Conspicuousness, not colour as foraging cue in plant–animal signalling

Abstract

The global prevalence of red and black fruits has still not been explained. Hypotheses based on innate consumer preferences have been tested and rejected. Though colour itself plays an important role in animal foraging, it is only one component of signals.Another major component are colour contrasts against background achieving the conspicuousness of signals. In order to evaluate which signal component determines consumers behaviour, we measured fruit colour and colour contrasts of 43 species against their natural background under ambient light conditions. Red and black fruits exhibit stronger contrasts and are therefore more conspicuous to consumers than fruits of other colours. Subsequently, trials were carried out to determine whether colour or conspicuousness influences avian food choice. Four bird species strongly preferred contrasting red Á/green or black Á/green over uni-coloured red, green, or black fruit displays, while no preference for particular hues was found. We therefore hypothesize that conspicuousness determines avian food selection and define the contrast hypothesis: Diurnal dispersers select fruit colours based on their conspicuousness and not their colour itself.Because colour vision is an ancient trait, the entire heterogeneous group of frugivorous birds most likely perceives conspicuousness uniformly over evolutionary time spans. Conspicuousness has thus the potential to explain the global prevalence of red and black fruits.

Geographic patterns in fruit colour diversity: do leaves constrain the colour of fleshy fruits?

“…Rather, fruit conspicuousness results from spectral contrasts between fruits and their backgrounds (Burns and Dalen 2002;Schmidt et al 2004). Selection by frugivores is therefore likely to favour colours that are perceived as strongly contrasting with their backgrounds, because strong fruitbackground contrasts are more conspicuous and increase the chances that fruits are detected (Schaefer et al 2006).…”

Camouflaging of seeds treated with pesticides mitigates the mortality of wild birds in wheat and rice crops

“…They tested blue and brown colors with relative success at avoiding the consumption of poisoned grains by birds, suggesting such colors would cause aversion. The notable ingestion of red seeds suggests that this color, attributed by rhodamine B, is not aversive, rebutting Avery and Mason (1997) and Nelms and Avery (1997), but attractive (Schmidt et al, 2004;Cuthill et al, 2005). This color's spectral behavior, in the wavelengths visible by the birds, is similar to yellow and orange (Schmidt et al, 2004), which are contrasting and conspicuous and similar to beige.…”

“…In addition, powder camouflaged coating yields an opaque surface and, for this reason, shines less than seeds treated with rhodamine, being less conspicuous (Schmidt et al, 2004;Cuthill et al, 2005).…”

“…However, the ingestion of small grits to help crush foods is a common aspect in the behavior of several bird Families (Sick, 1997). So, it is possible that brown-dyed seeds are less removed due to a set of factors linked to the camouflaging phenomenon, like background match, avoiding well defined contours and color contrasts (Merilaita 2003, Frankel et al, 2004Schmidt et al, 2004), and not only because of the aversion to the Table 5 -Student-Newman-Keuls a posteriori test for multiple average comparisons, applied to the mortality of birds in rice planting experiment 2.…”

“…According to the Brazilian Federal Decree 4074, January 4, 2002, agrochemical defensives must be added to the seeds in association with dyes, to reduce the risk of human ingestion and poisoning. Rhodamine B dye is frequently used with that purpose, and its purple-reddish hue contrast with soil is striking to wild birds (Hartley et al, 2000;Schmidt et al, 2004).…”