Many plants that rely on animals for seed dispersal have evolved brightly colored fruit to attract their seed distributors. But some fruits are not colorful, and some fruit-eating animal species have limited color vision. A new study has shown that plants can use scent to attract animals when color doesn’t work as a signal.
Omer Nevo of the University of Ulm in Germany and colleagues on three continents studied the properties of fruits eaten by lemurs, a primate endemic to Madagascar. Most lemurs have dichromatic vision and cannot distinguish between red and green. Thus, for lemurs, bright red fruits blend in with their surrounding green foliage.
The researchers found that fruits that depend on lemurs for propagation communicate their level of ripeness through scent. When the chemical compounds that produce scent were measured in ripe and unripe fruit, a set of compounds called aliphatic esters was almost exclusively found in ripe fruits eaten by lemurs. Furthermore, the fruits eaten by lemurs increased their scent production as they ripened. When researchers observed the sniffing behavior of wild red-bellied lemurs, Eulemur rubriventer, they found that the lemurs’ sniffing rate increased for fruits that increase their scent when ripe. This suggests that lemurs are sensitive to which fruits signal ripeness through changing scent.
As a control, the researchers also studied fruits in Madagascar that are primarily dispersed by birds, which can see four colors but have a poor sense of smell. Compared to fruits eaten by lemurs, the fruits eaten by birds showed less change in scent compounds upon ripening, and they did not produce significantly stronger scents when ripe.
Nevo and his colleagues conclude that the change and increase in fruit scent upon ripening is not an inevitable byproduct of maturation and does not characterize all fleshy fruits. Rather, it has evolved as a mechanism to attract dispersers that have a strong sense of smell and limited color vision. (Science Advances)