Agriculture is rightly recognized as one of the turning points in human history. The practice of tending to specific animals and plants to maintain and even increase their utility helped drive humans into city-building and, from there, into the large, complex, settled societies we know today. Humans, however, are not the only animals that have discovered agriculture. Everything from snails to elephants has some ability to foster and guide the evolution of another creature for its own use. Agriculture, it turns out, is a subset of ecosystem engineering, and a lot of creatures are engineers.
Savanna for Me but not for Thee
African elephants live in many habitats, from inland grasslands to coastal forests. Savanna elephants (Loxodonta africana), a distinct species from those that dwell in forests (L. cyclotis), are keystone species for their home grasslands. By being enormous and traveling long distances, elephants maintain a presence throughout their territory and affect its ecology in numerous ways. One of the more controversial ideas surrounding elephant activity in the savanna attempts to explain why savanna elephants are often seen stripping, trampling, uprooting, and otherwise destroying trees in their territory, usually without eating them. This behavior may be a mere release of aggressive energy, but it also has the result of pruning trees to more convenient heights for feeding on later growth. More extreme tree destruction by elephants prevents nearby forests from expanding into the savanna at all, preserving its character as a grassland. Given the intelligent behavior already observed among elephants in numerous other settings and the fact that intelligence is not necessary for ecosystem engineering, it is possible that elephants are deliberately maintaining these trees and the savanna ecosystem surrounding them—that is, farming the savanna.
Farming is not a purely terrestrial endeavor. Even fish can become farmers, specifically the damselfish Stegastes nigricans. These damselfish maintain gardens of one specific algal species, weeding out all others, and defend these gardens with characteristic aggression. Their preferred alga is rare and easily overtaken when not so protected, so these gardens are the only way for them to access a large amount of it at once. They even protect mysid shrimps that visit their gardens, having figured out that the shrimps’ excrement fertilizes their gardens. This is especially noteworthy because even herbivorous damselfish do not normally pass up a meal of planktonic invertebrates like mysid shrimp, but they protect this species to the point that it forms swarms only when so protected. Not only has S. nigricans figured out how to grow its own food, but it is in the process of domesticating farm animals to help.
Fungi are popular subjects for animal agriculture. It is easy to see how it starts: an herbivore or detritivore discovers that fungi growing on decaying material or injured plants provide more concentrated nourishment than their previous diet and begins preferentially eating the fungus, and the relationship grows from there. Ambrosia beetles (two subfamilies of weevils, family Curculionidae) dig tunnels in wood like many weevils, but unlike their long-faced cousins, they do not feed on the wood or seed pulp they find inside. Rather, their tunnels spread distinctive fungus species throughout the wood, which the beetles then eat. Ambrosia fungi comprise numerous species, none of which have been discovered outside the care of ambrosia beetles. Ambrosia beetles even take bits of this fungi from each other if their fungus farms run out, restarting cultivation from this new stock.
Leafcutter ants, dozens of species in in the genera Atta and Acromyrmex, are perhaps the most famous of all animal farmers. Their parades of workers carrying chunks of leaf across the forest floors of Central and South America are well known and eminently photogenic, and it was easy for observers to believe this was ordinary hoard-building behavior until it was investigated in more detail. The ants do not eat the leaves, but use them as fodder for fungus gardens. The symbiosis between the ants and their fungi is profound, emulating the changes that occur during domestication. The ants harbor bacteria inside their bodies whose antimicrobial secretions they use to keep other fungi and bacteria from harming their fungus gardens, and choose their leafy substrate based on their fungus’s preferences. Some ants’ fungus gardens no longer produce spores, spreading only by ant-induced fragmentation of their mycelia, and also produce swollen structures specifically for the ants to eat, similar to how the edible parts of domestic plants are many times larger than those of their wild cousins.
Lest anyone imagine that it takes at least an insect’s mind to become a fungus farmer, semiaquatic snails have also taken up this practice. Marsh periwinkles (Littoraria irrorata) graze on cordgrass plants, but they are not eating the grass. Rather, they are wounding it with their razored tongues called radulas, introducing fungi into the wounds. As these fungi grow, the snail returns to previous wounds to feed on them, and the periwinkles may even fertilize these gardens with their feces.
Perhaps the most fascinating of the fungal gardeners are Macrotermes termites. Like leafcutter ants and ambrosia beetles, these termites cultivate specific varieties of fungus that are not found outside of insect care, but unlike the previous two, they also do not eat the fungus. Rather, they feed the fungus on wood pulp that they collect from the area around their nests, and then feed on the resulting decay products. The fungi can break down cellulose and lignin, two plant compounds that few animals can digest, and their leftovers therefore have much higher nutritional value than the raw wood and other plant material. This has been described as a kind of “extracorporeal digestion,” outsourcing some of the work of turning food into nutrition to another body, but one might also call it something else: cooking.
Meat on the Tarsus
Ants are noteworthy farmers in one other way. Many ants maintain herds of aphids and similar insects for their honeydew, sugary secretions they release from their anuses. In some cases, they even remove the wings of their aphid charges to keep them near, effectively an early step in domestication.
But ants take their agricultural tendencies one step farther. Melissotarsus ants farm scale insects, relatives of aphids, but the scale insects that these ants protect produce neither honeydew nor the waxy “scale” compound for which their lineage is named. It is difficult to see what the ants gain from this association, and one possibility is that they are farming the scale insects to eat them. Further study will reveal whether the scale insects are farm animals or whether, like paussid beetles and certain spiders, they are instead farming the ants for protection.
I Am the Farm
Some animals skip the “farmland” part of farming and use themselves as fertile soil. These symbiotic relationships are, if anything, far more abundant than external agriculture, but some examples straddle the line between the two. The top of that list is yeti crabs, Kiwa hirsuta, which grow symbiotic bacteria on their trademark hairy arms and wave those arms in the outflows of hydrothermal vents. The bacteria feed on the energy-rich chemicals wafting out of the vents, and the crabs feed on the bacteria. Given the difficulty of accessing yeti crabs in their native habitat, whether the relationship between these two organisms is as complex as that between ants and their fungi has yet to be studied.
Next to all of these, the various soft-bodied invertebrates that house algae within their tissues to feed on its sugars seem almost tame.
If this tour of animal agriculture provides any lesson, it is that the natural world is full of wonders biochemical, behavioral, and everything in between, and few broad concepts are truly distinct to humans. Animals can be anything, including farmers.