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 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 50–100% 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.

Pollen nutritional content and digestibility for animals

Pollen ranges from 25% to 61% protein content Most pollen proteins are likely to be enzymes that function during pollen tube growth and subsequent fertilization, but the vast range of protein quantity may not reflect only pollen–pistil interactions Because numerous vertebrate and invertebrate floral visitors consume pollen for protein, protein content may influence floral host choice Additionally, many floral visitors pollinate their host plants If protein content influences pollinator visitation, then pollinators are hypothesized to select for increased protein content of host plants We analyzed or gleaned from the literature crude pollen protein concentrations of 377 plant species from 93 plant families Using this database, we compared pollen protein concentration with (1) pollination mode, (2) pollen collection by bees, and (3) distance from stigma to ovule, after accounting for phylogeny through paired phylogenetic comparisons and a nested ANOVA including taxonomic rank We found that pollen pr

https://www.jstor.org/stable/info/2657188

What governs protein content of pollen: pollinator preferences, pollen–pistil interactions, or phylogeny?

Bees are phytophagous insects that exhibit recurrent ecological specializations related to factors generally different from those discussed for other phytophagous insects. Pollen specialists have undergone extensive radiations, and specialization is not a always a derived state. Floral host associations are conserved in some bee lineages. In others, various species specialize on different host plants that are phenotypically similar in presenting predictably abundant floral resources. The nesting of solitary bees in localized areas influences the intensity of interactions with enemies and competitors. Abiotic factors do not always explain the intraspecific variation in the spatial distribution of solitary bees. Foods stored by bees attract many natural enemies, which may shape diverse facets of nesting and foraging behavior. Parasitism has evolved repeatedly in some, but not all, bee lineages. Available evidence suggests that cleptoparasitic lineages are most speciose in temperate zones. Female parasites f...

Floral resource utilization by solitary bees (Hymenoptera: Apoidea) and exploitation of their stored foods by natural enemies.

Nutritional content of fresh, bee-collected and stored pollen of Aloe greatheadii var. davyana (Asphodelaceae).

Bees require large amounts of pollen for their own reproduction. While several morphological flower traits are known to have evolved to protect plants against excessive pollen harvesting by bees, little is known on how selection to minimize pollen loss acts on the chemical composition of pollen. In this study, we traced the larval development of four solitary bee species, each specialized on a different pollen source, when reared on non-host pollen by transferring unhatched eggs of one species onto the pollen provisions of another species. Pollen diets of Asteraceae and Ranunculus (Ranunculaceae) proved to be inadequate for all bee species tested except those specialized on these plants. Further, pollen of Sinapis (Brassicaceae) and Echium (Boraginaceae) failed to support larval development in one bee species specialized on Campanula (Campanulaceae). Our results strongly suggest that pollen of these four taxonomic groups possess protective properties that hamper digestion and thus challenge the general view of pollen as an easy-to-use protein source for flower visitors.

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Specialized bees fail to develop on non-host pollen: do plants chemically protect their pollen?

The protein and amino acid contents of bee-collected pollens from 10 indigenous and 16 exotic Australian plants were determined. Crude protein content showed a large variation with a range from 9.5% for Pinus radiata pollen, to 36.9% for Banksia ornata pollen. Pollens which were found to have high protein contents are those which are also highly favoured by apiarists whereas those with low protein content are considered to be nutritionally inferior pollens. The amino acid pattern for pollens from both indigenous and exotic plants indicated that, generally, the levels of amino acids in the pollens were above the bees' requirements. Tryptophan appeared to be the first limiting amino acid in Australian pollens for honeybees.

DF Langridge

Papers

Amino acids in bee-collected pollens from Australian indigenous and exotic plants