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Bill shape reflects divergent prey consumption for island compared to mainland American kestrels ( Falco sparverius )
Abstract
Feeding morphology permits animals to adapt to changing environments and is often under strong selection. We evaluated if bill shape varies according to differences in dietary prey taken across geographical ranges (North America, Central America, South America, and Caribbean islands) in a ubiquitous, New World raptor species, the American kestrel (Falco sparverius). Specifically, we predicted that bills in geographies where kestrels consumed a larger proportion of vertebrates would be shorter and wider, with a larger tomial tooth. We reviewed the literature on kestrel diets across their range and quantified potential differences in bill shape using geometric morphometric methods for 245 museum specimens. The literature review revealed that most prey consumed by kestrels in North, South, and Central America were invertebrates (51.6–69.1%), whereas Caribbean kestrels consumed mostly vertebrates (61.4%), most of which were reptiles (77.3%). Morphometric analyses agreed with these findings; bill shape differed for island versus mainland kestrels but not among mainland regions. Bill shape on islands reflected a more robust bill with a larger tomial tooth, but had a longer hook, which we suggest is adaptive for consumption of lizard prey, more available on islands due to reduced competition with other raptors compared to mainland regions.
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Morphological variation in biological structures may be driven by genetic and environmental factors, such as inter-and intraspecific competition for resources. In seabirds, although the bill is also involved in vocalization, olfaction, sexual selection and defence, the main drivers of high morphological plasticity in bill size and shape appear to relate primarily to diet and thus to niche differentiation. Here, we combined geometric morphometrics and comparisons of linear measurements as a precise tool for measuring shape variation in anatomical features, to investigate the differences among species and populations (island groups) in bill shape of three planktivorous petrels (Antarctic prion Pachyptila desolata, blue petrel Halobaena caerulea and thin-billed prion Pachyptila belcheri). Fieldwork was carried out in South Georgia (54° 0′ S, 38° 3′ W), Falkland (51° 42′ S, 57° 51′ W), Diego Ramírez (56° 31′ S, 68° 44′ W) and Kerguelen (49° 20′ S, 69° 20′ E) Islands, from 2010 to 2021. Results show that the bills of Antarctic prions were more robust and shorter, appropriate for filtering large amounts of small prey. Blue petrels and thin-billed prions had narrower and longer bills, effective for catching and tearing large single prey. Also, Antarctic prions and blue petrels from Kerguelen had longer and narrower bills than conspecifics from other colonies, which could potentially be explained by geographic variation in diet. In conclusion, prey availability and diversity appear to be important factors influencing variation in bill morphology. This study highlights the utility of geometric morphometrics for investigating bill shape variation in seabirds. Nevertheless, further studies are needed to better understand selective pressures leading to morphological variation of biological structures.
Tropical forests are renowned for their astonishing diversity of life, but the fundamental question of how many species occur in tropical forests remains unanswered. Using geographic range maps and data on species habitat associations, we determined that tropical forests harbor 62% of global terrestrial vertebrate species, more than twice the number found in any other terrestrial biome on Earth. Up to 29% of global vertebrate species are endemic to tropical forests, with more than 20% of these species at risk of extinction. Humid tropical forests (also known as tropical rainforests) and the Neotropics dominate as centers of species diversity, harboring more than 90% and nearly half of all tropical forest vertebrates, respectively. To maintain the biodiversity that underpins the ecosystem functions and services essential for human well‐being, we emphasize the critical importance of environmental policies aimed at reducing tropical deforestation and mitigating deleterious anthropogenic pressures on these imperiled ecosystems.
Body and head shape among fishes both vary between environments influenced by water velocity and across ontogeny. Although the shape changes associated with variation in average water velocity and ontogeny are well documented, few studies have tested for the interaction between these two variables (i.e., does ontogenetic shape variation differ between velocity environments). We use geometric morphometrics to characterize shape differences in Trichomycterus areolatus, a freshwater catfish found in high and low-velocity environments in Chile. We identify a significant interaction between velocity environment and body size (i.e., ontogeny). Ontogenetic patterns of shape change are consistent with other studies, but velocity environment differentially affects the ontogenetic trajectory of shape development in T. areolatus. Shape change over ontogeny appears more constrained in high-velocity environments compared to low-velocity environments.
Competition has long been recognized as a central force in shaping evolution, particularly through character displacement. Yet research on character displacement is biased, as it has focused almost exclusively on pairs of interacting species while ignoring multispecies interactions. Communities are seldom so simple that only pairs of species interact, and it is not clear whether inferences from pairwise interactions are sufficient to explain patterns of phenotypes in nature. Here, we test for character displacement in a natural system of freshwater fishes in western Mexico that contains up to four congeneric species of the genus Poeciliopsis. We analyzed body shape differences between populations with different numbers of competitors while accounting for confounding environmental variables. Surprisingly, we found evidence for convergent character displacement in populations of P. prolifica, P. viriosa, and P. latidens. We also found that the convergence in body shape was not consistently in the same direction, meaning that when three or more competitors co-occurred, we did not find more extreme body shapes compared with when there were only two competitors. Instead, when three or more competitors co-occurred, body shape was intermediate between the shape found with a pair of species and the shape found with no competitor present. This intermediate shape suggests that evolution in multispecies communities likely occurs in response to several competitors rather than to simple pairwise interactions. Overall, our results suggest that competition among multiple species is more complex than simple pairwise competitive interactions.
During the Palaeozoic, a diversification in modes of life occurred that included a wide range of predators. Major macroecological events include the Cambrian Explosion (including the Agronomic Substrate Revolution and the here introduced 'Ediacaran-Cambrian Mouthpart Armament'), the Great Ordovician Biodiversification Event, the Palaeozoic Plankton Revolution, the Siluro-Devonian Jaw Armament (newly introduced herein) and the Devonian Nekton Revolution. Here, we discuss the evolutionary advancement in oral equipment, i.e. the Palaeozoic evolution of mouthparts and jaws in a macroecological context. It appears that particularly the latest Neoproterozoic to Cambrian and the Silurian to Devonian were phases when important innovations in the evolution of oral structures occurred.
Natural selection often results in profound differences in body shape among populations from divergent selective environments. Predation is a well-studied driver of divergence, with predators having a strong effect on the evolution of prey body shape, especially for traits related to escape behavior. Comparative studies, both at the population level and between species, show that the presence or absence of predators can alter prey morphology. Although this pattern is well documented in various species or population pairs, few studies have tested for similar patterns of body shape evolution at multiple stages of divergence within a taxonomic group. Here, we examine morphological divergence associated with predation environment in the livebearing fish genus Brachyrhaphis. We compare differences in body shape between populations of B. rhabdophora from different predation environments to differences in body shape between B. roseni and B. terrabensis (sister species) from predator and predator free habitats, respectively. We found that in each lineage, shape differed between predation environments, consistent with the hypothesis that locomotor function is optimized for either steady swimming (predator free) or escape behavior (predator). Although differences in body shape were greatest between B. roseni and B. terrabensis, we found that much of the total morphological diversification between these species had already been achieved within B. rhabdophora (29% in females and 47% in males). Interestingly, at both levels of divergence we found that early in ontogenetic development, females differed in shape between predation environments; however, as females matured, their body shapes converged on a similar phenotype, likely due to the constraints of pregnancy. Finally, we found that body shape varies with body size in a similar way, regardless of predation environment, in each lineage. Our findings are important because they provide evidence that the same source of selection can drive similar phenotypic divergence independently at multiple divergence levels.
Presentamos el primer análisis cuantitativo de la dieta de Falco sparverius en el sur de Argentina. Identificamos 1169 ítems de presas en un total de 272 egagrópilas y siete restos de presas de cuatro sitios de estudio en la Patagonia. En esta área, F. sparverius se alimentó principalmente de escorpiones (Bothruridae), escarabajos (Coleoptera), hormigas (Formicidae), saltamontes (Acrididae), gorgojos (Curculionidae) y roedores (Rodentia). La dieta difirió significativamente entre los sitios de estudio en términos de la frecuencia de varios ítems entre las presas, pero no en términos de la biomasa. De acuerdo a la frecuencia, los insectos conformaron la mayor parte de la dieta en la mayoría de los sitios, pero los vertebrados contribuyeron más en términos de biomasa.
This short review points out some of the major physical problems faced by insects feeding on plants, and some of the kinds of morphological adaptations that have been noted to date. Major emphasis is given to two factors: the nature of the plant surface, and the difficulty of dealing with hard or tough food. The surface provides a great variety of terrains that require specialization for maximizing tenacity and agility, especially for small insects. It is suggested that natural enemies may provide significant selection for the relevant morphologies. The difficulty of feeding upon certain plant tissues is shown to be overcome in different ways by different herbivore groups. In the case of tough leaves for example, grasshopper mandible adaptations appear to have evolved to maximize efficiency of processing. On the other hand, in the case of caterpillars, mandible adaptations for tough food appear to be minimizing handling time with a concomitant reduction in risk of predation. Convergent evolution is shown in both grasshoppers and caterpillars for dealing with leaves of similar design and an example is given of rapid evolution of mouthpart morphology in response to differences in host characteristics. These examples are used to indicate the risks of using such characters in establishing phylogenetic relationships. Finally, it is pointed out that plant chemical qualities and plant ecological factors can influence insect morphological features.
Superimposition methods for comparing configurations of landmarks in two or more specimens are reviewed. These methods show
differences in shape among specimens as residuals after rotation, translation, and scaling them so that they align as well
as possible. A new method is presented that generalizes Siegel and Benson's (1982) resistant-fit theta-rho analysis so that
more than two objects can be compared at the same time. Both least-squares and resistant-fit approaches are generalized to
allow for affine transformations (uniform shape change). The methods are compared, using artificial data and data on 18 landmarks
on the wings of 127 species of North American mosquitoes. Graphical techniques are also presented to help summarize the patterns
of differences in shape among the objects being compared.
Objective: To carry out a comparative study of the mouthparts and the diet of eight isopod fish
parasites. Methods: A description of the mouthparts, together with their diet nature, was derived
both by direct observation and an interpretation of their structure. The three-dimensional study of
the mouthparts of the isopod parasites was done to reveal their morphology. Results: Observations
revealed that these species are wholly carnivorous. Result shows how they are adapted for tearing
and bolting fish food material. The mouthparts consist of a labrum, paragnaths, paired mandibles,
maxillules, maxillae and maxillipeds. The labrum and the paragnaths are the least developed but
peculiarly the mandibles are asymmetrical, large, stout and highly modified. The analysis of gut
contents indicated that Cymothoa indica and Joryma brachysoma diet consisted of 90% to 95% of
animal blood. The diet of Mothocya renardi, Ryukyua circularis and Joryma hilsae were mainly
composed of mucus (80%-90%). The stomach contents of Nerocila phaeopleura and Nerocila
sundaica, were dominated by body muscles (75%-83%). Conclusions: The possible functions of
the mouthparts, especially in feeding are discussed in light of their structure. The morphology of
the mouthparts of the isopod parasites are heavily modified with their feeding behavior.
El dimorfismo sexual en el tamaño se presenta a través del reino animal, y sus causas e implicancias ecológicas y evolutivas han sido intensamente estudiadas. Las diferencias específicas de los sexos en la curvatura del pico son conocidas en varias especies des aves, incluyendo algunos picaflores tropicales. A pesar de la importancia de la forma del pico para forrajear, los estudios comparativos de dimorfismo sexual en la forma del pico son escasos. Cuantificamos la forma del pico en dos especies de picaflores de la zona templada, Archilocus alexandri y A. colubris, y comparamos los patrones de dimorfismo sexual en la forma. Varios indices de curvatura del pico comúnmente usados arrojaron resultados contrastantes: uno encontró diferencias entre especies y sexos, otro no identificó diferencias en la curvatura y un enfoque de círculo-curvatura reveló diferencias en la forma entre especies y entre los sexos. En contraste, los métodos de morfometría geométrica basados en puntos de referencia identificaron diferencias significativas en el dimorfismo sexual en la forma y también revelaron que A. colubris exhibió diferencias sexuales significativas en la forma, mientras que A. alexandri no. Las hembras de A. colubris exhibieron curvaturas de los picos relativamente mayores que los machos, un patrón que concuerda con observaciones de algunos picaflores tropicales. Aunque las causas de las diferencias en el dimorfismo en la forma del pico entre A. alexandri y A. colubris permanecen poco claras, hipotetizamos que podrían ser atribuibles a diferencias en la estructura de la comunidad en la cual cada especic cria y a la interacción entre competencia inter- e intra-específica por los recursos en estas comunidades. Finalmente, recomendamos que los estudios futuros de la forma del pico incluyan enfoques de morfometria geométrica debido a que son más adecuados que los enfoques univariados para identificar diferencias más complejas en la forma dentro de especies y entre especies.
Predation can cause morphological divergence among populations, while ontogeny and sex often determine much of morphological diversity among individuals. We used geometric morphometrics to characterize body shape in the livebearing fish Brachyrhaphis rhabdophora to test for interactions between these three major shape-determining factors. We assessed shape variation between juveniles and adults of both sexes, and among adults for populations from high- and low-predation areas. Shape differed significantly between predation regimes for all juveniles regardless of sex. As males grew and matured into adults, ontogenetic shape trajectories were parallel, thus maintaining shape differences in adult males between predation environments. However, shape of adult females between predation environments followed a different pattern. As females grew and matured, ontogenetic shape trajectories converged so that shape differences were less pronounced between mature females in predator and nonpredator environments. Convergence in female body shape may indicate a trade-off between optimal shape for predator evasion versus shape required for the livebearing mode of reproduction.
The vangas of Madagascar exhibit extreme diversity in morphology and ecology. Recent studies have shown that several other Malagasy species also are part of this endemic radiation, even as the monophyly of the clade remains in question. Using DNA sequences from 13 genes and representatives of all 15 vanga genera, we find strong support for the monophyly of the Malagasy vangids and their inclusion in a family along with six aberrant genera of shrike-like corvoids distributed in Asia and Africa. Biogeographic reconstructions of these lineages include both Asia and Africa as possible dispersal routes to Madagascar. To study patterns of speciation through time, we introduce a method that can accommodate phylogenetically non-random patterns of incomplete taxon sampling in diversification studies. We demonstrate that speciation rates in vangas decreased dramatically through time following the colonization of Madagascar. Foraging strategies of these birds show remarkable congruence with phylogenetic relationships, indicating that adaptations to feeding specializations played a role in the diversification of these birds. Vangas fit the model of an 'adaptive radiation' in that they show an explosive burst of speciation soon after colonization, increased diversification into novel niches and extraordinary ecomorphological diversity.
We are now in the midst of a revolution in morphometric methodology. The new approaches are more effective in capturing information about the shape of an organism and result in more powerful statistical procedures for testing for differences in s shape. They are also more effective in enabling a researcher to visualize differences in shape and in suggesting simple traditional measurements that could be used in future studies. In this review we emphasize applications to exploratory studies in taxonomy and evolution.
Charles Darwin's travels on HMS Beagle taught him that islands are an important source of evidence for evolution. Because many islands are young and have relatively few species, evolutionary adaptation and species proliferation are obvious and easy to study. In addition, the geographical isolation of many islands has allowed evolution to take its own course, free of influence from other areas, resulting in unusual faunas and floras, often unlike those found anywhere else. For these reasons, island research provides valuable insights into speciation and adaptive radiation, and into the relative importance of contingency and determinism in evolutionary diversification.
Unambiguous examples of ecological causes of animal sexual dimorphism are rare. Here we present evidence for ecological causation of sexual dimorphism in the bill morphology of a hummingbird, the purple-throated carib. This hummingbird is the sole pollinator of two Heliconia species whose flowers correspond to the bills of either males or females. Each sex feeds most quickly at the flower species approximating its bill dimensions, which supports the hypothesis that floral specialization has driven the evolution of bill dimorphism. Further evidence for ecological causation of sexual dimorphism was provided by a geographic replacement of one Heliconia species by the other and the subsequent development of a floral dimorphism, with one floral morph matching the bills of males and the other of females.
Phylogenetic relationships of the family Vangidae and representatives of several other passeriform families were inferred from 882 base positions of mitochondrial DNA sequences of 12S and 16S rRNA genes. Results indicated the monophyly of the Vangidae, which includes the genus Tylas, hitherto often placed in the family Pycnonotidae. Our results also revealed the Malagasy endemic Newtonia, a genus never previously assigned to the Vangidae, to be a member of this family. These results suggest the occurrence of an extensive in situ radiation of this family within Madagascar, and that the extant high diversity of this family is not the result of multiple colonizations from outside. The extremely high morphological and ecological diversification of the family seems to have been enhanced through the use and ultimate occupancy of vacant niches in this island.
Sexual dimorphism in bill morphology and body size of the Caribbean purple-throated carib hummingbird is associated with a reversal in floral dimorphism of its Heliconia food plants. This hummingbird is the sole pollinator of H. caribaea and H. bihai, with flowers of the former corresponding to the short, straight bills of males, the larger sex, and flowers of the latter corresponding to the long, curved bills of females. On St. Lucia, H. bihai compensates for the rarity of H. caribaea by evolving a second color morph with flowers that match the bills of males, whereas on Dominica, H. caribaea evolves a second color morph with flowers that match the bills of females. The nectar rewards of all Heliconia morphs are consistent with each sex's choice of the morph that corresponds to its bill morphology and energy requirements, supporting the hypothesis that feeding preferences have driven their coadaptation.
Geometric morphometrics is widely used to quantify morphological variation between biological specimens, but the fundamental influence of operator bias on data reproducibility is rarely considered, particularly in studies using photographs of live animals taken under field conditions. We examined this using four independent operators that applied an identical landmarking scheme to replicate photographs of 291 live Atlantic salmon ( Salmo salar L.) from two rivers. Using repeated measures tests, we found significant inter-operator differences in mean body shape, suggesting that the operators introduced a systematic error despite following the same landmarking scheme. No significant differences were detected when the landmarking process was repeated by the same operator on a random subset of photographs. Importantly, in spite of significant operator bias, small but statistically significant morphological differences between fish from the two rivers were found consistently by all operators. Pairwise tests of angles of vectors of shape change showed that these between-river differences in body shape were analogous across operator datasets, suggesting a general reproducibility of findings obtained by geometric morphometric studies. In contrast, merging landmark data when fish from each river are digitised by different operators had a significant impact on downstream analyses, highlighting an intrinsic risk of bias. Overall, we show that, even when significant inter-operator error is introduced during digitisation, following an identical landmarking scheme can identify morphological differences between populations. This study indicates that operators digitising at least a sub-set of all data groups of interest may be an effective way of mitigating inter-operator error and potentially enabling data sharing.
Superimposition methods for comparing configurations of landmarks in two or more specimens are reviewed. These methods show differences in shape among specimens as residuals after rotation, translation, and scaling them so that they align as well as possible. A new method is presented that generalizes Siegel and Benson's (1982) resistant-fit theta-rho analysis so that more than two objects can be compared at the same time. Both least-squares and resistant-fit approaches are generalized to allow for affine transformations (uniform shape change). The methods are compared, using artificial data and data on 18 landmarks on the wings of 127 species of North American mosquitoes. Graphical techniques are also presented to help summarize the patterns of differences in shape among the objects being compared.
Evolution typically occurs in response to a suite of selective pressures. Yet, many studies of natural selection in the wild only investigate a single selective agent at a time. This can be problematic when selective agents act in non- additive ways. Here we evaluate the interactive effects of diet and predation on the evolution of body shape in the cyprinid fish Utah chub (Gila atraria). We found that both factors and the interaction between them are significant predictors of body shape. This interaction is likely a result of different forms of selective pressures, where predation is a stabilizing selective force and diet is a disruptive selective force. Utah chub with more herbivorous diets exhibited a distended abdomen and shorter, shallower caudal peduncle relative to Utah chub with more carnivorous diets. These shapes correspond with common patterns of ecological divergence between limnetic and benthic morphotypes, and likely evolved due to diet specialization. Utah chub from predation environments are generally more streamlined and have larger caudal peduncles than Utah chub in non-predation environments, which may be an adaptive response to allow greater burst-swimming capability. However, Utah chub in predation environments also have deeper bodies than more-carnivorous Utah chub in non-predation environments. Therefore, Utah chub in predation environments exhibit an intermediate phenotype with an intermediate depth but larger caudal peduncle. Our results suggest that predation constrains the range of body shape variation that is expressed in response to diet. Hence, the interactive effects
The decomposition of deformations by principal warps is demonstrated. The method is extended to deal with curving edges between landmarks. This formulation is related to other applications of splines current in computer vision. How they might aid in the extraction of features for analysis, comparison, and diagnosis of biological and medical images is indicated.
Modern raptors feed predominantly on animals and exhibit a wide range of feeding behaviors and prey preferences. The following six ecomorphs, based on feeding preferences served to classify members from all families of diurnal birds of prey: (1) avivores, which feed primarily on avian prey; (2) mammalivores, which feed primarily on mammalian prey; (3) herpetivores, which feed primarily on reptiles and amphibians; (4) piscivores, which feed primarily on fishes; (5) scavengers, which are primarily carrion feeders; and (6) generalists, which are opportunistic feeders on all of the above prey and on invertebrates. A series of skull-based indices were created to reflect functional aspects of prey preferences-calculated from measures of the cranium, maxilla (beak) and mandible-and were analyzed separately and with all units combined. The same indices were determined for 13 raptor species, including some extinct genera unearthed from the Pleistocene Rancho La Brea tar pits in California. A discriminant function analysis was used to assess the functional predictability of the indices for the extant raptors and also to predict the ecomorphs of the fossils. Results indicate that all of the indices combined provide the best predictor of prey preferences, especially for scavengers and avivores. Cranial and mandibular indices provide a better resolution than maxillary indices when considering only isolated elements. Eleven fossil species were assigned to their dietary categories with high probabilities and two were less definitely classified.
The main potential avian predators, pearly-eyed thrasher Margarops fuscatus and American kestrel Falco sparverius, were studied on Anguilla, St. Martin and St. Eustatius. Anoles on Anguilla were quantitatively unaffected by avian predation; thrashers did not eat vertebrates, and the kestrel density was too low to have a significant impact on the lizard population. Thrashers in moist habitat on St. Martin ate vertebrates including anoles, and thrasher predation potentially accounted for the anole decline to the dry-season abundance low. An unanticipated thrasher niche shift was discovered. Thrasher body size and diet appear to be related to the abundance of the congeneric M. fuscus. One Anguilla, where M. fuscus was absent, M. fuscatus was smaller and had a non-vertebrate diet. On St. Eustatius, M. fusces was abundant while M. fuscatus was larger and preyed on anoles. M. fuscus was rare on St. Martin, where M. fuscatus had an intermediate body size and a diet comparable to that on St. Eustatius. -from Authors
Gape morphology has been linked to feeding and breeding ecology in raptors, according to the ingestion rate hypothesis. Mammal feeding raptors have larger gapes, allowing them to ingest prey more rapidly than bird feeders, which have evolved smaller average body sizes and gapes to capture more agile prey. One highly derived raptor, however, the Bat Hawk (Macheiramphus alcinus), specializes on colonial bats and swiftlets concentrated daily in a limited temporal window by capturing and swallowing them whole in flight. We hypothesized that the gape of the Bat Hawk evolved to feed rapidly on agile vertebrates limited temporally. We predicted that the gape of the Bat Hawk would be significantly larger than the gape of other raptors, more closely resembling the gapes of swifts (Apodidae), swallows (Hirundinidae), and goatsuckers (Caprimulgiformes). We measured gape area of the lower mandible in museum specimens representing 138 bird species in six orders. We also compared gape area by prey type in over 100 raptor species in three orders. We predicted that insectivorous raptors would exhibit gapes similar to mammal feeders but would differ from bird feeders because insects are not agile prey. The Bat Hawk had the largest gape of any raptor and more closely resembled the gape of insectivorous birds, which also swallow prey whole in flight. The evolution of an enlarged gape may have permitted the Bat Hawk to exploit a previously unrealized ecological niche. Gapes of bird feeding raptors were smaller than in mammal and insect feeders, supporting the ingestion rate hypothesis.
A powerful metaphor for evolutionary diversification is a tree. A typical spreading tree has a single trunk, two or more major branches, several minor branches and many twigs. Its foundations--its roots and the remnants of the original seed--remain hidden from sight. The metaphor applies to the full set of living things on Earth--"the tree of life"--as well as to small groups of species, whether they are marsupial mammals or Hawaiian Drosophila. We can think of the typical crown of a tree as resembling the shape of an umbrella, with twig-bearing branches radiating in all directions (Figure 2, top). The ends of the twigs represent organisms adapted to separate microenvironments; the tree overall represents an adaptive radiation. Darwin's finches are a prime example of an adaptive radiation. Fourteen or perhaps 15 species, all derived from a common ancestor, occupy individual ecological niches to
The technique used by birds of prey to kill their quarry has rarely been described in
detail. Although it is common to watch a raptor hunting, distance and natura1
obstacles normally prevent the observer from observing what actually happens to the
prey after being grasped by the bird. Therefore, we studied the predatory sequence
and killing technique displayed by a sample of ten wild Eurasian Kestrels temporarily
held in captivity because of various injuries. They were offered individually either a
laboratory mouse or young laboratory rat. Direct observation was complemented by
necropsy of the prey confirming that bill strikes frequently observed after grasping
the prey are not used to kill it but merely to damage its centra1 nervous system
thereby minimising escape attempts. Furthermore, talons are not used as weapons at
all, but only as tools that elongate the toes to hold the quarry more firmly. In
contrast, several signs indicating death by suffocation were regularly found and these
were confirmed by inspection of rodents sacrificed by CO2. We also discuss a
hypothesis explaining the adaptive and evolutionary significance of the kestrel's use
of toe pressure to kill the prey rather than using its obvious weapons.
Predation can drive morphological divergence in prey populations, although examples of divergent selection are typically limited to nonreproductive individuals. In livebearing females, shape often changes drastically during pregnancy, reducing speed and mobility and enhancing susceptibility to predation. In the present study, we document morphological divergence among populations of nonreproductive female livebearing fish (Brachyrhaphis rhabdophora) in predator and nonpredator environments. We then test the hypothesis that shape differences among nonreproductive females are maintained among reproductive females between predator and nonpredator environments. Nonreproductive females in predator environments had larger caudal regions and more fusiform bodies than females in nonpredator environments; traits that are associated with burst speed in fish. Shape differences were maintained in reproductive females, although the magnitude of this difference declined relative to nonreproductive females, suggesting morphological convergence during pregnancy. Phenotypic change vector analysis revealed that females in predator environments became more similar to females in nonpredator environments in the transition from nonreproductive to reproductive. Furthermore, the level of reproductive allocation affected shape similarly between predator environments. These results suggest a life-history constraint on morphology, in which predator-driven morphological divergence among nonreproductive B. rhabdophora is not maintained at the same level during pregnancy. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104, 386–392.
Most raptors take large prey for their size compared to other birds, but tear the prey apart into small morsels before swallowing. Little is known about how the efficiency of this prey handling varies among raptors, and how it relates to their feeding niches, diets and gape dimensions. We offered 202 mammalian and 224 avian prey items to 37 wild raptors kept temporarily in captivity, representing ten species and three orders. Feeding efficiency was measured as the proportion of a prey item that was ingested. The proportion of a prey item ingested was larger for mammalian than for avian prey, declined with prey size, increased with raptor size, and was larger for typical vole feeders (with shorter and wider bills) than typical bird feeders (with longer and narrower bills). The proportion of a prey item ingested was not related to raptor sex when controlling for focal raptor body mass. The probability that the head of a prey was ingested was higher if the prey was a mammal than if it was a bird, whereas the opposite was the case for the entrails of a prey. The results suggest that the traditional use of prey remains to estimate raptor diets may lead to severe biases, the magnitude of which would depend on prey type and size, raptor sex, species and order, and raptor feeding niche. Failure to correct for uneaten remains of a prey would lead to overestimating the profitability of large prey, and in particular of large avian prey. The results are consistent with the idea that vole feeders can afford to have a well-developed digestive tract, and thus swallow prey in large pieces, because they pounce on prey from above. Bird feeders, however, must ingest food in smaller pieces because they have smaller guts as a result of selection to keep the body mass low to capture agile prey.
Woodlice (Isopoda: Oniscoidea) are well defended by a heavy armour and many species have noxious secretions. Oniscophagy (feeding on woodlice) may be an important part of the biology of at least some members of the spider genus Dysdera (Dysderidae). Yet there little is known about the diet and possible specializations for feeding on woodlice in these spiders. Dysdera has unusual variability in mouthpart morphology, which may be related to the diet. Here, we investigate five species of Dysdera and consider the relationship between mouthpart morphology, capture frequency and prey-capture behaviour. We show that species with unmodified chelicerae readily capture a variety of arthropods, but refuse woodlice as prey, whereas species with modified chelicerae feed on woodlice and reject most of the other prey. Among the oniscophagic species, the grasping methods used during the capture of woodlice varied. Species with elongate chelicerae inserted one chelicera into the soft ventral side of the woodlouse and placed the other chelicera on the dorsal side of the woodlouse. Species with dorsally concave chelicerae quickly tucked their chelicerae under the woodlouse and bit the ventral side of the woodlouse's body. Species with flattened chelicerae inserted their chelicerae between the sclerites into the armour of the woodlouse.
Recent evidence of genetic homogeneity across morphologically diverse crossbill taxa Loxia spp. suggests that strong directional natural selection sustains morphological differences. If so, we would expect that, in sympatry, persistent crossbill morphs will be associated with the ecological circumstances that select for particular features. Here we report on a field study of niche differentiation in sympatric crossbills, showing correlation between bill size and habitat use, foraging and movements. In Deeside, north-east Scotland, crossbills occupied three ecological niches. Small-billed birds L. curvirostra were itinerant and migratory. They switched between conifer species in relation to the phenology of cone ripeness, feeding on spruce or larch from summer through winter and Scots Pine Pinus sylvestris in spring and early summer. Large-billed birds L. pytyopsittacus were more sedentary, feeding on pine seed year round in semi-natural pine forest. Birds with intermediate bills L. scotica were also sedentary but switched seasonally between conifer species. Deeside crossbills thus occupied three niches in line with the current designation of three species, but in the study years (1990–1997) there was no shortage of conifer seed and no evidence of strong selection for optimal bill size. Bill sizes did not fall precisely into three distinct modes so other factors were involved. These could have included the immigration of two sizes of L. curvirostra, and introgression (and possibly phenotypic plasticity) amongst the more sedentary larger-billed birds. The origin of L. scotica is discussed within the context of novel habitat, introgression, niche shift and competition for pine seed.
Rodents are important components of nearly every terrestrial ecosystem and display considerable ecological diversity. Nevertheless, a lack of data on the ecomorphology of rodents has led to them being largely overlooked in palaeoecological reconstructions. Here, geometric and linear morphometrics are used to examine how cranial and dental shapes reflect the diets of living rodent species. Although most rodents are omnivores or generalist herbivores, some species have evolved highly specialized carnivorous, insectivorous, and herbivorous diets. Results show that living rodents with similar diets display convergent morphology, despite their independent evolutionary histories. Carnivores have relatively elongate incisors, elongate and narrow incisor blades, orthodont incisor angles, reduced cheek tooth areas, and enlarged temporal fossae. Insectivores display relatively degenerate dentition, elongate rostra, narrow and thin zygomatic arches, and smaller temporal fossae. Herbivores are characterized by relatively broader incisor blades, longer molar tooth rows, larger cheek tooth areas, wider skull and rostrum, thicker and broader zygomatic arches, and larger temporal fossae. These results suggest that cranial and dental morphology can be used to accurately infer extinct rodent diets regardless of ancestry. Application to extinct beavers suggests that most had highly specialized herbivorous diets.
Compared to other birds, most raptors take large prey for their size, and feeding bouts are extended. However, ingestion rate has largely been overlooked as a constraint in raptors′ foraging and breeding ecology. We measured ingestion rate by offering avian and mammalian prey to eighteen wild raptors temporarily kept in captivity, representing seven species and three orders. Ingestion rate was higher for small than for large prey, higher for mammalian than for avian prey, higher for large than for small raptors, and higher for wide-gaped than for narrow-gaped raptors. Mammalian prey were ingested faster by raptors belonging to species with mainly mammals in their diet than by raptors with mainly birds in their diet, but the drop in ingestion rate with increasing prey size was more rapid for the former than for the latter. We argue that the separate sex roles found in raptors, i.e. the male hunting and the female feeding the young, is a solution of the conflict between the prolonged feeding bouts at the nest, and the benefit of rapid resumption of hunting in general, and rapid return to the previous capture site in particular (the prey size hypothesis). Thus, the sex roles differ more when prey takes longer to feed, i.e. from insects to mammals to birds. We then argue that the reversed sexual size dimorphism in raptors, i.e. smaller males than females, results from a conflict between the benefit of being small during breeding to capture the smallest items with the highest ingestion rate among these agile prey types (mammals and bird), and the benefit of being large outside the breeding season to ensure survival by being able to include large items in the diet when small items are scarce (the ingestion rate hypothesis). This hypothesis explains the observed variation in reversed sexual size dimorphism among raptors in relation to size and type of prey, i.e. increasing RSD from insects to mammals to birds as prey.
Phylogenetic relationships among Neotropical honeycreepers were studied by using cytochrome b sequence data. Although honeycreepers were once placed in their own family (Coerebidae), these species did not form a monophyletic group in any of the phylogenies in this study. Thus, our results indicate that nectar-feeding evolved independently multiple times among these birds. The Neotropical honeycreepers are best considered disparate members of a larger radiation of tanagers and finches, in which bill size and shape, and associated feeding behaviors have changed frequently to fill a variety of niches. Our phylogenies also provide specific taxonomic recommendations on the placement of each honeycreeper genus. The use of the taxon Coerebidae is no longer warranted given the well-supported relationships between different honeycreeper species and a variety of tanagers and finches.
Differences in predation intensity experienced by organisms can lead to divergent natural selection, driving evolutionary change. Western mosquitofish (Gambusia affinis) exhibit larger caudal regions and higher burst-swimming capabilities when coexisting with higher densities of predatory fish. It is hypothesized that a trade-off between steady (constant-speed cruising; important for acquiring resources) and unsteady (rapid bursts and turns; important for escaping predators) locomotion, combined with divergent selection on locomotor performance (favouring steady swimming in high-competition scenarios of low-predation environments, but unsteady swimming in high-predation localities) has caused such phenotypic divergence. Here, I found that morphological differences had a strong genetic basis, and low-predation fish required less hydromechanical power during steady swimming, leading to increased endurance. I further found individual-level support for cause-and-effect relationships between morphology, swimming kinematics and endurance. Results indicate that mosquitofish populations inhabiting low-predation environments have evolved increased steady-swimming abilities via stiffer bodies, larger anterior body/head regions, smaller caudal regions and greater three-dimensional streamlining.
Raptors exhibit a diversity of strategies to procure their prey but ultimately kill using their beaks and/or talons. Thus, bite and grip forces are ecologically important variables that have direct survival implications. Whereas hawks rely primarily on their feet for killing prey, falcons tend to employ their beaks. Consequently, falcons are expected to achieve relatively greater bite forces, and hawks are expected to generate relatively greater grip forces. Force estimates predicted from musculoskeletal morphology in a previous study indicated that falcons (Falco spp.) possess greater jaw force capabilities than accipiters (Accipiter spp.) but there were no clear differences in predicted grip-force capacity outside of differences in scaling. The objective of this study was to complement those results with measurements of in vivo forces by inducing captive and wild accipiters and falcons to bite and grasp force transducers. Bite force increased isometrically in both groups whereas grip force tended toward positive allometry. After adjusting for body mass, falcons produced greater bite forces, and accipiters produced greater grip forces. Thus, previous anatomical estimates of forces predicted the expected direction and magnitude of differences in bite forces but the overall greater in vivo grip forces of accipiters deviated from the pattern obtained from biomechanical estimates. Although the scaling relationships were similar between data sets, forces generated by live birds were consistently lower than those predicted from biomechanics. Estimated and in vivo jaw and digital forces were nevertheless correlated, and therefore provide an important link between morphology and killing behavior in these raptors.
When multiple groups of organisms experience similar environmental gradients, their patterns of differentiation might exhibit both shared and unique features. Here, we investigated the relative importance of three factors in generating body shape variation in a livebearing fish, Gambusia caymanensis, inhabiting the Cayman Islands: (i) shared patterns of divergent selection between predator regimes (presence/absence of piscivorous fish) driving replicated morphological differentiation, (ii) historical island effects yielding different morphologies across the three islands and (iii) unique effects of predation on morphological differentiation within each island. Shared effects of predation proved much more important than historical or unique effects. Populations coexisting with piscivorous fish exhibited larger caudal regions and smaller heads than conspecifics found in the absence of predatory fish. These results match a priori predictions, and mirror recent findings in a number of fish species, suggesting predation might often drive predictable morphological trends in disparate fishes. However, interestingly, the sexes achieved this morphological pattern through different means: head depth, caudal peduncle length and depth in males; head length, caudal peduncle depth in females. In G. caymanensis, we quantitatively confirmed that predation intensity represents a primary driver of body shape differentiation.
Restricted maximum likelihood (REML) is now well established as a method for estimating the parameters of the general Gaussian linear model with a structured covariance matrix, in particular for mixed linear models. Conventionally, estimates of precision and inference for fixed effects are based on their asymptotic distribution, which is known to be inadequate for some small-sample problems. In this paper, we present a scaled Wald statistic, together with an F approximation to its sampling distribution, that is shown to perform well in a range of small sample settings. The statistic uses an adjusted estimator of the covariance matrix that has reduced small sample bias. This approach has the advantage that it reproduces both the statistics and F distributions in those settings where the latter is exact, namely for Hotelling T2 type statistics and for analysis of variance F-ratios. The performance of the modified statistics is assessed through simulation studies of four different REML analyses and the methods are illustrated using three examples.
Evolution can be predicted in the short term from a knowledge of selection and inheritance. However, in the long term evolution is unpredictable because environments, which determine the directions and magnitudes of selection coefficients, fluctuate unpredictably. These two features of evolution, the predictable and unpredictable, are demonstrated in a study of two populations of Darwin's finches on the Galápagos island of Daphne Major. From 1972 to 2001, Geospiza fortis (medium ground finch) and Geospiza scandens (cactus finch) changed several times in body size and two beak traits. Natural selection occurred frequently in both species and varied from unidirectional to oscillating, episodic to gradual. Hybridization occurred repeatedly though rarely, resulting in elevated phenotypic variances in G. scandens and a change in beak shape. The phenotypic states of both species at the end of the 30-year study could not have been predicted at the beginning. Continuous, long-term studies are needed to detect and interpret rare but important events and nonuniform evolutionary change.