Networks of prey specialization in an Arctic monomorphic seabird

Generalist predator populations are sometimes made up of individuals that specialize on particular prey items. To examine specialization in thick‐billed murres Uria lomvia during self‐feeding we obtained stomach contents and muscle stable isotope values for 213 birds feeding close to five colonies in the Canadian Arctic. Adults were less specialized during self‐feeding than during chick‐provisioning. Nonetheless, particular specialists clustered together within the foraging network. While sexes showed similar levels of specialization, individuals of the same sex clustered together within the foraging network. The significant degree of clustering regardless of sex showed that individuals specializing on one prey item tend to also specialize on another, although network topology varied from colony to colony. Adult muscle stable isotope values correlated with the stable isotope values of the prey found in stomachs, at least at the one colony with relevant prey data, suggesting that specializations are maintained over time. Degree of specialization increased with niche width across the five colonies, but similarity in gastro‐intestinal and bill morphology was independent of dietary similarity. Thus, although individual specialization is thought to play a key role in sympatric speciation through trophic specialization, we found no support for an association between morphology and foraging patterns in our species. We conclude that self‐feeding murres show clustered dietary specialization, and that specialization is highest where diet is most diverse.     

Fine-scale spatial age segregation in the limited foraging area of an inshore seabird species,the little penguin

Competition for food resources can result in spatial and dietary segregation among individuals from the same species. Few studies have looked at such segregations with the combined effect of sex and age in species with short foraging ranges. In this study we examined the 3D spatial use of the environment in a species with a limited foraging area. We equipped 26 little penguins (Eudyptula minor) of known age, sex, and breeding output with GPS (location) and accelerometer (body acceleration and dive depth) loggers. We obtained dietary niche information from the isotopic analysis of blood tissue. We controlled for confounding factors of foraging trip length and food availability by sampling adults at guard stage when parents usually make one-day trips. We observed a spatial segregation between old (>11 years old) and middle-aged penguins (between 5 and 11 years old) in the foraging area. Old penguins foraged closer to the shore, in shallower water. Despite observing age-specific spatial segregation, we found no differences in the diving effort and foraging efficiency between age classes and sexes. Birds appeared to target similar prey types, but showed age-specific variation in their isotopic niche width. We hypothesize that this age-specific segregation was primarily determined by a “cohort effect” that would lead individuals sharing a common life history (i.e. having fledged and dispersed around the same age) to forage preferentially together or to share similar foraging limitations.     

Segregation in space and time explains the coexistence of two sympatric sub‐Antarctic petrels

Biological communities are shaped by competition between and within species. Competition is often reduced by inter‐ and intraspecific specialization on resources, such as differencet foraging areas or time, allowing similar species to coexist and potentially contributing to reproductive isolation. Here, we examine the simultaneous role of temporal and spatial foraging segregation within and between two sympatric sister species of seabirds, Northern Macronectes halli and Southern Macronectes giganteus Giant Petrels. These species show marked sexual size dimorphism and allochrony (with earlier breeding by Northern Giant Petrels) but this is the first study to test for differences in foraging behaviours and areas across the entire breeding season both between the two species and between the sexes. We tracked males and females of both species in all breeding stages at Bird Island, South Georgia, to test how foraging distribution, behaviour and habitat use vary between and within species in biological time (incubation, brood‐guard or post‐brood stages) and in absolute time (calendar date). Within each breeding stage, both species took trips of comparable duration to similar areas, but due to breeding allochrony they segregated temporally. Northern Giant Petrels had a somewhat smaller foraging range than Southern Giant Petrels, reflecting their greater exploitation of local carrion and probably contributing to their recent higher population growth. Within species, segregation was spatial, with females generally taking longer, more pelagic trips than males, although both sexes of both species showed unexpectedly plastic foraging behaviour. There was little evidence of interspecific differences in habitat use. Thus, in giant petrels, temporal segregation reduces interspecific competition and sexual segregation reduces intraspecific competition. These results demonstrate how both specialization and dynamic changes in foraging strategies at different scales underpin resource division within a community.     

Fast carnivores and slow herbivores: differential foraging strategies among grizzly bears in the Canadian Arctic

Categorizing animal populations by diet can mask important intrapopulation variation, which is crucial to understanding a species’ trophic niche width. To test hypotheses related to intrapopulation variation in foraging or the presence of diet specialization, we conducted stable isotope analysis (δ¹³C, δ¹⁵N) on hair and claw samples from 51 grizzly bears (Ursus arctos) collected from 2003 to 2006 in the Mackenzie Delta region of the Canadian Arctic. We examined within-population differences in the foraging patterns of males and females and the relationship between trophic position (derived from δ¹⁵N measurements) and individual movement. The range of δ¹⁵N values in hair and claw (2.0–11.0‰) suggested a wide niche width and cluster analyses indicated the presence of three foraging groups within the population, ranging from near-complete herbivory to near-complete carnivory. We found no linear relationship between home range size and trophic position when the data were continuous or when grouped by foraging behavior. However, the movement rate of females increased linearly with trophic position. We used multisource dual-isotope mixing models to determine the relative contributions of seven prey sources within each foraging group for both males and females. The mean bear dietary endpoint across all foraging groups for each sex fell toward the center of the mixing polygon, which suggested relatively well-mixed diets. The primary dietary difference across foraging groups was the proportional contribution of herbaceous foods, which decreased for both males and females from 42–76 to 0–27% and 62–81 to 0–44%, respectively. Grizzlies of the Mackenzie Delta live in extremely harsh conditions and identifying within-population diet specialization has improved our understanding of varying habitat requirements within the population.     

Individual specialization in diet by a generalist marine predator reflects specialization in foraging behaviour

1. We studied chick diet in a known-age, sexed population of a long-lived seabird, the Brünnich's guillemot (Uria lomvia), over 15 years (N = 136; 1993-2007) and attached time-depth-temperature recorders to examine foraging behaviour in multiple years (N = 36; 2004-07). 2. Adults showed specialization in prey fed to offspring, described by multiple indices calculated over 15 years: 27% of diet diversity was attributable to among-individual variation (within-individual component of total niche width = 0.73); average similarity of an individual's diet to the overall diet was 65% (mean proportional similarity between individuals and population = 0.65); diet was significantly more specialized than expected for 70% of individuals (mean likelihood = 0.53). These indices suggest higher specialization than the average for an across-taxa comparison of 49 taxa. 3. Foraging behaviour varied along three axes: flight time, dive depth and dive shape. Individuals showed specialized individual foraging behaviour along each axis. These foraging strategies were reflected in the prey type delivered to their offspring and were maintained over scales of hours to years. 4. Specialization in foraging behaviour and diet was greater over short time spans (hours, days) than over long time spans (years). Regardless of sex or age, the main component of variation in foraging behaviour and chick diet was between individuals. 5. Plasma stable isotope values were similar across years, within a given individual, and variance was low relative to that expected from prey isotope values, suggesting adult diet specialized across years. Stable isotope values were similar among individuals that fed their nestlings similar prey items and there was no difference in trophic level between adults and chicks. We suggest that guillemots specialize on a single foraging strategy regardless of whether chick-provisioning and self-feeding. With little individual difference in body mass and physiology, specialization likely represents learning and memorizing optimal feeding locations and behaviours. 6. There was no difference in survival or reproductive success between specialists and generalists, suggesting these are largely equivalent strategies in terms of evolutionary fitness, presumably because different strategies were advantageous at different levels of prey abundance or predictability. The development of individual specialization may be an important precursor to diversification among seabirds.     

Trophic niche width,offspring condition and immunity in a raptor species

Strategies developed by organisms to maximize foraging efficiency have a strong influence on fitness. The way in which the range of food resources is exploited has served to classify species, populations and individuals from more specialist (narrow trophic niche) to more generalist (broad trophic niche). Recent studies have provided evidence that many of the considered generalist species/populations are actually composed of different specialist individuals (individual specialization). Even the existence of generalism as an adaptive strategy has been questioned. In this study, we investigated the relationship between trophic niche width, individual quality and offspring viability in a population of common kestrel Falco tinnunculus during 4 years. We showed that the diet of kestrels varied significantly among years and that individuals of better quality fed their offspring with a higher diversity of prey species and a higher amount of food. Moreover, body condition and immune response of nestlings were positively correlated with diversity of prey delivered by parents. Our study suggests that generalism has the potential to increase fitness and that broadening the trophic niche may be an adaptive strategy in unpredictable environments.     

Individual food niche specialization in stream-dwelling charr

Food niche variation within a population of white-spotted charr, Salvelinus leucomaenis, was investigated by means of mark and recapture to examine the simplification that conspecific individuals are ecologically equivalent. Food diversity measured on the basis of living form of dietary organisms demonstrated variability in food niche in incidence, degree, and stability. As inferred from body size distribution and fish movement, niche variation was inexplicable by social rank hierarchy or foraging habitat rotation, and occasional niches were deemed to be realized individually. Higher annual growth rate shown by those with a larger niche width indicated the adaptive significance in niche variation. Assuming trade-off between feeding efficiency and predation risk, phenotypic plasticity was suggested to determine the food niche specialization of individuals to maximize lifetime fitness.     

Behavioural plasticity and trophic niche shift: How wintering geese respond to habitat alteration

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Individual specialization in a shorebird population with narrow foraging niche

Individual specialization in resource use is a widespread driver for intra-population trait variation, playing a crucial evolutionary role in free-living animals. We investigated the individual foraging specialization of Black-tailed Godwits (Limosa limosa islandica) during the wintering period. Godwits displayed distinct degrees of individual specialization in diet and microhabitat use, indicating the presence of both generalist and specialist birds. Females were overall more specialist than males, primarily consuming polychaetes. Specialist males consumed mainly bivalves, but some individuals also specialized on gastropods or polychaetes. Sexual dimorphism in bill length is probably important in determining the differences in specialization, as longer-billed individuals have access to deep-buried polychaetes inaccessible to most males. Different levels of specialization within the same sex, unrelated to bill length, were also found, suggesting that mechanisms other traits are involved in explaining individual specialization. Godwits specialized on bivalves achieved higher intake rates than non-specialist birds, supporting the idea that individual foraging choices or skills result in different short-term payoffs within the same population. Understanding whether short-term payoffs are good indicators of long-term fitness and how selection operates to favour the prevalence of specialist or generalist godwits is a major future challenge.     

The Long and the Short of It: No Dietary Specialisation between Male and Female Western Sandpipers Despite Strong Bill Size Dimorphism

Many bird species show spatial or habitat segregation of the sexes during the non-breeding season. One potential ecological explanation is that differences in bill morphology favour foraging niche specialisation and segregation. Western sandpipers Calidris mauri have pronounced bill size dimorphism, with female bills averaging 15% longer than those of males. The sexes differ in foraging behaviour and exhibit partial latitudinal segregation during the non-breeding season, with males predominant in the north and females in the south. Niche specialisation at a local scale might account for this broad geographic pattern, and we investigated whether longer-billed females and shorter-billed males occupy different foraging niches at 16 sites across the non-breeding range. We used stable-nitrogen (δ¹⁵N) isotope analysis of whole blood to test for dietary specialisation according to bill length and sex. Stable-nitrogen isotope ratios increase with trophic level. We predicted that δ¹⁵N values would increase with bill length and would be higher for females, which use a greater proportion of foraging behaviour that targets higher-trophic level prey. We used stable-carbon (δ¹³C) isotope analysis to test for habitat segregation according to bill length and sex. Stable-carbon isotope ratios vary between marine- and freshwater-influenced habitats. We predicted that δ¹³C values would differ between males and females if the sexes segregate between habitat types. Using a model selection approach, we found little support for a relationship between δ¹⁵N and either bill length or sex. There was some indication, however, that more marine δ¹³C values occur with shorter bill lengths. Our findings provide little evidence that male and female western sandpipers exhibit dietary specialisation as a function of their bill size, but indicate that the sexes may segregate in different habitats according to bill length at some non-breeding sites. Potential ecological factors underlying habitat segregation between sexes include differences in preferred habitat type and predation risk.     

Individual foraging specialisation in a social mammal: the European badger (Meles meles)

Individual specialisation has been identified in an increasing number of animal species and populations. However, in some groups, such as terrestrial mammals, it is difficult to disentangle individual niche variation from spatial variation in resource availability. In the present study, we investigate individual variation in the foraging niche of the European badger (Meles meles), a social carnivore that lives in a shared group territory, but forages predominantly alone. Using stable isotope analysis, we distinguish the extent to which foraging variation in badgers is determined by social and spatial constraints and by individual differences within groups. We found a tendency for individual badgers within groups to differ markedly and consistently in their isotope values, suggesting that individuals living with access to the same resources occupied distinctive foraging niches. Although sex had a significant effect on isotope values, substantial variation within groups occurred independently of age and sex. Individual differences were consistent over a period of several months and in some instances were highly consistent across the two years of the study, suggesting long-term individual foraging specialisations. Individual specialisation in foraging may, therefore, persist in populations of territorial species not solely as a result of spatial variation in resources, but also arising from individuals selecting differently from the same available resources. Although the exact cause of this behaviour is unknown, we suggest that specialisation may occur due to learning trade-offs which may limit individual niche widths. However, ecological factors at the group level, such as competition, may also influence the degree of specialisation.     

Disturbance-dependent spatial distribution of sexes in a gynodioecious understory shrub

Several ecological conditions and processes occurring naturally in plant populations may lead to spatial aggregation of sexes within populations of sexually polymorphic species. In addition, ecological disturbances such as forest management or fire could also affect the spatial distribution of sexes within populations. Spatial aggregation of sexes can have important consequences for the fitness of the individuals in sex-biased patches through increased pollen limitation and/or variation in the male fitness of hermaphrodites. Therefore, spatial aggregation of sexes could be relevant for the maintenance of the sexual polymorphism in plant species. Here, we used point pattern analysis to study the spatial distribution of female and hermaphrodite individuals within a single population of the gynodioecious understory shrub Daphne laureola, inhabiting a young pine reforestation and the adjacent natural undisturbed area. In the undisturbed area, we found that hermaphrodites were distributed randomly whereas females were spatially aggregated at short distances. Such aggregation could result from narrower ecological amplitude, cyto-nuclear determination of sex and limited dispersal and/or increased cloning in females. In the reforested area, females were slightly more abundant and the two sexes were randomly distributed among all plants. Recolonization processes after certain forest disturbances could thus favour the establishment of females and alter the spatial distribution of sexes in this understory species.     

Examining the link between dietary specialization and foraging modes of stream‐dwelling brown trout Salmo trutta

The aim of this study was to explore differences in dietary specialization across two foraging modes (benthic v. surface‐drift foraging) of stream‐dwelling brown trout Salmo trutta. The degree of inter‐individual niche variation within each foraging mode was high, but the dietary specialization was maintained between foraging modes. This study supports the view that if aquatic invertebrates are more abundant and accessible than surface prey, the individuals will not specialize on surface prey (surface‐drift foraging).     

Optimality modelling and quantitative genetics as alternatives to study the evolution of foraging behaviours in insect herbivores.

Summary Although the evolution of large-scale dispersal has received considerable attention, we know very little about how natural selection influences foraging behaviours in herbivorous insects. Host-selection behaviours and within-habitat movements jointly determine foraging behaviours, since host selection affects the allocation of time spent on a particular host versus moving between these hosts. However, host selection is generally a labile trait, whose expression is influenced by the physiological state of the forager and hence, by characteristics of the habitat. We discuss how the quantitative genetic concepts can be used to study the evolution of such labile behaviours. Since host responses depend on the physiological state of the forager, it is argued that the state of the forager must be explicitly considered when estimating the additive genetic basis of host-selection behaviours. The lability of foraging behaviours increases the difficulty of measuring the fitness consequence of variation in the foraging phenotype in specific habitats. Therefore, it may be difficult to rely exclusively on quantitative genetic methods to test hypotheses about adaptive change in foraging behaviours across different habitats. We provide a novel approach based on optimality modelling to calculate the fitness consequence of variation in the foraging phenotype across different habitats. This method, in conjunction with quantitative genetics, can be used to test hypotheses concerning the evolution of foraging behaviours.     

Effects of season, sex and body size on the feeding ecology of turtle-headed sea snakes (Emydocephalus annulatus) on IndoPacific inshore coral reefs

In terrestrial snakes, many cases of intraspecific shifts in dietary habits as a function of predator sex and body size are driven by gape limitation and hence are most common in species that feed on relatively large prey and exhibit a wide body-size range. Our data on sea snakes reveal an alternative mechanism for intraspecific niche partitioning, based on sex-specific seasonal anorexia induced by reproductive activities. Turtle-headed sea snakes (Emydocephalus annulatus) on coral reefs in the New Caledonian Lagoon feed entirely on the eggs of demersal-spawning fishes. DNA sequence data (cytochrome b gene) on eggs that we palpated from stomachs of 37 snakes showed that despite this ontogenetic stage specialization, the prey comes from a taxonomically diverse array of species including damselfish (41 % of samples, at least 5 species), blennies (41 %, 4 species) and gobies (19 %, 5 species). The composition of snake diets shifted seasonally (with damselfish dominating in winter but not summer), presumably reflecting seasonality of fish reproduction. That seasonal shift affects male and female snakes differently, because reproduction is incompatible with foraging. Adult female sea snakes ceased feeding when they became heavily distended with developing embryos in late summer, and males ceased feeding while they were mate searching in winter. The sex divergence in foraging habits may be amplified by sexual size dimorphism; females grow larger than males, and larger snakes (of both sexes) feed more on damselfish (which often lay their eggs in exposed sites) than on blennies and gobies (whose eggs are hidden within narrow crevices). Specific features of reproductive biology of coral reef fish (seasonality and nest type) have generated intraspecific niche partitioning in these sea snakes, by mechanisms different from those that apply to terrestrial snakes.     

Intersexual trophic niche partitioning in an ant-eating spider (Araneae: Zodariidae)

Background

Divergence in trophic niche between the sexes may function to reduce competition between the sexes (“intersexual niche partitioning hypothesis”), or may be result from differential selection among the sexes on maximizing reproductive output (“sexual selection hypothesis”). The latter may lead to higher energy demands in females driven by fecundity selection, while males invest in mate searching. We tested predictions of the two hypotheses underlying intersexual trophic niche partitioning in a natural population of spiders. Zodarion jozefienae spiders specialize on Messor barbarus ants that are polymorphic in body size and hence comprise potential trophic niches for the spider, making this system well-suited to study intersexual trophic niche partitioning.

Methodology/Principal Findings

Comparative analysis of trophic morphology (the chelicerae) and body size of males, females and juveniles demonstrated highly female biased SSD (Sexual Size Dimorphism) in body size, body weight, and in the size of chelicerae, the latter arising from sex-specific growth patterns in trophic morphology. In the field, female spiders actively selected ant sub-castes that were larger than the average prey size, and larger than ants captured by juveniles and males. Female fecundity was highly positively correlated with female body mass, which reflects foraging success during the adult stage. Females in laboratory experiments preferred the large ant sub-castes and displayed higher capture efficiency. In contrast, males occupied a different trophic niche and showed reduced foraging effort and reduced prey capture and feeding efficiency compared with females and juveniles.

Conclusions/Significance

Our data indicate that female-biased dimorphism in trophic morphology and body size correlate with sex-specific reproductive strategies. We propose that intersexual trophic niche partitioning is shaped primarily by fecundity selection in females, and results from sex-differences in the route to successful reproduction where females are selected to maximize energy intake and fecundity, while males switch from foraging to invest in mating effort.     

Variance‐Sensitive Green Woodhoopoes: A New Explanation for Sex Differences in Foraging?

Studies of cooperatively breeding birds rarely benefit from the extensive research on adaptive foraging behaviour, despite the potential for concepts such as state‐dependent foraging to explain many aspects of behaviour in social groups. For example, sex differences in preferred foraging techniques used by green woodhoopoes, Phoeniculus purpureus, have previously been explained by sexual dimorphism in bill length and the benefits afforded by foraging specialization and niche differentiation within cooperative groups. Contrary to this argument, there were no sex differences in mean foraging success and/or prey size captured when males and females used the same foraging techniques. Subordinates of both sexes did experience lower and more varied foraging success compared with dominants, but probably only as a consequence of competition or inexperience. However, dominant males experienced greater variance in individual foraging success compared with dominant females, and dominant males also experienced greater variances in prey size when using their preferred foraging techniques. Dominant males therefore appeared to specialize in foraging techniques that provided more variable rewards, whilst dominant females consistently chose to minimize variation in reward. Dominant females also experienced less variance in foraging returns when using the same techniques as males, suggesting a possible link with sexual dimorphism in bill length. Partitioning of foraging niches in dominant green woodhoopoes therefore appears to be better explained by sex differences in variance (risk) sensitivity to foraging rewards. We suggest that this kind of detailed analysis of state‐dependent foraging has the potential to explain many of the crucial age and sex differences in behaviour within cooperative groups.     

Balancing Selection in Species with Separate Sexes: Insights from Fisher’s Geometric Model

How common is balancing selection, and what fraction of phenotypic variance is attributable to balanced polymorphisms? Despite decades of research, answers to these questions remain elusive. Moreover, there is no clear theoretical prediction about the frequency with which balancing selection is expected to arise within a population. Here, we use an extension of Fisher’s geometric model of adaptation to predict the probability of balancing selection in a population with separate sexes, wherein polymorphism is potentially maintained by two forms of balancing selection: (1) heterozygote advantage, where heterozygous individuals at a locus have higher fitness than homozygous individuals, and (2) sexually antagonistic selection (a.k.a. intralocus sexual conflict), where the fitness of each sex is maximized by different genotypes at a locus. We show that balancing selection is common under biologically plausible conditions and that sex differences in selection or sex-by-genotype effects of mutations can each increase opportunities for balancing selection. Although heterozygote advantage and sexual antagonism represent alternative mechanisms for maintaining polymorphism, they mutually exist along a balancing selection continuum that depends on population and sex-specific parameters of selection and mutation. Sexual antagonism is the dominant mode of balancing selection across most of this continuum.     

Diet composition and foraging success in generalist predators: Are specialist individuals better foragers?

Factors affecting individual diet specialization in generalist populations and the relationship between diet and foraging success remain poorly studied, particularly in terrestrial wide-ranging predators. We studied whether individual variations in diet in Montagu's harrier males (determined through a combination of direct foraging observations and pellet analysis) were associated with patterns of foraging habitat selection and foraging success of 12 radiotracked males during the breeding period. We found important differences in diet composition and breadth between individuals. Diet diversity was negatively related to hunting success: the most efficient individuals in terms of hunting success had the most specialized diet. This study also suggests an important role of individual foraging habitat selection in explaining individual diet, as the proportion of different prey types in the diet was associated with habitat composition within the home range, with higher proportion of those habitats that held higher abundances of their more frequent prey. This study thus provides evidence of individual diet specialization having a knock-on effect on foraging efficiency in a wide-ranging raptor and highlights the role of individual behaviour as a driving force of intra-population niche variation.     

Scale dependence of sex ratio in wild plant populations: implications for social selection

Social context refers to the composition of an individual''s social interactants, including potential mates. In spatially structured populations, social context can vary among individuals within populations, generating the opportunity for social selection to drive differences in fitness functions among individuals at a fine spatial scale. In sexually polymorphic plants, the local sex ratio varies at a fine scale and thus has the potential to generate this opportunity. We measured the spatial distribution of two wild populations of the gynodioecious plant Silene vulgaris and show that there is fine‐scale heterogeneity in the local distribution of the sexes within these populations. We demonstrate that the largest variance in sex ratio is among nearest neighbors. This variance is greatly reduced as the spatial scale of social interactions increases. These patterns suggest the sex of neighbors has the potential to generate fine‐scale differences in selection differentials among individuals. One of the most important determinants of social interactions in plants is the behavior of pollinators. These results suggest that the potential for selection arising from sex ratio will be greatest when pollen is shared among nearest neighbors. Future studies incorporating the movement of pollinators may reveal whether and how this fine‐scale variance in sex ratio affects the fitness of individuals in these populations.