Combinations of rewards in the individual foraging on resources varying in both quality and quantity experiment.

<p>Quantity and quality of sugar solution and pollen rewards offered simultaneously in four series of tests, each of them comprising five tests with a varied reward – different quantities or qualities – of one resource and a standard reward of the other resource.</p

Summary of foraging behaviour for 48 tagged bumblebee workers.

<p><b>A</b>: Mean amount of days that the bumblebee workers perform specific foraging tasks. Each bumblebee was observed on 20–41 days (mean number of days observed  = 26.52±1.29 SE). Note that while collecting pollen, bumblebees also gathered small amounts of sugar solution to pack the pollen; the column “pollen” thus excludes foraging bouts just for sugar solution. <b>B</b>: Summary of all foraging tasks performed by each bumblebee in the course of the observation period. 7 bumblebees only ever collected sugar solution, 41 bumblebees collected sugar solution, pollen, or both sugar solution and pollen on different days.</p

Bumblebees collecting resources from artificial feeders.

<p><b>A</b>: Bumblebee drinking sugar solution from a 5 ml syringe with a cut tip. More than one worker can collect sugar solution from one feeder simultaneously. <b>B</b>: Bumblebee gathering pollen from a modified Eppendorf tube. By buzzing while placed underneath the tube, the worker loosens pollen from the tube which accumulates on her ventral side from whence she is able to pack it into her corbiculae.</p

Bumblebees gathering resources from artificial flowers.

<p><b>A</b>: Bumblebee collecting sugar solution from an artificial flower. The worker gathers sugar solution from a transparent plastic tube right above the orange-coloured feathers. <b>B</b>: Bumblebee buzz-collecting pollen from a bunch of yellow feathers; note the blurring of the wings caused by vibrations.</p

Statistical Analysis of the individual foraging on resource varying in quality and quantity experiment.

<p>Results of the repeated measures Anova applied to the series of tests of individual foraging on resources varying in quality and quantity experiment shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091900#pone-0091900-g006" target="_blank">Figs. 6</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091900#pone-0091900-g007" target="_blank">7</a>.</p

Average amount of resources collected from each feeder when the first feeder was emptied and when bees ceased foraging.

<p><b>A</b>: Bumblebees emptied the first feeder: n = 76 trials with sugar solution; n = 12 trials with bee-collected pollen (Anova: F_4;355 = 2.130,9; p<0,001 for sugar solution; F_4;55 = 138,17; p<0,001 for pollen). <b>B</b>: Bumblebees stopped foraging: n = 11 trials with sugar solution; n = 3 trials with bee-collected pollen (Anova: F_4;105 = 146,39; p<0,001 for sugar solution; no statistical analysis of data for pollen due to small sample size). Both sugar and pollen rewards were offered in 5 different qualities: sugar solution quality 1 = 0%; 2 = 9%; 3 = 18%; 4 = 27%; 5 = 36% sugar diluted in water; pollen quality 1 = 0%; 2 = 25%; 3 = 50%; 4 = 75%; 5 = 100% washed honeybee-collected pollen blended with dyed cellulose powder. Data presented are mean values and upper standard deviations. Different letters above columns indicate significant differences at p<0.001 from Tukey's Hsd-test.</p

Comparison of reflectance properties of some yellow-coloured <i>Crocus</i> species.

<p>Colour photograph, UV-photograph and relative spectral reflectance of tepals are shown for five exemplary yellow-flowering <i>Crocus</i> species. Note that only in <i>C</i>. <i>korolkowii</i> parts of the tepals are distinctly UV-reflecting.</p

A Matter of Contrast: Yellow Flower Colour Constrains Style Length in <i>Crocus</i> species

<div><p>Most flowers display distinct colour patterns comprising two different areas. The peripheral large-area component of floral colour patterns attracts flower visitors from some distance and the central small-area component guides flower visitors towards landing sites. Whereas the peripheral colour is largely variable among species, the central colour, produced mostly by anthers and pollen or pollen mimicking floral guides, is predominantly yellow and UV-absorbing. This holds also for yellow flowers that regularly display a UV bull’s eye pattern. Here we show that yellow-flowering <i>Crocus</i> species are a noticeable exception, since yellow-flowering <i>Crocus</i> species–being entirely UV-absorbing–exhibit low colour contrast between yellow reproductive organs and yellow tepals. The elongated yellow or orange-yellow style of <i>Crocus</i> flowers is a stamen-mimicking structure promoting cross-pollination by facilitating flower visitors’ contact with the apical stigma before the flower visitors are touching the anthers. Since <i>Crocus</i> species possess either yellow, violet or white tepals, the colour contrast between the stamen-mimicking style and the tepals varies among species. In this study comprising 106 <i>Crocus</i> species, it was tested whether the style length of <i>Crocus</i> flowers is dependent on the corolla colour. The results show that members of the genus <i>Crocus</i> with yellow tepals have evolved independently up to twelve times in the genus <i>Crocus</i> and that yellow-flowering <i>Crocus</i> species possess shorter styles as compared to violet- and white-flowering ones. The manipulation of flower visitors by anther-mimicking elongated styles in <i>Crocu</i>s flowers is discussed.</p></div

Phylogenetic tree of the genus <i>Crocus</i> with superimposed floral characters.

<p>Phylogenetic tree obtained by Bayesian phylogenetic inference of the nuclear rDNA ITS region. The floral characters indicated are flower colour and relative style length as compared to the relative stamens length and branching of the style for 106 <i>Crocus</i> and 3 <i>Romulea</i> species.</p

Reflectance properties of differently coloured <i>Crocus</i> flowers.

<p>Relative spectral reflectance of tepals in the yellow-flowering <i>C</i>. <i>chrysanthus</i> and <i>C</i>. <i>flavus</i> and the violet- or white-flowering <i>C</i>. <i>vernus</i>.</p