It’s a lovely notion, but tricky to prove. Still, lemurs sniffing around wild fruits in Madagascar are bolstering the idea that animal noses contributed to the evolution of aromas of fruity ripeness.
The idea sounds simple, says evolutionary ecologist Omer Nevo of the University of Ulm in Germany. Plants can use mouth-watering scents to lure animals to eat fruits, and thus spread around the seeds. But are those odors really advertising, or are they just the way fruits happen to smell as they ripen?
For some wild figs and a range of other fruits in eastern Madagascar, a strong scent of ripeness does seem to have evolved in aid of allure, Nevo and his colleagues argue October 3 in Science Advances. A lot of fruit collecting and odor chemistry suggest that fruits dispersed by lemurs, with their sensitive noses, change more in scent than fruits that rely more on birds with acute color vision.
Earlier studies had sniffed around several species, such as figs. But for a broader look, Nevo and his colleagues analyzed scents from 25 other kinds of fruits as well as five kinds of figs. All grew wild in a “really magnificent” mountainous rainforest preserved as a park in eastern Madagascar, Nevo says.
The researchers classified 19 of the plants as depending largely on red-bellied and other local lemurs to spread seeds. Most of these lemurs are red-green color-blind, not great for spotting the ripe fruits among foliage. But the researchers following some lemurs foraging in daylight noticed that sniffing at fruits was a big deal for the primates.Nevo doesn’t think the lemurs need scents to locate a promising tree. Instead the animals climbing around a tree often sniffed at fruits before biting into some of them, possibly using scent as a clue for what to eat right then.
Nevo and colleagues also picked fruits — 434 unripe and 428 ripe — and analyzed their scents. The odors proved complex, with blends of a total of 389 compounds. Some ingredients floated into the air in just traces, and some in strong whiffs.
Comparing ripe versus unripe fruit scents from individual trees, the researchers concluded that species dependent on lemurs for seed dispersal had ripening scents more than twice as strong as other fruits. An animal with a nose for fruit should be able to tell what’s ripe.
In contrast, plants that depended entirely or partly on birds had fruit that didn’t change much in odor strength. Likewise the chemical composition of their scents didn’t change as much during ripening, according to an index for making such comparisons. Those fruits did ripen; birds and other animals ate them with gusto. But a dramatically different scent wasn’t a big clue to ripeness. Nevo concludes that the noses of fruit-eating animals, maybe including primate ancestors of humans, appear to have played a role in the evolution of fruitiness.
Lemur noses could also matter in the evolution of Madagascar plants’ leaf scent, says primatologist Giuseppe Donati of Oxford Brookes University in England. “Plants overall do not really want their leaves to be eaten — it’s the opposite perspective.” One of his students is analyzing leaf odors from plants in woolly lemur territory to see if there’s any sign of “don’t eat me” version of the fruity “eat this now.”
SOMETHING TO SNIFF AT Red-bellied lemurs are red-green color-blind, but have fine noses. They do a lot of sniffing as they choose among a plant’s fruits to eat. This female picks and chooses her way through a fruit-laden tree in Madagascar’s Ranomafana National Park.
Editor’s note: This story was updated on October 3, 2018 to clarify the prevalence of red-green colorblindness among certain lemurs in Madagascar.