Acorns, and other nuts but particularly acorns, also contain tannins and other bitter/astringent substances to discourage animals from eating them. Humans have learned how to extract this from the nuts, and a few like squirrels have learned to tolerate them, but many critters simply won’t eat them because to those critters they taste bad.
Surely the number of coconuts which fall into water is too miniscule to propagate the species? 
In nature, coconut trees only grow along beaches, at the top of the high tide line where the nuts can be brought in by the sea. They are specifically adapted to long-distance transport by sea. Coconut trees that grow inland have been planted by humans (or are descended from ones that were).
Huh. Thanks.
First, excellent work Colibri!
Second, some plants have evolved defenses to protect their seeds from digestion by the carrier animal species. As a result, these seeds will not germinate unless eaten and then excreted by that animal.
There’s been some very interesting work done here in Panama about seed dispersal by agoutis, a large forest rodent. Agoutis not only bury palm nuts, they steal buried nuts from one another and bury them elsewhere. Because of this, seeds can move far beyond the home range of a single agouti. One tracked seed was moved 36 times and traveled more than 200 meters from its original location.
There are a couple of interesting corollaries:
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Agoutis move seeds from areas of high palm density to areas of low palm density. Because there are fewer agoutis in areas of low palm density (because there’s less food), the seeds are less likely to be stolen. This obviously benefits the palms because the seeds will have less competition in these areas.
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Agoutis in areas of low palm density are more likely to be eaten by ocelots, thus allowing their buried seeds to germinate. Agoutis are active by day, and ocelots mainly by night. Agoutis in areas of low palm density are hungrier and get up before dawn to look for food, when they are more vulnerable to being eaten. So again, this is a feedback loop that results in more seeds germinating in areas with low palm density.
The early bird gets the worm, but the early agouti gets eaten by the late ocelot.
200 meters is beyond the range of a single agouti?
Home ranges are around 1-2 hectares. While an agouti can move that distance, the radius of most home ranges is smaller than that. What the researchers found was that marked nuts placed in the home range of a single agouti were moving well beyond its boundaries.
So did squirrels develop their burying habits because there was a large supply of hard-shelled nuts that could safely be buried for long periods of time or did trees develop hard-shelled nuts because those were the most likely to be buried by squirrels?
And in many cases, the edible part of the fruit is eaten, but the actual seed is discarded (thus giving it s chance to grow). Think of examples like apples, oranges, pears, avocados, melons, olives, etc.
And not just by humans!
I have seen horses eat a whole apple, but spitting out the seeds, which don’t taste as good.
Neither and both at the same time, unless it was some other animal first, or some other food, or neither and both.
It’s also not a one-to-one relationship - lots of different animals are competing for lots of different kinds of seed - a network of overlapping dependencies and conflicts of varying strengths.
It’s probably a coevolutionary relationship, that is to say the characteristics gradually developed in tandem. With regard to the walnut/hickory family specifically, the family evolved in the late Cretaceous but at that time had small seeds not specifically specialized for either wind or animals dispersal. Animal dispersal developed in the early Cenozoic, with the first direct evidence of a cache of hickory nuts presumably stored by squirrels from the Miocene (5-22 million years ago).
Sadly, the example that I’d heard about, the dodo tree, is apparently under contention, now. Some think that it’s been debunked. Not that there aren’t other seeds that require a digestive tract, just that this is the one I had heard of. It made such a great story, too.
Yes, it was an interesting story but apparently hasn’t held up. Temple, the researcher involved, was my ornithology TA when I was an undergraduate.
Dr. Kerry Barringer of the Brooklyn Botanic Garden sez that ordinary berry seeds also fall into this category:
There are artisinal pork products made from pigs that feed on acorns, and supposedly the acorn-based diet leads to better-tasting meat from those animals. Usually, an animal’s diet does influence the flavor of its meat (look at grass-fed vs. grain-fed beef, for starters) so why do acorns not impart a disagreeable flavor to those pigs?
Why do pigs like acorns so much if most animals find them too bitter or astringent to be tasty?
There is a lesson here.
Why would acorn-fed pigs’ flesh taste like acorns? Grass-fed cattle’s flesh doesn’t taste like grass. Grain-fed poultry’s flesh doesn’t taste like grain, and so forth.
I don’t know why pigs like the taste of acorns, though.
There’s another interesting evolutionary development relating to fruits and seeds – at least, so I read some time back.
Earlier plants, even the biggest, were more fernlike and had tiny little spores (or whatever they’re called) that had no practical nutritional value. Animals that lived on plants had to be enormous (the dinosaurs) and have big digestive tracts and enormous diets (like elephants and cattle) to get enough calories, and they tended to be slow to conserve energy.
The development of seeds brought with it faster paced animals, like (eventually) us, who seek out more concentrated sources of calories. This was a big turning point in the evolution of all the big species.
I read an article about it somewhere years ago. Can’t remember to cite, and don’t have anything more.
And very delicious, I’m told.