Next time you find yourself reaching for an orange from a fruit bowl, or taking a banana out of your lunchbox, just take a moment to appreciate the sweet and varied scents of the fruit we eat. According to a recent study, it could just be that you are inhaling an aroma which has evolved as an important signal to us primates over millions of years.
The smell of ripe fruit is created by volatile chemicals (‘volatile’ meaning they evaporate readily into the surrounding air) produced by the plant, known as ‘plant secondary metabolites’ or PSMs. These PSMs serve diverse functions, ranging from making the plant or fruit inedible to herbivores, preventing disease, and even controlling the digestive process. One largely consistent aspect of PSMs, however, is that they are thought to be one of the main ways in which plants may communicate with animals.
This is something many plants need to do for a number of reasons – either to show they’re inedible, to attract pollinators, and, later in the plant reproductive cycle, show off their seed-bearing fruits. Many fruit-eating animals (frugivores) such as bats, birds and primates play a crucial ecological role simply by gorging on fruit – in many plant species, the seeds are able to tolerate the passage through the digestive tract, and get dispersed further away from the parent plants (with a nice helping of fertiliser to help them along too!).
This relationship mutually benefits both the fruit-eaters and the plants, though on one condition – that the fruit is not picked until it is ripe (i.e. the seeds are ready to germinate). It is well known that fruits advertise their ripeness visually, showing off a range of vivid hues and shades. Being such an important food source, it’s even thought that the evolution of fruit colour may parallel the evolution of colour vision in primates.
However, when studying animal-plant communication, it is important to distinguish two terms often used – an animal may respond to a ‘cue’, exploiting information provided by the plant, which is simply a by-product of another process and not specifically for communicating with animals.
Conversely, a ‘signal’ is information provided by a plant which is under natural selection to fulfil that exact purpose – such as the shape, odour and colour of flowers, which is solely for the purpose of attracting pollinators. Bright colouring may certainly make fruit more appealing, but the recent behavioural study has suggested that important seed-dispersing primates such as spider monkeys (Ateles geoffroyi), which are capable of rapidly telling whether fruit is ripe or not, rely primarily on their sense of smell.
Researchers from the German Primate Centre, collaborating internationally with a team of botanists, have attempted to determine whether the odour of ripe fruit is an evolved signal rather than a cue exploited by primates. Though this has been suggested before, they are the first to empirically test the idea. To do this, they looked at the PSMs produced by four plant species – two that rely on primates for seed-dispersal, and two that relied on birds – thought to mainly use vision for foraging, relying less on smell.
Under the assumption that all the fruits would produce PSMs regardless (due to their multiple functions), a rapid change in the PSMs produced would indicate that the plant is ‘signaling’ primates. The authors created a profile of the PSMs produced by the fruit, identifying the different compounds, and measured how these profiles varied according to species and to the ripeness of the fruit. They found that the profile was significantly different in ripe fruit in the case of the two primate-dispersed species, though not so in the bird-dispersed species.
Moreover, the chosen primate-dispersed species are more closely related to the bird-dispersed ones than each other, suggesting this may be a repeated evolutionary trend, perhaps an inevitable consequence of recruiting animals which rely on their sense of smell so heavily for dispersing seeds.
The researchers caution that the results are only from four species, and that much more work needs to be done to see whether this relationship exists across the animal (and plant) kingdom – but for the time being, next time you find yourself snacking on a piece of fruit, you can consider why it smells so sweet.
Nevo, O., Heymann, E. W., Schulz, S., & Ayasse, M. (2016). Fruit Odor as A Ripeness Signal for Seed-Dispersing Primates? A Case Study on Four Neotropical Plant Species. Journal of Chemical Ecology, 1-6. DOI: 10.1007/s10886-016-0687-x
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