in zoology , a nectarivore is an animal which derives its energy and nutrient requirements from a diet consisting mainly or exclusively of the sugar-rich nectar produced by flowering plants .

nectar as a food source presents a number of benefits as well as challenges .

it is essentially a solution of ( as much as 80 % ) the simple sugars sucrose , glucose and fructose , which are easily ingested and digested , representing a rich and efficient source of nutrition .

this solution is often diluted either by the plant that produces it or by rain falling on a flower and many nectarivores possess adaptations to effectively rid themselves of any excess water ingested this way .

however , nectar is an incomplete source of nutrition .

while it does contain proteins and amino acids , these are found in low quantities , and it is severely deficient in minerals and vitamins .

very few organisms consume nectar exclusively over their whole life cycle , either supplementing it with other sources , particularly insects ( thus overlapping with insectivores ) or only consuming it exclusively for a set period .

many species are nectar robbers or nectar thieves , performing no pollination services to a plant while still consuming nectar .

nectar is produced by flowering plants to attract pollinators to visit the flowers and transport pollen between them .

flowers often have specialized structures that make the nectar accessible only for animals possessing appropriate morphological structures , and there are numerous examples of coevolution between nectarivores and the flowers they pollinate .

for example , hummingbirds and hawkmoths have long narrow beaks that can reach nectar at the bottom of long tubular flowers .

bats , meanwhile , visit open flowers where the nectar is not as deeply hidden .

nectarivore

nectarivore <EOT> nitrogen requirements of an old world nectarivore , the orange - tufted sunbird nectarinia osea .

nitrogen requirements of an old world nectarivore , the orange - tufted sunbird nectarinia osea . - pubmed - ncbi

nectarivore - ( from the greek work nektar , the drink of the gods ) a creature deriving its energy from the sugar rich nectar of flowering plants .

…that feed on nectar ( nectarivore s ) and have life spans longer than the flowering time of one plant species have mutualistic relationships with a succession of pollinating species in order to survive .

during the same time that a [ pis ] m [ ellifera ] adansonii encounters on flowers were registered , we noted the type of floral products collected by this bee . this parameter was measured to determine if a . m . adansonii is strictly a pollinivore , nectarivore or pollinivore and nectarivore . this could give us an idea of its implication as a cross - pollinator of g [ ossypium ] hirsutum .

citation : welch kc jr , allalou a , sehgal p , cheng j , ashok a ( 2013 ) glucose transporter expression in an avian nectarivore : the ruby - throated hummingbird ( archilochus colubris ) . plos one 8 ( 10 ) : e77003 . urltoken

colour me in . it ' s nectar of a new sort . nectarivore certifed organic ipa , a big hoppy , colourful cocktail of organic new zealand hops , pineapple sage and hibiscus . there ' s oodles of colour inside , but we ' ve left the outside to you . stay in the lines , or not - it ' s up to you . happy 25th birthday common sense organics .

another possible route for elimination of surplus water is through evaporation . evaporative water loss ( ewl ) of anna ' s hummingbird is high because of its high mass - specific metabolic rate , but high excretory losses mean that ewl still represents a far smaller proportion of daily water loss compared with other birds and mammals ( powers , 1992 ) . estimates calculated from the difference between water gain and cloacal fluid output have also yielded high ewl values for sunbirds and honeyeaters ( collins , 1981 ; fleming and nicolson , 2003 ; lotz and nicolson , 1999 ) . nothing is known of the partitioning of evaporation between cutaneous and respiratory routes , but cutaneous ewl is influenced by hydration state in other birds ( williams et al . , 2012 ) . unfortunately , the high excretory output of avian nectarivores complicates the direct measurement of ewl , and the pharmacokinetic method has proved unreliable for its estimation ( purchase et al . , 2013b ) . further research in this area is required to determine the role of evaporation in osmoregulation for the three avian nectarivore lineages .

sugar preferences and sucrase activity in nectar - feeding birds . ( a ) concentration - dependent sugar preferences of red wattlebirds , anthochaera carunculata ( fleming et al . , 2008 ) . birds were offered pairs of sucrose and hexose ( fructose + glucose ) solutions of varying concentrations from 0 . 075 to 2 mol l −1 sucrose equivalents ( se ) . diets where birds did not achieve energy balance are indicated with increasingly lighter shaded symbols . values are means ± 1 s . d . index of sugar preference ( i . e . hexose ingested as a proportion of total sugar ingested ) . asterisks indicate significant preferences for either hexose or sucrose diets ( * p < 0 . 05 ) and different letters indicate diets that were significantly different from each other in terms of the index of sugar preference ( anova and post hoc analysis ) . ( b ) relationship between degree of hexose preference ( i . e . minimum concentration where no sugar preference was indicated ) and standardized intestinal sucrase activity in 11 nectarivore species ( napier et al . , 2013 ) . data are average values for each species . white symbols denote diet generalists and grey symbols nectar specialists .

whole - kidney glomerular filtration rate ( gfr ; a measure of the rate at which the kidneys filter the plasma ) is much more responsive to hydration state in birds than in mammals , decreasing with dehydration or salt loading and increasing with water loading ( goldstein and skadhauge , 2000 ) . studies carried out in a range of bird species indicate that this is due to release of arginine vasotocin ( avt ) , which acts mainly on gfr , while having a relatively modest effect on the water permeability of the collecting ducts ( goldstein , 2006 ; nishimura and fan , 2003 ) . the reduction of gfr by avt occurs through a decrease in filtration by reptilian - type nephrons , with mammalian - type nephrons being less affected ( goldstein and skadhauge , 2000 ) . in nectarivores , plasma avt concentration increases with increasing sugar concentration , presumably reflecting a hormonal response to increased plasma osmotic concentration ( gray et al . , 2004 ) . surprisingly , in view of the high variation in water intake , gfr does not vary with water loading for the three avian nectarivore lineages ( goldstein and bradshaw , 1998 ; hartman bakken and sabat , 2006 ; purchase et al . , 2013b ) , although gfr is greatly reduced at night ( see below ) .

high water fluxes are a problem for salt conservation , and avian nectarivores are very efficient at minimizing electrolyte losses in the cloacal fluid : for example , whitebellied sunbirds feeding on 0 . 25 mol l −1 sucrose solutions void cloacal fluid with an osmolality of only 6 mosmol kg −1 ( fleming and nicolson , 2003 ) . despite similar abilities to produce copious volumes of dilute excreta across the three nectarivore lineages , the birds handle dilute diets differently . extremely dilute , sucrose diets devoid of electrolytes cause hummingbirds to stop feeding and go into torpor , while honeyeaters and sunbirds suffer decreased plasma sodium levels and are unable to maintain energy balance ( fleming et al . , 2004c ; goldstein and bradshaw , 1998 ; lotz and martínez del rio , 2004 ) . nectarivores thus appear to be constrained in their intake of dilute diets due to hyponatremia , i . e . low plasma sodium concentration ( fleming and nicolson , 2003 ) , and the addition of even small amounts of sodium ( 5–10 mmol l −1 ) to very dilute sucrose diets results in a marked decrease in plasma aldosterone concentration ( the active principle of the renin–angiotensin–aldosterone system , stimulating renal reabsorption of sodium ) ( fleming et al . , 2004a ) and enables whitebellied sunbirds and new holland honeyeaters to increase consumption , up to an extraordinary eight times their body mass daily ( purchase et al . , 2010 ) .

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parallel adaptations to nectarivory in parrots , key innovations and the diversification of the loriinae . - pubmed - ncbi

warning : the ncbi web site requires javascript to function . more . . .

parallel adaptations to nectarivory in parrots , key innovations and the diversification of the loriinae .

schweizer m 1 , güntert m 1 , seehausen o 2 , leuenberger c 3 , hertwig st 1 .

naturhistorisches museum der burgergemeinde bern bernastrasse 15 , ch 3005 , bern , switzerland .

aquatic ecology and macroevolution , institute of ecology & evolution , university of bern baltzerstrasse 6 , ch 3012 , bern , switzerland ; fish ecology and evolution , eawag seestrasse 79 , ch 6047 , kastanienbaum , switzerland .

department of quantitative economics , university of fribourg boulevard de pérolles 90 , ch 1700 , fribourg , switzerland .

specialization to nectarivory is associated with radiations within different bird groups , including parrots . one of them , the australasian lories , were shown to be unexpectedly species rich . their shift to nectarivory may have created an ecological opportunity promoting species proliferation . several morphological specializations of the feeding tract to nectarivory have been described for parrots . however , they have never been assessed in a quantitative framework considering phylogenetic nonindependence . using a phylogenetic comparative approach with broad taxon sampling and 15 continuous characters of the digestive tract , we demonstrate that nectarivorous parrots differ in several traits from the remaining parrots . these trait - changes indicate phenotype - environment correlations and parallel evolution , and may reflect adaptations to feed effectively on nectar . moreover , the diet shift was associated with significant trait shifts at the base of the radiation of the lories , as shown by an alternative statistical approach . their diet shift might be considered as an evolutionary key innovation which promoted significant non - adaptive lineage diversification through allopatric partitioning of the same new niche . the lack of increased rates of cladogenesis in other nectarivorous parrots indicates that evolutionary innovations need not be associated one - to - one with diversification events .

longitudinal section through the epithelium at the border ( arrow ) between the esophagus ( right ) and the proventriculus ( left ) of psittrichas fulgidus . the compound glands ( cg ) of the proventriculus can be clearly distinguished from the mucous glands ( mg ) of the lower part of the esophagus .

schematic illustration of a caudal view and a transverse section along the median plane between the tendineal centers ( right ) of the gizzard . the measurements taken in this study are indicated . modified from ziswiler ( ) .

schematic illustration of a transverse section through the gizzards along the median plane between the tendineal centres of a nectarivorous ( vini australis , left ) and a granivorous parrot ( neophema chrysostoma , right ) .

best - scoring maximum likelihood tree including bootstrap values above 70 % indicated at nodes . this tree was used for the formulation of the hypothesis of the variance - covariance matrix of the error terms in the regression analyses . nectarivorous species are shown in red .

natural - log transformed values of the different independent variables against natural - log transformed body masses including the regression lines of the best - fitting model . for all these traits , the data were best explained by an allometric relationship between the dependent and independent variables .

natural - log transformed values of the different independent variables against natural - log transformed body masses including the regression lines of the best - fitting models . for all these traits , a model including the nectarivorous diet as a covariate with different intercepts but the same slope was considered as the best - fitting model .

out of the 40 . 000 or more species of spiders found up to date , every single one of them is a predator , capturing their prey using a complicated web , hunting them directly , or using other creative methods ; all except one , that is . bagheera kiplingi ( odd choice for a veggie spider ) is the first species that primarily feasts on something else than meat .

the type of plants it enjoys is called beltian bodies , and it basically represents leaf - tip structures that are produced by acacia . what’s really interesting is that another animal that fancies the same food is the ant . some ants have an almost symbiotic relationship with the acacia shrubs , living in the hollow spines and eating an amount that also allows the acacia to thrive , and protecting the plants from other invaders . this relationship has been studied and described in numerous studies .

“this is really the first spider known to specifically ‘hunt’ plants ; it is also the first known to go after plants as a primary food source , ” said christopher meehan of villanova university , who took notice of the spiders during a field course in mexico .

even more , this species of spider is the first known to eat solid vegetative food at all . they do occasionally prey on small invertebrates , but in an overwhelming proportion , their food is vegetal .

“i’ve done the math several times , and even the most conservative estimates point to near - total vegetarianism , ” meehan said . “almost all of the prey that the spiders do eat are acacia - defending ant larvae . ”

“spiders aren’t really thought to be capable of eating solid food at all , ” meehan said . spiders digest their prey externally , he explained , and any matter bigger than about one micrometer gets filtered out of the vital juices in the spider’s pharynx . beltian bodies , however , are 80 percent structural fiber , and quite large by spider standards . “the acacia spiders consume these nutrient - enriched—but not particularly nutrient - rich—’vegetables’ completely , often in less than five minutes . ”

but eating the food of an ant colony… that’s definitely not something you should be eager to do , so how does the spider get away with it ? ? well , according to meehan , they have what can only be called “sheer wit” .

“jumping spiders in general possess incredibly advanced sensory - cognitive skills and eight - legged agility , and bagheera is no exception , ” he said . “individuals employ diverse , situation - specific strategies to evade ants , and the ants simply cannot catch them . ”

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avenues of water gain and loss in bees are examined here at two levels of organisation : the individual and the colony . compared with the majority of terrestrial insects , bees have a high water turnover . this is due to their nectar diet and , in larger species , substantial metabolic water production during flight , counteracted by high evaporative and excretory losses . water fluxes at the colony level can also be very high . when incoming nectar is dilute , honeybees need to remove large volumes of water by evaporation . on the other hand , water is not stored in the nest and must be collected for evaporative cooling and for feeding the brood . water regulation has many similarities at individual and colony levels . in particular , manipulation of nectar or water on the tongue is extensively used by bees to increase evaporation for either food - concentrating or cooling purposes .

bees are not normally model insects for studying water balance . bee physiology is characterised by endothermy in even small species ( stone and willmer , 1989 ) and is complicated by the social context . bees show the full spectrum of sociality : although most are solitary it is the social bumblebees and honeybees whose physiology and behaviour are most familiar to us ( willmer and stone , 2004 ) . in addition , the sophisticated thermal physiology of bees has received much more attention ( e . g . heinrich , 1985 ; kovac et al . , 2007 ) than their water regulation . here i briefly examine the water balance physiology of bees , considering the usual avenues of water gain and loss , at two levels of organisation : the individual and the colony , sometimes termed a ` superorganism ' ( moritz and southwick , 1992 ) . it is accepted that social homeostasis can be explained by the coordinated activity of bees as individuals , responding to their internal or external environment , without the need to invoke centralised control ( jones and oldroyd , 2007 ; seeley , 1995 ) .

water fluxes of honeybees can be very high at the colony level . for example , seeley estimated average annual requirements of 120 kg of nectar , 20 kg of pollen and 25 litres of water for a single wild colony ( seeley , 1995 ) . this applies to honeybees in cold temperate conditions , and the estimate for nectar includes substantial energy stores required for over - wintering . the seasonal cycles of african honeybees are limited by rainfall , not temperature , and better foraging weather means that in comparison massive honey stores are not needed ( hepburn and radloff , 1998 ) .

while male bees of most species take care of their own energetic needs , all female bees forage far beyond their individual needs , collecting pollen and nectar as provisions for the offspring they raise ; therefore activity patterns of the sexes may be very different ( willmer and stone , 2004 ) . where comparisons have been made , male xylocopa capitata and anthophora plumipes carry much smaller crop loads than do females ( louw and nicolson , 1983 ; stone , 1995 ) . the impermeable crop expands greatly for storage and transport of nectar and protects the haemolymph from osmotic shock . apis mellifera workers can carry close to their body mass in nectar , although crop loads tend to be much smaller ( schmid - hempel et al . , 1985 ) and are positively correlated with the nectar concentration ( hunt et al . , 1995 ) . this suggests that under natural conditions honeybees might carry less than during experiments in which they are fed concentrated sugar syrup ( e . g . park , 1932 ; waller , 1972 ) . their feeding system is designed to retain nectar in the crop for as long as possible . sugar leaves the crop at a rate dependent on the metabolic rate of the individual , but the fluid - emptying rate ( and the rate of rectal filling ) are inversely proportional to nectar concentration ( roces and blatt , 1999 ) .

female allodapula variegata concentrating the dilute ( 14 % ) nectar of aloe arborescens by evaporation . the regurgitated droplet , held under the tongue , is repeatedly sucked in and out and may be very large in relation to the size of the bee ( body length 7 mm ) . photo , michael ellis .

fresh pollen is relatively dehydrated after exposure at anthesis , but its water content increases after collection by bees due to the addition of nectar and glandular secretions ( human and nicolson , 2006 ) . the larval diets of bees vary greatly in water content ( roubik , 1989 ) . the water content of royal jelly is around 67 % ( wongchai and ratanavalachai , 2002 ) , but the larval food of stingless bees contains less water , a thicker consistency being necessary for larvae floating on top of mass provisions ( hartfelder and engels , 1989 ) . large carpenter bees provision their nests with semi - solid masses of pollen combined with nectar , giving a final water content of only 20 % in the provisions of x . capitata ( louw and nicolson , 1983 ) . in xylocopa mordax , nectar is pre - concentrated on the tongue for this purpose ( corbet and willmer , 1980 ) .

honeybees prefer sugar concentrations of 30–50 % ( sugar concentrations here are given as % w / w as in refractometer measurements ) under experimental conditions ( waller , 1972 ) , but in practice they collect from a much wider range of nectars . seeley measured 15–65 % in nectar loads being brought into a single colony ( seeley , 1986 ) , and hunt and colleagues recorded a similar range of concentrations in incoming loads ( hunt et al . , 1995 ) . the choice of nectar concentration depends on the ecological context , i . e . on the other food sources available at the time . this has long been a complicating factor in experimental studies of honeybee foraging behaviour . for example , lindauer found that the threshold sucrose concentration for eliciting recruitment behaviour declined from 55 % to 4 % as the german summer progressed ( lindauer , 1948 ) . empirical measurements of energy intake rate in bees show peak values at sucrose concentrations around 60 % in bumblebees , stingless bees and honeybees ( harder , 1986 ; roubik and buchmann , 1984 ) . note that for orchid bees ( euglossini ) , which use suction feeding rather than a lapping mechanism , optimal concentrations are lower and more dilute nectars are collected ( borrell , 2004 ) .

communal food storage requires that the osmolality of honey is high enough to inhibit microbial growth ( pusey , 1999 ) . in honeybees this is achieved first by hydrolysis of nectar sucrose to glucose and fructose , through the addition of hypopharyngeal gland enzymes , and then through evaporative processing by food - handling bees to reach a concentration of about 82 % . these bees evaporate nectar on their tongues before placing droplets in open cells for further evaporation , accelerated by fanning ( park , 1925 ) . among stingless bees , workers drink water condensed in the nest during honey ripening and regurgitate it outside the entrance ( roubik , 2006 ) . their ripened honey is around 70 % in concentration and tends to ferment ( cortopassi - laurino et al . , 2006 ; roubik , 2006 ) . note that uncapped honey is hygroscopic and absorbs water , so can be both a sink and source of water in the nest .

trophallaxis is the regurgitation of the crop contents of a donor bee for ingestion by receiver bees . extremely rapid distribution of incoming nectar was demonstrated by nixon and ribbands , who fed radiolabelled food to six foragers and were able to detect the label in 62 % of all foragers after only 4 h and in all large larvae in unsealed cells after 48 h ( nixon and ribbands , 1952 ) . trophallactic interactions ensure that homeostasis is achieved in the ` collective stomach ' of all workers , which is a nectar reserve for the colony ( schmickl and crailsheim , 2004 ) . similarly , colonies preparing to swarm store concentrated food in their crops , comprising 20–30 % of the mass of individuals and of the swarm ( combs , 1972 ) . in addition to its nutritional significance , liquid transfer between adults is a means of exchange of information about the quality and quantity of food reserves in the colony ( crailsheim , 1998 ) .

it is not clear whether solitary bees drink water for their own needs , as distinct from seeking dilute nectar . large aggregations of bees of various genera can be seen foraging at wet soil substrates in the tropics , then regurgitating and reimbibing fluid , and this may be a means of obtaining salts ( roubik , 1989 ) . although bumblebees are not expected to drink , marked bombus terrestris were observed drinking repeatedly from a water trough during warm conditions ( ferry and corbet , 1996 ) : it is unlikely that individual bumblebees would have a water deficit so this was probably for the benefit of the colony . when groups of 100 honeybees are confined in cages and provided with 67 % sugar , they drink about 10 μl of water daily at t a of 35°c and 40°c ( free and spencer - booth , 1958 ) .

honeybee colonies collect water for two reasons , related to different types of weather : for cooling of the brood area by evaporation on hot days , and for feeding the larval brood when foraging is limited on cool days ( lindauer , 1955 ; seeley , 1995 ) . the classic studies of lindauer showed how bees regulate the hive temperature in hot conditions ( lindauer , 1955 ) . water is collected by water foragers , then distributed around the hive and in cells containing eggs and larvae ; fanning accelerates its evaporation , as does regurgitation and evaporation on the tongue ( lindauer , 1955 ) . visscher and colleagues measured mean water loads of 44 mg in honeybees collecting water under desert conditions ( visscher et al . , 1996 ) . paper wasps and hornets also use water for cooling their nests , but the highly social stingless bees do not ( jones and oldroyd , 2007 ; roubik , 2006 ) .

the second need for water – for consumption by nurse bees when feeding the brood – is an aspect of water use by honeybees that tends to be underestimated ( johansson and johansson , 1978 ) . nurse bees feed young larvae a secretion from their hypopharyngeal glands ; for worker larvae after the third day this jelly is supplemented with honey and pollen ( crailsheim , 1998 ) . as already mentioned , the water content of royal jelly is high , so nurse bees have a great need for water when brood rearing is intensive ; this water cannot always be obtained from nectar .

the regulation of water collection in honeybees is discussed in detail by seeley ( seeley , 1995 ) . in essence , the rate of unloading of water foragers indicates the colony demand for water ( i . e . the feedback system is similar to that for nectar ) . in this way the balance between collection and consumption of water is maintained . importantly , water collection does not interfere with the collection of concentrated nectar by the colony ( kuhnholz and seeley , 1997 ) . the first bees to start water collection may be stimulated by the collective increase in crop sugar concentration of all bees in the nest , due to trophallaxis ( lindauer , 1955 ; seeley , 1995 ) , or possibly by the collective increase in haemolymph osmolality . apart from environmental factors , the tendency of honeybee foragers to collect water , nectar or pollen has a genetic component ( hunt et al . , 1995 ) . workers with the lowest sucrose response thresholds , i . e . those able to distinguish low sucrose concentrations from water in proboscis extension response tests , become water foragers ( pankiw and page , 2000 ) .

water foraging is regulated according to current demand and water is not stored in combs by temperate honeybee colonies : this is because nectar availability fluctuates widely and water sources usually do not ( seeley , 1995 ) . for african honeybees , occasional water storage has been recorded in wild bee nests , as after summer rain in the kalahari desert ( eksteen and johannsmeier , 1991 ) . park recorded temporary storage of water in the crops of ` reservoir bees ' ( park , 1923 ) .

studies on the mechanisms by which bees thermoregulate in flight have yielded data on the relative magnitudes of metabolic water gains and evaporative water losses for bees as individuals . metabolic water production by large flying bees ( xylocopa and bombus ) is substantial ( bertsch , 1984 ; nicolson and louw , 1982 ) , especially at low or moderate t a . honeybees ferrying large water loads to the hive at high t a produce enough metabolic water to offset their evaporative water losses ( louw and hadley , 1985 ) . however , it cannot be assumed that metabolic gains will balance evaporative losses in flight . because variation in metabolic heat production is used by bees ( apis and centris ) as a primary mechanism of thermoregulation in flight ( roberts and harrison , 1999 ; roberts et al . , 1998 ) , at high t a metabolic water production will decrease as evaporative water loss increases . for individual bees , the thermal environment is crucial to water balance in flight .

bees engaged in brood warming generate metabolic heat using their flight muscles . at low t a , the metabolic rates of incubating bumblebees are extremely high ( heinrich , 1974 ) . honeybee brood nest temperatures are maintained constant at about 35°c by bees that ` shiver ' on the comb surface or inside empty cells in the brood area ( kleinhenz et al . , 2003 ) , and high metabolic water production can be assumed during this energy - intensive heating activity . however , the general colony heat production to which all workers contribute does not require much increase in metabolism ( harrison , 1987 ) .

in flying a . mellifera and centris pallida there is a negative relationship between water balance and t a , the bees being in negative water balance at t a above 31°c ( roberts and harrison , 1999 ; roberts et al . , 1998 ) . although the cuticular permeability of both species increases with t a , neither cuticular nor respiratory water losses are sufficient to explain the very high evaporative water losses at high t a . these probably involve cooling mechanisms such as regurgitation of crop contents onto the proboscis ( heinrich , 1980 ) . this ` tongue lashing ' at high t a has been observed in a variety of bees ( roberts and harrison , 1998 ) and in honeybees has been shown to increase evaporative losses dramatically ( louw and hadley , 1985 ) . as a result , the thoracic temperatures of nectar and water foragers are significantly lower than those of pollen foragers ( cooper et al . , 1985 ; feuerbacher et al . , 2003 ) .

the amount of water that has to be evaporated from dilute nectar is enormous ( fig . 2 ) . in order to increase the sugar concentration from 20 % to 82 % , bees must evaporate 0 . 75 g water for every 1 g of nectar collected , and the mass of honey produced from a given mass of nectar is correspondingly reduced . recently we have shown that foragers of a . mellifera scutellata collecting dilute nectar of aloe greatheadii var . davyana in dry winter air begin to concentrate the nectar before returning to the hive ( nicolson and human , 2008 ) . because the crop is impermeable to both sugar and water , we can only explain the doubling of crop sugar concentration , from 20 % to 40 % , by evaporation on the tongue . this contradicts the conventional wisdom that the concentration of nectar is unchanged during its transport by bees between flowers and the hive ( park , 1932 ) . the advantage for the bees lies in reducing the water load that has to be carried and the amount of evaporation needed in the hive ( fig . 2 ) ; the cooling effect is less desirable for individual foragers but disturbances of heat or water balance can be corrected in the hive .

relative masses of sugar and water in nectars of various concentrations and in honey ( horizontal red line ) . the figure shows the nectar - processing advantage for apis mellifera scutellata workers concentrating the nectar of aloe greatheadii var . davyana from 20 % to 40 % on their tongues : two - thirds of the necessary evaporation is achieved before the bees return to the hive ( nicolson and human , 2008 ) .

the various processes involved in cooling the honeybee nest – collecting water , spreading it within the brood comb , and speeding its evaporation by fanning and regurgitation – have been mentioned above . this enables the temperature in the brood area to be precisely regulated at 35°c , but humidity in the hive is less constant ( human et al . , 2006 ) . air in the hive will generally be more humid than outside , as a result of transpiration of the inhabitants and evaporation during nectar flows . while high humidity is necessary for brood development , a dry atmosphere favours nectar ripening . we have measured absolute humidity ( thus excluding temperature effects ) in various regions of the hive , and found higher values in the brood area than in nectar stores , suggesting adjustments by the bees ( human et al . , 2006 ) . however , trade - offs with regulation of temperature and respiratory gases will disrupt the establishment of optimum humidity levels .

colony level respiration is important in social homeostasis . periodic synchronised fanning leads to tidal ventilation in honeybees and stingless bees when only one nest entrance is present ( moritz and crewe , 1988 ; southwick and moritz , 1987 ) . measurement of cyclic fluctuations in water vapour pressure and temperature at the nest entrance would enable estimation of evaporative water losses at the colony level .

the non - random disposal of the excreta of social insects , such as ejecting faeces late in larval development , is assumed to be for hygienic reasons ( weiss , 2006 ) . in honeybee larvae , the midgut–hindgut junction is occluded until the end of the larval stage , and defaecation coincides with cocoon formation , the excreta being incorporated into the structure of the cocoon ( jay , 1964 ) . healthy adult apis do not defaecate in the nest , even during overwintering in temperate climates ( but defaecation in or around the nest is a sign of infection with the midgut parasite nosema ) . defaecation flights occur when weather permits , and until then rectal fluid may be stored for prolonged periods , the distended rectum occupying much of the abdominal cavity ( fig . 3 ) . obviously an individual honeybee ' s water content fluctuates enormously depending on the volume of crop or rectal contents . an inverse relationship between crop and rectal volumes has been measured in honeybees confined for varying times after feeding ( roces and blatt , 1999 ) .

mass defaecation flights are conspicuous in tropical honeybees , and led to the ` yellow rain ' scare , due to the high proportion of pollen exines in the faeces ( mardan and kevan , 1989 ) . heat - shedding benefits have been suggested for the giant honeybee apis dorsata , which builds exposed combs and engages in mass flights involving half the colony , each bee jettisoning 20 % of its body mass ( mardan and kevan , 1989 ) . however , more comprehensive recordings for the same species show that mass flight activity is highest during maximum brood production , and timed so that brood temperature is minimally affected by the temporary absence of the protective curtain of bees ( woyke et al . , 2004 ) . water shedding may be a more important function of mass flights than heat shedding .

bees are less subject to desiccation than most terrestrial insects . this is because their nectar diet and high metabolic water production during flight frequently generate excess water . water fluxes in the honeybee colony are also high , due to honey ripening and periodic water demand for evaporative cooling and for feeding the brood . very importantly , the favourable microclimate created by the nest architecture and its densely aggregated inhabitants reduces evaporative water losses and provides a hydric and thermal refuge for returning foragers .

honeybee abdomen showing full crop ( a ) or full rectum ( b ) . adapted from plate 8 in dade ( dade , 1962 ) .

workers of apis mellifera do not forage for themselves and in social bees there is a blurring between the individual and colony in terms of water balance physiology . common to water regulation at both the individual and colony level is the regurgitation of nectar or water on the tongue for evaporative purposes . this is involved in water elimination from nectar both in the hive and during foraging , and water is evaporated in the same way to cool either the hive or the individual bee in flight . heinrich has previously drawn attention to the similarities in individual cooling , nest cooling and food storage behaviours ( heinrich , 1985 ) . perhaps these all originate in bubbling ( a term i prefer to tongue lashing ) , which appears to function as a nectar - concentrating mechanism in a variety of bees , and may have a profound influence on the water balance physiology of solitary bees in addition to social homeostasis . the high propensity of bees for regurgitation is important in both trophallaxis , which is not confined to highly social species ( kukuk and crozier , 1990 ) , and bubbling to evaporate water .

i thank julian dow for inviting me to contribute to this volume , connal eardley for identifying allodapine bees , and christian pirk for commenting on the manuscript . the paper is dedicated to simon maddrell and his enduring fascination with water in insects .

cooper , p . d . , schaffer , w . m . and buchmann , s . l .

) . pollination of the yellow passionfruit : nectar , pollen and carpenter bees .

cortopassi - laurino , m . , inmperatriz - fonseca , v . l . , roubik , d . w . , dollin , a . , heard , t . , aguilar , i . , venturieri , g . c . , eardley , c . and nogueira - neto , p .

feuerbacher , e . , fewell , j . h . , roberts , s . p . , smith , e . f . and harrison , j . f .

) . effects of nectar concentration and flower depth on flower handling efficiency of bumble bees .

) . the composition of larval food in stingless bees : evaluating nutritional balance by chemosystematic methods .

hendrichs , j . , cooley , s . s . and prokopy , r . j .

) . post - feeding bubbling behaviour in fluid - feeding diptera : concentration of crop contents by oral evaporation of excess water .

human , h . , nicolson , s . w . and dietemann , v .

hummon , a . b . , richmond , t . a . , verleyen , p . , baggerman , g . , huybrechts , j . , ewing , m . a . , vierstraete , e . , rodriguez - zas , s . l . , schoofs , l . , robinson , g . e . et al .

hunt , g . j . , page , r . e . , fondrk , m . k . and dullum , c . j .

johansson , t . s . k . and johansson , m . p .

kleinhenz , m . , bujok , b . , fuchs , s . and tautz , j .

kovac , h . , stabentheiner , a . , hetz , s . k . , petz , m . and crailsheim , k .

) . uber die einwirkung von duft - und geschmackstoffen sowie anderer faktoren auf die tänze der bienen .

moritz , r . f . a . and crewe , r . m .

moritz , r . f . a . and southwick , e . e .

) . the importance of osmosis in nectar secretion and its consumption by insects .

) . simultaneous measurement of evaporative water loss , oxygen consumption , and thoracic temperature during flight in a carpenter bee .

) . effect of nectar on microbial antagonists evaluated for use in control of fire blight of pome fruits .

roberts , s . p . , harrison , j . f . and hadley , n . f .

( hymenoptera : apidae ) and the ecology of nectar intake by bee colonies in a tropical forest .

) . inner nest homeostasis in a changing environment with special emphasis on honey bee brood nursing and pollen supply .

schmid - hempel , p . , kacelnik , a . and houston , a . i .

) . social foraging by honeybees : how colonies allocate foragers among patches of flowers .

southwick , e . e . and moritz , r . f . a .

) . warm - up rates and body temperatures in bees : the importance of body size , thermal regime and phylogeny .

visscher , p . k . , crailsheim , k . and sherman , g .

) . evaluating responses of honeybees to sugar solutions using an artificial - flower feeder .

) . behavioral , ecological , and physiological determinants of the activity patterns of bees .

woyke , j . , kruk , c . , wilde , j . and wilde , m .

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regulation of energy and water are by necessity closely linked in avian nectarivores , because the easily available sugars in nectar are accompanied by an excess of water but few electrolytes . in general , there is convergence in morphology and physiology between three main lineages of avian nectarivores that have evolved on different continents – the hummingbirds , sunbirds and honeyeaters . these birds show similar dependence of sugar preferences on nectar concentration , high intestinal sucrase activity and rapid absorption of hexoses via mediated and paracellular routes . there are differences , however , in how these lineages deal with energy challenges , as well as processing the large volumes of preformed water ingested in nectar . while hummingbirds rely on varying renal water reabsorption , the passerine nectarivores modulate intestinal water absorption during water loading , thus reducing the impact on the kidneys . hummingbirds do not generally cope with salt loading , and have renal morphology consistent with their ability to produce copious dilute urine ; by contrast , as well as being able to deal with dilute diets , honeyeaters and sunbirds are more than capable of dealing with moderately high levels of added electrolytes . and finally , in response to energy challenge , hummingbirds readily resort to torpor , while the passerines show renal and digestive responses that allow them to deal with short - term fasts and rapidly restore energy balance without using torpor . in conclusion , sunbirds and honeyeaters demonstrate a degree of physiological plasticity in dealing with digestive and renal challenges of their nectar diet , while hummingbirds appear to be more constrained by this diet .

nectar has been described as ‘the simplest food on earth’ ( martínez del rio et al . , 2001 ) , but nevertheless may pose significant physiological challenges for animals feeding on it . since we reviewed the topic a decade ago ( nicolson and fleming , 2003b ) , researchers in the americas , africa and australia have made substantial progress towards comparing the physiology of the three main lineages of specialized avian nectarivores : the hummingbirds ( trochilidae ) , sunbirds ( nectariniidae ) and honeyeaters ( meliphagidae ) ( fig . 1 ) . numerous other bird families show varying reliance on nectar ( nicolson and fleming , 2003b ; symes et al . , 2008 ) , but this commentary will focus on how the main radiations deal with the challenges of nectarivory , highlighting both convergence and differences in their responses .

the relatively low concentration of bird nectars [ compared with insect nectars ( pyke and waser , 1981 ) ] is the fundamental reason for the physiological problems associated with a nectar diet . flowers visited by hummingbirds and sunbirds produce nectar averaging 10–30 μl in volume and 15–25 % w / w ( 0 . 5–0 . 8 mol l −1 sucrose equivalents ) in concentration [ while flowers visited by generalist birds produce nectars that are even more copious and dilute ( johnson and nicolson , 2008 ) ] . nectar can also be diluted by rainfall events . the challenge of drinking dilute diets is that the birds must handle large volumes of preformed water , necessitating fast transit times ( i . e . time for food to pass through the gut , thus minimizing mass gain , which would interfere with flight ) , and yet maintain high assimilation efficiency , while also warming this large volume of fluid to body temperature . more concentrated nectars are likely to be less problematic , because switching between plants would enable birds to access more dilute nectar and thus balance water intake . the other major challenge is that nectar also contains very little by way of other solutes ; nectarivorous birds therefore have to deal with low electrolyte intake as well as low nitrogen , although their nitrogen requirements are relatively low ( tsahar et al . , 2006 ) .

it might be predicted that the physiological constraints of a ‘simple’ nectar diet would result in convergence in physiological responses of the different lineages of nectarivorous birds . however , recent findings indicate that this is not always the case and we now know that these birds show interesting differences in response to their nectar diets ( fig . 2 ) . in the first half of this commentary , we examine some common strategies for dealing with the challenges of a nectar diet . in the second half , we examine available data that indicate differences in how hummingbirds , sunbirds and honeyeaters deal with these challenges .

) , this field is receiving renewed attention , and not without controversy . rico - guevara and rubega (

) presented morphological measurements of hummingbird tongues and high - speed videos of drinking that showed that the tongue traps liquid as a result of surface tension ; this requires no energy expenditure by the bird and does not involve capillary action , which suggests that viscosity is not important . this finding was disputed by kim et al . (

) , who used a modeling approach to differentiate between different modes of nectar drinking , showing that the ‘capillary suction’ mechanism used by hummingbirds , sunbirds and honeyeaters is associated with optimal concentrations in the range 30–50 % ( 1 . 0–1 . 8 mol l

sucrose ; much higher than natural nectar concentrations ) . after further theoretical and experimental analysis of drinking in ruby - throated hummingbirds ,

the methods / techniques used to study the interactions between a substance and the body from administration to excretion ( absorption , distribution , metabolism ) .

in addition to concentration , nectar volumes are important for tongue loading . the volume of nectar determines its height in tubular floral corollas , and partial immersion of the tongue is common ( kim et al . , 2012 ) . when honeyeaters and hummingbirds feed from tubular artificial flowers , the amount of nectar loaded per lick increases with greater nectar volumes or shorter flowers , both of which increase the contact between tongue and nectar ( collins , 2008 ) . however , honeyeaters also commonly extract nectar from open , brush - type inflorescences and the effects of inflorescence shape on feeding rates have yet to be investigated .

specialized avian nectarivores from three lineages . ( a ) whitebellied sunbird , cinnyris talatala ( photo credit : rudi van aarde ) , ( b ) new holland honeyeater , phylidonyris novaehollandiae ( kathryn napier ) and ( c ) charming hummingbird , amazilia decora ( cole wolf ) .

some key physiological differences between the three specialist bird lineages . a representative hummingbird , honeyeater and sunbird ( note the differences in body size ) are shown feeding on nectar of an erythrina species , a major bird - pollinated genus that occurs in the natural range of all three lineages . all lineages show high rates of intestinal sucrase activity ( highest for the hummingbirds ; thicker yellow lines ) and rapid absorption of hexose sugars ( orange arrows ) , but the passerines are capable of shunting water directly through the gut ( blue dotted lines ) and therefore do not only rely on filtration by the kidneys ( blue solid lines ) . * hummingbirds show only a small medullary portion to their kidneys ( shaded green ) , which limits their abilities to produce hyperosmotic urine .

nectar - feeding birds are compensatory feeders over a wide range of nectar concentrations ( martínez del rio et al . , 2001 ; nicolson and fleming , 2003a ) . this means that they adjust their volumetric intake rapidly in response to changes in energy density ( fleming et al . , 2004b ; köhler et al . , 2008 ) . sugar concentration consequently has a marked effect on their sugar preferences . probably because of the rapid transit times and because hexose nectars do not require hydrolysis in order for the sugars to be assimilated , a hexose diet is preferred by both generalist and specialist nectar - feeding birds on dilute diets [ for references , see napier et al . ( napier et al . , 2013 ) ] .

interestingly , most species tested to date show a switch to no preference or even sucrose preference on concentrated diets ( e . g . data for the red wattlebird , anthochaera carunculata , are shown in fig . 3a ) . the concentration at which the switch occurs varies between species , and the minimum sugar concentration at which birds show no hexose preference is significantly correlated with intestinal sucrase activity for 11 bird species , even after phylogenetic correction ( fig . 3b ) ( napier et al . , 2013 ) . birds with no sucrase activity ( e . g . redwinged starlings , onychognathus morio ) or relatively low sucrase activity ( e . g . rainbow lorikeets , trichoglossus haematodus ) prefer hexoses at higher sugar concentrations , while birds with the greatest sucrase activity ( i . e . hummingbirds , but also sunbirds and honeyeaters ) either show no hexose preference or hexose preference on only the most dilute diets ( napier et al . , 2013 ) .