Sucrose digestion capacity in birds shows convergent coevolution with nectar composition across continents

Abstract

Summary The major lineages of nectar-feeding birds (hummingbirds, sunbirds, honeyeaters, flowerpiercers, and lorikeets) are considered examples of convergent evolution. We compared sucrose digestion capacity and sucrase enzymatic activity per unit intestinal surface area among 50 avian species from the New World, Africa, and Australia, including 20 nectarivores. With some exceptions, nectarivores had smaller intestinal surfaces, higher sucrose hydrolysis capacity, and greater sucrase activity per unit intestinal area. Convergence analysis showed high values for sucrose hydrolysis and sucrase activity per unit intestinal surface area in specialist nectarivores, matching the high proportion of sucrose in the nectar of the plants they pollinate. Plants pollinated by generalist nectar-feeding birds in the Old and New Worlds secrete nectar in which glucose and fructose are the dominant sugars. Matching intestinal enzyme activity in birds and nectar composition in flowers appears to be an example of convergent coevolution between plants and pollinators on an intercontinental scale.

Sweet solutions: nectar chemistry and quality

“…If nectar sucrose is not hydrolyzed in the flower, it must be hydrolyzed in the gut of the pollinator before absorption. The invertases in plant nectaries are β-fructosidases, while animals use α -glucosidases, present in honeybees and nectar-feeding birds [ 27 , 41 ]. Intestinal sucrase activity in nectar-feeding birds matches the proportion of sucrose in the nectars they consume: this convergent coevolution is seen across continents in hummingbirds, sunbirds and honeyeaters, and the flowers they pollinate [ 27 ].…”

“…The invertases in plant nectaries are β-fructosidases, while animals use α -glucosidases, present in honeybees and nectar-feeding birds [ 27 , 41 ]. Intestinal sucrase activity in nectar-feeding birds matches the proportion of sucrose in the nectars they consume: this convergent coevolution is seen across continents in hummingbirds, sunbirds and honeyeaters, and the flowers they pollinate [ 27 ]. In spite of this association, these birds do not prefer sucrose to hexose nectars, and even prefer hexose nectars at low concentrations [ 72 ].…”

“…Ultimately, the sugar composition of nectar may not be physiologically important, because pollinators digest sucrose rapidly, and nectar sugars are efficiently assimilated [ 74 , 75 ]. The few exceptions include starlings and their allies and some acacia ants, which lack sucrase and avoid sucrose-containing floral and extrafloral nectars respectively [ 27 , 76 ]. Hummingbirds and bats use both fructose and glucose directly to support their high metabolic rates in hovering flight [ 77 ].…”

“… nectar components effects on pollinator health references sugars energy source for flight, thermoregulation and development, including endotherms with high metabolic rates [ 26 ] some birds (e.g. starlings) cannot digest sucrose [ 27 ] glucose and fructose reduce oxidative damage in hawkmoths [ 28 ] amino acids non-essential amino acids often predominate and ratios vary greatly; nutritional benefit is largely unknown [ 29 ] may affect the taste of nectar [ 30 ] amino acid preferences may influence sugar intake [ 31 , 32 ] metabolized in flight [ 33 , 34 ] pharmacological effects of non-protein amino acids may benefit plant but not pollinator [ 4 , 35 ] proteins nectar preservatives [ 36 ] fatty acids metabolised by hawkmoths [ 34 ] salts contribute to salt balance [ 24 ] high K+ is deterrent (e.g. onion); may affect energy intake [ 37 , 38 ] vitamin C (ascorbic acid) reduces oxidative damage [ ...…”

A generalized bird pollination system in Schotia brachypetala (Fabaceae)

“…The diverse morphology and nectar properties of birdpollinated flowers reflect various behavioural, morphological and physiological traits of flower-visiting birds (Cronk & Ojeda, 2008;Pauw, 2019;McWhorter et al 2021). A longstanding distinction was made between hummingbird and passerine avian pollination systems (Baker & Baker, 1983;Mart ınez del Rio et al 1989), but it has been suggested that this taxonomic framework be replaced by a more functionally based system which distinguishes flower feeding birds as either specialist or generalist nectar feeders (Johnson & Nicolson, 2008).…”