Consequences of climate and body size on the foraging performance of seed‐eating ants

1. Changes in climatic factors could have major effects on the foraging performance of animals. To date, however, no study has attempted to examine the concurrent effect of different climatic factors on foraging performance of individual organisms. 2. In the present study, this issue was addressed by studying changes in foraging performance of seed‐eating ant colonies of the genus Messor in response to variation in precipitation and ambient temperature along a macroecological gradient. In addition, we examined the way three colony‐level attributes, foraging distance, forager number, and variance in worker‐size, could affect foraging performance in those ants. Foraging performance was measured as size matching, i.e. the correlation between forager size and load size. The study was carried out for 2 years in six sites along a south‐north productivity gradient in a semi‐arid region of the Eastern‐Mediterranean. 3. Size matching increased with increased precipitation as well as with an increase in worker‐size variability, but slightly decreased with increasing temperatures, as predicted by foraging‐decision models. In contrast, foraging distance had no effect on size matching. Interestingly, size matching showed a unimodal relationship with forager number. 4. These results indicate that interplay between climate and body size affects foraging performance either directly via physiological constraints, or indirectly through their effect on food availability. Moreover, this is one of the first evidences to support the assumption that ant colonies can differ in their ability to optimally allocate their workforce in natural environments. This emphasises the importance of studying the way foraging strategies vary across environmental gradients at macroecological scales.     

Natural variation in learning rate and memory dynamics in parasitoid wasps: opportunities for converging ecology and neuroscience

Although the neural and genetic pathways underlying learning and memory formation seem strikingly similar among species of distant animal phyla, several more subtle inter- and intraspecific differences become evident from studies on model organisms. The true significance of such variation can only be understood when integrating this with information on the ecological relevance. Here, we argue that parasitoid wasps provide an excellent opportunity for multi-disciplinary studies that integrate ultimate and proximate approaches. These insects display interspecific variation in learning rate and memory dynamics that reflects natural variation in a daunting foraging task that largely determines their fitness: finding the inconspicuous hosts to which they will assign their offspring to develop. We review bioassays used for oviposition learning, the ecological factors that are considered to underlie the observed differences in learning rate and memory dynamics, and the opportunities for convergence of ecology and neuroscience that are offered by using parasitoid wasps as model species. We advocate that variation in learning and memory traits has evolved to suit an insect's lifestyle within its ecological niche.     

Context-dependent specialisation drives temporal dynamics in intra- and inter-individual variation in foraging behaviour within a generalist bird population

Individual niche variation is common within animal populations, and has significant implications for a wide range of ecological and evolutionary processes. However, individual niche differences may also temporally vary as a result of behavioural plasticity. While it is well understood how niche variation is affected by changes in resource availability, comparatively little is known about the extent to which individual niche differences may vary within the annual cycle due to internal drivers. Here, we assess how time- and energy-constraints imposed by incubating and brood rearing affect inter- and intra-individual variation in the foraging behaviour of lesser black-backed gulls, a generalist seabird with strong individual niche variation. To this end, we compared daily foraging trips of 22 breeding and 23 non-breeding GPS-tracked adult gulls from two colonies in the Southern Bight of the North Sea over the course of the breeding season. We find that breeding birds, unlike non-breeding ones, did indeed alter their foraging behaviour during the breeding season. Both sexes reduced their searching effort by increasingly revisiting earlier foraging locations, allowing for shorter and more frequent foraging trips. Breeding females also showed pronounced shifts in their habitat use and strongly specialised on urbanised foraging habitats throughout the breeding season. Hence, while individual variation in habitat use remained largely consistent within non-breeders and in breeding males, individual variation among breeding females almost completely disappeared. Female lesser black-backed gulls are on average smaller, and therefore often outcompeted by males for the most profitable food sources. The temporal specialisation on spatially reliable anthropogenic food sources during breeding hence suggests a complex interplay between intrinsic competitive constraints, resource reliability and shifting time- and energy budges in shaping temporal dynamics in individual niche variation within our study population.     

Does foraging efficiency vary with colony size in the fairy martin Petrochelidon ariel?

Colonial breeding occurs in a wide range of taxa, however the advantages promoting its evolution and maintenance remain poorly understood. In many avian species, breeding colonies vary by several orders of magnitude and one approach to investigating the evolution of coloniality has been to examine how potential costs and benefits vary with colony size. Several hypotheses predict that foraging efficiency may improve with colony size, through benefits associated with social foraging and information exchange. However, it is argued that competition for limited food resources will also increase with colony size, potentially reducing foraging success. Here we use a number of measures (brood feeding rates, chick condition and survival, and adult condition) to estimate foraging efficiency in the fairy martin Petrochelidon ariel, across a range of colony sizes in a single season (17 colonies, size range 28–139 pairs). Brood provisioning rates were collected from multiple colonies simultaneously using an electronic monitoring system, controlling for temporal variation in environmental conditions. Provisioning rate was correlated with nestling condition, though we found no clear relationship between provisioning rate and colony size for either male or female parents. However, chicks were generally in worse condition and broods more likely to fail or experience partial loss in larger colonies. Moreover, the average condition of adults declined with colony size. Overall, these findings suggest that foraging efficiency declines with colony size in fairy martins, supporting the increased competition hypothesis. However, other factors, such as an increased ectoparasitise load in large colonies or change in the composition of phenotypes with colony size may have also contributed to these patterns.     

The evolution of social learning rules: Payoff-biased and frequency-dependent biased transmission

Humans and other animals do not use social learning indiscriminately, rather, natural selection has favoured the evolution of social learning rules that make selective use of social learning to acquire relevant information in a changing environment. We present a gene-culture coevolutionary analysis of a small selection of such rules (unbiased social learning, payoff-biased social learning and frequency-dependent biased social learning, including conformism and anti-conformism) in a population of asocial learners where the environment is subject to a constant probability of change to a novel state. We define conditions under which each rule evolves to a genetically polymorphic equilibrium. We find that payoff-biased social learning may evolve under high levels of environmental variation if the fitness benefit associated with the acquired behaviour is either high or low but not of intermediate value. In contrast, both conformist and anti-conformist biases can become fixed when environment variation is low, whereupon the mean fitness in the population is higher than for a population of asocial learners. Our examination of the population dynamics reveals stable limit cycles under conformist and anti-conformist biases and some highly complex dynamics including chaos. Anti-conformists can out-compete conformists when conditions favour a low equilibrium frequency of the learned behaviour. We conclude that evolution, punctuated by the repeated successful invasion of different social learning rules, should continuously favour a reduction in the equilibrium frequency of asocial learning, and propose that, among competing social learning rules, the dominant rule will be the one that can persist with the lowest frequency of asocial learning.     

Fitness costs associated with low genetic variation are reduced in a harsher environment in amphibian island populations

A basic premise of conservation geneticists is that low levels of genetic variation are associated with fitness costs in terms of reduced survival and fecundity. These fitness costs may frequently vary with environmental factors and should increase under more stressful conditions. However, there is no consensus on how fitness costs associated with low genetic variation change under natural conditions in relation to the stressfulness of the environment. On the Swedish west coast, natterjack toad Bufo calamita populations show a strong population genetic structure and large variation in the amount of within-population genetic variation. We experimentally examined the survival of natterjack larvae from six populations with different genetic variation in three thermal environments corresponding to (a) the mean temperature of natural ponds (stable, laboratory), (b) a high temperature environment occurring in desiccating ponds (stable, laboratory) and (c) an outdoor treatment mimicking the natural, variable thermal conditions (fluctuating, semi-natural). We found that larvae in the outdoor treatment had poorer survival than larvae in the stable environments suggesting that the outdoor treatment was more stressful. Overall, populations with higher genetic variation had higher larval survival. However, a significant interaction between treatments and genetic variation indicated that fitness costs associated with low genetic variation were less severe in the outdoor treatment. Thus, we found no support for the hypothesis that fitness costs associated with low genetic variation increase under more stressful conditions. Our results suggest that natural thermal stress may mask fitness losses associated with low genetic variation in these populations.     

Fine‐scale foraging habitat selection by two diving central place foragers in the Northeast Atlantic

Habitat selection and spatial usage are important components of animal behavior influencing fitness and population dynamic. Understanding the animal–habitat relationship is crucial in ecology, particularly in developing strategies for wildlife management and conservation. As this relationship is governed by environmental features and intra‐ and interspecific interactions, habitat selection of a population may vary locally between its core and edges. This is particularly true for central place foragers such as gray and harbor seals, where, in the Northeast Atlantic, the availability of habitat and prey around colonies vary at local scale. Here, we study how foraging habitat selection may vary locally under the influence of physical habitat features. Using GPS/GSM tags deployed at different gray and harbor seals’ colonies, we investigated spatial patterns and foraging habitat selection by comparing trip characteristics and home‐range similarities and fitting GAMMs to seal foraging locations and environmental data. To highlight the importance of modeling habitat selection at local scale, we fitted individual models to colonies as well as a global model. The global model suffered from issues of homogenization, while colony models showed that foraging habitat selection differed markedly between regions for both species. Despite being capable of undertaking far‐ranging trips, both gray and harbor seals selected their foraging habitat depending on local availability, mainly based on distance from the last haul‐out and bathymetry. Distance from shore and tidal current also influenced habitat preferences. Results suggest that local conditions have a strong influence on population spatial ecology, highlighting the relevance of processes occurring at fine geographical scale consistent with management within regional units.     

Water temperature, light intensity and zooplankton density and the feeding activity of juvenile brook charr (Salvelinus fontinalis)

SUMMARY 1. The objective of this study was to evaluate the effects of zooplankton biomass (as a measure of density), fish biomass, light intensity and water temperature on the attack rate and swimming characteristics (i.e. swimming speed and angle of turn) of juvenile (1+) brook charr (Salvelinus fontinalis) in field enclosures. We used a portable underwater camera system in a series of pelagic enclosures to quantify the feeding behaviour of brook charr over a gradient of natural conditions. 2. In simple linear or non‐linear regression models we found (i) that attack rate and angle of turn were positively related to water temperature, (ii) that attack rate and swimming speed were positively related to zooplankton biomass and light intensity and (iii) that attack rate was positively related to swimming speed. In multiple regression models, fish biomass, light intensity and variance of the angle of turn accounted for 87% of the variation in attack rate. Light intensity and water temperature accounted for 86% of the variation in swimming speed. Fish gut fullness and attack rate accounted for 83% of the variation in the variance of the angle of turn executed by fish. 3. The increase in the number of attacks as zooplankton biomass increases conforms to the general positive functional response observed in other fish species. Our results also support the hypothesis that swimming speed increases with prey biomass. We did not observe a plateau in attack rate as zooplankton biomass increased. As our experiments were performed under natural biotic and abiotic conditions, factors other than zooplankton biomass might affect or limit this response, such as water temperature and light intensity. 4. Because zooplankton biomass was correlated with water temperature and light intensity, it was not possible to evaluate the independent contribution of these factors on the attack rate and swimming characteristics (swimming speeds and angle of turn) of brook charr. However, this study highlighted the impact of these factors on the feeding behaviour of juvenile brook charr when feeding in the pelagic habitat under natural conditions, and their importance in future models of optimal foraging and fish habitat quality.     

Colony-specific architecture of shelter tubes by termites

Social insects build sophisticated and complex architectures such as huge nests and underground galleries based on self-organizing rules. The structures of these architectures vary widely in size and shape within a species. Some studies have revealed that the current environmental and/or social factors can cause differences in the architectures that emerge from collective building. However, little is known about the effect of colony-level variations on the architecture. Here, we demonstrate that termite colonies build colony-specific architecture using shelter-tube construction as a model system. When we divided a colony into multiple groups of individuals, groups drawn from the same colony performed similar patterns of construction, whereas groups from different colonies exhibited different patterns. The colony variations in shelter-tube construction are generally thought to reflect differences in foraging strategy, and this difference can have important fitness consequences depending on the distribution of wood resources in the environment. This is the first demonstration of colony variation in the architecture that emerges from collective behavior. Colony-specific architectural variations provide new insights into our understanding of the self-organization systems, which were previously assumed to provide each species with a species-specific construction mechanism.     

Casual movement speed but not maximal locomotor capacity predicts mate searching success

Maximal locomotor performance is often used as a proxy for fitness. Maximal speed may be important under high‐threat conditions, such as during predator escape. However, animals do not always move at a speed that reflects their maximal physiological capacities when undisturbed. The physiological factors that determine the movement speed chosen by animals, such as minimization of energy use, may be independent from maximal performance. As a result, the casual speed at which individuals move when undisturbed in a given context may better represent an individual's motivation to move. The casual speed may therefore be a better predictor of fitness in natural contexts than maximal performance capacity. We tested the hypothesis that casual movement speed rather than maximal speed predicts fitness in the golden orb‐web spider, Nephila plumipes. We measured fitness in two separate contexts, mate‐searching success and the positional rank near a female. We show that casual but not maximal locomotor speed predicted both aspects of fitness. Casual speed was linearly related to maximal speed, indicating that casual speed is determined by physiological optimization. Size and metabolic scope were not related to either maximal or chosen speeds, indicating that the supply of ATP does not limit locomotor performance in this species. Overall, our results demonstrate that locomotor performance is related to fitness, but suggest that different types of performance and not necessarily maximal physiological capacities are most relevant for particular ecologically relevant tasks.     

Colony variation in the collective regulation of foraging by harvester ants

This study investigates variation in collective behavior in a natural population of colonies of the harvester ant, Pogonomyrmex barbatus. Harvester ant colonies regulate foraging activity to adjust to current food availability; the rate at which inactive foragers leave the nest on the next trip depends on the rate at which successful foragers return with food. This study investigates differences among colonies in foraging activity and how these differences are associated with variation among colonies in the regulation of foraging. Colonies differ in the baseline rate at which patrollers leave the nest, without stimulation from returning ants. This baseline rate predicts a colony's foraging activity, suggesting there is a colony-specific activity level that influences how quickly any ant leaves the nest. When a colony's foraging activity is high, the colony is more likely to regulate foraging. Moreover, colonies differ in the propensity to adjust the rate of outgoing foragers to the rate of forager return. Naturally occurring variation in the regulation of foraging may lead to variation in colony survival and reproductive success.     

The influence of prior experience on preference and performance of a cryptoparasitoid Scleroderma guani (Hymenoptera: Bethylidae) on beetle hosts

1. Numerous studies have reported the effects of learning or experience on parasitoid host preference and location. However, the integration of pre‐imaginal and adult experiences on the subsequent host preference and adult/offspring performance has been rarely tested in host–parasite interactions. 2. We present direct evidence that theses two kinds of experiences affect host preference and related fitness in the polyphagous parasitoid, Scleroderma guani. Two colonies of parasitoids were reared on Monochamus alternatus and Saperda populnea (Cerambycidae: Lamiinae). Individuals from the two colonies were given host‐switching experience for one generation (pre‐imaginal experience) while other individuals were given prior ovipositing experience on the two species, respectively (adult experience). 3. Scleroderma guani females demonstrated that their experiences determined adult behavioural responses and their subsequent performance to hosts. Females maximised both adult fitness (fecundity and longevity) and offspring fitness (survival and sex ratio) when they encountered hosts similar to their maternal hosts. Behavioural plasticity in host choice was affected by adult experience, resulting in improved adult feeding and ovipositing behaviour and further modifying adult fecundity and the offspring sex ratio. There was a positive correlation between oviposition preference and adult fecundity. 4. The results indicated that S. guani exhibited positive preference–performance correlations. This is most likely due to an adaptation to maternal hosts over multiple generations. However, foraging potential of adults to available cues from hosts may be driven quickly by an experience‐induced learning process rather than by natural selection processes shaped over many generations.     

Recruitment to forage of bumblebees in artificial low light is less impaired in light sensitive colonies,and not only determined by external morphological parameters

Bumblebees of Bombus terrestris are essential pollinators in natural and managed ecosystems. Their foraging ability relies on the individual morphology, task allocation within the colony, and external factors, such as light intensity. The foraging activities of commercial bumblebees can sometimes be impaired, especially in the artificial and weak light intensities of greenhouses at high altitudes. Here we investigated whether the eagerness (or willingness) to forage of bumblebee colonies in different light conditions is correlated with the light sensitivity of bumblebees colonies and/or different external morphological parameters. The initial foraging capacity of bumblebee colonies correlated with their light sensitivity. However, light sensitive bumblebee colonies did not necessarily had a higher foraging activity at lower light intensities. Differences in initial foraging capacity and light sensitivity among colonies could not be explained by the external morphological parameters. In conclusion, our data illustrated that the recruitment to forage in artificial low light is less impaired in light sensitive colonies, and that not only the external morphology parameters determine the light sensitivity of bumblebees and their eagerness to forage in weak light conditions. The data obtained here create a better understanding of which criteria are able to select towards light sensitive bumblebees and their link with the foraging capacity of these bumblebees.     

Genetic variation and task specialization in the desert leaf-cutter ant, Acromyrmex versicolor

The genetic diversity in social insect colonies that is generated by multiple mating or multiple queens has been hypothesized to promote worker task specialization and therefore facilitate division of labour. However, few studies have actually examined the mechanisms by which genotype may influence individual worker behaviour. In this study, we dissect possible genetic effects on worker task performance in the desert leaf-cutter ant. We hypothesize that genotype could affect worker behaviour via (1) the rate of age-related task switching (age polyethism schedule), (2) individual task preference, and/or (3) task performance rates. To discriminate among these possible mechanisms, we generated composite colonies of workers from different genetic sources and followed the behaviour of individually marked workers over their lifetimes. We found significant differences among matrilines (offspring of different queens) in overall task performance. In particular, we found a negative covariance in likelihood of foraging versus tending fungus inside the nest. Workers of different matrilines also varied in the age of transition from inside the nest to foraging, but did not vary in task performance rates. Our results suggest that division of labour in this system is affected by genetic influences on individual task preference and age-related task choice, but not on variation in activity level.     

Introduced Drosophila subobscura populations perform better than native populations during an oviposition choice task due to increased fecundity but similar learning ability

The success of invasive species is tightly linked to their fitness in a putatively novel environment. While quantitative components of fitness have been studied extensively in the context of invasive species, fewer studies have looked at qualitative components of fitness, such as behavioral plasticity, and their interaction with quantitative components, despite intuitive benefits over the course of an invasion. In particular, learning is a form of behavioral plasticity that makes it possible to finely tune behavior according to environmental conditions. Learning can be crucial for survival and reproduction of introduced organisms in novel areas, for example, for detecting new predators, or finding mates or oviposition sites. Here we explored how oviposition performance evolved in relation to both fecundity and learning during an invasion, using native and introduced Drosophila subobscura populations performing an ecologically relevant task. Our results indicated that, under comparable conditions, invasive populations performed better during our oviposition task than did native populations. This was because invasive populations had higher fecundity, together with similar cognitive performance when compared to native populations, and that there was no interaction between learning and fecundity. Unexpectedly, our study did not reveal an allocation trade‐off (i.e., a negative relationship) between learning and fecundity. On the contrary, the pattern we observed was more consistent with an acquisition trade‐off, meaning that fecundity could be limited by availability of resources, unlike cognitive ability. This pattern might be the consequence of escaping natural enemies and/or competitors during the introduction. The apparent lack of evolution of learning may indicate that the introduced population did not face novel cognitive challenges in the new environment (i.e., cognitive “pre‐adaptation”). Alternatively, the evolution of learning may have been transient and therefore not detected.     

Nest site and weather affect the personality of harvester ant colonies

Environmental conditions and physical constraints both influence an animal's behavior. We investigate whether behavioral variation among colonies of the black harvester ant, Messor andrei, remains consistent across foraging and disturbance situations and ask whether consistent colony behavior is affected by nest site and weather. We examined variation among colonies in responsiveness to food baits and to disturbance, measured as a change in numbers of active ants, and in the speed with which colonies retrieved food and removed debris. Colonies differed consistently, across foraging and disturbance situations, in both responsiveness and speed. Increased activity in response to food was associated with a smaller decrease in response to alarm. Speed of retrieving food was correlated with speed of removing debris. In all colonies, speed was greater in dry conditions, reducing the amount of time ants spent outside the nest. While a colony occupied a certain nest site, its responsiveness was consistent in both foraging and disturbance situations, suggesting that nest structure influences colony personality.     

Learning Differences between Feral Pigeons and Zenaida Doves: The Role of Neophobia and Human Proximity

Learning differences predicted from ecological variables can be confounded with differences in wariness of novel stimuli (neophobia). Previous work on feral pigeons ( Columba livia ), as well as on group-feeding and territorial zenaida doves ( Zenaida aurita ), reported individual and social learning differences predicted from social foraging mode. In the present study, we show that speed of learning a foraging task covaries with neophobia and latency to feed from a familiar dish in the three types of columbids. Pigeons were much faster than either territorial or group-feeding zenaida doves on all tests conducted in captivity, but showed unexpectedly strong neophobia in some urban flocks during field tests. Human proximity strongly affected performance in group-feeding doves both in the field and in captivity. They were slightly faster at learning than their territorial conspecifics in cage tests. In multiple regressions, species identity, but not social foraging mode, significantly predicted individual variation in learning, as did individual variation in neophobia. Wariness of novel stimuli and species differences associated with artificial selection appear to be more important than foraging mode and wariness of humans in accounting for learning differences between these columbids.     

The relationship between the distribution of worker sizes and new worker production in the ant Formica neorufibarbis

While it is commonly assumed that variation in worker sizes within a single ant colony increases colony efficiency, there is little causal evidence of a link between worker size variation and colony performance. I tested whether the range of worker sizes within colonies of the ant species Formica neorufibarbis affected new worker production. Removing large workers from colonies lowered the rate of new worker production. A study of unmanipulated colonies indicated that colonies did not maintain a full range of worker sizes; mean worker head widths varied from 0.89-1.24 mm. Colonies naturally missing large workers did not have lower rates of worker production, suggesting that the relative size, not the absolute size, of workers within colonies was important. These are the first results to directly link the range of worker sizes to a component of colony fitness in a natural setting.