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Many fruits are not homologous, originating from different parts of a plant. Yet they all have similar properties:

  • Ripe fruits all have yellow to dark red color

  • They all have a lot of water and sugars, even in dry climate where water is in shortage.

  • Besides sugars they usually have acids making them sour to sweet in taste

I understand that these possibly developed as a contract between the plants and the animals in that the plant would provide beneficial fruit tissue so that the animal to swallow the seed so to propagate it for longer distances.

But I wonder, why there are no fruits that would imitate the taste of meat, nuts, seeds, foliage, grass, mushrooms, roots, milk and other food sources, common for the animals?

Why there are no salty fruits for instance or those rich in proteins rather than sugar and water, or having the smell of carrion?

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I appreciate the intent of your question but I suggest you think more broadly about what are fruits. Coconuts are fruits. Dandelions produce fruits. Durian fruit smells terrible to most people. Look at samaras. Any seed-bearing structure on flowering plants is a fruit. –  Mike Taylor Aug 8 at 0:38
    
@3cat under fruits I explicitely mean those tissues intended to be eaten by animals. I am not interested in the other varieties. I ate durian, and as it was fresh I did not notice any distractiong smell. Do you claim it is not intended to be eaten? What's the purpose of it then? –  Anixx Aug 8 at 6:08

2 Answers 2

Howe and Smallwood (1982) provide a nice review of the many methods of seed dispersal that have evolved in plants. The review is broad but they do have a section on frugivory. They highlight hypotheses developed by McKey, and Howe and Estabrook (see Howe and Smallwood for citations) that suggest plants may use one of two strategies.

One strategy is the "high investment model." Plants invest lots of resources to produce large seeds and nutrient rich fruits. THe hypothesis is that these types of fruits tend to attract relatively few but specialized frugivores that are will to invest the energy necessary to find these types of fruits and their associated nutritional reward.

The second strategy is called the "low investment model." In this model, plants invest little in individuals seeds and fruits but produce an abundance of them. The hypothesis is that these types of fruits will be eaten by as many different potential seed dispersers as possible. These tend to be very small or starchy although they can still be colored as you describe.

Other factors also come into play. For example, tropical fruits tend to have large seeds and nutrient rich pulp while temperate fruits tend to be smaller and offer less nutritional reward. All of this is placed in the context of the community diversity of frugivores. The Howe and Smallwood paper was an interesting read.

A paper by Gatier-Hion et al. (1985) looked at the characters of fruit choice and seed dispersal mechanisms by tropical forest vertebrates. The showed the relationships between fruit characteristics and the corresponding frugivores. They looked at 122 different fruit species and the frugivores that consumed them. Their results showed that fruits tended to separate along one of three axes:

  1. Heavy, indehiscent fruits with fibrous flesh and well protected seeds vs light, dehiscent fruits with unprotected seeds. Many of small fruits are red,
  2. Lots of seeds produced (typically the the large indehiscent) vs few seeds produced, and
  3. Juicy and brightly colored fruits vs dry and dull-colored fruits.

Figure 2 from Gautier-Hion et al., shown here, shows the relationship between different frugivores and the fruit types. Notice that small granivores, which are frugivores, don't eat large, juicy fruits. They consume lots of small green or brown grains that are easy to manipulate and produced in high quantities. Squirrels are similar.

enter image description here

Thus, as you suspected, fruits and seeds have evolved in response to their frugivores but the fruits do not always have the traits you listed. I think, but do not know for certain, that the fruit traits you list are the result of selection being driving by the frugivores. The sugars provide lots of quick energy, which is necessary for active animals. The carbs probably provide a much greater nutritional reward than salts or proteins (which are present anyway). The colors may help the fruits to stand out against the green foliage (for organisms that are not red-green colorblind). These traits probably provide the greatest reward (lots of energy) with the lowest energy investment by the frugivore (easy to find).

Throughout, I've assumed natural conditions and not fruits modified by artificial selection for human consumption.

Literature Cited

Gautier-Hion, A., et al. 1985. Fruit characters as a basis of fruit choice and seed dispersal in a tropical forest vertebrate community. Oecologia 65: 324-337.

Howe, H.F. and J. Smallwood. 1982. Ecology of seed dispersal. Annual Reviews in Ecology and Systematics 13: 201-228.

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Squirrels mostly eat nuts which are not intended to be eaten because their seeds are destructed in the process thus there is no surprise that nuts typically have no attractive color. –  Anixx Aug 8 at 15:27
    
On the other hand, squirrels often take the nuts and bury them but do not later eat them. Dispersal from the plant's perspective has occurred. –  Mike Taylor Aug 8 at 15:35

How are you defining fruits? Because if you count any seed body of a flowering plant, you must include several items commonly described as vegetables, including pumpkins, zucchini, squash, cucumber, peppers, etc. And there are other colors of fruit, such as blue berries. These all serve similar functions for the plant, something eats the fruit, then deposits the seeds somewhere else.

If you define a fruit by what the average person calls fruit, then you've biased the population because humans have selected for juicy sweet tasting fruits over thousands of years of breeding. Humans like large sweet tasting fruits, so they selected for those traits, driving most fruits to have those traits. Wild fruits probably have much more diversity, and there would much less distinction between fruit and vegetable.

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The most accurate way to define fruit is by using the botanical definition. Seeds are not fruits so nuts (in the sense of walnuts or nut mix that you buy at the store) are not fruits. Fruits are the seed-bearing part of the flowering plant. –  Mike Taylor Aug 8 at 0:46
    
I made an edit to remove the reference to nuts. By your definition many vegetables are still fruits. –  user137 Aug 8 at 0:59
    
Such as? Tomatos, widely perceived as vegetables, are fruits. Chili peppers (bell, jalapeno, habanero, etc.) are fruits. If it bears seeds, it is a fruit. Fruits, by botanical definition, bear seeds. –  Mike Taylor Aug 8 at 1:09
    
That's what I was getting at. I erroneously included nuts as fruits in my answer, and that reference is removed. Was trying to point out that fruits did not all go through the convergent evolution described in the question because not all fruits are sweet, many food items commonly considered vegetables are in fact fruits. –  user137 Aug 8 at 1:27
    
Cucumbers are green just because we eat not ripe cucumbers. Ripe cucumbers are yellow. Blue berries are just too dark red with a fungal film on the surface. I define fruits as the tissues intended by the plant to be eaten by the animals. –  Anixx Aug 8 at 6:05

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