Academic Reading — Test 55

Reading passage

Among the most remarkable examples of cooperation in the natural world is the relationship between leafcutter ants and the fungus they grow for food. These ants, found across the warmer regions of Central and South America, do not eat the leaves they so industriously gather. Instead, they use the plant material as a substrate on which to cultivate a particular fungus, and it is this fungus, not the foliage itself, that nourishes the colony. The arrangement is so tightly bound that neither partner can survive for long without the other. For this reason, biologists describe the leafcutter ant as one of the few creatures besides humans to have developed a genuine form of agriculture, a practice that the ants appear to have refined over tens of millions of years. A single colony may contain several million individuals, and the work of harvesting leaves is divided among ants of strikingly different sizes. The largest workers, sometimes called soldiers, defend the foraging trails and the nest, while medium-sized workers do most of the cutting. Using their sharp, serrated jaws, these cutters slice neat fragments from leaves, flowers and other vegetation, then carry the pieces back to the nest along well-worn trails that may stretch for many metres. A column of returning ants, each holding a green segment aloft, is a familiar sight in tropical forests, and the burden an individual carries can be many times its own body weight. Curiously, the very smallest workers often ride on the cut fragments as they are transported. Their presence is not idle: they guard the leaves against parasitic flies that would otherwise lay eggs on the carriers, and they help to clean the material before it enters the nest. Once inside, the leaves are not used directly. Worker ants chew the fragments into a moist pulp and press this paste into the spongy mass of the fungus garden, which is housed in specially excavated underground chambers. The fungus spreads its thread-like filaments, known as hyphae, through the decaying plant matter, breaking down compounds that the ants themselves cannot digest. At the tips of some of these filaments the fungus produces small swollen structures rich in nutrients, and it is chiefly these that the ants harvest and feed to their developing larvae. In effect, the fungus acts as an external digestive system, converting tough leaf material into food the colony can readily use. The ants, for their part, supply a constant flow of fresh substrate, maintain ideal conditions of warmth and humidity, and tend the garden with great care. That care is necessary because the garden is vulnerable. A parasitic mould of the genus Escovopsis poses a persistent threat and can devastate a fungus garden if left unchecked. To combat it, the ants weed their crop much as a human gardener might, removing diseased portions and carrying waste to refuse chambers set apart from the living garden. They also carry on their bodies colonies of beneficial bacteria that produce antibiotic compounds, and these substances help to suppress the harmful mould. The partnership is therefore not a simple pairing of ant and fungus but a more intricate web involving several organisms, each contributing to the stability of the whole. The origins of the new colonies depend on the queen. When a young queen leaves her birth nest to found a colony of her own, she carries a small pellet of the parental fungus tucked into a pocket within her mouth. After mating and shedding her wings, she digs a chamber, spits out the pellet and begins a new garden, nurturing it with her own bodily secretions until the first workers emerge. From that point the colony can grow rapidly, and a mature nest may extend several metres underground, with hundreds of chambers connected by a network of tunnels. The scale of the ants' activity is such that, in some regions, leafcutters are considered serious agricultural pests, capable of stripping a crop overnight. Scientists are drawn to these ants for reasons that go beyond curiosity. The antibiotic-producing bacteria that the ants cultivate may point towards new medicines at a time when resistance to existing drugs is a growing concern. The fungus's ability to break down plant fibre is of interest to researchers seeking efficient ways to convert plant waste into fuel. Above all, the leafcutter system offers a striking lesson in how separate species can become so interdependent that they function almost as a single organism, a partnership shaped not by design but by the slow pressure of evolution acting over an immense span of time.