerometers with an exhaustive fieldwork campaign to identify the individual diet of 55 griffon vultures (Gyps fulvus) from two Spanish populations that partially overlap in their foraging areas. We found that individuals from the more humanized population consumed more anthropic resources (e.g. stabled livestock or rubbish), resulting in more homogeneous diets. By contrast, individuals from the wilder population consumed more wild ungulates, increasing their dietary variability. Between sexes, we found that males consumed anthropic resources more than females did. Interestingly, in the shared foraging area, vultures retained the dietary preference of their original population, highlighting a strong cultural component. Overall, these results expand the role of cultural traits in shaping key behaviours and call for the need of including cultural traits in Optimal Foraging models, especially in those species that strongly rely on social information while foraging.

Uncertainty in foraging success and its consequences on fitness

Optimal foraging models are commonly used to determine the strategy maximizing the proxies for fitness, such as foraging success. The strategies maximizing the proxy for fitness and fitness are assumed to be the same. However, this study shows that this assumption can be invalid when the relationship between the proxy for fitness and fitness is nonlinear and the foraging success is uncertain. A well-known prey choice model that uses long-term energy intake rate as the proxy for fitness was used as an example. This model considers a situation where predators predate on two types of prey that differ in quality, that is, one (primary prey) has higher quality than the other (alternative prey). A strategy can be represented by the probability of attacking alternative prey upon encounter while always attacking primary prey. The probability of attacking alternative prey that maximizes the expected rate of energy intake is either 0 or 1 depending on the density of primary prey (known as the zero–one rule). Meanwhile, the density of alternative prey has no influence on the optimal strategy. A simulation model was used to characterize the stochastic outcomes in the rate of energy intake in a finite foraging duration. The results revealed that foraging strategy influences the expected rate of energy intake and the uncertainty around the expectation. Consequently, the strategies maximizing the rate of energy intake and fitness may not be the same when the relationship between the rate of energy intake and fitness is nonlinear due to Jensen’s inequality. Previous results such as the zero–one rule and the independence of the optimal strategy on the availability of alternative prey are no longer valid when fitness, rather than the proxy for fitness, is explicitly considered in a finite foraging duration.

An entrepreneur's dilemma: An optimal stopping rule in pivoting

Though pivoting is widely practiced by entrepreneurs, an optimal stopping rule in pivoting remains unexplored. Proposing an optimal foraging model and accounting for competition in exploiting an opportunity, we identify an optimal stopping rule of 30% gain‐to‐loss ratio. That is, additional gains are less feasible when 70% of the pivoting across future plausible versions of opportunities are traversed to. The findings have practical implications for entrepreneurs.

Pollinators adjust their behavior to presence of pollinator-transmitted pathogen in plant population

Abstract Interactions between pollinators and plants can be affected by presence of plant pathogens that substitute their infectious propagules for pollen in flowers and rely on pollinators for transmission to new hosts. However, it is largely unknown how pollinators integrate cues from diseased plants such as altered floral rewards and floral traits, and how their behavior changes afterwards. Understanding pollinator responses to diseased plants is crucial for predicting both pathogen transmission and pollen dispersal in diseased plant populations. In this study, we investigated pollinator responses to contact with plants of Dianthus carthusianorum diseased with anther smut (Microbotryum carthusianorum). We combined three approaches: 1) observation of individual pollinators foraging in experimental arrays of pre-grown potted plants; 2) measurements of floral rewards and floral traits of healthy and diseased plants; and 3) quantification of pollen/spore loads of pollinator functional groups. We found that pollinators showed only weak preferences for visiting healthy over diseased plants, but after landing on plants, they probed fewer flowers on the diseased ones. Since diseased flowers offered lower nectar and no pollen rewards, this behavior is consistent with the prediction of optimal foraging models that pollinators should spend less time exploring less rewarding patches or plants. Furthermore, pollen-foraging solitary bees and hoverflies responded to diseased plants more negatively than nectar-feeding butterflies did. Lastly, based on group-specific behavior and typical pollen/spore loads, we suggest that solitary bees and hoverflies contribute to both pollen and pathogen spore dispersal mainly over short distances, while butterfly visits are most important for long-distance dispersal.

Salad with Clams: Prey Choice of an Intentionally Carnivorous Turtle

Prey choice is the non-random foraging and consumption of prey species by their predators, and is therefore the basis for studies of topics as diverse as quantifying food webs, predator–prey relationships, and optimal-foraging models. Malaclemys terrapin (Diamond-back Terrapin) is a diet generalist with a large geographic distribution: the US Atlantic and Gulf coasts from Massachusetts to Texas. Individual terrapins have relatively small home ranges and feed primarily on local mollusc species. In feeding trials with 1 prey species and 2 prey species, wild-caught terrapins from New York readily consumed Mya arenaria (Soft-shelled Clam) and Geukensia demissa (Atlantic Ribbed Mussel), preferring them over a wide range of other prey species commonly eaten by terrapins at other sites. Our correlation test indicated that Ulva latuca (Sea Lettuce), common in the diets of this population, is ingested incidentally when terrapins forage for Soft-shelled Clams. The impact of consumption of algae by this primarily molluscivorous turtle is unexplored, but could have important impacts on their energy balance and contaminant intake.

Horizontal-vertical movement relationships: Ad\xe9lie penguins forage continuously throughout provisioning trips

BackgroundDiving marine predators forage in a three-dimensional environment, adjusting their horizontal and vertical movement behaviour in response to environmental conditions and the spatial distribution of prey. Expectations regarding horizontal-vertical movements are derived from optimal foraging theories, however, inconsistent empirical findings across a range of taxa suggests these behavioural assumptions are not universally applicable.MethodsHere, we examined how changes in horizontal movement trajectories corresponded with diving behaviour and marine environmental conditions for a ubiquitous Southern Ocean predator, the Adélie penguin. Integrating extensive telemetry-based movement and environmental datasets for chick-rearing Adélie penguins at Béchervaise Island, we tested the relationships between horizontal move persistence (continuous scale indicating low [‘resident’] to high [‘directed’] movement autocorrelation), vertical dive effort and environmental variables.ResultsPenguins dived continuously over the course of their foraging trips and lower horizontal move persistence corresponded with less intense foraging activity, likely indicative of resting behaviour. This challenges the traditional interpretation of horizontal-vertical movement relationships based on optimal foraging models, which assumes increased residency within an area translates to increased foraging activity. Movement was also influenced by different environmental conditions during the two stages of chick-rearing: guard and crèche. These differences highlight the strong seasonality of foraging habitat for chick-rearing Adélie penguins at Béchervaise Island.ConclusionsOur findings advance our understanding of the foraging behaviour for this marine predator and demonstrates the importance of integrating spatial location and behavioural data before inferring habitat use.

Optimal foraging by a large ungulate in an extreme environment: Wild mountain reindeer select snow-free feeding habitats in winter.

Optimal foraging models predict that individual animals will optimize net energy gain by intensifying forage activity and/or reducing forage energy cost. Then, the free distribution model predicts an animal's distribution in a patchy landscape will match the distribution of the resources. If not modified by other factors, such patterns may be expected to be particularly explicit in variable and extreme, forage‐limited, and patchy environments, notably alpine and Arctic environments during winter. The large ungulate wild mountain reindeer (Rangifer tarandus tarandus) surviving in such environments is used as a model during the forage‐limited winter season. The largest wild reindeer area in Western Europe (Hardangervidda, 8130 km2) is actively managed to sustain 10,000–12,000 wild reindeer. Since 2001, 104 different individuals have been GPS‐tracked at 3‐hr intervals. In winter, mountain reindeer may either choose to seek out and forage in patchy snow‐free habitats, typically on top of wind‐blown ridges, or use energy‐demanding digging through the snow to reach ground forage (cratering). We use late April satellite data from Landsat 5 and 8 (30 × 30 m), airborne laser scanning subsampling (processed to 1 × 1 m grid), and topographic information (1 m resolution) derived from digital aerial photographs (0.25 × 0.25 m resolution) to delineate snow‐free patches, constituting less than 694 km2. By overlaying recorded wild reindeer GPS positions winters 2001–2017 (188,942 positions), we document a strong positive selection for snow‐free patches, which were used about four times more frequently than expected from a “random walk” model. On a daily basis, the preference for snow‐free areas was slightly stronger in the evenings. In the sustainable management of wild mountain reindeer, the area of snow‐free patches is an important predictor of winter forage availability and important winter source areas. It may be derived from remote sensing data.

Human choices respond to added costs according to the energy budget rule

The energy budget rule is a risk-sensitive optimal foraging model that predicts choice should be risk averse when need levels are relatively easy to meet within a foraging period (positive budget conditions), and risk prone when need levels are more difficult to meet (negative budget conditions). Although research with nonhumans has produced only limited support for the energy budget rule, analogue research with humans has shown that risky choice for monetary outcomes is typically consistent with the model’s predictions. Most human studies have tested the energy budget rule by manipulating minimum requirements, rates of gain, or reserves. The energy budget model also predicts, however, that energy expenditures (costs) can affect risk sensitivity. Two experiments therefore investigated whether monetary costs for choices would affect risk sensitivity. Adults were presented with choices between monetary options in blocks of five trials. Block earnings were paid only if earnings exceeded a minimum amount. Each trial, a monetary cost was subtracted from earnings when a choice was made. Cost magnitude was manipulated across conditions to generate one positive- (cost $0.00) and two negative-budget (cost $0.01 or $0.02) conditions. In Experiment 1, choice options produced the same mean earnings, with one producing a constant amount of money and the other a variable amount. In Experiment 2, both choice options produced the same mean amounts of money but different variances. Similar to previous earnings budget studies, choice varied as a function of budget condition. Trial-by-trial choice patterns were inconsistently predicted by a dynamic optimization model.

Herbivore-induced plant volatiles do not affect settling decisions by synanthropic spiders

An underlying assumption of optimal foraging models is that animals are behaviorally, morphologically, and physiologically adapted to maximize their net energy intake. Here we explored whether this concept applies to web-building spiders in a multi-trophic context. If a spider were to build her web next to herbivore-fed-on plants that signal the herbivores’ enemies for help by emitting herbivore-induced plant volatiles (HIPVs), that spider may maximize web captures in the short term. However, she would also risk predation by generalist predators that “listen” to signaling plants to find both herbivore and spider prey, likely resulting in lower overall reproductive fitness for the spider. We tested the hypothesis that HIPVs trigger avoidance responses by web-building spiders. We selected seven common HIPVs and one HIPV elicitor, and in two-choice olfactometer bioassays tested their effect on four synanthropic spider species (false black widow, Steatoda grossa; common cellar spider, Pholcus phalangioides; hobo spider, Eratigena agrestis; western black widow, Latrodectus hesperus). The 8-component HIPV/HIPV elicitor blend had a weak deterrent effect on S. grossa, but the effect did not extend to P. phalangioides, E. agrestis, and L. hesperus. Our findings imply that there was insufficient selection pressure for these spiders to recognize HIPVs in a multi-trophic context, where spiders themselves could become prey if generalist predators or spider-hunting parasitoid wasps were to respond to signaling plants.

Honey robbing: could human changes to the environment transform a rare foraging tactic into a maladaptive behavior?

Human environmental modifications have outpaced honey bees' ability to evolve adaptive regulation of foraging tactics, possibly including a tactic associated with extreme food shortage, honey robbing. Honey robbing is a high risk, high reward, and understudied honey bee tactic whereby workers attack and often kill neighboring colonies to steal honey. Humans have exacerbated the conditions that provoke such robbing and its consequences. We describe robbing as an individual-level and colony-level behavioral syndrome, implicating worker bees specialized for foraging, food processing, and defense. We discuss how colony signaling mechanisms could regulate this syndrome and then explore the ecological underpinnings of robbing-highlighting its unusual prevalence in the commonly managed Apis mellifera and outlining the conditions that provoke robbing. We advocate for studies that identify the cues that modulate this robbing syndrome. Additionally, studies that apply behavioral ecology modeling approaches to generate testable predictions about robbing could clarify basic bee biology and have practical implications for colony management.

Interactions among Shade, Caching Behavior, and Predation Risk May Drive Seed Trait Evolution in Scatter-Hoarded Plants

Although dispersal is critical to plant life history, the relationships between seed traits and dispersal success in animal-dispersed plants remain unclear due to complex interactions among the effects of seed traits, habitat structure, and disperser behavior. We propose that in plants dispersed by scatter-hoarding granivores, seed trait evolution may have been driven by selective pressures that arise from interactions between seedling shade intolerance and predator-mediated caching behavior. Using an optimal foraging model that accounts for cache concealment, hoarder memory, and perceived predation risk, we show that hoarders can obtain cache-recovery advantages by placing caches in moderately risky locations that force potential pilferers to engage in high levels of vigilance. Our model also demonstrates that the level of risk needed to optimally protect a cache increases with the value of the cached food item. If hoarders perceive less sheltered, high-light conditions to be more risky and use this information to protect their caches, then shade-intolerant plants may increase their fitness by producing seeds with traits valued by hoarders. Consistent with this hypothesis, shade tolerance in scatter-hoarded tree species is inversely related to the value of their seeds as perceived by a scatter-hoarding rodent.

Unpacking chimpanzee (Pan troglodytes) patch use: Do individuals respond to food patches as predicted by the marginal value theorem?

The marginal value theorem is an optimal foraging model that predicts how efficient foragers should respond to both their ecological and social environments when foraging in food patches, and it has strongly influenced hypotheses for primate behavior. Nevertheless, experimental tests of the marginal value theorem have been rare in primates and observational studies have provided conflicting support. As a step towards filling this gap, we test whether the foraging decisions of captive chimpanzees (Pan troglodytes) adhere to the assumptions and qualitative predictions of the marginal value theorem. We presented 12 adult chimpanzees with a two-patch foraging environment consisting of both low-quality (i.e., low-food density) and high-quality (i.e., high-food density) patches and examined the effect of patch quality on their search behavior, foraging duration, marginal capture rate, and its proxy measures: giving-up density and giving-up time. Chimpanzees foraged longer in high-quality patches, as predicted. In contrast to predictions, they did not depress high-quality patches as thoroughly as low-quality patches. Furthermore, since chimpanzees searched in a manner that fell between systematic and random, their intake rates did not decline at a steady rate over time, especially in high-quality patches, violating an assumption of the marginal value theorem. Our study provides evidence that chimpanzees are sensitive to their rate of energy intake and that their foraging durations correlate with patch quality, supporting many assumptions underlying primate foraging and social behavior. However, our results question whether the marginal value theorem is a constructive model of chimpanzee foraging behavior, and we suggest a Bayesian foraging framework (i.e., combining past foraging experiences with current patch sampling information) as a potential alternative. More work is needed to build an understanding of the proximate mechanisms underlying primate foraging decisions, especially in more complex socioecological environments.

The slow rise of technology: Computer vision techniques in fish population connectivity

Abstract Technological advancements in data collection and analysis are producing a new generation of ecological data. Among these, computer vision (CV) has received increased attention for its robust capabilities for rapidly processing large volumes of digital imagery. In marine ecosystems, the study of fish connectivity provides fundamental information for assessing fisheries stocks, designing and implementing protected areas and understanding the impact of habitat loss. While the field of fish connectivity has benefited from technological advancements, the extent to which novel techniques, such as CV, have been utilized has not been assessed. To inform future directions and developments, this study reviewed the current use of CV in fish connectivity research, quantified how the implementation of such technology in fish connectivity research compared with other areas of marine research and described how this field could benefit from CV. The review found that the use of remote camera systems in fish connectivity research is increasing, but the implementation of automated analysis of digital imagery has been slow. Successful implementation and expansion of CV frameworks in aquaculture and coral reef ecology suggest that CV techniques could greatly benefit fish connectivity research. A case study of potential use of CV in fish connectivity research, scaling up optimal foraging models to predict marine population connectivity, highlights how beneficial it could be. The capacity for CV techniques to be adopted alongside traditional approaches, the unparalleled speed, accuracy and reliability of these approaches and the benefits of being able to study ecosystems along multiple spatial–temporal scales, all make CV a valuable tool for assessing connectivity. Ultimately, these technologies can assist data‐driven decisions that directly influence the health and productivity of marine ecosystems.

Dynamics of a discrete-time stoichiometric optimal foraging model

In this paper, we discretize and analyze a stoichiometric optimal foraging model where the grazer's feeding effort depends on the producer's nutrient quality. We systematically make comparisons of the dynamical behaviors between the discrete-time model and the continuous-time model to study the robustness of model predictions to time discretization. When the maximum growth rate of producer is low, both model types admit similar dynamics including bistability and deterministic extinction of the grazer caused by low nutrient quality of the producer. Especially, the grazer is benefited from optimal foraging similarly in both discrete-time and continuous-time models. When the maximum growth rate of producer is high, dynamics of the discrete-time model are more complex including chaos. A phenomenal observation is that under extremely high light intensities, the grazer in the continuous-time model tends to perish due to poor food quality, however, the grazer in the discrete-time model persists in regular or irregular oscillatory ways. This significant difference indicates the necessity of studying discrete-time models which naturally include species' generations and are thus more popular in theoretical biology. Finally, we discuss how the shape of the quality-based feeding function regulates the beneficial or restraint effect of optimal foraging on the grazer population.

Behavioural adjustments during foraging in two diving seabirds: king and macaroni penguins

Optimal foraging theory predicts that diving predators should maximise their energy intake while minimizing the costs of their foraging activities at depth. This hypothesis was tested in two breath-hold divers that foraged in the pelagic waters off Kerguelen Island, southern Indian Ocean. Macaroni penguins (Eudyptes chrysolophus) feed predominately on crustaceans at moderate depths (ca. 50 m), while king penguins (Aptenodytes patagonicus) feed on mesopelagic fish at deep depths (ca. 100-150 m). We instrumented 22 penguins with time-depth recorders during the breeding season of the 2014 southern summer, just before they went to sea. Birds were recaptured after one to five foraging trips and the dive data were analysed on the level of single dive cycles and entire dive bouts. Both species adjusted their diving behaviour and modulated their foraging activity in accordance with their experience in their previous dives. Foraging activity during a dive (in terms of number of wiggles per dive) was greater if birds targeted the same depth as the previous dive. The penguins spent less time at the surface between two prey capture attempt dives than when a prey capture attempt dive was preceded by a non-prey capture attempt dive. Dive bout analyses showed that foraging activity increased with bout duration, while transit time during dives decreased with bout duration. Our findings are in general agreement with predictions based on optimal foraging models. The results suggest that penguins anticipated the depth of their next dive based on their encounter of a prey patch in the previous dive and optimized their chances of feeding success by spending minimal time at the surface and in transit to the patch depth. Once penguins detected a prey patch, these behavioural adjustments allowed them to rapidly return to that patch, increasing their chances of feeding again before the prey disperses.

Hunters' preferences and perceptions as hunting predictors in a semiarid ecosystem

Game meat is a resource widely exploited by rural populations in various parts of the world. In recent decades, the growth in the number of people living near conserved areas has increased the demand for game meat. In this work, based on the Social-ecological Theory of Maximization, we seek to verify the influence of cost-benefit ratio, availability, and subjective preferences (flavor) in the selection of hunted species. We interviewed game-eating people in seven communities in the Brazilian semiarid region, recording information on hunting strategies, flavor preferences, and relative abundance of game fauna. We found that people hunt for the most available species regardless of the cost-benefit of this choice. Also, flavor preference can increase the odds of a species being hunted almost 100%. Our data show that hunters may prefer species that require less capture effort, even though they have energy-efficient alternatives. We found that flavor preference is proportionally the variable with the most significant effect on the chances of a species being hunted, suggesting that traditional optimal foraging models are too simple to cover the complexity involved in the selection of game species.

Egg laying rather than host quality or host feeding experience drives habitat estimation in the parasitic wasp Nasonia vitripennis.

In variable environments, sampling information on habitat quality is essential for making adaptive foraging decisions. In insect parasitoids, females foraging for hosts have repeatedly been shown to employ behavioral strategies that are in line with predictions from optimal foraging models. Yet, which cues exactly are employed to sample information on habitat quality has rarely been investigated. Using the gregarious parasitoid Nasonia vitripennis (Walker; Hymenoptera: Pteromalidae), we provided females with different cues about hosts to elucidate, which of them would change a wasp's posterior behavior suggesting a change in information status. We employed posterior clutch size decisions on a host as proxy for a female's estimation of habitat quality. Taking into account changes in physiological state of the foraging parasitoid, we tested whether different host qualities encountered previously change the subsequent clutch size decision in females. Additionally, we investigated whether other kinds of positive experiences—such as ample time to investigate hosts, host feeding, or egg laying—would increase a wasp's estimated value of habitat quality. Contrary to our expectations, quality differences in previously encountered hosts did not affect clutch size decisions. However, we found that prior egg laying experience changes posterior egg allocation to a host, indicating a change in female information status. Host feeding and the time available for host inspection, though correlated with egg laying experience, did not seem to contribute to this change in information status.

Compensatory Foraging in Stoichiometric Producer-Grazer Models.

Nutritional constraints are common as food resources are rarely optimally suited for grazing species. Elemental mismatches between trophic levels can influence population growth and foraging behaviors. Grazing species, such as Daphnia, utilize optimal foraging techniques, such as compensatory feeding. Here, we develop two stoichiometric producer-grazer models, a base model that incorporates a fixed energetic foraging cost and an optimal foraging model where energetic foraging costs depend on food nutritional content. A variable energetic foraging cost results in cell quota-dependent predation behaviors. Analyzing and comparing these two models allows us to investigate the potential benefits of stoichiometric compensatory foraging behaviors on grazer populations. Optimal foraging strategies depend on environmental conditions, such as light and nutrient availability. In low-light conditions, fixed energetic foraging appears optimal regardless of the nutrient loads. However, in higher light conditions and intermediate nutrient loads, grazers utilizing compensatory foraging strategies gain an advantage. Overall, grazers can benefit from compensatory feeding behaviors when the food nutrient content of their prey becomes low or high.

Resource intensification and zooarchaeological record in the southern margins of pre-Hispanic Andean agriculture

Central Western Argentina is an area archeologically defined as one of the southernmost limits of farming in the pre-Hispanic Americas. Optimal foraging models help to predict important changes in faunal use prior to the adoption of agriculture related to resource depression and environmental changes. This paper evaluates changes in pre-Hispanic human subsistence from a zooarchaeological perspective and explores archeological record trends from the northern area of Central Western Argentina. Using zooarchaeological information in Northern Mendoza (Central Western Argentina), trends related to the human exploitation of faunal resources were explored in connection with time and space. Two high-ranked resource indexes were calculated: the artiodactyl index and a high-ranked resources index. The results show a decrease in the dependence on high-ranked resources by human groups after ca. 2000 bp and an increase in richness, indicating a diet breath expansion. Long-term trends in the density of radiocarbon dates suggest a considerable population growth after ca. 2000 bp. These trends are consistent with the contemporary changes reported by other archaeological proxies in Northern Mendoza. The results of the zooarchaeological analysis confirm changes in human organization around 2000 years bp that reflect an intensification process.

Risk sensitivity of a forager with limited energy reserves in stochastic environments

AbstractLong‐term environmental stochasticity is known to affect the adaptive evolution of life history traits. In stochastic environments, there are two different levels of behavioral optimization, as follows: Level 1, the optimal strategy under an intrageneration stochastic environment and Level 2, the optimal strategy under an intergeneration stochastic environment. This article presents a simple optimal foraging model under predation risks and verified the effect of behavioral optimization on the foraging time ratio. In this model, foragers are exposed to predation risks during foraging but are safe if they stay in their nests without any food. The foraging time allocation strategies that optimize the geometric mean fitness (Level 2) were compared with the arithmetic mean fitness (Level 1) to verify the effects of intergenerational stochasticity, whereby there is an alternation in good/bad environments across generations. As in previous studies, risk‐averse strategies (a shorter foraging time is adopted for Level 2 than for Level 1) were commonly observed using this model. Unexpectedly, the model showed a tendency toward a preference for risk‐prone strategies. This qualitative difference became prominent when food was abundant and the maximum energy reserves were small. Theoretical studies have shown that risk‐averse strategies are commonly adopted during food shortages and result in starvation. However, the current results indicate that risk‐prone strategies may become optimal under a limited reserve capacity. Thus, the optimal strategy depends not only on the individual status and environmental conditions, but also on the detailed selection regimes.