Age-related shifts in the diet composition of southern elephant seals expand overall foraging niche

Southern elephant seals are important apex predators in a highly variable and unpredictable marine environment. In the presence of resource limitation, foraging behaviours evolve to reduce intra-specific competition increasing a species’ overall probability of successful foraging. We examined the diet of 141 (aged 1–3 years) juvenile southern elephant seals to test the hypotheses that differences between ages, sexes and seasons in diet structure occur. We described prey species composition for common squid and fish species and the mean size of cephalopod prey items for these age groups. Three cephalopod species dominated the stomach samples, Alluroteuthis antarcticus, Histioteuthis eltaninae and Slosarczykovia circumantarcticus. We found age-related differences in both species composition and size of larger prey species that probably relate to ontogenetic changes in diving ability and haul-out behaviour and prey availability. These changes in foraging behaviour and diet are hypothesised to reduce intra-specific food competition concomitant with the increase in foraging niche of growing juveniles.     

Experimental examination of behavioural interactions between free-ranging wild and domestic canids

The structure of mammalian carnivore communities is strongly influenced by both intraguild competition and predation. However, intraguild interactions involving the world’s most common carnivore, the domestic dog (Canis familiaris), have rarely been investigated. We experimentally examined the behavioural responses of a small canid, the Indian fox (Vulpes bengalensis), to the presence of dogs and dog odours. Resource competition between dogs and Indian foxes is low, so it is unclear whether foxes perceive dogs as interference competitors. To test this, we exposed foxes to neutral, live dog, and animal odour stimuli at food trays, and recorded the time spent at food trays, the amount of food eaten, and vigilance and non-vigilance behaviours. When dogs were visible, foxes continued to visit the food trays, but reduced the amount of time spent and food eaten at those trays. Foxes were more vigilant during dog trials than during neutral and odour trials and also exhibited lower levels of non-vigilance behaviour (resting and playing). In contrast, dog odours did not affect fox foraging and activity. These results show that vigilance/foraging trade-offs due to interference competition can occur between native and domestic carnivores despite low dietary overlap. These negative effects of dogs on a smaller member of the carnivore guild raise conservation concerns, especially for endangered carnivores. In many parts of the world, free-ranging dog densities are high due to human subsidies, and these subsidized predators have the potential to exacerbate the indirect effects of human presence.     

A geometrical model for the effect of interference on food intake

Interference competition is often due to kleptoparasitism (food stealing). In which case, the attack distance, the distance over which one animal attacks another in an attempt to steal food, determines to a large extent the competitor density range over which interference significantly affects the intake rate of foraging animals.We develop a simple model of kleptoparasitism containing three parameters: attack distance, the density of foraging animals and a single dimensionless parameter α which summarizes the non-geometrical aspects of the interference process. Dominant and subdominant animals are not considered separately. The model predicts that the average intake rate will decrease exponentially with animal density and that a measure of the strength of interference depends on attack distance squared.The simple model is compared with a much more detailed individual-based foraging model from the literature. Simulated average intake rates are indeed well approximated by an exponential decrease with competitor density. Also the measure of interference behaves in the way expected from the simple model. By explaining the shape of the relationship between intake rate and animal density, the simple model provides insight into the behaviour of the detailed behavioural model.Insight into the role of geometry is important in the interpretation of field results and in the further development of detailed foraging models.     

Geographical variation in the behaviour of a central place forager: Antarctic fur seals foraging in contrasting environments

Foragers show adaptive responses to changes within their environment, and such behavioural plasticity can be a significant driving force in speciation. We investigated how lactating Antarctic fur seals, Arctocephalus gazella, adapt their foraging within two contrasting ecosystems. Location and diving data were collected concurrently, between December 2003 and February 2004, from 43 seals at Bird Island, where krill, Euphausia superba, are the main prey, and 39 at Heard Island, where mostly fish are consumed. Seals at Heard Island were shorter and lighter than those at Bird Island and they spent longer at sea, dived more frequently and spent more time in the bottom phase of dives. Generalized additive mixed effects models showed that diving behaviours differed between the islands. Both populations exploited diel vertically migrating prey species but, on average, Heard Island seals dived deeper and exceeded their estimated aerobic dive limits. We propose that the recovery of the Heard Island population may be limited by the relative inaccessibility and scarcity of food, whereas at Bird Island, the presence of abundant krill resources helps sustain extremely high numbers of seals, even with increased intra- and inter-specific competition. Both populations of fur seals appear to be constrained by their physiological limits, in terms of their optimal diving behaviour. However, there does appear to be some flexibility in strategy at the level of trip with animals adjusting their time at sea and foraging effort, in order to maximize the rate of delivery of energy to their pups.     

Intra-sexual variability in feeding behaviour of a mountain ungulate: size matters

Many studies comparing the behaviour of individuals of different genders or species showed that animal body mass and forage quality/quantity are key elements of the foraging ecology of herbivores. Since body mass could also influence the animal’s sensitivity to predation risk, its vigilance behaviour should consequently be affected. Alpine ibex (Capra ibex) is characterised by a strong dimorphism among males of different ages, thus representing an ideal case study for testing the Jarman-Bell principle, avoiding possibly misleading effects resulting from the comparison between different species or genders. We analysed the fine-scale foraging behaviour of male ibex in order to assess the effect of body mass and the effects of vegetation quality/quantity on both foraging and vigilance behaviour. Our results showed that smaller males were more selective than larger ones, on account of their lower capability of digesting plant. Smaller males scanned the environment more frequently than larger ones. Male ibex grazed more selectively in sites with high quality forage and their bite rate increased as forage biomass decreased. Vigilance frequency increased with increasing forage biomass as, under these circumstances, ibex are able to prolong anti-predator vigilance while chewing bites that have already been cropped. Our findings highlight the effects of body mass per se on both foraging and anti-predator behaviours in herbivores, thus supporting the Jarman-Bell principle. Foraging can arguably be considered a very flexible behaviour with high evolutionary relevance as it enables herbivores to optimally adjust their total energy intake under varying conditions of food resources.     

Social context influences cue-mediated recruitment in an invasive social wasp

Social insects often serve as model systems for communication and recruitment studies, and yet, it remains controversial whether social vespid wasps can reliably communicate resource information to nestmates. In this study, I present empirical evidence that foraging strategies depend on the initial assessment of resource size and potential competition by foraging yellowjackets. The context dependent foraging behavior of Vespula pensylvanica provides a potential explanation for the inconsistent reports of the existence of recruitment communication in vespid wasps. Furthermore, life history traits may influence yellowjacket foraging behavior; annual V. pensylvanica colonies, whose foragers routinely patrol near the nest, exhibited increased bait visitation in response to the return of successful foragers, whereas perennial colonies did not. These behavioral disparities provide insight into how foraging strategies and search patterns may shift with colony size and longevity. In experiments that investigate the effects of visual cues of conspecifics and bait dispersion, foraging decisions corresponded with expectations of yellowjackets integrating resource quantity and access into a perception of demand. When resource competition could be assessed as high, V. pensylvanica foragers quickly exploited the bait closest to their colony regardless of occupation by other wasps; however, foragers preferred visiting unoccupied baits in situations where competition could be perceived as low. Moreover, a meta-analysis revealed that context-dependent, cue-mediated recruitment was widespread in Vespidae, where such foraging behaviors changed with habitat and the potential for resource competition. Such plastic foraging strategies may contribute to the invasion success of some vespid wasps.     

Spatio-temporal ranging behaviour and its relevance to foraging strategies in wide-ranging wolverines

Conservation of carnivores in an increasingly changing environment is greatly helped by understanding the decision-making processes underlying habitat patch choice. Foraging theory may give us insight into spatio-temporal search patterns and consequent foraging decisions that carnivores make in heterogeneous and fluctuating environments. Constraints placed on central-place foragers in particular are likely to influence both foraging decisions and related spatio-temporal movement patterns. We used discrete choice models to investigate the spatio-temporal ranging behaviour of GPS collared female wolverines (Gulo gulo) with dependent cubs in south-central Norway. Activity patterns, home range use and selection for elevation were analyzed in relation to spatial and temporal covariates (daily and seasonal) and related to different foraging behaviours. In spring, wolverines showed restricted movement patterns around rendezvous sites at high elevations by day, whereas during the night animals were active at lower elevations. Over the summer, this daily pattern in intensity of use diminished and their overall selectiveness for elevation decreased as cubs grow more mobile and independent. At the onset of autumn, wolverines showed intensive use of the profitable forest-alpine tundra ecotone. We argue that reproducing wolverines deployed a foraging strategy attuned to altering their movement patterns throughout the summer to address a continuous, but diminishing, trade-off between providing both food and shelter for their offspring. Incorporating spatially and temporally explicit activity patterns and home range use in discrete choice resource selection models thus enhances the understanding of the motives behind wolverine resource utilization in space and time. Such knowledge may provide guidance to managers designing regional-scale zoning, in order to facilitate carnivore recovery and to minimize conflicts with human activities.     

δ15N and δ13C values in dental collagen as a proxy for age- and sex-related variation in foraging strategies of California sea lions

We assessed the foraging habits of California sea lions, Zalophus californianus, from Isla Santa Margarita, BCS, Mexico, by analyzing δ¹³C and δ¹⁵N values of dentin collagen. Since dentin is deposited annually in growth layer groups (GLGs), it can be subsampled to construct ontogenetic isotopic profiles at the individual level. We drilled 20 canine teeth and obtained 141 samples for isotopic analysis that were assigned to age-specific categories from GLG-based estimated ages. Pups’ GLGs had the highest mean δ¹⁵N values and the lowest mean δ¹³C values, a pattern likely driven by the consumption of milk. Juveniles had δ¹⁵N values between those of pups and adult females, which may reflect continued nursing into the second year or preferential consumption of coastal benthic versus pelagic prey. Significant differences were observed between the sexes of adults; adult females had lower mean δ¹³C and δ¹⁵N values than adult males. Higher isotope values in adult males relative to females may reflect a higher trophic position, but differences in foraging grounds cannot be excluded as a potential explanation because tracking data are not available at this time. Evidence of intra-specific foraging diversification may be related to a strategy to reduce competition within and among age and sex categories.     

Marine aquaculture and bottlenose dolphins’ (<Emphasis Type="Italic">Tursiops truncatus</Emphasis>) social structure

In this study, we investigate association patterns of 249 bottlenose dolphin feeding groups off Sardinia Island (Italy) from January 2000–May 2007 and describe how their association behaviour is related to their response to food patches created by a marine fin fish farm. We also tested the hypothesis that dolphins have different social structures with different feeding activities: Associations should decrease during opportunistic feeding behaviours as it is easier to capture prey, and cooperation is not as necessary. Sixteen individually identified bottlenose dolphins were observed participating in both opportunistic and not opportunistic feeding activities, with a mean of 30 ± 8 times and 9.6 ± 1 times, respectively. Bottlenose dolphins show non-random social behaviour during feeding and this behaviour differs depending on their specific foraging activity. Dolphin associations during feeding can be divided into three categories: acquaintances, affiliates, and feeding associates. Association behaviour during fish farm feeding is consistent with our hypothesis that during opportunistic behaviours, benefits from cooperation decrease, as it is easier to capture prey. Group size homogeneity in both feeding activities demonstrates that the number of dolphins engaging in foraging is not necessarily related with cooperation levels. Moreover, an adult dolphin may prefer to associate with a specific individual, independent of the sex, who shares the same foraging priorities. This study is the first to show how aquaculture is not only directly affecting marine predators but could also indirectly affect their social structure and behaviour.     

Exogenous testosterone increases female aggression in the European starling (<Emphasis Type="Italic">Sturnus vulgaris</Emphasis>)

In the European starling, Sturnus vulgaris, optimal mating systems differ between males and females. Males gain from polygyny, whereas monogamy increases female fitness. The cost of polygyny to females lead to intense female–female competition, and it has previously been shown that the intensity of female aggression during the pre-breeding period can predict the realised mating system. The physiological regulation of such female aggression in starlings is not yet known. This study examines the role of testosterone in mediating aggressive behaviours involved in intra-specific reproductive competition in female starlings. Testosterone levels were experimentally elevated with testosterone implants in females during the pre-laying period. To simulate a situation in which an additional female tried to mate with the focal female’s mate, a caged female was presented close to a nest-site to which the male could attract a secondary female. Testosterone was significantly related to several behaviours involved in female–female interactions. Females with testosterone implants spent significantly more time close to the caged female and produced more song bouts than control females. In contrast, male behaviour was unrelated to the experimental status of the mate. Females mated to males that attracted a secondary female were less aggressive towards the caged female than those that remained monogamously mated. The effect of exogenous testosterone in this study indicates that androgens may mediate social behaviours in female starlings during the breeding season.     

Testing a stochastic version of the Beddington–DeAngelis functional response in foraging shore crabs

Current behaviour-based interference models assume that the predator population is infinitely large and that interference is weak. While the realism of the first assumption is questionable, the second assumption conflicts with the purpose of interference models. Here, we tested a recently developed stochastic version of the Beddington–DeAngelis functional response—which applies to a finite predator population without assuming weak interference—against experimental data of shore crabs (Carcinus maenas) foraging on mussels (Mytilus edulis). We present an approximate maximum likelihood procedure for parameter estimation when only one focal individual is observed, and introduce ‘correction factors’ that capture the average behaviour of the competing but unobserved individuals. We used the method to estimate shore crab handling time, interaction time, and searching rates for prey and competitor. Especially the searching rates were sensitive to variation in prey and competitor density. Incorporating constant parameter values in the model and comparing observed and predicted feeding rates revealed that the predictive power of the model is high. Our stochastic version of the Beddington–DeAngelis model better reflects reality than current interference models and is also amenable for modelling effects of interference on predator distributions.     

Analyses of limpet defense and predator offense in the field

Between November 1982 and August 1984 diving observations of two intertidal limpets' defensive behaviours in response to two predatory species were made near the Seto Marine Biological Laboratory of Kyoto University, in Wakayama Prefecture, Japan, for over 800 h. The focus of the observations was a simple isolated rock, 0.9 m² in area with an inclination of 75°. The pulmonate homing limpet Siphonaria sirius responded by fleeing from its home in response to the whelk Thais clavigera and by calmping tightly to its home scar in response to the starfish Coscinasterias acutispina. Both behaviours were successful for avoiding predation. After a whelk moved far away, limpets returned to their homes. The patellid limpet Cellana toreuma showed mantle folding behaviour or fled for an average distance of 8 cm vertically upwards in response to the whelk and for an average distance of 21 cm vertically upwards in response to the starfish. The amount of time predators spent foraging increased in the low intertidal zone relative to the mid and high zones, and most predator attacks were observed during the limpets' resting periods (submersion in late afternoon or nighttime). The upward directional flight displayed by C. toreuma was related to the upward orientation of the limpets' heads when they are in a resting position. Half of the limpets observed fleeing downward away from a starfish, a movement elicited by a downward attack, received a second attack. These limpets were preyed upon at the second encounter 67% of the time. The upward fleeing behaviour of C. toreuma resulted in the shifting of its resting site higher up the study site where fewer attacks occurred and where the mortality rate was at its lowest. It is suggested that the homing behaviour of S. sirius and the upward fleeing behaviour of C. toreuma are mechanisms by means of which interspecific competition between limpets is reduced and coexistence on a small rock surface is maintained.     

Towards an ecological solution to the folivore paradox: patch depletion as an indicator of within-group scramble competition in red colobus monkeys (Piliocolobus tephrosceles)

A number of socioecological models assume that within-group food competition is either weak or absent among folivorous primates. This assumption is made because their food resources are presumed to be superabundant and evenly dispersed. However, recent evidence increasingly suggests that folivore group size is food-limited, that the primates prefer patchily distributed high-quality foods, and display some of the expected responses to within-group scramble competition. To investigate this apparent contradiction between theoretical models and recent empirical data, we examined the foraging behaviour of red colobus monkeys (Piliocolobus tephrosceles) in Kibale National Park, Uganda. We found that red colobus monkeys foraged in a manner that suggests they deplete patches of preferred foods: intake rate slowed significantly during patch occupancy while movement rate, an index of foraging effort, increased. Furthermore, patch occupancy was related to the size of the feeding group and the size of the patch. These results suggest that within-group scramble competition occurs, may limit folivore group size, and should be considered in models of folivore behavioural ecology.     

Social influence on the correlation between behaviours in young-of-the-year perch

The connection between risk-taking behaviour and exploratory behaviour in young-of-the-year perch (Perca fluviatilis) was studied in aquarium experiments to see whether individual behaviour patterns could be identified in this species and also to investigate how individual behaviour is influenced by their social environment. Risk-taking was defined as the time spent foraging in an open area vs hiding in the vegetation in the presence of a piscivore. Explorative behaviour was measured as latency to enter a passage leading to an unknown area. Groups of four fish were used for the observations, and both behaviours measured were positively correlated with the mean scores of these behaviours in the other group members. Risk-taking and explorative behaviours were correlated only when data was adjusted for the behaviour of the other group members. Individuals that spent more time in the open than their companions also tended to be faster than the others to enter the passage to the unknown area and vice versa. The results indicate that there are consistent individual differences in boldness in perch, but also that behaviour could be modified according to the behaviour of group members.     

How wave exposure, group size and habitat complexity influence foraging and population densities in fishes of the genus Halichoeres (Perciformes: Labridae) on tropical rocky shores

Wave exposure and habitat complexity have been used to explain variations in the distribution patterns and behavior of many reef fishes. This study analyzed the influence of both factors on density and foraging activity, and the influence of group size on foraging in three species of the genus Halichoeres (Labridae) on tropical rocky shores. It was shown that initial phases (IP) and terminal phases (TP) Halichoeres, were influenced by wave exposure, although foraging in Halichoeres brasiliensis TP was not influenced by wave exposure. The IP in all three species were positively associated with rugosity and algal cover while the TP was positively associated with depth. Habitat complexity also influences foraging in these species. Group size influenced foraging activity, except in H. brasiliensis TP. We also found variations in microhabitat patches used for foraging between species and differences in the stomach contents between species and phases, showing that functional roles change in response to ontogenetic shifts.     

COLIWAVE a simulation model for survival of E. coli O157:H7 in dairy manure and manure-amended soil

A simulation model was developed to investigate the relative effects of temperature, oxygen concentration, substrate content and competition by autochthonous microbial community on the oscillatory behaviour and survival of Escherichia coli O157:H7 in manure and manure-amended soil. The overall decline in E. coli O157:H7 was primarily determined by competition with autochthonous copiotrophic bacteria simulated by an inter-specific competition term according to Lotka-Volterra. Oscillations of bacterial populations were attained by the relationships between relative growth and death rates with readily available substrate content. The model contains a logistic and exponential relation of relative growth and death rates, respectively, of E. coli O157:H7 and copiotrophic bacteria with temperature, resulting in optimum curves for net growth rates similar to the curves reported in the literature. The model has been both calibrated and validated on experimental data. The model was used to perform sensitivity analysis and to evaluate different manure and soil management scenarios in terms of survival of E. coli O157:H7. The relative effects of changes in temperature on simulated survival time of E. coli O157:H7 were more pronounced than changes in oxygen condition. Testing manure storage scenarios with realistic data revealed that manure stored in a heap that was turned every week resulted in almost 70% reduction of E. coli O157:H7 survival compared to unturned manure. At the surface of a heap with unturned manure, simulated survival time was the longest (2.4 times longer than inside the same heap). The simulation model provides a new approach to investigating dynamic changes of invasive microorganisms in natural substrates such as manure or manure-amended soil.     

A model for the evolution of communal foraging in hierarchically structured populations

A kin selection model is described for populations in which groups of interacting individuals (trait groups, sensu Wilson 1975) are spatially situated within larger aggregations. The model predicts the optimal foraging strategy when resources are shared with other trait group members and there is an individual risk in foraging. The ecological mechanism of variation in group fitness, differential resource accumulation, is explicitly incorporated into the model. The optimal foraging rate obtained from this model depends on the product of a benefit-to-cost ratio and a relatedness parameter. The appropriate definition of relatedness for the evolution of communal foraging is determined by the details of the ecological interaction between consumers and resources. When competition is purely intra-specific, the genetic correlation among interactants relative to other members of the local aggregation defines the relatedness parameter applicable to selection on foraging propensity. When competition is primarily inter-specific, the genetic correlation among trait group members relative to the entire population defines relatedness.     

Threat-sensitive foraging by larval threespine sticklebacks (Gasterosteus aculeatus)

Summary The threat-sensitive predator avoidance hypothesis predicts that prey can assess the relative threat posed by a predator and adjust their behaviour to reflect the magnitude of the threat. We tested the ability of larval threespine sticklebacks to adjust their foraging in the presence of predators by exposing them to conspecific predators of various sizes and recording their foraging and predator avoidance behaviours. Larvae (<30 days post-hatch) displayed predator escape behaviours only towards attacking predators. At 3 weeks post-hatch larvae approached the predator after fleeing, a behaviour which may be the precursor to predator inspection. Larvae reduced foraging and spent less time in the proximity of large and medium-sized predators compared to small predators. The reduction in foraging was negatively correlated to the predator/larva size ratio, indicating that larvae increased their foraging as they increased in size relative to the predator. We conclude that larval sticklebacks can assess the threat of predation early in their ontogeny and adjust their behaviour accordingly.Correspondence to: J.A. Brown     

Local interactions between predators and prey call into question commonly used functional responses

Commonly used functional response models (Holling’s type I and type II models) assume that the encounter rate of a predator increases linearly with prey density, provided that the predator is searching for prey. In other other words, aN (a is the baseline encounter rate and N is prey density) describes the encounter rate. This study examined whether the models are adequate when predators and prey interact locally by using a spatially explicit individual based model because local interactions affect the spatial distribution of predators and prey, which also affects the encounter rate. Predators were assumed to possess a spatial perception range that influenced their foraging behavior (e.g., if a prey is in the perception range, the predator moves towards the prey). The effect of antipredator behavior by prey was also examined. The results suggest that prey and predator densities as well as handling time affect the baseline rate (i.e., parameter a) as opposed to the common assumption that the parameter is constant. The nature of model deviations depended on both the antipredator behavior and the predators’ perception range. Understanding these deviations is important as they qualitatively affect community dynamics.     

Functional mechanisms underlying cetacean distribution patterns: hotspots for bottlenose dolphins are linked to foraging

Many studies have shown that the distribution of cetaceans can be closely linked to habitat, but the underlying function of the preferred habitats often remains unclear. Only when behavioural observations are made in relation to habitat types can functional mechanisms behind the habitat use be revealed. Within the range of a bottlenose dolphin (Tursiops truncatus) population off NE Scotland, dolphins show clear preferences for several discrete areas. If the observed patterns of distribution are related to foraging, we predict that behaviour patterns shown by dolphins would reflect this relationship. In this study we identify behaviours of dolphins at the water surface that were related to feeding events, evaluate whether the patterns of distribution were related to foraging and whether they were related the local submarine habitat characteristics. To investigate whether visible surface evidence of foraging behaviour varied spatially, we analysed data collected from 104 regular boat-based surveys made within the Moray Firth, NE Scotland, between 1990 and 2000. To determine whether underlying bathymetry had any influence on the surface behaviour of dolphins, a land-based observation study was carried out in the populations core region of use. The results of this study show that feeding behaviour by dolphins was significantly higher in areas used intensively by dolphins. Furthermore, there were clear relationships between feeding events and the submarine habitat characteristics; certain forms of feeding occur primarily over steep seabed gradients, and in deeper waters during June and July. These results quantitatively support the hypothesis that the distinctive patterns of distribution shown by these dolphins are related to foraging behaviour or opportunities, and that submarine habitat characteristics may be a significant factor in the foraging efficiency of dolphins. Future work should focus on collecting detailed information on the distribution patterns of prey within the study area to allow direct comparisons between predator and prey distributions.Communicated by J.P. Thorpe, Port Erin