Beak Size Research Articles

Global analysis of acoustic frequency characteristics in birds.

Animal communication plays a crucial role in biology, yet the wide variability in vocalizations is not fully understood. Previous studies in birds have been limited in taxonomic and analytical breadth. Here, we analyse an extensive dataset of >140 000 recordings of vocalizations from 8450 bird species, representing nearly every avian order and family, under a structural causal model framework, to explore the influence of eco-evolutionary traits on acoustic frequency characteristics. We find that body mass, beak size, habitat associations and geography influence acoustic frequency characteristics, with varying degrees of interaction with song acquisition type. We find no evidence for the influence of vegetation density, sexual dimorphism, range size and competition on our measures of acoustic frequency characteristics. Our results, built on decades of researchers' empirical observations collected across the globe, provide a new breadth of evidence about how eco-evolutionary processes shape bird communication.

Birds optimize fruit size consumed near their geographic range limits.

Animals can adjust their diet to maximize energy or nutritional intake. For example, birds often target fruits that match their beak size because those fruits can be consumed more efficiently. We hypothesized that pressure to optimize diet-measured as matching between fruit and beak size-increases under stressful environments, such as those that determine species' range edges. Using fruit-consumption and trait information for 97 frugivorous bird and 831 plant species across six continents, we demonstrate that birds feed more frequently on closely size-matched fruits near their geographic range limits. This pattern was particularly strong for highly frugivorous birds, whereas opportunistic frugivores showed no such tendency. These findings highlight how frugivore interactions might respond to stressful conditions and reveal that trait matching may not predict resource use consistently.

Body size shapes song in honeyeaters.

Birdsongs are among the most distinctive animal signals. Their evolution is thought to be shaped simultaneously by habitat structure and by the constraints of morphology. Habitat structure affects song transmission and detectability, thus influencing song (the acoustic adaptation hypothesis), while body size and beak size and shape necessarily constrain song characteristics (the morphological constraint hypothesis). Yet, support for the acoustic adaptation and morphological constraint hypotheses remains equivocal, and their simultaneous examination is infrequent. Using a phenotypically diverse Australasian bird clade, the honeyeaters (Aves: Meliphagidae), we compile a dataset consisting of song, environmental, and morphological variables for 163 species and jointly examine predictions of these two hypotheses. Overall, we find that body size constrains song frequency and pace in honeyeaters. Although habitat type and environmental temperature influence aspects of song, that influence is indirect, likely via effects of environmental variation on body size, with some evidence that elevation constrains the evolution of song peak frequency. Our results demonstrate that morphology has an overwhelming influence on birdsong, in support of the morphological constraint hypothesis, with the environment playing a secondary role generally via body size rather than habitat structure. These results suggest that changing body size (a consequence of both global effects such as climate change and local effects such as habitat transformation) will substantially influence the nature of birdsong.

Investigating the teratogenic potential of diclofenac sodium on chick embryos: A warning for pregnant women

Teratogenic and embryotoxic effect of diclofenac sodium (DS) on different developmental stages of the chick-embryos was investigated by examining different parameters such as its mortality rate, hatching, morphological measurements, weighing its internal organs and calculation of different indices. Experiment was divided into four trials with different dose (0.1 mL, 0.2 mL, 0.3 mL in groups A, B, and C, respectively and group D received 0.3 mL saline solution (0.9% NaCl) and group E remained un-injected) administration and observation. Results of first and second trial showed statistically (p<0.01) significant difference in bodyweight, body-length, forelimb and hindlimb length between experimental and control groups. In third trial, diclofenac sodium administration showed a statistically (p<0.01) significant difference in the bodyweight, body-length, forelimb, hindlimb length, liver weight, egg weight (EE ratio) and kidney somatic index (KSI). The beak-size, heart weight, kidney weight, cardiac somatic index (CSI) and hepato somatic index (HSI) were not significant (p>0.05) when compared with the control groups. In trial 4, forelimb, hindlimb length, heart weight, CSI and HSI were statistically (p<0.01) significant. Body-length and liver weight were significant (p<0.05). While bodyweight, beak size, kidney weight and KSI were non-significant (p>0.05). The mortality rate was increased with increase dose of DS and also affected the hatching. DS effect on chick embryos can be applied to humans because the early development of mammals and birds are closely related. So, it was concluded that DS should be used with caution during pregnancy especially during first trimester of pregnancy.

From beak to fruit: An Asian hornbill database for frugivory and seed dispersal research

Hornbills (family Bucerotidae) are important frugivores and seed dispersers of Asian tropical forests. While much is known about the diets and their role as seed dispersers at the local or landscape scale, the lack of a comprehensive, standardised, and digital database on hornbill frugivory and seed dispersal interactions hampers broader and comparative studies across hornbill species and with other avian frugivore groups. To fill this crucial gap, we compiled a species-level database of plant-hornbill interactions in tropical Asia from primary sources throughout the literature; the database consisting of 2365 interaction records for 26 Asian and New Guinea hornbill species (out of the 32, or 80% of, species), a testament to the efforts of numerous hornbill biologists and ecologists. Fruits or seeds from 471 plant species, 240 genera and 84 families were reported to be consumed. Significant gaps in data coverage, however, remain. Rarefaction curves based on interaction records are not strongly saturating, revealing the scale of the “Eltonian shortfall”, that is, the paucity of species interaction information for many species. Our database also reveals a dearth in information on seed fate of hornbill consumed fruits and seeds. While approximately 58% of the interaction records provided were implied or reported to be associated with seed removal, only about 26% and 1% of studies provide any indication of seed deposition and post-depositional seed viability, respectively. To conclude, we briefly illustrate the value of the dataset for investigating broad patterns in hornbill frugivory. By combining our dataset with complementary hornbill beak and plant trait data, we show that hornbills consume and potentially disperse seeds of many plant species that would be beyond the physical capabilities of other avian frugivores. We were also able to recover a positive relationships between hornbill beak sizes and the size of the fruits and seeds they consumed though the patterns were generally weak, possibly due to the low sample size and afore-mentioned gaps in data coverage. In light of these data gaps, it is clear that more hornbill frugivory and seed dispersal research is needed, and we hope that our database will be a valuable, central resource for future hornbill research.

From microcosm to macrocosm: adaptive radiation of Darwin’s finches

Abstract In this Perspective we show the value of studying living organisms in the field to understand their history. Darwin’s finches are an iconic example of the early stages of speciation in a young adaptive radiation that produced 18 species in little more than a million years. The question they pose is how and why so many species originated and diversified rapidly. A long-term study of four species of finches on the small island of Daphne Major, combined with genomic investigations, provide some answers in terms of extrinsic and intrinsic factors. Beak size and shape, as well as body size, are key heritable features involved in both ecological and reproductive isolation, and their evolution by natural selection was caused by competitor species during prolonged droughts. Introgressive hybridization of related species is rare but recurring, apparently widespread, increases genetic variation, and does not incur a fitness cost. Hybridization can produce a new species. We use a phylogeny based on whole genome sequences of the four finches to infer morphological transitions in their radiation. Several lines of evidence indicate that some species are missing from the early phase of the radiation due to extinction. Combining these results, we re-cast the classical allopatry-then-sympatry theory of adaptive radiation as a competition-selection-hybridization process that generates a diversity of species.

Community-wide genome sequencing reveals 30 years of Darwin's finch evolution.

A fundamental goal in evolutionary biology is to understand the genetic architecture of adaptive traits. Using whole-genome data of 3955 of Darwin's finches on the Galápagos Island of Daphne Major, we identified six loci of large effect that explain 45% of the variation in the highly heritable beak size of Geospiza fortis, a key ecological trait. The major locus is a supergene comprising four genes. Abrupt changes in allele frequencies at the loci accompanied a strong change in beak size caused by natural selection during a drought. A gradual change in Geospiza scandens occurred across 30 years as a result of introgressive hybridization with G. fortis. This study shows how a few loci with large effect on a fitness-related trait contribute to the genetic potential for rapid adaptive radiation.

Changes in the acoustic structure of Australian bird communities along a habitat complexity gradient

Abstract Avian vocalizations have evolved in response to a variety of abiotic and biotic selective pressures. While there is some support for signal convergence in similar habitats that are attributed to adaptation to the acoustic properties of the environment (the “acoustic adaptation hypothesis,” AAH), there is also evidence for character displacement as a result of competition for signal space among coexisting species (the “acoustic niche partitioning hypothesis”). We explored the acoustic space of avian assemblages distributed along six different habitat types (from herbaceous habitats to warm rainforests) in southeastern Queensland, Australia. We employed three acoustic diversity indices (acoustic richness, evenness, and divergence) to characterize the signal space. In addition, we quantified the phylogenetic and morphological structure (in terms of both body mass and beak size) of each community. Acoustic parameters showed a moderately low phylogenetic signal, indicating labile evolution. Although we did not find meaningful differences in acoustic diversity indices among habitat categories, there was a significant relationship between the regularity component (evenness) and vegetation height, indicating that acoustic signals are more evenly distributed in dense habitats. After accounting for differences in species richness, the volume of acoustic space (i.e., acoustic richness) decreased as the level of phylogenetic and morphological resemblance among species in a given community increased. Additionally, we found a significantly negative relationship between acoustic divergence and divergence in body mass indicating that the less different species are in their body mass, the more different their songs are likely to be. This implies the existence of acoustic niche partitioning at a community level. Overall, while we found mixed support for the AAH, our results suggest that community-level effects may play a role in structuring acoustic signals within avian communities in this region. This study shows that signal diversity estimated by diversity metrics of community ecology based on basic acoustic parameters can provide additional insight into the structure of animal vocalizations.

A novel qPCR-based technique for identifying avian sex: An illustration within embryonic craniofacial bone.

Sex is a biological variable important to consider in all biomedical experiments. However, doing so in avian embryos can be challenging as sex can be morphologically indistinguishable. Unlike humans, female birds are the heterogametic sex with Z and W sex chromosomes. The female-specific W chromosome has previously been identified in chick using a species-specific polymerase chain reaction (PCR) technique. We developed a novel reverse transcription quantitative PCR (RT-qPCR) technique that amplifies the W chromosome gene histidine triad nucleotide-binding protein W (HINTW) in chick, quail, and duck. Accuracy of the HINTW RT-qPCR primer set was confirmed in all three species using species-specific PCR, including a novel quail-specific HINTW PCR primer set. Bone development-related gene expression was then analyzed by sex in embryonic lower jaws of duck and quail, as adult duck beak size is known to be sexually dimorphic while quail beak size is not. Trends toward sex differences were found in duck gene expression but not in quail, as expected. With these novel RT-qPCR and PCR embryo sexing methods, sex of chick, quail, and duck embryos can now be assessed by either/both RNA and DNA, which facilitates analysis of sex as a biological variable in studies using these model organisms.

Genomic architecture underlying morphological and physiological adaptation to high elevation in a songbird.

Organisms often acquire physiological and morphological modifications to conquer ecological challenges when colonizing new environments which lead to their adaptive evolution. However, deciphering the genomic mechanism of ecological adaptation is difficult because ecological environments are often too complex for straightforward interpretation. Thus, we examined the adaptation of a widespread songbird-the rufous-capped babbler (Cyanoderma ruficeps)-to a relatively simple system: distinct environments across elevational gradients on the mountainous island of Taiwan. We focused on the genomic sequences of 43 birds from five populations to show that the Taiwan group split from its sister group in mainland China around 1-2 million years ago (Ma) and colonized the montane habitats of Taiwan at least twice around 0.03-0.22 Ma. The montane and lowland Taiwan populations diverged with gene flow between them, suggesting strong selection associated with different elevations. We found that the montane babblers had smaller beaks than the lowland ones, consistent with Allen's rule, and identified candidate genes-COL9A1 and SOX11-underlying the beak size changes. We also found that altitudinally divergent mutations were mostly located in noncoding regions and tended to accumulate in chromosomal inversions and autosomes. The altitudinally divergent mutations might regulate genes related to haematopoietic, metabolic, immune, auditory and vision functions, as well as cerebrum morphology and plumage development. The results reveal the genomic bases of morphological and physiological adaptation in this species to the low temperature, hypoxia and high UV light environment at high elevation. These findings improve our understanding of how ecological adaptation drives population divergence from the perspective of genomic architecture.

Variability in Alternaria lini causing leaf blight disease of linseed (Linum usitatissimum L.)

Alternaria blight is an important disease of linseed that hampers its productivity and oil content. The pathogen is geneticallydiverse showing variability in respect of cultural, morphological and pathogenic characters. Variability in seventeen isolates of Alternaria lini was carried out. Studies on cultural variability showed considerable variation among the isolates. The colony diameter varied from 46.67- 71.67 mm after 96 hrs. of incubation and most of the isolates recorded medium growth rate (50-64mm). The isolates Alt-6,Alt-7 Alt-12 showed fast growth habit (&gt;65mm). The colony colour varied from grayish white, grey, dark olive green to grayish black. Sporulation in most of the isolates were late and concentric ring was present only in four isolates (Alt 3, Alt 8, Alt 13, Alt 14) while zonation was completely absent in other isolates. Morphologically different isolates of A. lini revealed variation with respect to mycelium width, size, shape, colour and septation of conidia. Mycelial width of most of the isolates having moderate to long width. Conidial length ranged from 24.87-33.68 mm similarly breadth of conidia ranged from7.54-10.67 mm. Transverse septation were more than longitudinal septation. Variation in beak size ranged from 3.5-6.4 mm while colour of conidia varied from brown to dark brown colour and most of the isolates exhibited obclavate, oval to obclavate shaped conidia while one isolate appeared as oval shaped. Studies on pathogenic variations revealed that the variation observed in accordance with the latent period, lesion size, lesion shape and plant disease index. The latent period of A. lini isolates varied from4.75 to 6.25 days with shortest (4.75 days) latent period recorded by isolates Alt-3 and Alt-11 and longest latent period of 6.25 days was observed in isolates Alt-6, Alt-14, Alt-16. The size of lesion varied from 2.25 to 4.75 mm and maximum lesions size (4.75 mm) was observed in isolate Alt-4 whereas minimum lesion size (2.25 mm) was observed in isolate Alt-5. The lesion produced after inoculation were elongated, oval and irregular in shape. The average number of lesions ranged from 2.75 to 4.00 with maximum number (4.00) of lesion recorded in isolate Alt-13. Plant disease index revealed that Meera and Nagarkot were more severe/virulent compare to Divya and Priyam in all the isolates.

On the predictability of phenotypic divergence in geographic isolation.

Do related populations that are separated by barriers predictably evolve differences from one another over time, or is such divergence idiosyncratic and unpredictable? We test these alternatives by investigating patterns of trait evolution for 54 sister pairs of Andean forest birds that live in similar environments on either side of the arid Marañón Gap, a strong dispersal barrier for humid montane species. We measured divergence in both sexual (song and plumage) and ecological (beak size and beak shape) traits. Sexual traits evolve in a clock-like fashion, with trait divergence positively correlated with genetic distance (r = 0.6-0.7). In contrast, divergence in ecological traits is uncorrelated or only loosely correlated with genetic distance (r = 0.0-0.3). Thus, for geographically isolated Andean montane forest birds that live in similar environments, divergence is predictable in sexual traits, but not for ecological traits. This means that sexual trait divergence occurs independently of adaptive ecological divergence within the mega-diverse tropical Andean avifauna. Last, we show that variation in genetic divergence across a biogeographic barrier is associated with traits that are proxies for species' opportunities for dispersal (low elevation limit and elevational niche breadth), but not with traits that are proxies for species' dispersal abilities (hand-wing index and foraging strata).

The terroir of the finch: How spatial and temporal variation shapes phenotypic traits in DARWIN'S finches.

The term terroir is used in viticulture to emphasize how the biotic and abiotic characteristics of a local site influence grape physiology and thus the properties of wine. In ecology and evolution, such terroir (i.e., the effect of space or “site”) is expected to play an important role in shaping phenotypic traits. Just how important is the pure spatial effect of terroir (e.g., differences between sites that persist across years) in comparison to temporal variation (e.g., differences between years that persist across sites), and the interaction between space and time (e.g., differences between sites change across years)? We answer this question by analyzing beak and body traits of 4388 medium ground finches (Geospiza fortis) collected across 10 years at three locations in Galápagos. Analyses of variance indicated that phenotypic variation was mostly explained by site for beak size (η 2 = 0.42) and body size (η 2 = 0.43), with a smaller contribution for beak shape (η 2 = 0.05) and body shape (η 2 = 0.12), but still higher compared to year and site‐by‐year effects. As such, the effect of terroir seems to be very strong in Darwin's finches, notwithstanding the oft‐emphasized interannual variation. However, these results changed dramatically when we excluded data from Daphne Major, indicating that the strong effect of terroir was mostly driven by that particular population. These phenotypic results were largely paralleled in analyses of environmental variables (rainfall and vegetation indices) expected to shape terroir in this system. These findings affirm the evolutionary importance of terroir, while also revealing its dependence on other factors, such as geographical isolation.

Resource-dependent investment in male sexual traits in a viviparous fish.

Exaggerated and conspicuous sexually selected traits are often costly to produce and maintain. Costly traits are expected to show resource-dependent expression, since limited resources prevent animals from investing maximally in multiple traits simultaneously. However, there may be critical periods during an individual's life where the expression of traits is altered if resources are limited. Moreover, costly sexual traits may arise from sexual selection acting both before (pre-copulatory) and after mating (post-copulatory). Gaining a robust understanding of resource-dependent trait expression therefore requires an approach that examines both episodes of sexual selection after resource limitation during different times in an individual's life. Yet few studies have taken such an approach. Here, we examine how resource restriction influences a set of pre- and post-copulatory traits in male pygmy halfbeaks (Dermogenys collettei), which invest in sexual ornaments and routinely engage in male-male contests and sperm competition. Critically, we examined responses in males when resources were restricted during development and after reaching sexual maturity. Both pre- and post-copulatory traits are resource-dependent in male halfbeaks. Body size, beak size, courtship behavior, and testes size were reduced by diet restriction, while, unexpectedly, the restricted-diet group had a larger area of red color on the beak and fins after diet treatment. These patterns were generally consistent when resources were restricted during development and after reaching sexual maturity. The study reinforces the role of resource acquisition in maintaining variation among sexual traits.

Ontogenetic niche shifts in the Mesozoic bird Confuciusornis sanctus

Paleontological evidence reveals that the rapid growth characteristic of living birds evolved close to the origin of the crown-group Neornithes, as more stemward birds experienced protracted growth until becoming fully grown.1 Research on Mesozoic confuciusornithids, the earliest divergence of fully beaked birds, has revealed a complex life cycle in which these birds experienced multiple growth phases.2-4 Such a life-history pattern calls for the exploration of the role that ontogenetic niche shifts may have played in size-structuring confuciusornithid populations.5,6 Here, by analyzing the skeletal morphometrics of a dense sample of fossil individuals of Confuciusornis sanctus (n= 171, all fledged), we show that the youngest individuals of this confuciusornithid species experienced a precocious burst of beak growth, probably facilitating access to novel food resources that helped them meet the high energetic demands of their initial growth spurt. Such an early burst of facial (i.e., snout) growth resembles that of young crocodilians.7 However, in these reptiles, facial growth slows down soon thereafter, and the matching of snout scaling between mid-sized and larger individuals instigates demographic competence and the dispersion of the former.8 In contrast, our results reveal that beak growth in C.sanctus continued steadily. We hypothesized that the protracted facial growth of older individuals led to ontogenetic niche shifts by dietary segregation among size classes within populations. Our study thus confirms that the life cycle of C.sanctus was notably different from that of modern birds, and it reveals that beak size allometry may have facilitated population cohesiveness between coinhabiting age classes.

Is bigger always better? Neither body size nor aggressive behavior predicts specialization of hummingbirds in a rocky outcrop

AbstractInterspecific competition can strongly influence community structure and shape niche breadth and overlap. One of the main factors that determines hummingbird community structure is competition for food. Hummingbirds’ functional attributes, such as beak length and body mass, influence nectar acquisition in flowers. This study evaluates how functional and behavioral attributes of hummingbirds influence their specialization, as measured by pollen transportation. We tested the following hypothesis: H1) smaller and short‐billed hummingbirds will carry higher pollen richness and relatively more pollen grains than long‐billed hummingbirds; H2) dominant and territorial hummingbirds carry a lower richness of pollen types and fewer pollen grains when compared to subordinate hummingbirds, and H3) the similarity in the composition of pollen morphotypes, between the hummingbirds species, will be low because we expect to find high specialization in the use of plant species. We conducted the study between September 2018 and March 2019 in a Campo Rupestre (rocky outcrops) in Southeastern Brazil. Hummingbirds were captured with a trap. The pollen adhered to the body parts was collected, counted, and identified in the laboratory. We recorded hummingbirds' beak sizes and body masses. Behavioral responses of hummingbirds to artificial feeders were collected regarding the time and duration of each visit and the outcome of aggressive interactions. An interaction network was built based on the pollen grains found on each hummingbird. Our results showed that neither body size nor aggressive behaviors influenced pollen richness on hummingbirds’ bodies. Beak length was the most important hummingbird attribute that influenced pollen richness, but not pollen relative abundance. Short‐billed hummingbirds carried the greatest richness of pollen grains. The hummingbird pollen‐grain interaction network was generalized in the pollen grains transported. We conclude that hummingbirds' beak length is the central morphological variable related to pollen grain transport in rocky outcrops.

Allometric conservatism in the evolution of bird beaks.

Evolution can involve periods of rapid divergent adaptation and expansion in the range of diversity, but evolution can also be relatively conservative over certain timescales due to functional, genetic‐developmental, and ecological constraints. One way in which evolution may be conservative is in terms of allometry, the scaling relationship between the traits of organisms and body size. Here, we investigate patterns of allometric conservatism in the evolution of bird beaks with beak size and body size data for a representative sample of over 5000 extant bird species within a phylogenetic framework. We identify clades in which the allometric relationship between beak size and body size has remained relatively conserved across species over millions to tens of millions of years. We find that allometric conservatism is nonetheless punctuated by occasional shifts in the slopes and intercepts of allometric relationships. A steady accumulation of such shifts through time has given rise to the tremendous diversity of beak size relative to body size across birds today. Our findings are consistent with the Simpsonian vision of macroevolution, with evolutionary conservatism being the rule but with occasional shifts to new adaptive zones.

Quantifying adaptive divergence of the snowfinches in a common landscape

AbstractAimSpecies living in a shared environment face similar selective pressures and often evolve adaptive divergence to avoid competition. Quantifying phenotypic divergence and its genetic parallelism among sympatric species is important for understanding of ecologically moderated biodiversity. Here, we integrate ecologic, phenotypic and genomic datasets to study to what extent three sympatrically snowfinches (Montifringilla adamsi, Pyrgilauda ruficollis and Onychostruthus taczanowskii) differ in their adaptations in order to co‐exist in a shared environment.LocationQinghai–Tibetan Plateau.MethodsWe used principal component analysis to summarize and compare environmental and phenotypic divergence. We compared phenotypes relevant to body and beak sizes (n = 68) because they are indicators of niche and food segregation, thus critical for establishing co‐existence of sympatric birds. We used comparative genomics (n = 33) to identify genetic loci that are highly divergent between species as well as loci unique for each of species. Using vector analyses, we integrated correlation and permutation to quantify parallelism between phenotypic and genetic divergences.ResultsWe found that body and beak sizes are significantly different among three snowfinches. The phenotypic differentiations are greater in species that share similar ecological conditions than in those that do not. We showed that genes related to developmental process are over‐represented within highly divergent genomic regions and unique genetic loci of each species. We found that the extent of phenotypic divergence between snowfinch pairs is more strongly correlated with the magnitude of divergence in developmental genes than in the whole genome.Main conclusionsAdaptive divergence of sympatric snowfinches is highly constrained on developmental genes. As this genetic divergence is strongly correlated with divergence of the traits related to segregation in niche and food resources, this correlation reflects either causal effects or indirect consequences of ecological mediated changes. Our study provides novel insights into the mechanisms underlying evolutionary versatility and ecological success among sympatric species.

Innovative mark-recapture experiment shows patterns of selection on transcript abundance in the wild.

A fundamental aspect of evolutionary biology is natural selection on trait variation. Classically, selection has been estimated primarily on external morphological traits such as beak size and coloration, or on easily assayable physiological traits such as heat-tolerance. As technologies and methods improved, evolutionary biologists began examining selection on molecular traits such as protein sequences and cellular processes. In a From the Cover paper in this issue of Molecular Ecology, Ahmad et al. continue this trend by estimating parasite-driven selection on the molecular trait of transcript abundance in a wild population of brown trout (Salmo trutta) by uniquely combining a mark-recapture experimental design with noninvasive RNA sampling. Using transcript abundance to estimate selection allows for many different traits (each unique gene's transcript counts) to be tested in a single experiment, providing the opportunity to examine trends in selection. Ahmad et al. find directional selection strength on transcript counts is generally low and normally distributed. Surprisingly, transcripts under nonlinear selection showed a disruptive selection bias, contradicting previous comparative studies and theoretical work. This highlights the importance of within-generation selection studies, where mechanisms may differ from longer time frames. Their paper also highlights the benefits of a cost-effective 3' RNA sequencing technique to measure gene expression.

Testing the influence of crushing surface variation on seed-cracking performance among beak morphs of the African seedcracker Pyrenestes ostrinus.

Extreme phenotypic polymorphism is an oft-cited example of evolutionary theory in practice. Although these morphological variations are assumed to be adaptive, few studies have biomechanically tested such hypotheses. Pyrenestes ostrinus (the African seedcracker finch) shows an intraspecific polymorphism in beak size and shape that is entirely diet driven and allelically determined. Three distinct morphs feed upon soft sedge seeds during times of abundance, but during lean times switch to specializing on three different species of sedge seeds that differ significantly in hardness. Here, we test the hypothesis that beak morphology is directly related to consuming seeds of different hardness. We used a novel experimental analysis to test how beak morphology affects the efficiency of cracking sedge seeds of variable hardness, observing that neither mandibular ramus width nor crushing surface morphology had significant effects on the ability to crack different seed types. It is likely that feeding performance is correlated with other aspects of beak size and shape, such as beak depth and strength, muscle force or gape. Our results highlight how even seemingly straightforward examples of adaptive selection in nature can be complex in practice.