Nectar sugar and amino acid preferences of Battus philenor (Lepidoptera, Papilionidae)

Abstract.

• 1 Butterflies of Battus philenor were tested for their preferences for nectar sugars and amino acids in an outdoor cage experiment.
• 2 The butterflies clearly preferred both sucrose and fructose over glucose. They also preferred sucrose over fructose.
• 3 No other preferences were found to be statistically significant, although male butterflies tended to prefer a plain sugar solution over a sugar solution containing a mixture of amino acids: females consumed both of these solutions in almost equal proportions.
• 4 The results are discussed with respect to nectar composition of butterfly pollinated flowers, flower preferences, physiological and reproductive aspects of butterflies.

     

Nectar sugar preferences and absorption in a generalist African frugivore, the Cape White-eye Zosterops pallidus

Sugar preferences and absorption efficiencies were investigated in the Cape White-eye Zosterops pallidus , a generalist frugivore. Unlike the mainly American frugivorous passerines previously studied, Cape White-eyes preferred 20% (weight: weight) solutions of sucrose to glucose or fructose, and apparently possess high sucrase activity because absorption efficiencies for all three sugars were close to 100%. Cape White-eyes rejected 20% solutions containing a fourth nectar sugar, xylose, recently found in the nectar of Protea and Faurea (Proteaceae). Absorption efficiencies for xylose averaged 61%, but we do not know whether this sugar is utilized by the birds. Cape White-eyes thus resemble specialized southern African nectarivores (Nectariniidae, Promeropidae) in their responses to nectar sugars.     

Nectar concentration preferences and sugar intake in the white-bellied sunbird, <Emphasis Type="Italic">Cinnyris talatala</Emphasis> (Nectariniidae)

Plant nectar is a simple food consumed by many different animals. Preferences regarding its components, especially sugars, have been studied for many species, but the preferences of nectar-feeding birds for different sugar concentrations are less well known than their sugar type preferences. The concentration preferences of white-bellied sunbirds, Cinnyris talatala, were examined using paired solutions of either sucrose or equicaloric 1:1 mixtures of glucose and fructose. Preferences were tested over a broad concentration range of 0.25–2.5 M sucrose equivalents (using 0.25 or 0.5 M differences between pairs). On both sucrose and hexose diets, the higher concentration was preferred up to 1 M, but there were no significant preferences above this concentration, except that birds preferred 1.5 to 2 M sucrose. As with other nectar-feeding vertebrates, the laboratory preferences of sunbirds do not explain the low concentration of their natural nectars. We recorded apparent excess sugar consumption during 6 h preference tests involving concentrated hexose diets; this could be due to digestive constraints or viscosity differences between sucrose and hexose solutions.     

Sucrose hydrolysis does not limit food intake by Pallas's long-tongued bats

Nectarivorous bats include very dilute nectar in their natural diet, and recent work with Pallas's long-tongued bat Glossophaga soricina showed that sugar (energy) intake rate decreased at dilute sucrose solutions. However, chiropterophillous nectar is composed mainly of the hexoses glucose and fructose. Because bats fed hexose nectar would save the delay of hydrolyzing sucrose, we hypothesized that sugar intake rate should be higher on this diet than on sucrose nectar. We compared intake response in Pallas's long-tongued bats offered 1 : 1 glucose-fructose (hexose) and sucrose diets at 5%, 10%, 20%, 30%, and 40% (mass/volume) sugar solutions. We also tested the hypothesis that sucrose hydrolysis limits food intake in bats. Intake response was the same in bats fed both types of diet: sugar intake rate was lower in dilute solutions and then increased with sugar concentration. Similar intake responses in both diets indicate that sucrose hydrolysis alone does not limit food intake and support the idea that the burden of processing excess water in dilute solutions plays a major role.     

Nectar sugars enhance fitness in male Coenonympha pamphilus butterflies by increasing longevity or realized reproduction

The principal components of floral nectar are water and the sugars sucrose, fructose and glucose. Several studies have shown the importance of nectar sugars for female butterfly fecundity, whereas to date little attention has been paid to the effect of nectar sugars on male butterfly reproduction. Clear evidence for an effect of nectar sugars on male realized reproductive success is still missing. In this study, we fed male Coenonympha pamphilus butterflies nectar mimics with low (5%), medium (20%) or high (30%) total sugar concentrations with a sucrose:glucose:fructose ratio of 2.7:1.1:1. Sugar solutions were made mimicking Knautia arvensis, an essential nectar plant for C. pamphilus and many other European butterflies. Realized male reproductive success for each treatment was measured indirectly via nuptial gifts, by recording reproductive parameters and by characterizing time patterns over the oviposition period of their female partner. Male butterflies fed high‐concentrated nectar sugars had a longer lifespan than males fed low‐concentrated nectar sugars. In contrast, offspring of males fed medium‐concentrated nectar sugars had a higher hatching mass than progeny of males fed low‐concentrated nectar sugars, indicating a tradeoff between somatic maintenance and reproduction in the use of nectar sugars. Thus, allocation patterns of nectar sugars differed according to sugar concentrations in adult food. The method used in this experiment took into account the indispensable role of female butterflies in passing male nutrients to offspring. With this comprehensive approach, we can show the general importance of nectar sugars for male butterfly fitness and support previous findings suggesting a coevolutionary process between butterflies and flowers dependent on butterfly pollination.     

The intake responses of three species of leaf-nosed Neotropical bats

Flower-visiting bats encounter nectars that vary in both sugar composition and concentration. Because in the new world, the nectars of bat-pollinated flowers tend to be dominated by hexoses, we predicted that at equicaloric concentrations, bats would ingest higher volumes of hexoses than sucrose-containing nectars. We investigated the intake response of three species of Neotropical bats, Leptonycteris curasoae, Glossophaga soricina and Artibeus jamaicensis, to sugar solutions of varying concentrations (292, 438, 584, 730, 876, and 1,022 mmol L⁻¹) consisting of either sucrose or 1:1 mixtures of glucose and fructose solutions. Bats did not show differences in their intake response to sucrose and 1:1 glucose–fructose solutions, indicating that digestion and absorption in bat intestines are designed under the principle of symmorphosis, in which no step is more limiting than the other. Our results also suggest that, on the basis of energy intake, bats should not prefer hexoses over sucrose. We used a mathematical model that uses the rate of sucrose hydrolysis measured in vitro and the small intestinal volume of bats to predict the rate of nectar intake as a function of sugar concentration. The model was a good predictor of the intake responses of L. curasoae and G. soricina, but not of A. jamaicensis.     

Sugar Preferences in Nectar‐ and Fruit‐Eating Birds: Behavioral Patterns and Physiological Causes1

Sucrose, glucose, and fructose are the three sugars that commonly occur in floral nectar and fruit pulp. The relative proportions of these three sugars in nectar and fruit in relation to the sugar preferences of pollinators and seed dispersers have received considerable attention. Based on the research of Herbert and Irene Baker and their collaborators, a dichotomy between sucrose‐dominant hummingbird‐pollinated flowers and hexose‐dominant passerine flowers and fruits was proposed. Data on sugar preferences of several hummingbird species (which prefer sucrose) vs. a smaller sample of passerines (which prefer hexoses) neatly fitted this apparent dichotomy. This hummingbird–passerine dichotomy was strongly emphasized until the discovery of South African plants with sucrose‐dominant nectars, which are pollinated by passerines that are able to digest, and prefer sucrose. Now we know that, with the exception of two clades, most passerines are able to assimilate sucrose. Most sugar preference studies have been conducted using a single, relatively high, sugar concentration in the nectar (ca 20%). Thus, we lack information about the role that sugar concentration might play in sugar selection. Because many digestive traits are strongly affected not only by sugar composition, but also by sugar concentration, we suggest that preferences for different sugar compositions are concentration‐dependent. Indeed, recent studies on several unrelated nectar‐feeding birds have found a distinct switch from hexose preference at low concentrations to sucrose preference at higher concentrations. Finally, we present some hypotheses about the role that birds could have played in molding the sugar composition of plant rewards.     

Sugar gustatory thresholds and sugar selection in two species of Neotropical nectar-eating bats

Nectar-feeding bats play an important role in natural communities acting as pollinators; however, the characteristics that affect their food selection are unclear. Here we explore the role that sugar gustatory thresholds and sugar concentration play on sugar selection of Glossophaga soricina and Leptonycteris yerbabuenae. We offered bats paired feeders containing sugar solutions of sucrose (S), glucose (G) or fructose (F) vs. pure water, and sucrose vs. 1:1 equicaloric solutions of glucose–fructose at 5, 15 and 35% (wt./vol.). To see the effect of sweetness on sugar selection, we habituated the bats with a diet containing either sucrose or hexoses and subsequently evaluated sugar preferences. Sugar thresholds were S < G,F for G. soricina and G < S < F for L. yerbabuenae. These thresholds did not match with sugar preferences when the bats fed on dilute nectars. L. yerbabuenae changed its sugar preferences with concentration while G. soricina did not. Finally, the bats consistently preferred the sugar they were habituated to. Our results suggest that bats become accustomed to the sugar found in the most abundant plants they use, and thus prefer the most common sugars included in their diet. This could confer an advantage by allowing them shifting sugar preferences on the most common food present in their environment.     

Larval starvation reduces responsiveness to feeding stimuli and does not affect feeding preferences in a butterfly

It is commonly assumed that holometabolic insects such as Lepidoptera rely primarily on larval storage reserves for reproduction. Recent studies though have documented a prominent role of adult-derived carbohydrates for butterfly reproduction. Moreover, a few studies have shown that adult butterflies may also benefit from adult-derived amino acids, at least when larval storage reserves are reduced. Given that in holometabolous insects larval deficiencies are carried over into the adult stage, reduced storage reserves have the potential to modulate adult feeding preferences and responses in order to allow for a successful compensation. We tested this hypothesis here in the fruit-feeding butterfly Bicyclus anynana using larval food stress to manipulate storage reserves. Alcohols (methanol, ethanol, butanol, propanol), sugars (maltose, glucose, fructose, sucrose), and acetic acid acted as feeding stimuli, while butterflies did not respond to other substances such as amino acids, yeast, salts, or vitamins. Contrary to expectations, stressed butterflies showed a weaker response than controls to several feeding stimuli. In preference tests, butterflies preferred sugar solutions containing proline, arginine, glutamic acid, acetic acid, or ethanol over plain sugar solutions, but discriminated against salts. However, there were no general differences among starved and control butterflies. We conclude that larval food-stress does not elicit compensatory feeding behavior such as a stronger preference for amino acids or other essential nutrients in B. anynana. Instead, the stress imposed by a period of starvation yielded negative effects.     

Sugar preferences of some southern African nectarivorous birds

Three southern African nectarivorous passerine birds, Gurney's Sugarbird Promerops gur-neyi , the Malachite Sunbird Nectarinia famosa and the Black Sunbird Nectarinia amethystina , were tested to determine their hexose and sucrose preferences. All three species preferred sucrose when offered a choice of 0.25 M solutions of glucose, fructose and sucrose. However, when the concentrations were increased to 0.73 M, the three species showed no preference for any of the three sugars. The choice made at low concentrations (equivalent to the lower limit of the range of nectar concentrations of preferred nectar-producing plants) may reflect preference for the sugar with the highest energy reward. We also examined the proposition that birds offered a choice of different concentrations of one sugar would show ranked preferences and maximize their rate of energy return by selectivity. In contrast to expectations, Gurney's Sugarbird and the Malachite Sunbird showed no preference for the highest concentrations. We suggest that dietary choices in these species indicate the birds had either reached a limit where they had sufficient energy intake or were affected by post-ingestion constraints.     

FACULTATIVE MUTUALISM BETWEEN WEEDY VETCHES BEARING EXTRAFLORAL NECTARIES AND WEEDY ANTS IN CALIFORNIA

Vetches (Vicia spp.) were studied in the San Francisco Bay Area of California in the spring of 1978. The stipular nectaries of the vetches are visited by the Argentine ant, Iridomyrmex humilis Mayr. The nectaries were removed to exclude ants in controlled experiments to determine if these ants protect the vetches from herbivores or seed predators. Plants with excised nectaries suffered substantially greater damage to their foliage than control plants, indicating that ants protect the foliage. There was no indication that ants protect the vetches from seed predators, but fruit set was substantially lower in plants with excised nectaries. Analysis of sugar and amino acid composition of extrafloral nectar served as a basis for feeding tests with Argentine ants by using artificial nectar solutions. Ants preferred sucrose and glucose solutions over fructose. They showed no preference for any one sugar mixture over another, nor did they exhibit differential recruitment to artificial nectar solutions containing only sugars or sugars and amino acids.     

Feeding responses by female Pieris brassicae butterflies to carbohydrates and amino acids

Most Lepidoptera feed during the adult stage on carbohydrate‐rich food sources, primarily floral nectar. However, little is known about the factors leading to the acceptance of a possible food source. It is reported that butterflies select for nectar rich in sucrose and amino acids. This suggests that the insects have developed a sensitivity to these nectar compounds. We tested females of the large cabbage white, Pieris brassicae L. (Lepidoptera: Pieridae) for their responses to 10 different nectar‐ or honeydew‐sugars after either tarsal or proboscis stimulation. In no‐choice experiments, food‐deprived P. brassicae showed the strongest response to sucrose, followed by fructose. Other sugars, including galactose, glucose, maltose, mannose, melezitose, melibiose, raffinose and trehalose, did not elicit a feeding response. Mixtures of essential or common non‐essential amino acids did not stimulate feeding. In a choice situation, P. brassicae preferred sucrose over fructose, whereas they accepted a sucrose and amino acid solution equally to a plain sucrose solution. The results indicate that for P. brassicae, feeding is mainly elicited by sucrose and fructose.     

A cell wall invertase controls nectar volume and sugar composition

Nectar volume and sugar composition are key determinants of the strength of plant–pollinator mutualisms. The main nectar sugars are sucrose, glucose and fructose, which can vary widely in ratio and concentration across species. Brassica spp. produce a hexose-dominant nectar (high in the monosaccharides glucose and fructose) with very low levels of the disaccharide sucrose. Cell wall invertases (CWINVs) catalyze the irreversible hydrolysis of sucrose into glucose and fructose in the apoplast. We found that BrCWINV4A is highly expressed in the nectaries of Brassica rapa. Moreover, a brcwinv4a null mutant: (i) has greatly reduced CWINV activity in the nectaries; (ii) produces a sucrose-rich nectar; but (iii) with significantly less volume. These results definitively demonstrate that CWINV activity is not only essential for the production of a hexose-rich nectar, but also support a hypothetical model of nectar secretion in which its hydrolase activity is required for maintaining a high intracellular-to-extracellular sucrose ratio that facilitates the continuous export of sucrose into the nectary apoplast. The extracellular hydrolysis of each sucrose into two hexoses by BrCWINV4A also likely creates the osmotic potential required for nectar droplet formation. These results cumulatively indicate that modulation of CWINV activity can at least partially account for naturally occurring differences in nectar volume and sugar composition. Finally, honeybees prefer nectars with some sucrose, but wild-type B. rapa flowers were much more heavily visited than flowers of brcwinv4a, suggesting that the potentially attractive sucrose-rich nectar of brcwinv4a could not compensate for its low volume.     

Sugar preferences, absorption efficiency and water influx in a Neotropical nectarivorous passerine, the Bananaquit (Coereba flaveola)

Nectarivory has evolved repeatedly in a number of unrelated bird taxa throughout the world and nectar feeding birds, regardless of their taxonomic affiliation, display convergent foraging and food processing adaptations that allow them to subsist on weak sugar solutions. However, phylogeny influences sugar type preferences of nectarivores. We investigated sugar preferences, assimilation efficiency and water flux in a Neotropical honeycreeper, the Bananaquit (Coereba flaveola; Coerebidae), a member of a radiation of tanagers and finches. Bananaquits showed no preference for nearly equicaloric (25% w/v) sucrose, glucose, fructose or glucose-fructose mixtures in pair-wise choice tests. In agreement with this lack of preference, they were equally efficient at absorbing sucrose and both hexoses. Apparent assimilation efficiency of these sugars was around 97.5%. In pair-wise tests, Bananaquits displayed a strong preference for the most concentrated sucrose solution when the lowest concentration ranged from 276 to 522 mM. Between 522 and 1120 mM sucrose solution concentrations, Bananaquits were able to adjust their volumetric food intake in order to maintain a constant energy intake. At solution concentration of 276 mM, birds could not maintain their rate of energy intake by increasing food consumption enough. We consider that at low sugar concentrations, Bananaquits faced a physiological constraint; they were unable to process food at a fast enough rate to meet their energy needs. We also explored the possibility that dilute nectars might be essential to sustain high water needs of Bananaquits by allowing them to control osmolarity of the food. Between 276 and 1120 mM sucrose solution concentrations, average amount of free water drunk by Bananaquits was independent of food concentration. They drank very little supplementary water and did not effectively dilute concentrated nectars. The evidence suggests that water bulk of dilute nectars is a burden to Bananaquits.     

Nectar formation and floral nectary anatomy of Anigozanthos flavidus: a combined magnetic resonance imaging and spectroscopy study

Metabolic processes underlying the formation of floral nectar carbohydrates, especially the generation of the proportions of fructose, glucose, and sucrose, are important for understanding ecological plant-pollinator interactions. The ratio of sucrose-derived hexoses, fructose and glucose, in the floral nectar of Anigozanthos flavidus (Haemodoraceae) was observed to be different from 1:1, which cannot be explained by the simple action of invertases. Various NMR techniques were used to investigate how such an unbalanced ratio of the two nectar hexoses can be formed. High-resolution (13)C NMR spectroscopy in solution was used to determine the proportion of carbohydrates in vascular bundles of excised inflorescences fed with (13)C-labelled carbohydrates. These experiments verified that feeding did not affect the metabolic processes involved in nectar formation. In vivo magnetic resonance imaging (e.g. cyclic J cross-polarization) was used to detect carbohydrates in vascular bundles and (1)H spin echo imaging non-invasively displayed the architecture of tepal nectaries and showed how they are connected to the vascular bundles. A model of the carbohydrate metabolism involved in forming A. flavidus floral nectar was established. Sucrose from the vascular bundles is not directly secreted into the lumen of the nectary but, either before or after invertase-catalysed hydrolyses, taken up by nectary cells and cycled at least partly through glycolysis, gluconeogenesis, and the pentose phosphate pathway. Secretion of the two hexoses in the cytosolic proportion could elegantly explain the observed fructose:glucose ratio of the nectar.     

Sugar preferences and xylose metabolism of a mammal pollinator, the Namaqua rock mouse (Aethomys namaquensis).

The sugar preferences of 10 Namaqua rock mice, Aethomys namaquensis, were assessed using pairwise combinations of 30% (w/w) solutions of sucrose, glucose, fructose, xylose, and a mixture of equal parts of glucose and fructose. The tests were designed to control for side biases that were apparent in preliminary experiments. The mice preferred sucrose to fructose and glucose. Xylose, although the least preferred sugar, was willingly consumed by the rodents (up to 5.8 mL in 24 h). This contrasts with the strong rejection of xylose by nectarivorous birds on which similar preference tests were performed. The efficiency of xylose absorption and metabolism by A. namaquensis was investigated by measuring dietary intake, blood xylose levels, and urinary and fecal xylose output. Again in contrast to the birds, the apparent absorption efficiency of xylose was found to be very high at 97%, but exactly how the xylose is metabolized requires further study. Xylose is thought to be only slowly metabolized by mammals, and it is possible that intestinal bacteria may serve this purpose, like the ruminal bacteria that break down xylans in plant tissue.     

Do Peacock butterflies (Inachis io L.) detect and prefer nectar amino acids and other nitrogenous compounds?

The preferences for nectar amino acids, urea and ammonium ions of peacock butterflies, Inachis io, were tested experimentally. Females clearly preferred a mimic of Lantana camara nectar containing amino acids to an otherwise similar plain sugar solution, whereas males did not discriminate between these test solutions. Neither males nor females discriminated between the full mixture of amino acids in a mimic of L. camara nectar and similar test solutions containing only the single amino acids arginine or proline. Furthermore, the butterflies were not able to detect methionine in the test solutions. Both sexes detected and preferred ammonium ions in test solutions but showed no response to urea. These results support the hypothesis that butterflies can select for high amino acid concentrations in floral nectar. However, it seems unlikely that they select for particular amino acids. The rather unspecific response of I. io males to the nectar constituents tested may result from their relatively low demand for nitrogen for spermatophore and sperm production, while their high activity may make energy supply (i.e. sugar) more important. The preference for ammonium ions suggests that I. io could also acquire nitrogen from ammonium-contaminated soil by puddling, as has been shown for sodium in swallowtail butterflies. Received: 15 August 1997 / Accepted: 14 September 1998     

Sugar preferences and feeding strategies in the hawkmoth Macroglossum stellatarum

Hummingbird hawkmoths were tested for their preferences for different types of sugar. In triple choice tests, moths sucked for longer periods from sucrose than from fructose and glucose. Naive moths released in a large flight cage and monitored over 24 days, drank on average 137 l sucrose, 67 l fructose and 7 l glucose daily. In an independent test, moths spent more time feeding from sucrose than from fructose and more time feeding from fructose than from glucose. Animals in hibernation made less but longer feeding bouts, whereas animals that fed every day and newly eclosed animals were more likely to make more but shorter visits to feeders. The hawkmoths learned to associate colour with the preferred sugar. In a dual choice test, flower-naive moths chose blue rather than yellow artificial flowers. After the initial test, these animals received sucrose from the yellow and glucose from the blue feeders. A week later they chose yellow more frequently than blue indicating that they had learned to associate a colour with the preferred type of sugar.     

The sweet side of life: nectar sugar type and concentration preference in Wahlberg's epauletted fruit bat

Whether nectarivores or frugivores place selective pressure on the plants they feed on, in terms of nectar or fruit traits, is much debated. Globally sugar preferences, concentration preference and digestive ability of avian nectarivores have been extensively researched. In contrast, relatively little is known about mammalian nectarivores or frugivores in terms of these, particularly Old World species. Consequently effect of sugar type and concentration on food preference in Wahlberg's epauletted fruit bat Epomophorus wahlbergi was investigated. Pair-wise choice tests were conducted using equicaloric hexose and sucrose solutions at five different concentrations (5%-25%). It was expected that they would prefer hexose sugars as these are dominant in available indigenous fruits. However, bats preferred hexoses only when offered dilute (5%) concentrations. From 10% to 25% they showed a decrease in volume intake. Their body mass was generally higher and similar after feeding during the night with the exception of 5% concentration where the mean body mass decreased. When E. wahlbergi were offered a range of sucrose or hexose solutions (10%-25%) respectively, they showed no concentration preference in terms of total volume consumed, nor energy intake. These findings suggest that these fruit bats do not appear to act as a selective pressure on sugar composition in Old World fruit. In fruit bats with high energy requirements, dietary flexibility may be an advantage when faced with seasonal and unpredictable fruit availability.     

Sugar preferences and digestive efficiency in an opportunistic avian nectarivore, the Dark-capped Bulbul Pycnonotus tricolor

It has recently been recognized that flowers pollinated by generalist opportunistic nectarivores tend to have different nectar properties to those pollinated by specialist nectarivores (including both hummingbirds and specialist passerines). While renewed interest in specialist avian nectarivore sugar preferences and digestive physiology has helped explain the concentrated sucrose-dominated nectar of plants they feed on, there has been little progress in understanding why generalist or occasional nectar-feeding birds tend to be associated with flowers that have dilute hexose-dominated nectar. We examined sugar preferences and assimilation efficiencies over a range of concentrations, and concentration preferences, in Dark-capped Bulbuls Pycnonotus tricolor, one of the more common occasional avian nectarivores in southern Africa. Dark-capped Bulbuls showed significant preference for hexose sugar solutions, irrespective of concentration, when given a choice between hexose and sucrose solutions in equicaloric pair-wise choice tests conducted at five different concentrations (5–25%). This contrasts with results from specialist nectarivore groups which generally show a significant concentration-dependant switch in preference from hexose at low concentrations to sucrose at high concentrations for equicaloric solutions. In addition, Dark-capped Bulbuls showed an unusual lack of preference for solutions of higher sugar concentration when simultaneously offered four solutions varying in concentration from 10 to 25%. Dark-capped bulbuls also showed a unique effect of concentration on sugar assimilation efficiency, assimilating relatively more energy on 5% diets than on 25% diets. Although able to assimilate sucrose effectively, assimilation rates of hexose sugars were marginally higher. These results shed new light on pollination systems involving occasional nectarivores and, in particular, help to explain the prevalence of low concentration hexose-dominated nectars in flowers pollinated by these birds.