Plants color their fruits to identify good animals

Why are raspberries red and yellow bananas?

Scientific debate has raged for decades over whether plants evolved to produce particular colored fruits in order to attract particular animals to help spread their seeds. A new study seems to have finally settled.

The researchers examined similar plants in two similar mountain rainforests in Uganda and Madagascar and found that the color palette of the fruits in both forests was adapted to the way local fruit animals viewed the world.

In Uganda, most fruits ripen in bright red – standing out on green leaves when they are spotted by local birds and monkeys, which have a color vision similar to ours.

A red-bellied lemur feeds on a forest in Madagascar. Lemurs can not tell the difference between red and green. (Christmas Rowe)

The research team was led by Omer Nevo, postdoctoral researcher at the University of Ulm in Germany, and Canadian biologist Kim Valenta, who is currently an assistant professor of research at Duke University. from Durham, North Carolina. .

The premise that they were trying to confirm is quite simple. Because plants can not move, many of them rely on animals to spread and plant their seeds. For the animals to do this, the plants lock the seeds into a tasty fruit. The animals eat the fruit, fly or fly to another part of the forest, then poach the seeds.

But the system only works for plants if the animals eat the seeds when they are ready to be planted, that is to say when the fruits are ripe.

As for animals, Nevo said: "TThey do not want to waste time and energy trying a fruit that turns out to be immature and not at all tasty or nutritious at all. "

Plants talking to animals

Plants usually report to animals that the fruit is ripe by changing color – what scientists consider a form of communication.

Thus, it might seem obvious that the plants would evolve and produce fruits of the right color to attract the right animals to eat.

But scientists have found that proving this theory is not so easy, and many previous studies show no evidence of this type of evolution.

In addition, there are other possible explanations for which fruits are the colors that they are. Maybe it's just genetic, and so all the plants associated with that lineage have the same color. Or maybe because the plant produces pigments of this color for other parts of the plant.

Eye of the spectator

One of the challenges has been that color vision varies greatly among animals and that most animals see colors very differently from the way humans – including scientists – do.

Conscious of this, Valenta began measuring fruit color using a machine capable of objectively quantifying and comparing all colors – a spectrometer that she says she "bought" through a Ph.D. University of Toronto.

These are some of the fruits harvested in Madagascar by researcher Omer Nevo. They are mainly greenish, but contrast with the color of the leaves when they are seen with the "lemur vision". (Omer Nevo)

The fruit colors measured by the spectrometer could then be compared to the color vision of different animals, which has been measured and tested for years by Amanda Melin, a researcher at the University of Calgary who collaborated on the study. .

Melin, a Canada Research Chair in Anthropology, Archeology and Medical Genetics, said she could accurately predict how an animal sees colors by examining the DNA of its excrement. .

Another problem is that it is not easy to know which animals disperse the seeds of which plant. And anyway, some scientists argue that the best strategy for a plant is to get as many different animals as possible to disperse its seeds, rather than depending on a specific species.

Nevo and Valenta felt that if this were the case, it is probably best to look at all the animals that eat fruit in the community together rather than individually.

"Natural experience"

While chatting, they realized that they both collected fruit samples from tropical forests with very similar plants but very different animals.

"It's kind of a natural experience," Nevo said.

A Milne-Edwards type sifaka, a type of lemur, crunches on certain fruits. Plants depend on fruit-eating animals to disperse and plant their seeds. (Omer Nevin)

He worked among lemurs in the lush rainforests of Ranomafana National Park in Madagascar, studying the smells and nutritional content of various fruits ranging in color from dark red to blue to violet and white.

"I am always impressed by the fruits that are not good at all, the fruits that look brown or greenish, really very boring and dull, but their smell is amazing. "

Valenta collected the types of fruit that monkeys, chimpanzees and baboons – and sometimes elephants – enjoyed in the "spectacular" tropical forests of the mountains of Kibale National Park in Uganda.

When Nevo and Valenta compared fruit colors and animal color vision in both areas, they teamed up as planned.

Nevo said that he was surprised by how "pretty" the results were.

"It is rare that you have a hypothesis and that you test it, and you get exactly what you expect. "

He said the results show how dependent animals and plants are on each other and how complex their relationships are – and, therefore, how important it is to protect whole ecosystems.

"Once you start hurting plants or animals, you risk interfering with a very complex network of interactions."

The researchers' work was supported by the German Science Foundation, the Natural Sciences and Engineering Research Council of Canada, and the Canada Research Chairs Program.

Jedediah Brodie, a biologist from the University of Montana who published an earlier study that found that fruit color was not related to the diversity of fruit-eating animals, said the new study was "really cool" .

"It provides some of the first evidence that fruit-eating animals can affect the evolution of fruit color in large geographical areas."