Head morphology and piscivory of European eels,Anguilla anguilla,predict their probability of infection by the invasive parasitic nematode Anguillicoloides crassus

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Short‐term prey field lability constrains individual specialisation in resource selection and foraging site fidelity in a marine predator

Spatio‐temporally stable prey distributions coupled with individual foraging site fidelity are predicted to favour individual resource specialisation. Conversely, predators coping with dynamic prey distributions should diversify their individual diet and/or shift foraging areas to increase net intake. We studied individual specialisation in Scopoli's shearwaters (Calonectris diomedea) from the highly dynamic Western Mediterranean, using daily prey distributions together with resource selection, site fidelity and trophic‐level analyses. As hypothesised, we found dietary diversification, low foraging site fidelity and almost no individual specialisation in resource selection. Crucially, shearwaters switched daily foraging tactics, selecting areas with contrasting prey of varying trophic levels. Overall, information use and plastic resource selection of individuals with reduced short‐term foraging site fidelity allow predators to overcome prey field lability. Our study is an essential step towards a better understanding of individual responses to enhanced environmental stochasticity driven by global changes, and of pathways favouring population persistence.     

Induction of phenotypic plasticity in rattlesnake trophic morphology by diet manipulation

Gape‐limited predators are restricted in the shape and size of prey items they can ingest by their trophic morphology. Evolutionary theory predicts that gape‐limited predators, such as rattlesnakes, should possess plasticity in their trophic morphology to allow them to respond to environmental cues about their prey base. This study examined the effects of two possible influences over trophic morphology in the pit‐viper Crotalus viridis viridis. Snakes from six litters were exposed to diet manipulations performed over 480 days. By day 480, snakes from two prey‐size treatments exhibited significantly different head shapes. Snakes reared on whole rodents had broader heads, whereas snakes force‐fed homogenized prey had narrower heads. Shape differences varied among litters, suggesting that not all litters responded the same to diet manipulations. Results suggest that trophic morphology of rattlesnakes is plastic, at least in some litters, and can be induced by prey items. J. Morphol. 275:1339–1348, 2014. © 2014 Wiley Periodicals, Inc.     

Beyond trophic morphology: stable isotopes reveal ubiquitous versatility in marine turtle trophic ecology

The idea that interspecific variation in trophic morphology among closely related species effectively permits resource partitioning has driven research on ecological radiation since Darwin first described variation in beak morphology among Geospiza. Marine turtles comprise an ecological radiation in which interspecific differences in trophic morphology have similarly been implicated as a pathway to ecopartition the marine realm, in both extant and extinct species. Because marine turtles are charismatic flagship species of conservation concern, their trophic ecology has been studied intensively using stable isotope analyses to gain insights into habitat use and diet, principally to inform conservation management. This legion of studies provides an unparalleled opportunity to examine ecological partitioning across numerous hierarchical levels that heretofore has not been applied to any other ecological radiation. Our contribution aims to provide a quantitative analysis of interspecific variation and a comprehensive review of intraspecific variation in trophic ecology across different hierarchical levels marshalling insights about realised trophic ecology derived from stable isotopes. We reviewed 113 stable isotope studies, mostly involving single species, and conducted a meta‐analysis of data from adults to elucidate differences in trophic ecology among species. Our study reveals a more intricate hierarchy of ecopartitioning by marine turtles than previously recognised based on trophic morphology and dietary analyses. We found strong statistical support for interspecific partitioning, as well as a continuum of intraspecific trophic sub‐specialisation in most species across several hierarchical levels. This ubiquity of trophic specialisation across many hierarchical levels exposes a far more complex view of trophic ecology and resource‐axis exploitation than suggested by species diversity alone. Not only do species segregate along many widely understood axes such as body size, macrohabitat, and trophic morphology but the general pattern revealed by isotopic studies is one of microhabitat segregation and variation in foraging behaviour within species, within populations, and among individuals. These findings are highly relevant to conservation management because they imply ecological non‐exchangeability, which introduces a new dimension beyond that of genetic stocks which drives current conservation planning. Perhaps the most remarkable finding from our data synthesis is that four of six marine turtle species forage across several trophic levels. This pattern is unlike that seen in other large marine predators, which forage at a single trophic level according to stable isotopes. This finding affirms suggestions that marine turtles are robust sentinels of ocean health and likely stabilise marine food webs. This insight has broader significance for studies of marine food webs and trophic ecology of large marine predators. Beyond insights concerning marine turtle ecology and conservation, our findings also have broader implications for the study of ecological radiations. Particularly, the unrecognised complexity of ecopartitioning beyond that predicted by trophic morphology suggests that this dominant approach in adaptive radiation research likely underestimates the degree of resource overlap and that interspecific disparities in trophic morphology may often over‐predict the degree of realised ecopartitioning. Hence, our findings suggest that stable isotopes can profitably be applied to study other ecological radiations and may reveal trophic variation beyond that reflected by trophic morphology.     

Temporal stability of niche use exposes sympatric Arctic charr to alternative selection pressures

There is now strong evidence that foraging niche specialisation plays a critical role in the very early stages of resource driven speciation. Here we test critical elements of models defining this process using a known polymorphic population of Arctic charr from subarctic Norway. We test the long-term stability of niche specialisation amongst foraging predators and discuss the possibility that contrasting foraging specialists are exposed to differing selection regimes. Inter-individual foraging niche stability was measured by combining two time-integrated ecological tracers of the foraging niche (each individual’s δ¹³C and δ¹⁵N stable isotope (SI) signatures and their food borne parasite fauna) with a short-term measure of foraging niche use (stomach contents composition). Three dietary subgroups of predators were identified, including zooplankton, gammarid and benthivore specialists foragers. Zooplanktivorous specialists had muscle low in δ ¹³C, a high abundance of parasites transmitted from pelagic copepods, a smaller head, longer snout and a more slender body-form than gammaridivorous specialist individuals which had muscle more enriched in δ ¹³C and high abundance of parasites transmitted from benthic Gammarus. Benthivorous individuals were intermediate between the other two foraging groups according to muscle SI-signals (δ¹³C) and loadings of parasites transmitted from both copepods and Gammarus. The close relationship between subgroups identified by stomach contents, time-integrated tracers of niche use (SI and parasites) and functional trophic morphology (niche adaptations) demonstrate a long-term temporally stable niche use of each individual predator. Differential habitat use and contrasting parasite communities and loadings, show differential exposure to different suites of selection pressures for different foraging specialists. Results also show that individual specialisation in trophic behaviour and thus exposure to different suites of selection pressures are stable over time, and thus provide a platform for disruptive selection to operate within this sympatric system.     

Niche segregation of coexisting Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) constrains food web coupling in subarctic lakes

1. Generalist fish species are recognised as important couplers of benthic and pelagic food‐web compartments in lakes. However, interspecific niche segregation and individual specialisation may limit the potential for generalistic feeding behaviour. 2. We studied summer habitat use, stomach contents and stable isotopic compositions of the generalist feeder Arctic charr coexisting with its common resource competitor brown trout in five subarctic lakes in northern Norway to reveal population‐level and individual‐level niche plasticity. 3. Charr and trout showed partial niche segregation in all five lakes. Charr used all habitat types and a wide variety of invertebrate prey including zooplankton, whereas trout fed mainly on insects in the littoral zone. Hence, charr showed a higher potential to promote habitat and food‐web coupling compared to littoral‐dwelling trout. 4. The level of niche segregation between charr and trout and between pelagic‐caught and littoral‐caught charr depended on the prevailing patterns of interspecific and intraspecific resource competition. The two fish species had partially overlapping trophic niches in one lake where charr numerically dominated the fish community, whereas the most segregated niches occurred in lakes where trout were more abundant. 5. In general, pelagic‐caught charr had substantially narrower dietary and isotopic niches and relied less on littoral carbon sources compared to littoral‐caught conspecifics that included generalist as well as specialised benthivorous and planktivorous individuals. Despite the partially specialised planktivorous niche and thus reduced potential of pelagic‐dwelling charr to promote benthic–pelagic coupling, the isotopic compositions of both charr subpopulations suggested a significant reliance on both littoral and pelagic carbon sources in all five study lakes. 6. Our study demonstrates that both interspecific niche segregation between and individual trophic specialisation within generalist fish species can constrain food‐web coupling and alter energy mobilisation to top consumers in subarctic lakes. Nevertheless, pelagic and littoral habitats and food‐web compartments may still be highly integrated due to the potentially plastic foraging behaviour of top consumers.     

The evolution of habitat specialisation in a group of marine triplefin fishes

There has been considerable interest in the directionality of resource specialisation during the diversification of lineages. We developed a quantitative method to investigate habitat specialisation in a radiation of New Zealand triplefin fishes, as habitat use appears to be an important axis of diversification in this marine group. The degree of specialisation in 15 species was calculated by comparing each individual to all other individuals of a species, thus allowing for quantitative distinction between species. Species differed in habitat specialisation, but Bayesian comparative methods found no directional trend in the evolution of resource use. Further analyses showed that specialisation had evolved gradually and was phylogenetically constrained, with most differences between species arising toward the tips of the tree. No correlation between the degree of specialisation and body size was detected in this group, suggesting that habitat specialisation evolved independently of body size. Habitat specialisation does not appear to have been an impediment to ecological diversification in this group. Rather, diversification in these fishes appears to have followed different evolutionary trajectories in habitat specialisation, one in which species have sub-partitioned available resources, and another in which species have expanded their use of resources. These findings support recent studies suggesting that diversification does not necessarily proceed from generalised ancestors to specialised descendants.     

The influence of habitat accessibility on the dietary and morphological specialisation of an aquatic predator

Individual diet and habitat specialisation are widespread in animal taxa and often related to levels of predation and competition. Mobile consumers such as predatory fish can stabilise lake food webs by ranging over a larger area than their prey, thereby switching between habitats. Although, this switching assumes that the predator has equal preference for the available prey, individual diet specialisation and morphological adaptations to different habitats could potentially prevent individuals from switching between habitats. In this study, we assessed the niche width and individual specialisation in Eurasian perch Perca fluviatilis in response to a shift in habitat use by manipulating the ability for this top predator to couple habitats. We ran an eight weeks pond experiment, to test the effect of habitat switching on diet and morphological specialisations. We show that habitat coupling influenced individual diet specialisation and niche use in expected directions where specialisation increased with decreasing habitat switching. In contrast to expectations, the morphological variation decreased with increasing diet specialisation. Our results expand on previous work and suggest that individual specialisation and niche width can impact the ability of mobile predators to couple habitats. Furthermore, it shows the importance of individual specialisations in relation to habitat coupling.     

Questioning assumptions of trophic behavior in a broadly ranging marine predator guild

We evaluated whether existing assumptions regarding the trophic ecology of a poorly‐studied predator guild, northwest (NW) Atlantic skates (family: Rajidae), were supported across broad geographic scales. Four hypotheses were tested using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope values as a proxy for foraging behavior: 1) species exhibit ontogenetic shifts in habitat and thus display a shift in ¹³C with differential use of the continental shelf; 2) species exhibit ontogenetic prey shifts (i.e. from smaller to larger prey items) and become enriched in ¹⁵N; 3) individuals acquire energy from spatially confined local resource pools and exhibit limited displacement; and 4) species exhibit similarly sized and highly overlapping trophic niches. We found some evidence for ontogenetic shifts in habitat‐use (δ¹³C) for thorny and little skate and diet (δ¹⁵N) of thorny and winter skate and hypothesize that individuals exhibit gradual trophic niche transition, especially in δ¹⁵N space, rather than a clear and distinct shift in diet throughout ontogeny. Spatial isoscapes generated for little, thorny, and winter skate highlighted distinct spatial patterns in isotopic composition across the coastal shelf. For little and thorny skate, patterns mimicked expected spatial variability in the isotopic composition of phytoplankton/POM, suggesting limited displacement and utilization of spatially confined resource pools. Winter skate, however, exhibited a much narrower range of δ¹³C and δ¹⁵N values, suggesting individuals may use resources from a more confined latitudinal range. Although high total trophic niche overlap was observed between some species (e.g. little and thorny skate), sympatric species (e.g. little and winter skate) exhibited a degree of trophic niche separation. These findings offer new insight into the trophic dynamics of a poorly‐studied, vulnerable group of predators, and highlight a need to re‐examine assumptions pertaining to aspects of their ecology.     

Lifelong foraging and individual specialisation are influenced by temporal changes of resource availability

Resource availability largely determines the distribution and behaviour of organisms. In plant–pollinator communities, availability of floral resources may change so rapidly that pollinator individuals can benefit from switching between multiple resources, i.e. different flowering plant species. Insect pollinator individuals of a given generation often occur in different time windows during the reproductive season. This temporal variation in individual occurrences, together with the rapidly changing resource availability, may lead individuals of the same population to encounter and use different resources, resulting in an apparent individual specialisation. We hypothesized, that 1) individual pollinators change their resource use (flower visitation) during their lifetime according to the changing availability of floral resources, and that 2) temporal variation in individual occurrences of pollinators and in resource availability will partly explain individual specialisation. To test these hypotheses, we observed flower visitations of individually marked clouded Apollo butterflies Parnassius mnemosyne during one reproductive season. We found temporal changes in lifetime individual resource use that followed the changes in resource availability, indicating that butterflies can adjust foraging to varying resource availability. Individuals differed considerably in their resource use. This variation was partly explained by temporal variation in both floral resource availability and temporal occurrence of individual butterflies. We suggest the butterfly as a sequential specialist, i.e. short‐term specialist and long‐term generalist. This foraging plasticity can be essential for short‐living insect pollinators in rapidly changing environments. Although flowering dynamics do not fully explain the variability in foraging, our results highlight the importance of temporal dimension in resource use studies. Ultimately, the relative pace of environmental change compared to individual lifespan may be a key factor in resource use plasticity.     

Individual feeding specialisation in shorebirds: population consequences and conservation implications

Individual feeding specialisation in shorebirds is reviewed, and the possilble mechanisms involved in such specialisations. Any specialisation can he seen as an individual strategy, and the optimum strategy for any given individual will be conditional upon its specific priorities and constraints. Some specialisations are related to social status and some to individual skills. Some are also probably frequency-dependent. However, most shorebird specialisations are constrained to a large extent by individual morphology, particularly bill morphology. For example, larger birds are able to handle larger prey, and birds with longer bills are able to feed on more deeply buried prey. Sex differences in bill length are uncommon in the Charardriidae, which are surface peckers, but are common in the Scolopacidae, which feed by probing in soft substrates. Sex differences in bill morphology are frequently associated with sex differences in feeding specialisation. There is evidence that different feeding specialisations are associated with different payoffs, in which case the probability of failing to reproduce or of dying will not be distributed equally throughout the population. I consider the population consequences of such feeding specialisations, particularly the different risks and benefits associated with different habitats or diets. I also consider the way in which individuals may differ in their response to habitat loss or change. I suggest that population models designed to predict the effect of habitat loss or change on shorebirds should have the ability to investigate the differential response of certain sections of the population, particularly different ages or sexes, that specialise in different diets or feeding methods.     

Beyond ecological opportunity: Prey diversity rather than abundance shapes predator niche variation

1. Ecological opportunity (i.e. the diversity of available resources) has a pivotal role in shaping niche variation and trophic specialisation of animals. However, ecological opportunity can be described with regard to both diversity and abundance of resources. The degree to which these two components contribute to niche variation remains unexplored.
2. To address this, we used an extensive dataset on fish diet and benthic invertebrate diversity and density from 73 sampling events in three Norwegian rivers in order to explore realised trophic niches and the response of dietary niche variation along gradients of resource diversity (potential trophic niches), resource density (as a proxy of resource abundance) and fish density (as a proxy of inter‐ and intra‐specific competition) in a freshwater top predator (the brown trout, Salmo trutta L.).
3. Linear models indicated that individual and population niche variation increased with increasing ecological opportunity in terms of prey diversity. However, no simple cause‐and‐effect associations between niche indices and prey abundance were found. Our multiple regression analyses indicated that the abundance of certain resources (e.g. Chironomidae) can interact with prey diversity to determine individual and population realised trophic niches. Niche variation (within‐individual component and inter‐individual diet variation) decreased with increasing inter‐ and intra‐specific competition.
4. This study extends prevailing trophic ecology theory by identifying diversity, rather than density, of available prey resources as a primary driver of niche variation in fish of temperate riverine systems with no extensive resource limitation. The study also shows that ecological opportunity may mask the direction of the effect (compression or expansion) of competition on niche variation when food resources are diverse.
5. Our study provides novel empirical insight to the driving forces behind niche variation and reveals that diversity, rather than density, of available prey resources may be a primary driver of niche variation in freshwater fish. Our study supports the view that a broader potential trophic niche promotes broader realised trophic niche variation by individuals, which leads to individual niche diversification by opening access to alternatives resources, resulting in a concomitant rise in the realised trophic niche width of the population.

     

Trophic Niche in a Raptor Species: The Relationship between Diet Diversity,Habitat Diversity and Territory Quality

Recent research reports that many populations of species showing a wide trophic niche (generalists) are made up of both generalist individuals and individuals with a narrow trophic niche (specialists), suggesting trophic specializations at an individual level. If true, foraging strategies should be associated with individual quality and fitness. Optimal foraging theory predicts that individuals will select the most favourable habitats for feeding. In addition, the “landscape heterogeneity hypothesis” predicts a higher number of species in more diverse landscapes. Thus, it can be predicted that individuals with a wider realized trophic niche should have foraging territories with greater habitat diversity, suggesting that foraging strategies, territory quality and habitat diversity are inter-correlated. This was tested for a population of common kestrels Falco tinnunculus. Diet diversity, territory occupancy (as a measure of territory quality) and habitat diversity of territories were measured over an 8-year period. Our results show that: 1) territory quality was quadratically correlated with habitat diversity, with the best territories being the least and most diverse; 2) diet diversity was not correlated with territory quality; and 3) diet diversity was negatively correlated with landscape heterogeneity. Our study suggests that niche generalist foraging strategies are based on an active search for different prey species within or between habitats rather than on the selection of territories with high habitat diversity.     

Head shape dimorphism in European glass eels (Anguilla anguilla)

The life cycle of the European eel (Anguilla anguilla) remained a mystery until the 20th century, when Schmidt discovered that the Sargasso Sea was its spawning area. However, many aspects of the eel's life cycle remain poorly understood. Among these is the bimodal distribution in head shape, with broad- and narrowheaded phenotypes reported in the yellow eel stage. Although this has been linked to dietary preferences of the yellow eels, very little is known about why, how and when this dimorphism arises during their ontogeny. To determine whether this dimorphism indeed appears in relation to trophic niche segregation, we examined head shape variation at an earlier ontogenetic stage, the glass eel stage, as at this stage eels are considered to be non-feeding. Head shape was studied in a large dataset, containing glass eels captured from the Yser river mouth, the Leopold Canal (Belgium) and from the rivers Severn, Trent and Parret (UK), by both taking measurements (head width/head length) and using an outline analysis. Our results show that there is already considerable variation in broadness and bluntness of the head at the glass eel stage. In most cases, equal support for a unimodal and bimodal head shape distribution is found, whereas some cases support head shape bimodality in glass eels, suggesting that glass eel head shape might be shifting from a unimodal to a bimodal distribution. This, in combination with the observation that variation in head width/head length ratios in non-feeding glass eels shows a similar range as in feeding yellow eels, indicates that head shape in European eel might be at least partially determined through other mechanisms than trophic segregation.     

A generalized habitat matching rule

Summary When fitness of a resource-limited animal depends only on that individual's share of the total resource in a habitat patch and individuals are free to move to the patch where their gains are highest, population density matches resource availability under the simple assumption that individual fitness increases with resource use. Previous theory on habitat matching required the stronger assumption that individual fitness was directly proportional to (rather than monotonically increasing with) resource use. The basic theory suggests conditions under which population density empirically indicates habitat quality. Extensions of this basic theory apply when individuals that are free to move among resource patches interact by interfering with each other's resource extraction or by competing unequally. Analysis of existing models of such ideal free competition yields conditions for a single general matching rule in which the logarithm of crowding is a linear function of the logarithm of resource abundance. Double logarithmic plots of empirical data on habitat use and habitat quality based on this rule furnish possible graphical indicators of the occurrence and intensity of competition in nature.     

Stable resource polymorphism along the benthic littoral–pelagic axis in an invasive crayfish

Although intraspecific variability is now widely recognized as affecting evolutionary and ecological processes, our knowledge on the importance of intraspecific variability within invasive species is still limited. This is despite the fact that understanding the linkage between within‐population morphological divergences and the use of different trophic or spatial resources (i.e., resource polymorphism) can help to better predict their ecological impacts on recipient ecosystems. Here, we quantified the extent of resource polymorphism within populations of a worldwide invasive crayfish species, Procambarus clarkii, in 16 lake populations by comparing their trophic (estimated using stable isotope analyses) and morphological characteristics between individuals from the littoral and pelagic habitats. Our results first demonstrated that crayfish occured in both littoral and pelagic habitats of seven lakes and that the use of pelagic habitat was associated with increased abundance of littoral crayfish. We then found morphological (i.e., body and chelae shapes) and trophic divergence (i.e., reliance on littoral carbon) among individuals from littoral and pelagic habitats, highlighting the existence of resource polymorphism in invasive populations. There was no genetic differentiation between individuals from the two habitats, implying that this resource polymorphism was stable (i.e., high gene flow between individuals). Finally, we demonstrated that a divergent adaptive process was responsible for the morphological divergence in body and chela shapes between habitats while difference in littoral reliance neutrally evolved under genetic drift. These findings demonstrated that invasive P. clarkii can display strong within‐population phenotypic variability in recent populations, and this could lead to contrasting ecological impacts between littoral and pelagic individuals.     

Habitat use throughout migration: linking individual consistency, prior breeding success and future breeding potential

1.?Habitat use can influence individual performance in a wide range of animals, either immediately or through carry-over effects in subsequent seasons. Given that many animal species also show consistent individual differences in reproductive success, it seems plausible that individuals may have consistent patterns of habitat use representing individual specializations, with concomitant fitness consequences. 2.?Stable-carbon isotope ratios from a range of tissues were used to discern individual consistency in habitat use along a terrestrial-aquatic gradient in a long-distance migrant, the Bewick's swan (Cygnus columbianus bewickii). These individual specialisations represented <15% of the isotopic breadth of the population for the majority of individuals and were seen to persist throughout autumn migration and overwintering until aquatic habitats were no longer available. 3.?Individual foraging specialisations were then used to demonstrate two consecutive carry-over effects associated with macroscale habitat segregation: consequences of breeding season processes for autumn habitat use; and consequences of autumn habitat use for future reproductive success. Adults that were successful breeders in the year of capture used terrestrial habitats significantly more than adults that were not successful, revealing a substantial cost of reproduction and extended parental care. Use of aquatic habitats during autumn was, however, associated with increased body condition prior to spring migration; and increased subsequent breeding success in adults that had been unsuccessful the year before. Yet adults that were successful breeders in the year of capture remained the most likely to be successful the following year, despite their use of terrestrial habitats. 4.?Our results uniquely demonstrate not only individual foraging specializations throughout the migration period, but also that processes during breeding and autumn migration, mediated by individual consistency, may play a fundamental role in the population dynamics of long-distance migrants. These findings, therefore, highlight the importance of long-term consistency to our understanding of habitat function, interindividual differences in fitness, population dynamics and the evolution of migratory strategies.     

Built to bite? Differences in cranial morphology and bite performance between narrow‐ and broad‐headed European glass eels

The presence of two phenotypes in a single species is a widespread phenomenon, also observed in European eel (Anguilla anguilla). This dimorphism has been related to dietary differences in the subadult elver and yellow eel stages, with broad‐heads generally feeding on harder and/or larger‐bodied prey items than narrow‐heads. Nevertheless, both broad‐ and narrow‐headed phenotypes can already be found among glass eels, the stage preceding the elver eel stage. As these glass eels are considered nonfeeding, we investigate here to what degree the observed variation in head width is reflected in variation in the musculoskeletal feeding system, as well as whether this reflects the same variation observed in the older, dimorphic yellow eels. Additionally, we investigate whether musculoskeletal differences between broad‐ and narrow‐headed glass eels have implications on their feeding performance and could thus impact prey preference when eels start feeding. Therefore, we compared the cranial musculoskeletal system of five broad‐ and narrow‐headed glass eels using 3D‐reconstructions and simulated the glass eel's bite force using the data of the muscle reconstructions. We found that the variation in the musculoskeletal system of glass eels indeed reflects that of the yellow eels. Broader heads were related to larger jaw muscles, responsible for mouth closure. Accordingly, broad‐heads could generate higher bite forces than narrow‐headed glass eels. In addition, broader heads were associated with higher coronoid processes and shorter hyomandibulae, beneficial for dealing with higher mechanical loadings and consequently, harder prey. We, thus, show that head width variation in glass eels is related to musculoskeletal differences which, in turn, can affect feeding performance. As such, differences in prey preference can already take place the moment the eels start feeding, potentially leading to the dimorphism observed in the elver and yellow eel stage.     

Brain size,head size and behaviour of a passerine bird

A recent increase in comparative studies of the ecological and evolutionary consequences of brain size in birds and primates in particular have suggested that cognitive abilities constitute a central link. Surprisingly, there are hardly any intraspecific studies investigating how individuals differing in brain size behave, how such individuals are distributed and how brain size is related to life history and fitness components. Brain mass of the barn swallow Hirundo rustica was strongly predicted by external head volume, explaining 99.5% of the variance, allowing for repeatable estimates of head volume as a reflection of brain size. Repeatability of head volume within and between years was high, suggesting that measurement errors were small. In a 2 years study of 501 individual adult barn swallows, I showed that head volume differed between sexes and age classes, with yearlings having smaller and more variable heads than older individuals, and females having smaller and more variable heads than males. Large head volume was not a consequence of large body size, which was a poor predictor of head volume. Birds with large heads arrived early from spring migration, independent of sex and age, indicating that migratory performance may have an important cognitive component. Head volume significantly predicted capture date and recapture probability, suggesting that head volume is related to learning ability, although morphological traits such as wing length, aspect ratio and wing area were unimportant predictors. Intensity of defence of offspring increased with head volume in females, but not in males. Barn swallows with large heads aggregated in large colonies, suggesting that individuals with large heads were more common in socially complex environments. These results suggest that brain size is currently under natural and sexual selection, and that micro‐evolutionary processes affecting brain size can be studied under field conditions.