Different figs look and smell way they do to attract birds or bats — not because of a family resemblance, a new study shows.
The research published Monday in the Proceedings of the National Academy of Sciences provides evidence of something scientists have suspected for a long time but were unable to prove — that different fruits adopt certain groups of traits while evolving to attract specific types of animals to spread their seeds.
"It explains why fruits are so diverse and so different in their characteristics," said McMaster University biostatistician Ben Bolker, who co-authored the paper. "Go to the supermarket — look around."
The study, led by researcher Silvia Lomascolo while she was a PhD student at the University of Florida, looked at 54 different types of figs collected in Papua New Guineau's Madang Province. The fruits are hugely varied and the plants rely on animals to disperse their seeds.
Scientists had long suspected that fruits that are spread by:
- Birds would evolve to be small, soft and coloured red, purple or black easily seen against a leafy background because birds have colour vision, a poor sense of smell and no teeth.
- Bats would be larger, harder, more strongly scented and less strikingly coloured, since bats have teeth, a good sense of smell, and usually look for food at night when colours are hard to see.
However, the grouping of certain traits might also be due to a family resemblance among related plants, said Bolker, and until now no one has been able to show otherwise, except in the case of one or two traits.
Lomascolo and her collaborators measured the size and texture of the fruits, recorded their colour precisely using a spectrometer and then extracted smelly compounds from them using a vacuum pump. They measured the quantity of scented compounds using a technique called gas chromatography. They also used video to observe and record whether the fruits were eaten by birds, bats, both, or other animals, and researched published reports about which animals ate which fruits.
"That is arguably the reason why no one has found this before," Bolker said. "It's a hell of a lot of work to do that."
Lomascolo asked Bolker, who was at the University of Florida at the time, whether he would help use statistical techniques to find the patterns in the data, and he agreed.
Their results showed that small fruits are indeed redder, darker coloured, softer and less smelly, and tend to be eaten by birds. Large fruits tend to be green or yellow, paler, and smellier and tend to eaten by bats. The pattern held regardless of whether figs were closely related, and explained 42 per cent of the variation in fig traits.
Bolker said the research answers interesting questions about how the world works. But if plant diversity depends on animal diversity, as the study shows, he said, that also raises questions about the fate of plants in forests where fruit-eating animals have been declining, especially in Africa and South America.
"What will happen to the plants that co-evolved with them?" he asked. The demise of an animal that co-evolved with a plant that relies on the animal to spread its seeds doesn't necessarily mean the plant will suffer, he added. "But it's certainly suggestive."