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A new dimension to hummingbird-flower relationship
Abstract
The close correspondence between the bills of hummingbirds and the lengths of the flowers they feed from has been interpreted as an example of coadaptation. Observations of birds feeding at flowers longer and shorter than their bills, however, and the lack of experimental evidence for any feeding advantage to short bills, seem to contradict this interpretation. I address this problem by considering a little-studied dimension of floral morphology: corolla diameter. In laboratory experiments on female ruby-throated hummingbirds (Archilochus colubris), probing abilities (maximum extraction depths) increased with increasing corolla diameter. Handling times increased with decreasing corolla diameter, resulting in handling time equivalents, i.e., flowers having the same handling times but different lengths and diameters. Longer-billed birds had greater maximum extraction depths and shorter handling times than shorter-billed birds at all corolla diameters greater than the width of the bill. In contrast, shorter-billed birds made fewer errors inserting their bills into narrow flowers. Hence, differences in bill lengths apparently are associated with trade-offs in foraging abilities, whereby longer-billed birds are able to feed at long flowers and may do so more quickly, whereas shorter-billed birds are able to feed more successfully at narrow flowers.
... As foraging efficiency (caloric value obtained relative to the caloric costs for obtaining food) can act as a selective pressure (Schoener, 1971;Tullock, 1971;Houston and McNamara, 2014), individuals of a given hummingbird species (or sex) would be expected to be more efficient (minimizing handling time) while feeding on flowers matching their bill length and shape (Wolf et al., 1972;Temeles and Roberts, 1993;Temeles et al., 2000Temeles et al., , 2010. However, tests of this 'bill-corolla matching increasing efficiency' hypothesis have produced conflicting results; it would be expected that long-billed species would be more efficient with long flowers and short-billed hummingbirds would be more efficient with short flowers, but in fact previous research has failed to support the second prediction (Hainsworth, 1973;Montgomerie, 1984;Temeles and Roberts, 1993;Temeles, 1996;Temeles et al., 2002Temeles et al., , 2009. Specifically, experiments have found that longer-billed birds feed faster from longer flowers than shorter-billed birds, but shorterbilled birds do not feed faster from shorter flowers than longerbilled ones (reviewed in Temeles, 1996). ...
... However, tests of this 'bill-corolla matching increasing efficiency' hypothesis have produced conflicting results; it would be expected that long-billed species would be more efficient with long flowers and short-billed hummingbirds would be more efficient with short flowers, but in fact previous research has failed to support the second prediction (Hainsworth, 1973;Montgomerie, 1984;Temeles and Roberts, 1993;Temeles, 1996;Temeles et al., 2002Temeles et al., , 2009. Specifically, experiments have found that longer-billed birds feed faster from longer flowers than shorter-billed birds, but shorterbilled birds do not feed faster from shorter flowers than longerbilled ones (reviewed in Temeles, 1996). Given that regardless of corolla length, longer bills seem to be able to achieve smaller distances between the bill tip and the nectar than shorter bills, achieving elevated nectar extraction efficiency (Hainsworth, 1973;Hainsworth and Wolf, 1976;Montgomerie, 1984;Grant and Temeles, 1992;Temeles and Roberts, 1993;Temeles, 1996), it may seem that evolving a longer bill will be a functionally selected adaptive peak. ...
... Specifically, experiments have found that longer-billed birds feed faster from longer flowers than shorter-billed birds, but shorterbilled birds do not feed faster from shorter flowers than longerbilled ones (reviewed in Temeles, 1996). Given that regardless of corolla length, longer bills seem to be able to achieve smaller distances between the bill tip and the nectar than shorter bills, achieving elevated nectar extraction efficiency (Hainsworth, 1973;Hainsworth and Wolf, 1976;Montgomerie, 1984;Grant and Temeles, 1992;Temeles and Roberts, 1993;Temeles, 1996), it may seem that evolving a longer bill will be a functionally selected adaptive peak. Longer-billed hummingbirds could potentially access a wider floral spectrum; however, they may not be able to find enough nectar on small flowers (Snow and Snow, 1980) because they are preferentially visited by short-billed hummingbirds (unless they nectar rob). ...
Hummingbirds are the most speciose group of vertebrate nectarivores and exhibit striking bill variation in association with their floral food sources. To explicitly link comparative feeding biomechanics to hummingbird ecology, deciphering how they move nectar from the tongue to the throat is as important as understanding how this liquid is collected. We employed synced, orthogonally positioned, high-speed cameras to describe the bill movements, and backlight filming to track tongue and nectar displacements intraorally. We reveal that the tongue base plays a central role in fluid handling, and that the bill is neither just a passive vehicle taking the tongue inside the flower nor a static tube for the nectar to flow into the throat. Instead, we show that the bill is actually a dynamic device with an unexpected pattern of opening and closing of its tip and base. We describe three complementary mechanisms: (1) distal wringing: the tongue is wrung out as soon as it is retracted and upon protrusion, near the bill tip where the intraoral capacity is decreased when the bill tips are closed; (2) tongue raking: the nectar filling the intraoral cavity is moved mouthwards by the tongue base, leveraging flexible flaps, upon retraction; (3) basal expansion: as more nectar is released into the oral cavity, the bill base is open (phase-shifted from the tip opening), increasing the intraoral capacity to facilitate nectar flow towards the throat.
... Illustrations are from Birds of the World, Cornell Lab of Ornithology and are not to scale. challenges include: efficiently extracting nectar that is often concealed within floral structures and can vary dramatically in volume and concentration (Figure 2a; Carothers, 1982;Gass & Roberts, 1992;Gill & Wolf, 1978;Stiles, 1981;Temeles, 1996;Temeles et al., 2002Temeles et al., , 2009; moving among nectar resources, which can be patchy and temporally unreliable (Figure 2b; Bogo et al., 2021;Carpenter, 1978;Ford & Paton, 1985;Franklin & Noske, 1999;Keast, 1968;McFarland, 1986;McGoldrick & Mac Nally, 1998;Parachnowitsch et al., 2019;Ramsay, 1989); and processing high volumes of water while efficiently extracting solutes (Figure 2c; Baker & Baker, 1983;Gartrell, 2000;T. McWhorter & Martinez del Rio, 1999;T. ...
... A decurved bill is thought to allow greater ease of access to a corolla when approaching from the side and/or below, which is often the case when feeding from a perched position as opposed to hovering, which permits feeding while facing the opening of the corolla (Johnson et al., 2020;Rocca & Sazima, 2010;Temeles et al., 2009;Westerkamp, 1990). There have been many studies examining how corolla traits affect feeding efficiency in hummingbirds (e.g., Collins, 2008;Montgomerie, 1984;Temeles, 1996;Temeles et al., 2002), and relatively few studies for sunbirds (Ngcamphalala et al., 2018) and honeyeaters (Collins, 2008). These studies have demonstrated that hummingbirds, sunbirds, and honeyeaters feed less efficiently (lower volumetric intake rate) at flowers longer than their bills (Collins, 2008;Hainsworth, 1973;Montgomerie, 1984;Ngcamphalala et al., 2018;Schlamowitz et al., 1976), but also that flower length and width interact to affect feeding efficiency such that long, wide flowers and short, narrow flowers can result in similar feeding efficiencies (Ngcamphalala et al., 2018;Temeles, 1996;Temeles et al., 2002). ...
... There have been many studies examining how corolla traits affect feeding efficiency in hummingbirds (e.g., Collins, 2008;Montgomerie, 1984;Temeles, 1996;Temeles et al., 2002), and relatively few studies for sunbirds (Ngcamphalala et al., 2018) and honeyeaters (Collins, 2008). These studies have demonstrated that hummingbirds, sunbirds, and honeyeaters feed less efficiently (lower volumetric intake rate) at flowers longer than their bills (Collins, 2008;Hainsworth, 1973;Montgomerie, 1984;Ngcamphalala et al., 2018;Schlamowitz et al., 1976), but also that flower length and width interact to affect feeding efficiency such that long, wide flowers and short, narrow flowers can result in similar feeding efficiencies (Ngcamphalala et al., 2018;Temeles, 1996;Temeles et al., 2002). ...
Nectar‐feeding birds provide an excellent system in which to examine form‐function relationships over evolutionary time. There are many independent origins of nectarivory in birds, and nectar feeding is a lifestyle with many inherent biophysical constraints. We review the morphology and function of the feeding apparatus, the locomotor apparatus, and the digestive and renal systems across avian nectarivores with the goals of synthesizing available information and identifying the extent to which different aspects of anatomy have morphologically and functionally converged. In doing so, we have systematically tabulated the occurrence of putative adaptations to nectarivory across birds and created what is, to our knowledge, the first comprehensive summary of adaptations to nectarivory across body systems and taxa. We also provide the first phylogenetically informed estimate of the number of times nectarivory has evolved within Aves. Based on this synthesis of existing knowledge, we identify current knowledge gaps and provide suggestions for future research questions and methods of data collection that will increase our understanding of the distribution of adaptations across bodily systems and taxa, and the relationship between those adaptations and ecological and evolutionary factors. We hope that this synthesis will serve as a landmark for the current state of the field, prompting investigators to begin collecting new data and addressing questions that have heretofore been impossible to answer about the ecology, evolution, and functional morphology of avian nectarivory. This article is protected by copyright. All rights reserved.
... Flowers that have shorter corollas and wider openings to nectar are easier to access and thus more efficient for hummingbirds to feed from (Grant and Temeles 1992). Studies show that handling time decreases as corolla length decreases relative to hummingbird bill length (Montgomerie 1984, Temeles and Roberts 1993, Temeles 1996, Temeles et al. 2009), suggesting that hummingbirds in general should prefer flowers with short or no corollas over long corollas when all else is equal. Profitability of the reward can be measured by nectar volume and concentration. ...
... Except for nectar concentration, we kept constant the same variables as Maglianesi et al. (2015), but we used a 25% wider corolla diameter. All corollas had an internal diameter of 5 mm, a diameter that should reduce the difficulty and energy expenditure for the hummingbirds to drink from long and thin corollas (Wolf and Hainsworth 1972, Temeles 1996, Temeles et al. 2002. ...
... In accordance with this prediction, we did not observe competitive behavior that appeared to affect feeding choice by hummingbirds based on body size or bill size. Hummingbirds with bill lengths of 26-27 mm, which should be physically able to reach the nectar source of the long-corolla flowers using their long tongues, also fed more frequently from short flowers, suggesting that the energetic cost incurred by a farther reach to the nectar source affects their visitation rates (Fenster 1991, Temeles 1996. ...
Specific factors that determine whether hummingbirds feed from flowers matching their bill morphology are not well understood. Here, we asked whether long-billed hummingbirds at tropical mid-elevations visit flowers that match their bill morphology more often when those flowers are more energetically profitable in terms of nectar concentration compared to short-corolla flowers. We measured visitation rates by hummingbirds in 3 experiments involving feeders with 5 flower morphologies and 2 nectar concentrations in the mountains of Costa Rica. Not surprisingly, all species tended to prefer higher nectar concentration when given the choice across all available flower morphologies. When nectar concentration was the same across all flower morphologies, hummingbirds with bills shorter than 28 mm generally avoided long-corolla flowers (30 mm) and fed more frequently from short-corolla flowers (10 mm), while species with longer bills did not show a preference. When nectar concentration was higher in long-corolla flowers compared to short-corolla flowers (30% vs 10% m/v), short-billed species showed the same visitation rates as above, but long-billed species (>28 mm) changed their visitation patterns and visited long-corolla flowers significantly more often than short-corolla flowers. Our results suggest that visitation rates of long-billed hummingbirds to long-corolla flowers might be influenced more by nectar properties than by flower morphology at mid-elevations in the tropics. Los factores especficos que determinan si los colibres se alimentan de flores que se ajustan a la morfologa de su pico no se conocen bien. Aqu, nos preguntamos si los colibres de pico largo en elevaciones medias tropicales visitan ms frecuentemente las flores que se ajustan a su morfologa de pico cuando stas son energticamente mas rentables en trminos de concentracin de nctar comparadas con flores de corola corta. Medimos la tasa de visitacin de colibres en 3 experimentos que involucraron comederos con 5 morfologas florales y 2 concentraciones de nctar en las montaas de Costa Rica. Sin que resultara sorprendente, todas las especies prefirieron concentraciones de nctar mas altas en todas las morfologas florales cuando se les dio la opcin. Cuando la concentracin de nctar fue la misma en todas las morfologas florales, los colibres con picos mas cortos que 28 mm generalmente evitaron flores de corola larga (30 mm) y se alimentaron mas frecuentemente de flores de corola corta (10 mm), mientras que colibres de pico mas largo no mostraron preferencia. Cuando la concentracin de nctar fue mas alta en flores de corola larga comparada con flores de corola corta (30 vs 10% m/v), las especies de pico corto mostraron los mismos patrones de visitacin mencionados arriba, pero las especies de pico largo (>28 mm) cambiaron los patrones de visitacin y visitaron las flores de corola larga significativamente con mas frecuencia que las flores de corola corta. Nuestros resultados sugieren que las tasas de visitacin de colibres de pico largo a flores de corola larga se pueden ver influenciadas ms por las propiedades del nctar que por la morfologa floral en elevaciones tropicales medias. Palabras clave: alimentacin de colibres, coevolucin, concentracin de nctar, flores artificiales, forma floral, sndrome de polinizacin.
... We found a significant relationship between plant-hummingbird trait-matching and plant fitness for the species-level network; an adequate fit between pollinator bills and plant corollas promotes better pollen deposition. Our results are consistent with previous studies documenting a close match between hummingbird and floral traits (e.g., Snow and Snow 1980, Stein 1992, Temeles et al. 2000. We evaluated corolla length but other traits, such as corolla width, can influence foraging behavior. ...
... We evaluated corolla length but other traits, such as corolla width, can influence foraging behavior. For instance, long flowers with a narrow width often exclude short-billed birds, while short-billed birds can assess long flowers with a wide width (Temeles, 1996). In our study, we only used the length of the structures. ...
... Thus far, important baselines on these nectar-feeding parameters have been gained from experiments with artificial feeders under ad-libitum conditions [e.g. [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. However, the feeding behavior of pollinators at real flowers may differ substantially from their behavior under experimental conditions [3]. ...
Animals should maximize their energy uptake while reducing the costs for foraging. For flower-visitors these costs and benefits are rather straight forward as the energy uptake equals the caloric content of the consumed nectar while the costs equal the handling time at the flower. Due to their energetically demanding lifestyle, flower-visiting bats face particularly harsh energetic conditions and thus need to optimize their foraging behavior at the flowers of the different plant species they encounter within their habitat. In flight cage experiments we examined the nectar-drinking behavior (i.e. hovering duration, nectar uptake, and the resulting feeding efficiency) of the specialized nectar-feeding bat Hylonycteris underwoodi and the more generalistic Glossophaga commissarisi at flowers of two plant species that constitute important nectar resources in the Caribbean lowland rainforests of Costa Rica and compared nectar-drinking behavior between both bat species and at both plant species. We hypothesized that the 1) specialized bat should outperform the more generalistic species and that 2) bats should generally perform better at flowers of the nectar-rich flowers of the bromeliad Werauhia gladioliflora than at the relatively nectar-poor flowers of the Solanaceae Merinthopodium neuranthum that has an extremely long flowering phase and therefore is an extremely reliable nectar resource, particularly for the specialized Hylonycteris. While we did not find substantial differences in the feeding efficiency of the generalist G. commissarisi, we observed an increased feeding efficiency of the specialized H. underwoodi at flowers of the nectar-poor M. neuranthum. This suggests that familiarity and ecological importance are more important determinants of the interaction than just morphological traits. Our results demonstrate that in addition to morphology, behavioral adaptations are also important drivers that determine the fitness of nectar-feeding bats. Both familiarity with and the ecological importance of a resource seem to contribute to shaping the interactions between pollinating bats and their plants.
... For specialized trophic guilds such as hummingbirds, which rely on floral nectar as food (Gass and Montgomerie 1981) and have high metabolic rates (Chai and Millard 1997;Suarez and Gass 2002), both the availability and diversity of food resources are of paramount importance. Hummingbirds can increase their energy intake by selecting plant species with floral traits (e.g., corolla length and curvature) that facilitate their access to nectar and reduce handling time (Temeles 1996;Temeles et al. 2009;Maglianesi et al. 2014). From an adaptive perspective, the higher the congruence between the morphological traits of hummingbirds and their visited flowers, the higher the efficiency of nectar acquisition, thereby improving their overall fitness (Cotton 1998a;Maglianesi et al. 2014;Weinstein and Graham 2017). ...
The limiting similarity theory predicts that ecological communities are formed by species with complementary traits in at least one niche dimension. However, studies using morphological traits to infer niche partitioning have found conflicting evidence regarding the coexistence of species with similar traits. In this study, we analyzed data from 39 hummingbird-plant ecological networks to test whether habitat productivity and morphological traits related to food acquisition (bill length and bill curvature) can explain dietary niche overlap in hummingbird communities. We found that communities characterized by species with traits associated with lower floral specialization (e.g., shorter or less curved bills) had greater dietary niche overlap. However, our findings did not support the hypothesis that habitat productivity influences the relationship between morphological traits and dietary niche overlap. These findings underscore the importance of morphological traits in shaping dietary niche overlap among hummingbird species. While habitat productivity did not emerge as a key factor in this context, it remains important to investigate other ecological variables that might contribute to the complex patterns of niche partitioning among hummingbirds.
... One of the greatest difficulties that nectar-feeding birds face is navigating plant morphologies to extract nectar efficiently and without damaging the flower (Gill and Wolf 1978;Stiles 1981;Carothers 1982;Gass and Roberts 1992;Temeles 1996;Temeles et al. 2002). Most birds typically consume liquids by scooping them up with the lower bill (Rico-Guevara et al. 2019), but the restrictive nature of floral corollas and small nectar volumes mean that the scooping strategy is not a feasible method for collecting nectar, therefore nectarfeeding birds must use their tongues to feed . ...
Synopsis
We investigated the kinematics and biomechanics of nectar feeding in five species of honeyeater (Phylidonyris novaehollandiae, Acanthagenys rufogularis, Ptilotula penicillata, Certhionyx variegatus, Manorina flavigula). There is abundant information on honeyeater foraging behaviors and ecological relationships with plants, but there has never been an examination of their nectar-feeding from kinematic and biomechanical perspectives. We analyzed high-speed video of feeding in captive individuals to describe the kinematics of their nectar feeding, with specific focus on describing tongue movements and bill-tongue coordination, and to characterize the mechanism of nectar uptake in the tongue. We found clear interspecific variation in kinematics and tongue filling mechanics. Species varied in lick frequency, tongue velocity, and protrusion and retraction duration, which, in some cases, are relevant for differences in tongue filling mechanisms. We found support for the use of capillary filling in Certhionyx variegatus only. By contrast, Phylidonyris novaehollandiae, Acanthagenys rufogularis, Ptilotula penicillata, and Manorina flavigula employed a modified version of the expansive filling mechanism seen in hummingbirds, as there was dorsoventral expansion of the tongue body, even the portions that remain outside the nectar, once the tongue tip entered the nectar. All species use fluid trapping in the distal fimbriated portion of the tongue, which supports previous hypotheses describing the honeyeater tongue as a “paintbrush.”
... Thus, on one hand, short-billed birds cannot drain long-tubed flowers legitimately simply because they cannot reach all of the nectar (Snow and Snow 1972;Feinsinger 1976;Rico-Guevara et al. 2019; but see Ewald and Williams 1982;Rengifo et al. 2006). On the other hand, strong arguments from functional morphology imply that long-billed birds are less efficient than short-billed birds at extracting nectar from short-tubed flowers (Temeles 1996;Temeles et al. 2002;Rico-Guevara et al. 2019, 2021, as first suggested by Snow and Snow (1972), Stiles (1975), and Wolf et al. (1976). Moreover, in many cases, short-billed birds aggressively exclude long-billed species from defended feeding territories, protecting the nectar sources that short-billed birds are most capable of exploiting (Feinsinger and Colwell 1978;Snow and Snow 1980;Skandalis et al. 2017), although there are exceptions in which longerbilled species, particularly males with weaponized bills, dominate shorter-billed species (Rico-Guevara et al. 2019). ...
Differences among hummingbird species in bill length and shape have rightly been viewed as adaptive in relation to the morphology of the flowers they visit for nectar. In this study we examine functional variation in a behaviorally related but neglected feature: hummingbird feet. We gathered records of hummingbirds clinging by their feet to feed legitimately as pollinators or illegitimately as nectar robbers-"unorthodox" feeding behaviors. We measured key features of bills and feet for 220 species of hummingbirds and compared the 66 known "clinger" species (covering virtually the entire scope of hummingbird body size) with the 144 presumed "non-clinger" species. Once the effects of phylogenetic signal, body size, and elevation above sea level are accounted for statistically, hummingbirds display a surprising but functionally interpretable negative correlation. Clingers with short bills and long hallux (hind-toe) claws have evolved-independently-more than 20 times and in every major clade. Their biomechanically enhanced feet allow them to save energy by clinging to feed legitimately on short-corolla flowers and by stealing nectar from long-corolla flowers. In contrast, long-billed species have shorter hallux claws, as plant species with long-corolla flowers enforce hovering to feed, simply by the way they present their flowers.
Background
Most bromeliad (Bromeliaceae) species have specialised flowers for specific pollinators, establishing a mutualistic relationship. However, other animals may also pollinate bromeliads, though it is logical to assume that specialised flowers are more compatible with one specific group of pollinators.
Aims
We compared the performance of a hummingbird (Thalurania glaucopis) and a butterfly (Heliconius erato) as pollinators of flowers classified as ornithophilous of the bromeliad Tillandsia stricta (Bromeliaceae).
Methods
We applied the single visit method with seed counting, and measured corolla tube length of flowers. We also conducted observations on plants to determine the effectiveness of pollination by the hummingbird and the butterfly.
Results
Both animal species pollinated flowers, although the hummingbird was a more frequent and effective pollinator, resulting in greater seed production. The corolla tube length of T. stricta was a significant predictor of seeds produced per flower following interactions with the hummingbird, but not with the butterfly.
Conclusion
Our results indicate a more intimate flower – pollinator relationship between T. stricta and the hummingbird than the butterfly in this population of the bromeliad in Rio de Janeiro, Brazil.
I measured phenotypic selection of floral traits through both male and female functions of the hermaphroditic flowers of Ipomopsis aggregata (Pursh) V. Grant subsp. aggregata (Polemoniaceae). Fluorescent powdered dyes were used to track movement of pollen by hummingbirds and to measure pollen delivery to individual plants as well as pollen receipt. A phenotypic selection analysis revealed that selection due to male-male competition during pollination was capable of delaying flowering date and widening corolla tubes by 0.22 and 0.24 standard-deviation units, respectively, in a single generation. Several floral traits were highly correlated with each other. Multivariate selection analysis suggested that selection through male function directly favored late flowering as well as a sexual expression characterized by a short pistillate phase and long corollas. Selection intensities through male and female functions were of similar overall magnitude during the pollination stage of the life cycle, but different traits were favored, and selection sometimes acted in opposing directions. In 1985, selection through female function favored increased time spent in the pistillate phase and exserted stigmas (unlike selection through male function). As a result, individual plants varied greatly in functional gender. Plants that had exserted stigmas and narrow corollas and that spent a disproportionately long time in the pistillate phase achieved greater pollination success as females, while plants with the opposite traits achieved greater success as males. Moreover, female pollination success tended to increase, and male pollination success to decrease, with time spent in the pistillate phase, supporting a critical assumption of sex-allocation theory. Selection in the populations studied fluctuated from year to year and was highly sex-specific.
Nine hummingbird-pollinated species of Heliconia occur together at Finca La Selva, in the wet Caribbean lowlands of Costa Rica. In forest habitats, Heliconia clumps (clones) are typically small; in more open areas, many clumps attain large size. This probably reflects differences in light intensity and degree of vegetative competition in these habitats. Nine species of hummingbirds regularly visit Heliconia flowers at La Selva. The four hermits are nonterritorial, traplining foragers with long, curved bills. Non-hermits frequently hold territories at Heliconia clumps, and have short, straight bills. Pollination by hermits tends to produce more cross-pollination; territorial hummingbirds increase self-pollination. Different Heliconia species appear to be specialized for pollination by either hermits on non-hermits, largely through components of the caloric phenotype: amount and timing of nectar production, rate of inflorescene and flower production, and morphological paramerters that affect the energetic efficiency of nectar-harvesting hummingbirds. Habitat may influence pollination systems through its effects on clump size and thus on the number of flowers a clump can have at any one time. Ultimately, specialization for hermits or non-hermits may depend on the degree of self-compatibility of the different Heliconia species. Hermit-pollinated Heliconia mostly show sequential and nonoverlapping flowering peaks, probably resulting from competition for pollinators and/or selection against hybridization. Two hermit-pollinated species bloom simultanesoulsy, thereby inducing the birds to utilize an other-wise little-used microhabitat. Heliconia species pollinated by non-hermits bloom in the early to middle rainy season, and are mostly separated by habitat. Isolating mechanisms among sympatric Heliconia species involve both spatial and temporal patterns of partioning available pollinators. Floral parameters include mechanical (different site of pollen deposition on the bird) and ethological (caloric and visual factors affecting flower choice) mechanisms. Selection for pollinator specificity may result in convergence of blooming peaks, provided that other isolating mechanisms are present. Human activity has broken down some habitat barriers by producing large areas of second growth.
Monarda didyma L. and M. clinopodia L. occasionally form introgressive populations in the mountains of eastern Tennessee and western North Carolina. Monarda didyma produces large red scentless nectar-rich flowers and is pollinated primarily by the Ruby-throated Hummingbird (Archilochus colubris [Linnaeus]). The smaller, white, scented flowers of Monarda clinopodia produce small amounts of nectar and are pollinated mainly by bumblebees (Bombus spp.). The two species are highly interfertile and have strongly overlapping flowering periods. Differences in pollinator sets and pollinator constancy appear to be only partial barriers to hybridization. Lack of suitable habitats for the establishment of hybrid progeny might be the most important isolating mechanism between the two species.
Sweet-flowered plants of Polemonium viscosum in Colorado are visited by a fly-dominated pollinator fauna at timberline (krummholz), but almost exclusively by bumblebees in higher-elevation tundra habitats. Significant increases in flower size and height are associated with increasing elevation along this habitat gradient. This paper presents the results of an experiment designed to test whether bumblebees exert sufficient selection on morphometric floral phenotypes to account for the clinal shifts seen in natural populations. Two populations of sweet-flowered plants of krummholz origin were established: one randomly pollinated, the other solely bumblebee-pollinated. I tested the effects of two independent axes of floral variation, obtained by principal-components analysis, on mean seed set per flower of plants in each population. PC1, with strong correlations to corolla diameter, corolla length, and stem height, explained a significant amount of variance in seed set for bumblebee-pollinated plants but had no bearing on that of randomly pollinated plants. PC2, with strong correlation to flower number, did not influence seed set in either population. Bumblebee behavior was correlated with variation in PC1 scores of the selected population, yielding positive directional selection on morphometric floral traits associated with PC1. Selection coefficients for PC1, corolla length, corolla diameter, and inflorescence height were estimated, respectively, as 0.11, 0.09, 0.07, and 0.06 (P < 0.025 in all cases). These results support the hypothesis that pollinator-mediated selection can bring about changes in floral form, and can explain shifts in floral morphology of P. viscosum along natural habitat gradients.
In this investigation of orchids, first published in 1862, Darwin expands on a point made in On the Origin of Species that he felt required further explanation, namely that he believes it to be 'a universal law of nature that organic beings require an occasional cross with another individual'. Darwin explains the method by which orchids are fertilised by insects, and argues that the intricate structure of their flowers evolved to favour cross pollination because of its advantages to the species. The book is written in Darwin's usual precise and elegant style, accessible despite its intricate detail. It includes a brief explanation of botanical terms and is illustrated with 34 woodcuts.
We studied temporal and spatial relationships among hummingbirds and their food flowers in a tropical dry forest on the western coast of Mexico. From June 1985 to July 1986 we recorded flowering phenology, nectar production, and hummingbird visits to flowers, and made morphological measurements of hummingbirds and plants. Twenty-three species of plants were studied, and ten of them were pollinated exclusively by hummingbirds. Out of the six hummingbird species we found, only two were permanent residents (Amazilia rutila and Cynanthus latirostris), two were altitudinal wanderers (Chlorostilbon canivetii and Heliomaster constantii), and two were winter visitors (Archilochus colubris and A. alexandri). The most abundant resident was A. rutila. It used clumped flowers and defended feeding territories around them. The other species in the area, subordinates to A. rutila, used less plentiful resources without the establishment of territories.
Sweet-flowered plants of Polemonium viscosum in Colorado are visited by a fly-dominated pollinator fauna at timberline (krummholz), but almost exclusively by bumblebees in higherelevation tundra habitats. Significant increases in flower size and height are associated with increasing elevation along this habitat gradient. This paper presents the results of an experiment designed to test whether bumblebees exert sufficient selection on morphometric floral phenotypes to account for the clinal shifts seen in natural populations. Two populations of sweet-flowered plants of krummholz origin were established: one randomly pollinated, the other solely bumblebee-pollinated. I tested the effects of two independent axes of floral variation, obtained by principalcomponents analysis, on mean seed set per flower of plants in each population. PC1, with strong correlations to corolla diameter, corolla length, and stem height, explained a significant amount of variance in seed set for bumblebee-pollinated plants but had no bearing on that of randomly pollinated plants. PC2, with strong correlation to flower number, did not influence seed set in either population. Bumblebee behavior was correlated with variation in PC1 scores of the selected population, yielding positive directional selection on morphometric floral traits associated with PC1. Selection coefficients for PC1, corolla length, corolla diameter, and inflorescence height were estimated, respectively, as 0.11, 0.09, 0.07, and 0.06 (P < 0.025 in all cases). These results support the hypothesis that pollinator-mediated selection can bring about changes in floral form, and can explain shifts in floral morphology of P. viscosum along natural habitat gradients.