Most of the world's animals eat plants, and plants have evolved a spectacular array of defenses against being eaten. They produce hairs and hooks, tough coverings, sticky traps, gooey sap, compounds that reduce digestibility, and a startling variety of toxins. But the herbivores are not to be outdone.
For every plant defense, there is likely to be some animal that has breached the defenses. This sets the stage for further evolution of the plant's defenses, to counter the inventive attackers. The constant interplay between plant protection and herbivore attack is an example of a co-evolutionary arms race: Every defense prompts the evolution of a new way of attacking, which in turn fosters the evolution of new defenses, then new attacks, in a continual escalation.
Here is a sample of some of the ways that vertebrate animals cope with plant defenses.
Some vertebrates begin to overcome the problems of coping with tough plant material by grinding it up with strong teeth (in mammals) or gizzards (in birds). Mammals that forage on grasses (horses, cows, bison) usually have tall, high-crowned, very hard teeth, because the silica (the mineral in sand, for example) in grasses is both totally indigestible and very abrasive, and the big teeth resist wear. Rabbits also have high-crowned teeth that grow continually. There is even a kangaroo in Australia that replaces its molars, one by one, as they wear out, but eventually it runs out of replacement teeth and can no longer eat. Mammals that regularly browse on woody stems (such as moose) also tend to have tall, hard teeth to crush and shred the twigs. But grinding, crushing, and shredding are just the first step in extracting nutrition from tough plants.
Most plants contain a variety of toxic compounds that must be detoxified by a consumer so that the animal is not poisoned by its food. Special organs such as livers contain high concentrations of detoxifying enzymes. Some detoxifying agents are permanent fixtures and others are induced only in response to the presence of a toxin. Herbivorous animals can differ greatly in their ability to detoxify plant material.
One of the most interesting ways of dealing with both plant toxins and tough fibrous molecules is the enlisting of aid from other organisms, and animals have found a variety of ways of getting help. Helpers come in the form of bacteria and protozoa - micro-organisms that can break down all kinds of chemical compounds into simpler and less harmful forms (and they often can synthesize vitamins too).
Many herbivores house billions and billions of these micro-organisms inside their digestive tracts, in some cases in a complex stomach, in other cases in a special outpocketing of the gut. Cud-chewing mammals such as cows and sheep not only chew their food twice but also use micro-organisms to break down large molecules such as cellulose and detoxify nasty compounds. Rabbits also process their food twice, but they do it by defecating two kinds of pellets and then re-ingesting the softer pellets that have concentrated the protein content of the food. (This is termed "refection," and I have fun with this, because a "refectory" is a dining hall, where monks and nuns go to eat!)
Herbivorous animals have longer digestive tracts than carnivores, whose food is more easily digested. And large herbivores not only eat large meals, they also hold the material longer in the gut, giving their gut micro-organisms more time to break down tough plant material. For example, a deer may hold a meal almost two days and bison may retain a meal for over three days. Even with a prolonged holding time, some foods remain largely undigested: A deer can digest only about 20 percent of blueberry stems and leaves.
In contrast, small herbivores eat smaller meals and retain the material for shorter times. They have a small gut capacity and little room for keeping material during long digestion times. That means, in general, that they need to be more picky of what they eat, selecting plant parts that are more easily digested.
Very small herbivores, such as insects, have a spectacular array of ways to deal with plant defenses. Of which, more anon.
Mary F. Willson is a retired professor of ecology and a Trail Mix board member.
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