The heart of my research is to understand how plants communicate with animals. Plants must do everything animals do, including competition for resources and proliferation. But they can’t move, so a big part of plant activity is signaling animals to help them do these things.
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All fleshy fruits therefore exist as a means of plant proliferation – they attract animals which consume the fruit and spread the seed. It is a mutualism in nature whereby a plant obtains pro-life evaluation service while an animal obtains nutrients. To signal this to animals, plants invest in different strategies, one of the most important of which is color.
THEIR EVOLVED INVITATION: Plants proliferate by signaling to pollinators, animals and birds, spreading their openings through their brightly colored flowers and fruits. Image Courtesy: iStock
Various factors determine fruit color, including the physiological presence of various compounds. There is also an evolutionary perspective because the different species involved in these mutualisms with plants have different ways of seeing colors. Some animals only see black and white. These are largely nocturnal and do not rely on the sense of color anyway.
Most diurnal active mammals are dichromatic – they can distinguish yellows and blues but not reds and greens. But birds can see colors that we can’t even imagine. In our work we quantify color using a technique called spectroscopy – it allows us to capture the wavelengths of light or color that a fruit reflects, if we can even see it, then we model whether the color change of this fruit upon ripening would be visually salient to an intended disperser. We found a really interesting match between the colors animals can detect against backgrounds like green leaves and the types of fruit they scatter. Thus, the color of the fruit is not random – it is a finely calibrated signal for the animals that are supposed to be attracted to it.
A banana is an interesting example – these are meant to be dispersed by birds. Open a banana and you’ll find tiny, perfect seeds right in the middle of the fruit. If you look at the banana, its seeds and its flesh under an ultraviolet sensitive lens, you will find that even though we see a banana that is completely yellow, birds see a lot of variation in that yellow since they can see the ultraviolet signal against the signal not ultraviolet.
The closest color is a vibrant electric blue. So, ultraviolet light would be a very flashy signal for them and in fact birds invest heavily in the proliferation of these fruits. In my research in Madagascar, I found that whenever fruits were dispersed by different species with varying visual abilities, the fruits tended to turn blue – this can be seen by all birds and mammals. It’s fascinating how plants have very different evolutionary histories, but many of them have converged around the color blue to make themselves as attractive as possible to as many different animals and birds.
THEY DON’T FEEL BLUE: In nature, the color blue, far from symbolizing despair, in fact signals life wishing to proliferate. Image Courtesy: iStock
As a biologist, I’m always thrilled to realize that everything in nature is so alive, that plants have such complex behavioral strategies, and that they constantly communicate with animals that respond to them. However, this colorful world of communication is facing threats now – in Madagascar, some fruits have lost the animals that were supposed to disperse them because they have disappeared. So we have “ghost fruits” – plants that somehow still exist even though they have lost the beings they once called.