Leafcutter ants don't eat the leaves. They farm a fungus that can't live without them.
The columns of ants carrying leaf fragments through the forests of the Americas are not hauling food for themselves. The leaves feed an underground fungus garden, which in turn feeds the ants' larvae. The partnership is so old that the fungus has been fully domesticated and no longer produces spores.
Leafcutter ants are at least 55 species in the genera Atta, Acromyrmex and Amoimyrmex, found from the southern United States to South America. After our own, their colonies are among the largest and most complex animal societies on Earth: a central nest mound can grow more than 30 metres across and hold millions of ants. Workers of different sizes divide the labour. The tiniest, minims, tend the fungus and the brood; mediae cut and carry leaf pieces, hauling up to 50 times their body weight; large majors defend the nest and clear trails.
The leaves are processed and fed to a fungus of the family Lepiotaceae, which the ants weed and tend. It grows swollen tips called gongylidia that exist only to feed the ants, and it is fed to the larvae, while adults drink leaf sap. The ants respond to the fungus's needs: if a kind of leaf proves toxic to it, the colony stops collecting that plant. Domestication is thought to have taken some 30 million years, completed around 15 million years ago. Only two other insect groups, ambrosia beetles and termites, practise fungus farming.
Like any farm, this one has pests. A parasitic fungus called Escovopsis can devastate a garden; one study found it in almost 60 per cent of colonies aged one to two years. The ants weed out infected pieces, groom off spores and take waste to dedicated dumps staffed by expendable workers. They also carry bacteria of the genus Pseudonocardia in glands on their bodies, which secrete antibiotics that hold the parasite in check: in effect, portable antimicrobials. Small workers even ride on leaf fragments to fend off parasitic flies.
A new colony begins with a single queen carrying a scrap of her mother's fungus in a pouch inside her mouth. During a mass nuptial flight she mates with several males, collecting the roughly 300 million sperm she needs. Only about 2.5 per cent of young queens go on to found a long-lived colony. Those that succeed can become serious agricultural pests; some Atta species can strip a citrus tree in less than 24 hours.
Source: Wikipedia — Leafcutter ant · Text summarised from Wikipedia (CC BY-SA 4.0)
