Diversification and introgression in four chromosomal taxa of the Pearson's horseshoe bat (Rhinolophus pearsoni) group.

Chromosomal variation among closely related taxa is common in both plants and animals, and can reduce rates of introgression as well as promote reproductive isolation and speciation. In mammals, studies relating introgression to chromosomal variation have tended to focus on a few model systems and typically characterized levels of introgression using small numbers of loci. Here we took a genome-wide approach to examine how introgression rates vary among four closely related horseshoe bats (Rhinolophus pearsoni group) that possess different diploid chromosome numbers (2n = 42, 44, 46, and 60) resulting from Robertsonian (Rb) changes (fissions/fusions). Using a sequence capture we obtained orthologous loci for thousands of nuclear loci, as well as mitogenomes, and performed phylogenetic and population genetic analyses. We found that the taxon with 2n = 60 was the first to diverge in this group, and that the relationships among the three other taxa (2n = 42, 44 and 46) showed discordance across our different analyses. Our results revealed signatures of multiple ancient introgression events between the four taxa, with evidence of mitonuclar discordance in phylogenetic trees and reticulation events in their evolutionary history. Despite this, we found no evidence of recent and/or ongoing introgression between taxa. Overall, our results indicate that the effects of Rb changes on the reduction of introgression are complicated and that these may contribute to reproductive isolation and speciation in concert with other factors (e.g. phenotypic and genic divergence).

Dietary Diversification and Specialization in Neotropical Bats Facilitated by Early Molecular Evolution.

Dietary adaptation is a major feature of phenotypic and ecological diversification, yet the genetic basis of dietary shifts is poorly understood. Among mammals, Neotropical leaf-nosed bats (family Phyllostomidae) show unmatched diversity in diet; from a putative insectivorous ancestor, phyllostomids have radiated to specialize on diverse food sources including blood, nectar, and fruit. To assess whether dietary diversification in this group was accompanied by molecular adaptations for changing metabolic demands, we sequenced 89 transcriptomes across 58 species and combined these with published data to compare ∼13,000 protein coding genes across 66 species. We tested for positive selection on focal lineages, including those inferred to have undergone dietary shifts. Unexpectedly, we found a broad signature of positive selection in the ancestral phyllostomid branch, spanning genes implicated in the metabolism of all major macronutrients, yet few positively selected genes at the inferred switch to plantivory. Branches corresponding to blood- and nectar-based diets showed selection in loci underpinning nitrogenous waste excretion and glycolysis, respectively. Intriguingly, patterns of selection in metabolism genes were mirrored by those in loci implicated in craniofacial remodeling, a trait previously linked to phyllostomid dietary specialization. Finally, we show that the null model of the widely-used branch-site test is likely to be misspecified, with the implication that the test is too conservative and probably under-reports true cases of positive selection. Our findings point to a complex picture of adaptive radiation, in which the evolution of new dietary specializations has been facilitated by early adaptations combined with the generation of new genetic variation.

Interrogating Phylogenetic Discordance Resolves Deep Splits in the Rapid Radiation of Old World Fruit Bats (Chiroptera: Pteropodidae).

The family Pteropodidae (Old World fruit bats) comprises $>$200 species distributed across the Old World tropics and subtropics. Most pteropodids feed on fruit, suggesting an early origin of frugivory, although several lineages have shifted to nectar-based diets. Pteropodids are of exceptional conservation concern with $>$50% of species considered threatened, yet the systematics of this group has long been debated, with uncertainty surrounding early splits attributed to an ancient rapid diversification. Resolving the relationships among the main pteropodid lineages is essential if we are to fully understand their evolutionary distinctiveness, and the extent to which these bats have transitioned to nectar-feeding. Here we generated orthologous sequences for $>$1400 nuclear protein-coding genes (2.8 million base pairs) across 114 species from 43 genera of Old World fruit bats (57% and 96% of extant species- and genus-level diversity, respectively), and combined phylogenomic inference with filtering by information content to resolve systematic relationships among the major lineages. Concatenation and coalescent-based methods recovered three distinct backbone topologies that were not able to be reconciled by filtering via phylogenetic information content. Concordance analysis and gene genealogy interrogation show that one topology is consistently the best supported, and that observed phylogenetic conflicts arise from both gene tree error and deep incomplete lineage sorting. In addition to resolving long-standing inconsistencies in the reported relationships among major lineages, we show that Old World fruit bats have likely undergone at least seven independent dietary transitions from frugivory to nectarivory. Finally, we use this phylogeny to identify and describe one new genus. [Chiroptera; coalescence; concordance; incomplete lineage sorting; nectar feeder; species tree; target enrichment.].

Single nucleotide polymorphisms (SNPs) provide unprecedented resolution of species boundaries, phylogenetic relationships, and genetic diversity in the mastiff bats (Molossus).

Mammals are one of the better known groups of animals, and in the Neotropics bats typically comprise about half of the mammalian species diversity. But, well resolved species-level phylogenies are still lacking for most taxa of bats. One broadly distributed genus is the mastiff bats, Molossus. Species within this genus are morphologically very similar, which results in a confusing and unstable taxonomy. In addition, low levels of genetic divergence among some clades make resolution of phylogenetic relationships difficult. Most authors recognize Molossus as being monophyletic, however, phylogenetic relationships within the genus remain poorly understood based on traditional Sanger sequencing of individual genes. We propose a more comprehensive framework based on large-scale genomic data derived from Next Generation Sequencing techniques to better understand evolutionary relationships within a group of closely related species with a rich taxonomic history. In this study, we utilized the NGS method of Genotype by Sequencing (GBS) to test the monophyly of the genus, understand evolutionary relationships within Molossus and investigate the genetic integrity of currently recognized species. Given that both de novo and reference genome pipelines are often used in the assembly of Single Nucleotide Polymorphism data from GBS, and that several tree inference methodologies have been proposed for SNP data, we test whether different alignments and phylogenetic approaches produce similar results. We also examined how the process of SNP identification and mapping can affect the consistency of the analyses. Our data provide the first high resolution phylogeny for the genus Molossus, bringing new insights into recognition of species boundaries and relationships among taxa. This study clarifies the taxonomy of Molossus and elevates the number of species in the genus from 11 to 14. We suggest the revalidation of the names M. nigricans, and M. fluminensis, which were synonymized under the name M. rufus; and M. bondae, previously synonymized under the name M. currentium. Different alignments and phylogenetic inferences produce consistent results, supporting use of SNP approach in addressing evolutionary questions on a macroevolutionary scale where the genetic distance among clades is low.

Functional Shifts in Bat Dim-Light Visual Pigment Are Associated with Differing Echolocation Abilities and Reveal Molecular Adaptation to Photic-Limited Environments.

Bats are excellent models for studying the molecular basis of sensory adaptation. In Chiroptera, a sensory trade-off has been proposed between the visual and auditory systems, though the extent of this association has yet to be fully examined. To investigate whether variation in visual performance is associated with echolocation, we experimentally assayed the dim-light visual pigment rhodopsin from bat species with differing echolocation abilities. While spectral tuning properties were similar among bats, we found that the rate of decay of their light-activated state was significantly slower in a nonecholocating bat relative to species that use distinct echolocation strategies, consistent with a sensory trade-off hypothesis. We also found that these rates of decay were remarkably slower compared with those of other mammals, likely indicating an adaptation to dim light. To examine whether functional changes in rhodopsin are associated with shifts in selection intensity upon bat Rh1 sequences, we implemented selection analyses using codon-based likelihood clade models. While no shifts in selection were identified in response to diverse echolocation abilities of bats, we detected a significant increase in the intensity of evolutionary constraint accompanying the diversification of Chiroptera. Taken together, this suggests that substitutions that modulate the stability of the light-activated rhodopsin state were likely maintained through intensified constraint after bats diversified, being finely tuned in response to novel sensory specializations. Our study demonstrates the power of combining experimental and computational approaches for investigating functional mechanisms underlying the evolution of complex sensory adaptations.

The role of ecological factors in shaping bat cone opsin evolution.

Bats represent one of the largest and most striking nocturnal mammalian radiations, exhibiting many visual system specializations for performance in light-limited environments. Despite representing the greatest ecological diversity and species richness in Chiroptera, Neotropical lineages have been undersampled in molecular studies, limiting the potential for identifying signatures of selection on visual genes associated with differences in bat ecology. Here, we investigated how diverse ecological pressures mediate long-term shifts in selection upon long-wavelength (Lws) and short-wavelength (Sws1) opsins, photosensitive cone pigments that form the basis of colour vision in most mammals, including bats. We used codon-based likelihood clade models to test whether ecological variables associated with reliance on visual information (e.g. echolocation ability and diet) or exposure to varying light environments (e.g. roosting behaviour and foraging habitat) mediated shifts in evolutionary rates in bat cone opsin genes. Using additional cone opsin sequences from newly sequenced eye transcriptomes of six Neotropical bat species, we found significant evidence for different ecological pressures influencing the evolution of the cone opsins. While Lws is evolving under significantly lower constraint in highly specialized high-duty cycle echolocating lineages, which have enhanced sonar ability to detect and track targets, variation in Sws1 constraint was significantly associated with foraging habitat, exhibiting elevated rates of evolution in species that forage among vegetation. This suggests that increased reliance on echolocation as well as the spectral environment experienced by foraging bats may differentially influence the evolution of different cone opsins. Our study demonstrates that different ecological variables may underlie contrasting evolutionary patterns in bat visual opsins, and highlights the suitability of clade models for testing ecological hypotheses of visual evolution.

Review of genetic diversification of bats in the Caribbean and biogeographic relationships to Neotropical species based on DNA barcodes.

DNA barcoding is helping in discovering high levels of cryptic species and an underestimation of biodiversity in many groups of organisms. Although mammals are arguably the most studied and one of the least speciose taxonomic classes, the rate of species discovery is increasing and biased for small mammals on islands. An earlier study found bats in the Caribbean as a taxonomic and geographic deficiency in the International Barcode of Life initiative to establish a genetic reference database to enable specimen identification to species. Recent surveys in Dominican Republic, Jamaica, and Martinique have documented and barcoded half of the 58 bat species known from the Caribbean. I analyze all available barcode data of Caribbean bats to investigate biogeography and cryptic species in the Neotropical region. Analysis of the mitochondrial DNA gene cytochrome c oxidase subunit 1 results in a phylogenetic tree with all but one species as well-supported and reciprocally monophyletic. With a broader sampling across the Neotropics, there are also divergent lineages that exhibit biogeographic structuring: (i) a phylogenetic split between northern and southern Dominican Republic in three species, (ii) two taxa with cryptic species associated with higher degree of island endemism, (iii) populations of two widely distributed species with deep divergence between the Caribbean and North and Central America, and (iv) one species in the Caribbean with affinities to taxa in South America.

Speciation processes in putative island endemic sister bat species: false impressions from mitochondrial DNA and microsatellite data.

Cases of geographically restricted co-occurring sister taxa are rare and may point to potential divergence with gene flow. The two bat species Murina gracilis and Murina recondita are both endemic to Taiwan and are putative sister species. To test for nonallopatric divergence and gene flow in these taxa, we generated sequences using Sanger and next-generation sequencing, and combined these with microsatellite data for coalescent-based analyses. MtDNA phylogenies supported the reciprocally monophyletic sister relationship between M. gracilis and M. recondita; however, clustering of microsatellite genotypes revealed several cases of species admixture suggesting possible introgression. Sequencing of microsatellite flanking regions revealed that admixture signatures stemmed from microsatellite allele homoplasy rather than recent introgressive hybridization, and also uncovered an unexpected sister relationship between M. recondita and the continental species Murina eleryi, to the exclusion of M. gracilis. To dissect the basis of these conflicts between ncDNA and mtDNA, we analysed sequences from 10 anonymous ncDNA loci with *beast and isolation-with-migration and found two distinct clades of M. eleryi, one of which was sister to M. recondita. We conclude that Taiwan was colonized by the ancestor of M. gracilis first, followed by the ancestor of M. recondita after a period of allopatric divergence. After colonization, the mitochondrial genome of M. recondita was replaced by that of the resident M. gracilis. This study illustrates how apparent signatures of sympatric divergence can arise from complex histories of allopatric divergence, colonization and hybridization, thus highlighting the need for rigorous analyses to distinguish between such scenarios.

A new species of broad-nosed bat Platyrrhinus Saussure, 1860 (Chiroptera: Phyllostomidae) from the Guianan Shield.

A new species of broad-nosed bat Platyrrhinus Saussure, 1860 (Chiroptera: Phyllostomidae: Stenodermatinae) from the Guianan Shield is described based on molecular and morphological data. Previously confused with P. helleri and P. recifinus, the new taxon is currently known from only Guyana and Suriname and is most closely related to P. recifinus from eastern Brazil and not to the two sympatric species (P. fusciventris and P. incarum) also recently recognized as distinct from P. helleri. Morphometrically the new taxon overlaps with the smaller species of the genus (P. angustirostris, P. brachycephalus, P. fusciventris, P. helleri, P. incarum, and P. matapalensis), but forms a different cluster from the larger P. recifinus. Morphologically the new taxon is distinguished from its congeners by a combination of external and craniodental characteristics. Platyrrhinus now includes 21 species making it the most speciose genus in the Neotropical family Phyllostomidae.

Systematics and biogeography of Anoura cultrata (Mammalia, Chiroptera, Phyllostomidae): a morphometric, niche modeling, and genetic perspective, with a taxonomic reappraisal of the genus.

The nectar-feeding bats of the genus Anoura are widely distributed in the Neotropics, but are most speciose in the Andes. Anoura cultrata is a rare mid-elevation bat occurring in South and Central America. It is thought to be one of the few bat species exemplifying a latitudinal cline in body size. We address three systematic and biogeographic questions: 1) is the geographic variation in A. cultrata continuous, as argued to justify its current monotypic status? 2) do ecogeographic barriers to dispersal affect such variation? and 3) how do the genetic divergence and biogeography of the species compare to those of other members of the genus? To answer these questions, we used morphometric analyses, ecological niche modeling, and DNA barcoding. We divided the samples of A. cultrata into six geographic groups, delimited by topographic depressions separating mountain systems. We did not find significant correlations between body size and the geographic coordinates within five groups. Therefore, we conclude that ecogeographic barriers to dispersal between the regions occupied by such groups influenced morphometric variation in A. cultrata, and that despite its general north to south reduction in body size, the species does not show continuous clinal variation. A recent phylogenetic study of the genus Anoura concluded that it contains seven valid species. Our DNA barcoding analysis and morphological examination indicated that at least 10 species should be recognized, including A. peruana distinct from A. geoffroyi, and A. aequatoris and A. luismanueli distinct from A. caudifer. Moreover, we show that Central and South American populations of A. cultrata differ from each other at least at the subspecific level, thus we respectively refer to them as A. cultrata cultrata and as A. c. brevirostrum. Similarly, we refer to Central American and Mexican populations of 'A. geoffroyi' as A. peruana lasiopyga, and to their South American counterparts as A. p. peruana. The range of the latter subspecies reaches northeastern Venezuela. The Andes from southern Colombia to northern Peru appear to be the ancestral range of the genus.

Hologenomic insights into mammalian adaptations to myrmecophagy.

Highly specialized myrmecophagy (ant- and termite-eating) has independently evolved multiple times in species of various mammalian orders and represents a textbook example of phenotypic evolutionary convergence. We explored the mechanisms involved in this unique dietary adaptation and convergence through multi-omic analyses, including analyses of host genomes and transcriptomes, as well as gut metagenomes, in combination with validating assays of key enzymes' activities, in the species of three mammalian orders (anteaters, echidnas and pangolins of the orders Xenarthra, Monotremata and Pholidota, respectively) and their relatives. We demonstrate the complex and diverse interactions between hosts and their symbiotic microbiota that have provided adaptive solutions for nutritional and detoxification challenges associated with high levels of protein and lipid metabolisms, trehalose degradation, and toxic substance detoxification. Interestingly, we also reveal their spatially complementary cooperation involved in degradation of ants' and termites' chitin exoskeletons. This study contributes new insights into the dietary evolution of mammals and the mechanisms involved in the coordination of physiological functions by animal hosts and their gut commensals.

Divergent lineage of a novel hantavirus in the banana pipistrelle (Neoromicia nanus) in Côte d'Ivoire.

Recently identified hantaviruses harbored by shrews and moles (order Soricomorpha) suggest that other mammals having shared ancestry may serve as reservoirs. To investigate this possibility, archival tissues from 213 insectivorous bats (order Chiroptera) were analyzed for hantavirus RNA by RT-PCR. Following numerous failed attempts, hantavirus RNA was detected in ethanol-fixed liver tissue from two banana pipistrelles (Neoromicia nanus), captured near Mouyassué village in Côte d'Ivoire, West Africa, in June 2011. Phylogenetic analysis of partial L-segment sequences using maximum-likelihood and Bayesian methods revealed that the newfound hantavirus, designated Mouyassué virus (MOUV), was highly divergent and basal to all other rodent- and soricomorph-borne hantaviruses, except for Nova virus in the European common mole (Talpa europaea). Full genome sequencing of MOUV and further surveys of other bat species for hantaviruses, now underway, will provide critical insights into the evolution and diversification of hantaviruses.

Bats and their vital ecosystem services: a global review.

Bats play crucial ecosystem services as seed dispersers, pollinators, controllers of insects, and nutrient recyclers. However, there has not been a thorough global review evaluating these roles in bats across all biogeographical regions of the world. We reviewed the literature published during the last two decades and identified 283 relevant studies: 78 dealt with the control of potential insect pests by bats, 80 related to the suppression of other arthropods, 60 on the dispersal of native or endemic seeds, 11 dealt with the dispersal of seeds of introduced plants, 29 on the pollination of native or endemic plants, 1 study on pollination of introduced plants, and 24 on the use of guano as fertilizer. Our literature search showed that queries combining the terms "seed dispersal," "insectivorous bats," "nectarivorous bats," "use of guano," and "ecosystem services" returned 577 studies, but half were experimental in nature. We found that the evaluation of ecosystem services by bats has been mostly conducted in the Neotropical and Palearctic regions. To detect differences across relevant studies, and to explain trends in the study of ecosystem services provided by bats, we performed generalized linear mixed models (GLMM) fitted with a Poisson distribution to analyze potential differences among sampling methods. We identified 409 bat species that provide ecosystem services, 752 insect species consumed by bats and 549 plant species either dispersed or pollinated by bats. Our review summarizes the importance of conserving bat populations and the ecological services they provide, which is especially important during the current pandemic.

Gene losses in the common vampire bat illuminate molecular adaptations to blood feeding.

Vampire bats are the only mammals that feed exclusively on blood. To uncover genomic changes associated with this dietary adaptation, we generated a haplotype-resolved genome of the common vampire bat and screened 27 bat species for genes that were specifically lost in the vampire bat lineage. We found previously unknown gene losses that relate to reduced insulin secretion (FFAR1 and SLC30A8), limited glycogen stores (PPP1R3E), and a unique gastric physiology (CTSE). Other gene losses likely reflect the biased nutrient composition (ERN2 and CTRL) and distinct pathogen diversity of blood (RNASE7) and predict the complete lack of cone-based vision in these strictly nocturnal bats (PDE6H and PDE6C). Notably, REP15 loss likely helped vampire bats adapt to high dietary iron levels by enhancing iron excretion, and the loss of CYP39A1 could have contributed to their exceptional cognitive abilities. These findings enhance our understanding of vampire bat biology and the genomic underpinnings of adaptations to blood feeding.

Foraging strategies, craniodental traits, and interaction in the bite force of Neotropical frugivorous bats (Phyllostomidae: Stenodermatinae).

Bats in the family Phyllostomidae exhibit great diversity in skull size and morphology that reflects the degree of resource division and ecological overlap in the group. In particular, the subfamily Stenodermatinae has high morphological diversification associated with cranial and mandibular traits that are associated with the ability to consume the full range of available fruits (soft and hard).We analyzed craniodental traits and their relationship to the bite force in 343 specimens distributed in seven species of stenodermatine bats with two foraging strategies: nomadic and sedentary frugivory. We evaluated 19 traits related to feeding and bite force in live animals by correcting bite force with body size.We used a generalized linear model (GLM) and post hoc tests to determine possible relationships and differences between cranial traits, species, and sex. We also used Blomberg's K to measure the phylogenetic signal and phylogenetic generalized least-squares (PGLS) to ensure the phylogenetic independence of the traits.We found that smaller nomadic species, A. anderseni and A. phaeotis , have a similar bite force to the large species A. planirostris and A. lituratus; furthermore, P. helleri registered a bite force similar to that of the sedentary bat, S. giannae. Our study determined that all the features of the mandible and most of the traits of the skull have a low phylogenetic signal. Through the PGLS, we found that the diet and several cranial features (mandibular toothrow length, dentary length, braincase breadth, mastoid breadth, greatest length of skull, condylo-incisive length, and condylo-canine length) determined bite force performance among Stenodermatiane.Our results reinforce that skull size is a determining factor in the bite force, but also emphasize the importance of its relationships with morphology, ecology, and phylogeny of the species, which gives us a better understanding of the evolutionary adaptions of this highly diverse Neotropical bat group.

Skull Morphology, Bite Force, and Diet in Insectivorous Bats from Tropical Dry Forests in Colombia.

In Neotropical bats, studies on bite force have focused mainly on differences in trophic ecology, and little is known about whether factors other than body size generate interspecific differences in bite force amongst insectivorous bats and, consequently, in their diets. We tested if bite force is related to skull morphology and also to diet in an assemblage of Neotropical insectivorous bats from tropical dry forests in the inter-Andean central valley in Colombia. It is predicted that the preference of prey types among insectivorous species is based on bite force and cranial characteristics. We also evaluated whether skull morphology varies depending on the species and sex. Cranial measurements and correlations between morphological variation and bite force were examined for 10 insectivorous bat species. We calculated the size-independent mechanical advantage for the mandibular (jaw) lever system. In all species, bite force increased with length of the skull and the jaw more than other cranial measurements. Obligate insectivorous species were morphologically different from the omnivorous Noctilio albiventris, which feeds primarily on insects, but also consumes fish and fruits. Our results show that bite force and skull morphology are closely linked to diets in Neotropical insectivorous bats and, consequently, these traits are key to the interactions within the assemblage and with their prey.

Nectar-feeding bats and birds show parallel molecular adaptations in sugar metabolism enzymes.

In most vertebrates, the demand for glucose as the primary substrate for cellular respiration is met by the breakdown of complex carbohydrates, or energy is obtained by protein and lipid catabolism. In contrast, a few bat and bird species have convergently evolved to subsist on nectar, a sugar-rich mixture of glucose, fructose, and sucrose.1-4 How these nectar-feeders have adapted to cope with life-long high sugar intake while avoiding the onset of metabolic syndrome and diabetes5-7 is not understood. We analyzed gene sequences obtained from 127 taxa, including 22 nectar-feeding bat and bird genera that collectively encompass four independent origins of nectarivory. We show these divergent taxa have undergone pervasive molecular adaptation in sugar catabolism pathways, including parallel selection in key glycolytic and fructolytic enzymes. We also uncover convergent amino acid substitutions in the otherwise evolutionarily conserved aldolase B (ALDOB), which catalyzes rate-limiting steps in fructolysis and glycolysis, and the mitochondrial gatekeeper pyruvate dehydrogenase (PDH), which links glycolysis and the tricarboxylic acid cycle. Metabolomic profile and enzyme functional assays are consistent with increased respiratory flux in nectar-feeding bats and help explain how these taxa can both sustain hovering flight and efficiently clear simple sugars. Taken together, our results indicate that nectar-feeding bats and birds have undergone metabolic adaptations that have enabled them to exploit a unique energy-rich dietary niche among vertebrates.

Does evolution of echolocation calls and morphology in Molossus result from convergence or stasis?

Although many processes of diversification have been described to explain variation of morphological traits within clades that have obvious differentiation among taxa, not much is known about these patterns in complexes of cryptic species. Molossus is a genus of bats that is mainly Neotropical, occurring from the southeastern United States to southern Argentina, including the Caribbean islands. Molossus comprises some groups of species that are morphologically similar but phylogenetically divergent, and other groups of species that are genetically similar but morphologically distinct. This contrast allows investigation of unequal trait diversification and the evolution of morphological and behavioural characters. In this study, we assessed the role of phylogenetic history in a genus of bat with three cryptic species complexes, and evaluated if morphology and behavior are evolving concertedly. The Genotype by Sequence genomic approach was used to build a species-level phylogenetic tree for Molossus and to estimate the ancestral states of morphological and echolocation call characters. We measured the correlation of phylogenetic distances to morphological and echolocation distances, and tested the relationship between morphology and behavior when the effect of phylogeny is removed. Morphology evolved via a mosaic of convergence and stasis, whereas call design was influenced exclusively through local adaptation and convergent evolution. Furthermore, the frequency of echolocation calls is negatively correlated with the size of the bat, but other characters do not seem to be evolving in concert. We hypothesize that slight variation in both morphology and behaviour among species of the genus might result from niche specialization, and that traits evolve to avoid competition for resources in similar environments.