Posted by: oikosasa | March 19, 2013

On the evolution of fruit colour

Ever thought about why an orange is orange while an apple is green? And a blueberry blue and blackberry black, while a raspberry is red? Well, one explanation – seasonality – is studied in the new Early View Paper  “Fruit color and contrast in seasonal habitats – a case study from a cerrado savanna” by Maria Gabriela G. Camargo and co-workers. Here is their short summary of the study:

Fig3

Examples of fruits with different colors. A. green; B. multicolored; C. yellow; D. black; E. multicolored; F. brown; G. multicolored; H. red.

Fruit color is an important signal for diurnal seed dispersers, mainly for birds, and the contrast between the fruit and the background is regarded as more important than the color per se for fruit detectability. However, the contrast between fruit displays and their background are not necessarily constant in seasonal habitats where part of leaves is shed in the dry season.

We thus hypothesized that the contrast between fruit displays and their background vary throughout the year in a seasonal habitat and if this variation is adaptive, we predicted higher contrasts between fruits and foliage during the fruiting season.

To verify our hypotheses we used reflectance measurements of fruits and leaves and contrast analysis. We also accessed a six-year data base of fruit ripening according to the fruit color (red, yellow, black, brown and multicolored) for a woody community in a cerrado-savanna vegetation, southeastern Brazil. The cerrado is subjected to a seasonal climate, with a wet summer between October and March and a dry winter between April and September, when the leaf background get yellowish.

Fig1

Circular histograms for the frequency of fruiting onset dates per fruit colors in a woody cerrado savanna community, southeastern Brazil.

We found that black, and particularly red fruits, that have a high contrast against the leaf background, were highly seasonal, peaking in the wet season. Multicolored and yellow fruits were less seasonal, not limited to one season, with a bimodal pattern for yellow ones, represented by two peaks, one in each season. We further supported the hypothesis that seasonal changes in fruit contrasts can be adaptive because fruits contrasted more strongly against their own foliage in the wet season, when most fruits are ripe. Hence, the seasonal variation in fruit colors observed in the cerrado-savanna may be, at least partly, explicable as an adaptation to ensure high conspicuousness to seed dispersers.

Fig2

Mean leaf reflectance curves during the dry and wet seasons for 49 species of the woody cerrado savanna community, Southeastern Brazil.

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