Reliable cues and signals of fruit quality are contingent on the habitat in black elder (Sambucus nigra)

Abstract

Communication mediates interactions between organisms and can be based on signals or cues. Signals are selected for their signaling function, whereas cues evolve for reasons other than signaling. To be evolutionarily stable, communication needs to be reliable on average, but the mechanisms that enforce reliability are hotly debated in light of strong environmental influence on signals and cues. While fruit quality in black elder (Sambucus nigra) is unrelated to fruit color, it is indicated by alternative pedicel phenotypes. Information on fruit quality has thus been transferred from the fruit to the developmentally associated pedicels, which are environmentally determined cues. Within each phenotype, color variation indicates fruit quality. Communication by black elder is thus reliable, but the proximate mechanisms enforcing reliability are habitat specific. High irradiance increases both the contrasts of the visual cue and fruit quality in the anthocyanin-based red pedicel phenotype, while shaded plants of the chlorophyll-based green phenotype apparently use signals by forgoing photosynthesis. This is because lower chlorophyll content in green pedicels creates contrasting pedicels, and higher contrasts indicate higher sugar content in the fruits of green pedicels. Because anthocyanins are light-induced, plants use cues when exposed to high irradiance, whereas they apparently use costly signals in the shade by reducing chlorophyll content in the pedicels. In behavioral field and laboratory experiments we document that avian seed dispersers select among pedicel phenotypes that indicate different fruit quality. Plants can thus increase their reproductive success by sending highly informative cues. Our results indicate how reliable information transfer can be maintained both in cues and signals in spite of substantial environmental influence on visual traits.

Reproductive phenology of Sambucus nigra subsp. canadensis in the Abies religiosa temperate forest of the Magdalena river basin, Mexico City

“…For example, in Sambucus nigra subsp. nigra and Sambucus racemosa, it has been reported that their establishment and fruit production increases in sites with the highest light availability (Atkinson & Atkinson 2002, Abe et al 2008, Schaefer & Braun 2009). These results coincide with what we found in this study, since in micro-environment one, which showed the highest canopy openness as well as light amount, the individuals of S. nigra subsp.…”

Signal and reward in wild fleshy fruits: Does fruit scent predict nutrient content?

“…Studies of fruit color have predominated research on this debate. In the black elder (Sambucus nigra), visual contrasts in the pedicels bearing the fruits are positively associated with sugar content (Schaefer & Braun, 2009). Chromatic and achromatic (brightness) properties of fruits are also sometimes correlated with nutritional content (Cazetta, Galetti, Rezende, & Schaefer, 2012;Valido, Schaefer, & Jordano, 2011).…”

Colour and odour drive fruit selection and seed dispersal by mouse lemurs

“…Numerous studies have demonstrated that plant signals and cues are critical to fruit selection by animals2345. While ripe fruit signals refer to traits such as colour and odour that are maintained by natural selection because of their ability to reliably convey information to other organisms6, ripe fruit cues refer to traits that evolved in a context unrelated to animal signalling (e.g. red anthocyanine pigmentation), that may nonetheless convey reliable information to dispersers7.…”

“…Plant signals and cues available to animals depend critically on three complex factors: first, the complete signal being broadcast, second, its arbitration by the local environment6, and third, animal sensory phenotypes, which mediate the detectability of plant signals and cues to potential seed dispersers and pollinators10. Plant signals and cues are highly variable8, and comprise visual components - chromaticity (hue, saturation)11, brightness or luminance12 - and odour components, including individual volatile compounds as well as overall odour plume13.…”