Evaluating Frugivore-fruit Interactions Using Avian Eye Modelling

Abstract

Fruit phenotypes are often hypothesised to be affected by selection by frugivores. Here, we tested two hypotheses concerning frugivore-fruit interactions from the perspective of fruit colours. We measured the spectral properties of 26 fruits and the associated leaves of plants from 2 islands in New Zealand. Visual observations were also performed to record the birds that fed on the fruits. First, we tested the fruit-foliage hypothesis, where fruit colours are assumed to be evolutionarily constrained by their own leaf colour to maximise colour contrast and fruit conspicuousness. We ran a null model analysis comparing fruit colour contrast using an avian eye model. Second, we tested the frugivore specificity hypothesis, where specific fruit colours are thought to be connected with a specific bird frugivore. We performed a regression on the number of bird visits against the fruit colour in tetrahedral colour space based on an avian eye calculation using Mantel's test. The results show that fruit colours are not constrained by their own leaf colours. There is also no relationship or pattern suggesting a link between a specific fruit colour and specific bird visitors. We suggest that although fruit colour is one of the most highly discussed components, it is not the most important single deciding factor in frugivore fruit selection.

Keywords: Avian Eye Model; Frugivore Interaction; Fruit Colour; Leaf Colour.

Figure 1:

The observed and expected values of fruit colour contrast: (a) chromatic contrast refers to the Euclidean distance in tetrahedral colour space; (b) achromatic contrast refers to the JND values from the avian eye model calculation.

Figure 2:

Multidimensional scaling of plant species based on bird visitation (refer to Table 1 for abbreviation descriptions; grey squares represent South Island plants and black diamonds represent North Island plants, most of the points overlap).

Figure 3:

A tetrahedral colour space representation of the fruit colours. Each point represents how the specific colour of a fruit is processed by the bird eye receptors (refer to Table 1 for abbreviation descriptions; black circles represent South Island plants and open circles represent North Island plants).

Figure 4:

Mantel’s test correlation results for North Island (black diamonds) and South Island (grey squares) between the distances from the multidimensional scaling based on bird visits and distances in the tetrahedral colour space.