Pollen nutritional content and digestibility for animals
This paper reviews the literature concerning digestion and nutrient content of pollen. Four topics are addressed in detail: 1) The mechanism of pollen digestion by animals; 2) The efficiency of mechanical and digestive removal of pollen content by various
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Abstract. This paper reviews the literature concerning digestion and nutrient content of pollen . Four topics are addressed in detail : I) The mechanism of pollen digestion by animals; 2) The efficiency of mechanical and digestive removal of pollen content by various animals; 3) Range and taxonomic distribution of pollen nutrients, and 4) Adaptive hypotheses proposed to associate pollen chemistry with pollinator reward. Studies on the mechanism(s) of pollen digestion remain inconclusive, but suggest that differences in digestibility among pollen types may reflect differences in pollen wall porosity, thickness, and composition. Although hummingbirds reportedly digest pollen very poorly, most animals studied, including those that do not regularly consume pollen, can digest 100% of ingested grains. Overlooked and recent research of pollen protein content shows that pollen grains may contain over 60% protein, double the amount cited in some studies of pollen-feeding animals. Adaptive hypotheses that associate pollen starch and pollen caloric content with pollinator reward remain unsubstantiated when critically viewed through the lens of phylogeny.
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Key words: Pollen chemistry, pollen digestion, pollination syndrome, palynology, bees, nutrition.
Pollen grain digestion Pollen walls so resist decay and digestion that they often survive intact and recognizable for
millions of years in bog and sediment deposits. Their durability lends paleoecologists a valuable tool for reconstructing ancient floras and paleoclimates. Their durability also presents pollen-feeding animals with a substantial digestive challenge : How to access the nutrientrich cytoplasm encased within the refractory pollen wall? Generalized structure of pollen grains
The outermost layer of the pollen wall is the pollenkitt, a semi-solid coating comprised primarily of neutral lipids, hydrocarbons, terpenoids, and carotenoid pigments (Dobson 1988) (see also Nepi and Franchi, and Hesse, this issue). Inside the pollenkitt is the exine (thin or lacking in some monocots, Kress et al. 1978),an often intricately-ridged matrix of the complex carbohydrate sporopollenin. The exine greatly resists decay and digestion, but is commonly perforated by one-to-several pores or slits (germination pores) that lead to the inner wall layer, known as the intine. The intine, composed primarily of cellulose and pectin, also resists decay and digestion , and forms the final barrier to the nutrient-rich cytoplasm. Thus, any animal consuming pollen contacts pollenkitt nutrients through external probing ofpollen grains or ingestion of pollen grains, but must penetrate or dismantle two resistant pollen wall layers in
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T. H . Roulston and J. H. Cane : Pollen nutrition and digestion
order to access cytoplasmic nutrients. Six basic methods have been suggested for animals to extract pollen contents: I Crack open the pollen wall mechanically II Pierce the pollen wall with sharp mouthparts III Dissolve the pollen wall with enzymes IV Induce germination or pseudo-germination V Bur
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