Nectar uptake rates and optimal nectar concentrations of two butterfly species

Summary The relationship between sucrose concentration of nectar and volume uptake rate by the butterflies Agraulis vanillae (Nymphalidae) and Phoebis sennae (Pieridae) was examined. Recent theoretical models simulating feeding energetics of nectarivores have assumed that this volume uptake rate is produced by a constant but undetermined pressure drop (the difference between pressure at the proximal and distal ends of the feeding channel) at all nectar concentrations. These models predict that nectar of 20–25% sucrose maximizes the rate of energy intake and should thus be preferred by nectarivores. Data collected for Agraulis and Phoebis falsify this pressure drop assumption; both species produce greater pressure drops with increasing nectar concentration. In addition, males of both species produce greater suction pressure and uptake rates than females. This results in greater rates of energy intake for males of both species. The volume uptake rates produced by each species differ from those predicted by the models. This produces a maximal rate of energy intake at 35–40% sucrose rather than 20–25%. The empirically determined relationship between energy intake rate and nectar concentration esembles that predicted for discontinuous nectar feeders such as hummingbirds more closely than the relationship predicted for continuous suction feeders, suggesting that other basic assumptions about the feeding mechanism of butterflies should be critically examined.     

On the mechanics and energetics of nectar feeding in butterflies.

A mechanistic model describing the mechanics and energetics of nectar-feeding in butterflies is developed. The butterflies Collas eurytheme and Danaus plexippus are used to illustrate the model. Simulation results indicate that there are mechanical limitations upon the range of nectar sugar concentrations and nectar extraction times available to butterflies. There is a unique optimum for net rate of energy gain at 20–25% nectar sugar concentration which is independent of the metabolic rate and of proboscis shape and size over the ranges found in butterflies. The optimal nectar extraction rate depends upon the size and shape of the proboscis. These results are discussed in relation to the design of nectar feeding structures, optimal foraging strategy, and the evolution of insect pollination.     

On the capability of birds for symbolization

The main approaches to studying the ability of animals to form concepts and symbolization are discussed. The available data on the capabilities of highly organized birds (parrots and corvids) to comprehend equivalence between signs and the concept of number are analyzed. The new own data on the ability of hooded crows for symbolization are described. The considered results confirm the concept that the capability for symbolization is inherent not only to higher mammals, but also to highly organized representatives of birds.     

Climate change and the optimal arrival of migratory birds

Recent climate change has sparked an interest in the timing of biological events, which is a general problem in life-history evolution. Reproduction in many organisms breeding in seasonal environments, e.g. migratory birds, is dependent on the exploitation of a short but rich food supply. If the seasonal timing of the food peak advances owing to climate change, then one would expect the bird to track those changes, hence, initiate migration and breeding earlier. However, when there is competition for territories and a risk of pre-breeding mortality, the optimal response to a shifting food distribution is no longer obvious. We develop a theoretical model to study how the optimal arrival time depends on the mean and variance of the food distribution, the degree of competition for territories and the risk of mortality. In general, the optimal shift in arrival date should never be as extreme as the shift in food peak date. Our results also show that we should expect the high variation of trends in arrival date observed among migratory birds, even if migration and information about climate change were unconstrained.     

On the mechanisms of nectar secretion: revisited

Background and Scope

Models of nectar formation and exudation in multilayered nectaries with modified stomata or permeable cuticle are evaluated. In the current symplasmic model the pre-nectar moves from terminal phloem through the symplasm into the apoplasm (cell walls and intercellular spaces) with nectar formation by either granulocrine or eccrine secretion and its diffusion outwards. It is concluded, however, that no secretory granules are actually produced by the endoplasmic reticulum, and that secretory Golgi vesicles are not involved in the transport of nectar sugar. Therefore, the concept of granulocrine secretion of nectar should be discarded. The specific function of the endomembrane system in nectary cells remains unknown. According to the apoplasmic model, the pre-nectar moves from the terminal phloem in the apoplasm and, on the way, is transformed from phloem sap into nectar. However, viewed ultrastructurally, the unloading (terminal) phloem of nectaries appears to be less active than that of the leaf minor veins, and is therefore not actively involved in the secretion of pre-nectar components into the apoplasm. This invalidates the apoplasmic model. Neither model provides an explanation for the origin of the driving force for nectar discharge.

Proposal

A new model is proposed in which nectar moves by a pressure-driven mass flow in the nectary apoplasm while pre-nectar sugars diffuse from the sieve tubes through the symplasm to the secretory cells, where nectar is formed and sugars cross the plasma membrane by active transport (‘eccrine secretion’). The pressure originates as the result of water influx in the apoplasm from the symplasm along the sugar concentration gradient. It follows from this model that there can be no combinations of apoplasmic and symplasmic pre-nectar movements. The mass-flow mechanism of nectar exudation appears to be universal and applicable to all nectaries irrespective of their type, morphology and anatomy, presence or absence of modified stomata, and their own vascular system.     

Testing the optimal defence hypothesis for two indirect defences: extrafloral nectar and volatile organic compounds

Many plants respond to herbivory with an increased production of extrafloral nectar (EFN) and/or volatile organic compounds (VOCs) to attract predatory arthropods as an indirect defensive strategy. In this study, we tested whether these two indirect defences fit the optimal defence hypothesis (ODH), which predicts the within-plant allocation of anti-herbivore defences according to trade-offs between growth and defence. Using jasmonic acid-induced plants of Phaseolus lunatus and Ricinus communis, we tested whether the within-plant distribution pattern of these two indirect defences reflects the fitness value of the respective plant parts. Furthermore, we quantified photosynthetic rates and followed the within-plant transport of assimilates with (13)C labelling experiments. EFN secretion and VOC emission were highest in younger leaves. Moreover, the photosynthetic rate increased with leaf age, and pulse-labelling experiments suggested transport of carbon to younger leaves. Our results demonstrate that the ODH can explain the within-plant allocation pattern of both indirect defences studied.     

Sugar preferences of nectar feeding birds – a comparison of experimental techniques

Experiments to determine sugar preferences of nectarivorous animals have been conducted using a wide variety of experimental procedures, all of which aim at ensuring that the solutions offered in choices are “equivalent”. Each method used historically has controlled for a particular variable, such as number of molecules in solution, weight of sugar in solution, or amount of energy in solution, depending on what question the researchers have tried to answer. Biologists interpreting these results in terms of bird sugar preference have seldom taken these differences into account. The consequences of using different experimental procedures for sugar preferences exhibited by a nectarivorous bird, the malachite sunbird Nectarinia famosa, were examined using paired sucrose and hexose sugar solutions made up to be either equimolar, equiweight or equicaloric. We found the effect of methodology on bird sugar preference to be quite distinct, especially at low concentrations, where malachite sunbirds showed either sucrose preference, no preference, or hexose preference, depending on the method used. This study highlights the need for researchers to consider methodology when interpreting, or comparing among, results from previous studies.     

On the calculation of sugar concentration in flower nectar

Summary There are several sources of potential error in calculating the concentration or energy value of floral nectar. Errors resulting from confusing data become substantial with increasing concentration. The different methods of expressing sugar concentration are here clarified, and the correct methods of converting from one to the other are provided. Refractometers in use in field studies usually read on a weight per total weight basis; this is recommended as the mode of statement. The perils of oversimplifying conversions from this mode, as is often done, are pointed out.     

Annual routines of non-migratory birds: optimal moult strategies

In a periodically changing environment it is important for animals to properly time the major events of their life in order to maximise their lifetime fitness. For a non-migratory bird the timing of breeding and moult are thought to be the most crucial. We develop a state-dependent optimal annual routine model that incorporates explicit density dependence in the food supply. In the model the birds' decisions depend on the time of year, their energy reserves, breeding status, experience, and the quality of two types of feathers (outer and inner primaries). Our model predicts that, under a seasonal environment, feathers with large effects on flight ability, higher abrasion rate and lower energetic cost of moult should be moulted closer to the winter (i.e. later) than those with the opposite attributes. Therefore, we argue that the sequence of moult may be an adaptive response to the problem of optimal timing of moult of differing feathers within the same feather tract. The model also predicts that environmental seasonality greatly affects optimal annual routines. Under high seasonality birds breed first then immediately moult, whereas under low seasonality an alternation occurs between breeding and moulting some of the feathers in one year and having a complete moult but no breeding in the other year. Increasing food abundance has a similar effect.     

On the optimal policies of cancer screening.

Some problems of optimal screening are considered. A screening strategy is allowed to be nonperiodic. Two approaches to screening optimization are used: the minimum delay time approach and the minimum cost approach. Both approaches are applied to the analysis of an optimization problem when the natural history of the disease is known and when it is unknown (a minimax problem). The structure of optimal screening policies is investigated as well as the benefit they can provide compared to the periodic screening policy. The detection probability is assumed to depend only on the stage of the disease, though it may not be constant throughout each stage. It is shown that periodic screening appears to be optimal when one has no information on the natural history of the disease, the minimum delay time criterion being used for optimization. Some applications to lung cancer screening are presented.     

A simpler, more general method of finding the optimal foraging strategy for Bayesian birds

Oaten's (1977) stochastic model for optimal foraging in patches has been solved for a number of particular cases. A few cases, such as Poisson prey distribution and either systematic or random search, are easy to solve. In other cases, such as binomial prey distribution and random search, the form of the optimal strategy may be found using a theorem of McNamara, although more work is required to find which particular rule of the proper form is actually best. More generally (but not completely generally), optimal strategies may be found using dynamic programming. This requires that the number of prey found up to a particular time is a sufficient statistic for the number of prey remaining in a patch. This requirement cannot be dispensed with, but other simplifying assumptions that were used in the past are not necessary. In particular, it is not necessary, even for the sake of convenience, to assume that prey distribution has a form convenient for Bayesian analysis, such as a beta mixture of binomials or a gamma mixture of Poissons. Any prey distribution may be used if whatever prey are in a patch are located at random, and if search either is systematic for discrete time or for continuous time, or is random for continuous time. In earlier work, some pains had to be taken to find the rate of finding prey achieved by a given candidate strategy, but this is not necessary if expected gains and expected times are calculated routinely for each potential stopping point during dynamic programming. A new, simple method of finding optimal strategies is illustrated for discrete time and systematic search. This paper is based on a talk given at the Fifth Hans Kristiansson Symposium held in Lund, Sweden in August, 2003. The subject of the symposium was Bayesian foraging.     

On the optimal compromise for a dispersing parasitoid

Vector optimization concepts are used to demonstrate that key characteristics of searching and oviposition by the parasitoid Anagrus delicatus (Mymaridae, Hymenoptera) are consistent with the idea that it forages to achieve an optimal compromise, in an uncertain environment, between maximizing ovipositions at suitable sites and dispersing its eggs as widely as possible.     

Field methods for sampling and storing nectar from flowers with low nectar volumes

Background and Aims

Although several methods of sampling and storing floral nectar are available, little information exists on sampling and storing nectar from flowers with low nectar volumes. Methods for sampling and storing nectar from the flowers of species with low floral nectar volumes (<1 µL) were investigated using the flowers of Eucalyptus species.

Methods

Sampling with microcapillary tubes, blotting up with filter paper, washing and rinsing were compared to determine masses of sugars recovered and differences in sugar ratios. Storage methods included room temperature, refrigeration and freezing treatments; the addition of antimicrobial agents benzyl alcohol or methanol to some of these treatments was also evaluated. Nectar samples were analysed using high-performance liquid chromatography and the masses of sucrose, glucose and fructose in each sample were determined.

Key Results

Masses of sugars varied significantly among sampling treatments, but the highest yielding methods, rinsing and washing, were not significantly different. A washing time of 1 min was as effective as one of 20 min. Storage trials showed that the sugar concentration measurements of nectar solutions changed rapidly, with the best results achieved for refrigeration with no additive (sucrose and fructose were stable for at least 2 weeks). Sugar ratios, however, remained relatively stable in most treatments and did not change significantly across 4 weeks for the methanol plus refrigerator and freezing treatments, and 2 weeks for the refrigeration treatment with no additive.

Conclusions

Washing is recommended for nectar collection from flowers with low nectar volumes in the field (with the understanding that one wash underestimates the amounts of sugars present in a flower), as is immediate analysis of sugar mass. In view of the great variation in results depending on nectar collection and storage methods, caution should be exercised in their choice, and their accuracy should be evaluated. The use of pulsed amperometric detection, more specific than refractive index detection, may improve the accuracy of nectar sugar analysis.Key words: Eucalyptus, flower with small nectar volume, nectar collection, nectar sampling, nectar storage, sugar ratio     

On the evolution of genome size of birds

We measured genome size (nuclear DNA content) by fluorescence flow cytometry in 55 species of birds representing 12 different orders. Similar studies were performed in approximately 100 species by laboratories using absorption cytophotometry of Feulgen-stained nuclei. Although there have been apparent discrepancies in the assigned values for the species used as a reference, the values obtained in the different laboratories are generally in agreement. When the data are standardized in relation to a diploid (2C) value of 2.5 picograms (pg) of DNA for the domestic chicken (Gallus gallus domesticus), the mean for DNA content in 135 species representing 17 orders is 2.82 +/- 0.33 (SD) pg with a range of 2.0-3.8 pg. Thus the genome size of birds is the most conservative of any vertebrate class and, all values considered, is smaller and more uniform in size than previous estimates would indicate. This could be explained by a previously unexplored hypothesis: that the genome of birds has evolved from a small ancestral genome that was reduced before emergence of the protoavian.     

On a probabilistic model for the numerical estimation of nocturnal migration of birds

The study of nocturnal bird migration by cone methods of observation has a century-long history but has continued to be used up to the present. To describe the flux and estimate the number of passing birds a probabilistic model is proposed. This model is based on the concept of dynamic Poisson ensemble of points in appropriate phase space and has two parameters. One is scalar and the other one is functional. We constructed consistent estimations of these parameters and discuss their use for the numerical estimation of the flux of birds observed in a narrow light cone generated by the bright lunar disk and formed by an open angle of telescope. Selection on the same type of birds was suggested as the necessary condition for the model application. Ground speed of each bird was introduced into the model as a new but obligatory value determining the quantification of the flux of bird.     

On optimal delivery of combination therapy for tumors

A mathematical model for the scheduling of angiogenic inhibitors in combination with a chemotherapeutic agent is formulated. Conditions are given that allow tumor eradication under constant infusion therapies. Then the optimal scheduling of a vessel disruptive agent in combination with a cytotoxic drug is considered as an optimal control problem. Both theoretical and numerical results on the structure of optimal controls are presented.