Thursday, March 17, 2016

Seed predators break into and eat nutritious foods that others can't

It's not even spring yet, technically, in the northern hemisphere, yet flowers are already starting to blossom. To be followed by fruits. We spoke about leaf-eating (folivory) awhile back, and now for St. Patrick's Day, we'll resume our discussion of eating (green, unripe) seeds.

In an earlier post, we learned how seeds and nuts have particularly high nutritional content, and that they provide us with numerous health benefits, despite their high fat content. Plus they are yummy.

Did someone say Yummy?  Nuts and seeds have a "high fat content"?
Does this mean that sweetened waffles with flavored whipped cream can substitute?
Answer: No. Photo: BridgettBlough

Such high concentrations of fats, proteins, fiber, and minerals in a food source were probably important to early humans (genus Homo), or really anyone who hunts and gathers for a living, as they would otherwise have to hunt down meat to obtain enough of these essential nutrients. This was not always possible, and it's more likely that our early ancestors evolved eating seeds, fruits, leaves, and insects or fish, depending, of course, on where they lived.

Some common nuts inside their shells --
nutcrackers help us make use of this energy-dense food source.
Photo: Kazvorpal, Wikimedia Commons

Bring in the seed predators!

Humans, of course, are just one of many animals that eat nuts and seeds. Animals are smart, and they like fat too. We share our penchant for noshing on nuts with a variety of species -- monkeys, birds, and insects in the trees themselves, and a variety of rodents and ungulates on the ground.

In fact, for many a small bird, rodent, deer, or other herbivorous animal ill-equipped to kill and eat another animal, nuts and seeds are key to a healthy diet.

Rodents may be small to catch prey, but they are well-equpped to crush seeds
Photo: MyKidsWay

Animals that eat seeds, rather than fruit pulp, leaves, or other animals, are called seed predators, or granivores.

Obviously any animal that eats whole fruits also takes in the seeds, but granivores chew up and destroy the seeds in the process, while traditional frugivores prefer the ripe sweet fruit pulp and often pass or spit out the seeds undamaged.

Needless to say, many trees have evolved mechanisms that spread their seeds far apart, where they can develop inconspicuously, and seed predators will have to search harder to find them all. One of those is making ripe fruits really attractive to helpful frugivores, who will do some of the work of dispersing seeds.

Janzen and Connell proposed that seed predation would be higher closer to a parent tree, as that high concentration of seeds in one place would allow specialist seed eaters to take out all or most of the crop. Seeds carried farther from the parent showed a better generation rate. Diagram: Study Blue
Animals that eat fruit, move away from the parent tree, and either regurgitate or poop out the undamaged seeds—that is, disperse the seeds—are a critical part of any ecosystem. Why, you might ask? You could ask researchers, starting with Drs. Dan Janzen and Joseph Connell, who studied the fate of seeds falling at various distances away from their parent tree. They developed a hypothesis that the area immediately around the mother tree will be targeted by specialized seed-eaters and diseases, so seedlings will fare better if they are carried far from their parent tree.

Any seed with a bit of weight can't be carried off by the wind, so it depends on animals to move it away to safer ground. The larger the seed, the larger the animal needed to disperse it. Killing off large animals (elephants, rhinos, tapir, giraffe, antelope, toucans, hornbills, etc) by hunting them or destroying their home forests and savannahs, prohibits tropical plant regeneration.

A saki money inspects a fruit in southeastern Peru.
Note the long bushy tail that resembles a tree branch.
Photo: Edgard Collado
Pre-dispersal seed predators eat seeds while the fruits are still on the plant (i.e. before the fruit has been dispersed, by wind or some other animal, or falls to the ground). These include monkeys, squirrels, birds, and insects.

Post-dispersal seed predators eat the seeds after fruits left the plant, so basically on the ground. These critters love it if the seeds are conveniently bunched beneath the parent tree.

Some pre-dispersal seed predators will also eat seeds from the ground, and they are joined in the feast by a host of non-climbing neighbors — rodents, pigs, tapirs, deer, and more insects.

A favorite pre-dispersal seed predator of mine is the bald-faced saki monkey, the subject of my PhD research. What a crazy hairy animal - looks a bit like a sloth and puts its cryptic coloration to good use by pretending to be part of the branches it sits on in large rainforest trees in the Amazon.

Another saki monkey uses its big strong canine teeth to dig into a fruit.
Photo: Edgard Collado
Sakis dine primarily on the seeds of unripe fruits — "green" seeds tend to be softer and have fewer nasty chemicals in them - and because they eat this resource that almost nobody else can eat (except a few parrots), they face little competition for food.

Why can sakis make use of otherwise indigestible food?

They have strong jaws and hard tooth enamel that enable them to break open hard husks to reach the seeds inside. And their apparently iron stomachs allow them to detoxify the secondary compounds in unripe fruits of over 200 different tree species. They don't ferment tough plant material like some species (cows), but their relatively slow digestion helps them process fibrous fruit parts.

Why don't all animals eat seeds, if they're so nutritious?

Good question.

It's probably because some aspect of their system couldn't handle all the defenses that seeds put out to avoid being eaten prematurely. For instance, a recent study suggests that, unlike saki monkeys, the jaw of one relative of early humans, Australopithicus sediba, might not have been strong enough to crack nuts or eat tough bark.

The spiky exterior and creamy (yet incredibly smelly) durien fruit.
Not readily available to people without a tool (and set of nose plugs)
Photo: مانفی-Wikimedia Commons
Other animals may have strong jaws but can't process the chemicals contained in seeds.

Plant-animal mortal combat, a.k.a. co-evolution

To counterbalance the threat of animals wanting to eat their seeds, plants have evolved defenses. Seeds can have coarse skins (like pineapples) or hard shells (like coconuts), or both (like walnuts or pecans), spines (like a durien) or even slimy coverings.

Besides the seed's shape or toughness, there are their chemical defenses (secondary compounds such as tannins and alkaloids) to defend against their seed predators by making the seed taste bad and difficult to digest, potentially making the animal sick. Think of the astringent taste of an unripe banana makes you want to avoid it.

A walnut still on the tree, with its hard shell and outer husk
(the seed you eat is inside!)
Photo: Böhringer Friedrich, Wikimedia Commons
In response to the defenses that plants come up with, seed predators have adapted to physical and chemical mechanisms to eat them anyway (e.g., strong teeth and jaws, strong digestive systems that can detoxify chemical compounds).

You can see where this is going — plants with certain features succeed and reproduce, and an animal with certain features succeeds in eating it, so then the plant may evolve to evade this new predator, which then evolves to make better use of the plant, etc.

The dynamic relationships this arms race produces are interesting examples of coevolution.


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