A lot of the tropical fruits – mangos, pineapples – are really sweet, esp. compared to fruits from more temperate climes, like apples and berries. Why so sweet? The amount of sunlight they get? the length of the growing season?
I don’t think there is a real difference in sweetness on average between tropical fruits and temperate fruits. You can’t really take cultivated fruits as an index, because their degree of sweetness has been greatly influenced by selective breeding.
There is a much greater variety of fruits in the tropics, most of which never make it to market in the temperate zone. In the tropics, fruit dispersal by animals (as opposed to wind or gravity) is much more important than it is elsewhere. This is because it is possible for different species of plants to bear fruit all year round, and hence support populations of specialized fruit-eaters. In the temperate zone, fruit eaters must either migrate or make it through the winter on the fruit that remains on the plant in the fall.
You’re right about cultivated fruits – I didn’t think of that factor. At the same time, however, although the various types of apples we’ve “created” are probably sweeter than their feral forebears (hm, I may go back and read that section of the oft-recommended [by me] Botany of Desire), I still think the “average” apple is not as sweet as the “average” pineapple – and that although some temperate fruits are extremely sweet (like peaches), and some tropical fruits aren’t (bananas), in general, tropical fruits are sweeter. Is there such a thing as a measure of sweetness, similar to the measure of hotness of peppers? I guess it would be the proportion of fructose. Hm.
Interesting point on animal dispersal – let me think about that a while, I’ll probably be back later with another question. 
← fruit-eating grin
More sun equals more sugar. It’s why grapes grown in a temperate zone like France tend to be less sweet than grapes grown in a sunnier climate like California, which is why French wines tend to be more subtle and less robust than California wines.
Thanks, Neurotik. (And I’ll be you knew this was coming…)
Why does more sun mean more sugar?
More sun means the plant has more energy available to manufacture carbon compounds such as sugar. However, the equation “more sun means more sugar” only holds if that is what the plant has been selected for. It could equally well put more energy into growth. Or, if the plant produces a non-sweet fruit, like an olive, it could put it into making the olive more oily.
BTW bananas a not a fruit. They are a giant herb in the grass family.
Generally, fruits have an outer skin,an inner juicy flesh or pulp and a seed or seeds in the center.
Wrong on every count.
The banana plant is a giant herb, but although it is a monocot it is not in the grass family (Poaceae). It is in the banana family, Musaceae.
The banana fruit is indeed a true fruit. The cultivated banana, however, is a sterile hybrid and does not develop seeds. The small black specks in the banana are the aborted seed remnants. It is dependent on humans for vegetative propagation. The ancestral species have seeds within the fruit.
Botanically speaking, a "“fruit” does not need to have a juicy flesh or pulp. There are lots of dry fruits.
Okay, Colibri, as promised, I’ve got a question on your animal dispersal point – is it possible that there was a coevolution thing going on where the fruits selected for sweetness because that increased the likelihood of seed dispersal via animals? (Note, I’m not talking intentionality here.)
Yes, absolutely. Fruits and their dispersers, like flowers and pollinators, are one of the classic illustrations of co-evolution. There are a variety of different “syndromes” of characteristics by which particular fruits are adapted to different kinds of dispersers. For example, small sugary fruits with lots of small seeds are often adapted for dispersal by fairly unspecialized small bird dispersers. The birds swallow the fruits whole and disperse them in their droppings. Some seeds need to be passed through an animals digestive tract before they will germinate.
However, you can’t disperse very heavy seeds this way. For that you may need larger dispersers, and you attract them by packing your fruits with lipids (oils) which are harder for the plant to make and harder for animals to get. Many tropical fruits that are dispersed by large, highly specialized birds like toucans or trogons are oily, like olives, rather than sweet. Rather than passing these fruits through the gut, these birds swallow them, strip off the flesh in the gizzard, and then regurgitate the large seed.
The largest seeds usually need to be dispersed by mammals. Fruits adapted for dispersal by bats often hang off the tree on long stalks so that they are only accessible to a hovering animal. Some fruits have heavy rinds so that they will be eaten only by animals large enough to break through such as primates, so that the flesh will not be eaten by animals too small to disperse the seeds. Some biologists think that the avocado, with its oily flesh and very large seed, is adapted for dispersal by jaguars.
Counterpoint hijack…
My favorite examples of a unique method of attracting pollination vectors are carrion flowers. They attract animals such as flies by smelling similar to rotting flesh. The flies land on blossoms hoping for a meal, and end up pollinating them.
Colibri – thanks for yet another of your always extremely interesting posts – can you recommend additional reading on this?
This site has an extensive discussion of fruit dispersal in rain forest.
One of the most interesting methods is dispersal by fish, which is especially common along the Amazon in areas where much of the forest is flooded seasonally, allowing fish to swim around below the canopy.
Another unusual syndrome is dispersal by ants. The seeds have small packets of nutrients attached to them and are carried to suitable germination sites by the ants before they strip the packet off.
One of the most interesting discussions of fruit-eating birds and their adaptations is The Web of Adaptation by David Snow. While it is out of print, you may be able to find it at a university library, and should be able to find a used copy on-line.
Thanks for the tip – I’ll have a nice used copy of Web of Adaptation in two to four business days.
Any recommendations on a book on coevolution in general? I’ve been reading a lot of natural history – just finished David Quammen’s Song of the Dodo, which I found fascinating.