Keller L., Gordon E. The Lives of Ants

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pollinators. With the aim of monopolizing the resources, they

took to chasing away any other insects that trespassed onto the

plant. This being of beneﬁt to the plant, it developed extraﬂoral

nectars and nutritive bodies to retain the services of its protect-

ors; and they, now that they had a plentiful supply of provisions,

took up residence in the plant.

In tropical forests, it may have been competition between

different species of tree-dwelling ants that made some of them

develop an association with a particular tree. Species that came

off second best and were evicted from the canopy would have

had to build their nests in the crowns of other trees which,

ﬁnding the presence of ants advantageous, adapted to the

needs of the insects.

However they came about, once these close partnerships were

formed, it was in the interest of both parties to remain united,

since they both found them extremely beneﬁcial.

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Attines and fungus getting

on famously

In Guadaloupe and throughout much of Latin America

they are called parasol ants; and when you see them

making for their nest holding a large leaf over their

heads, you could be forgiven for thinking they are trying

to keep the sun off. However, these leaf-cutting ants carry

their sunshades even when foraging at night, their sole

purpose being to harvest vegetable matter as fertilizer for

their gardens. Leaf-cutting ants of the genus

Atta

, found

in Central and South America, mostly feed on fungi,

which they grow with great care on the leaves they have

collected. This has led entomologists to claim that agri-

culture was ﬁrst invented by ants. It is true that fungus-

growing (attine) ants started to cultivate their fungi about

ﬁfty million years ago, well in advance of

Homo sapiens

who discovered the possibility of sowing and reaping only

about 10,000 years ago.

Here we have another example of real symbiosis of immense

beneﬁt to both parties. The interest of the fungus is served by its

being protected against fungivores and parasites. And for Atta

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ants the advantages of this mutualism are enormous, as they

have a staple diet of fungi, mainly basidiomycetes, which their

larvae consume almost exclusively.

The dependence of the ants on the fungus did not just spring

up o vernight. The original fungus-growing ants were not leaf-

cutters, but debris collectors, using bits of withered plants for

cultivation of a relativ ely unspecialized mycelial fungus that could

live independently from the ants. Nowadays, most of the more

than 200 species of fungus-growing ants still use leaf-litter debris

for fungal cultivation. These species are much less conspicuous

than leaf-cutting ants. They typically form small colonies compris-

ing a few hundreds workers and they lay out their little gardens

inside pretty unsophisticated nests, which are dug very close to

the surface or even protected by just a few stones. Though not

much larger than a golfball, such a garden supplies all their needs.

The evolutionary transition from debris-collector to leaf-cutter

was accompanied by a dramatic increase in worker numbers and

complexity of social organization. Some extant leaf-cutting nests

are estimated to live for ten to twenty years, contain ﬁve to ten

million workers and maintain 500 football-sized fungus gardens.

In these extended underground nests, as big as a bus, work is

highly organized and performed on the model of an assembly

line. Large workers go out to gather vegetation of all kinds,

leaves, fruit, blades of grass, whatever suits their species, and

bring it back to the nest. There they dump their loads of vege-

table matter, which is cut up into tiny fragments by smaller ants.

Then comes the turn of even smaller workers who crush and

mould the plant debris into damp balls. The end result of this

sequence of operations is that vegetation is turned into a spongy

paste consisting mostly of microscopic leaf fragments and plant

juices. This is how, like Voltaire’s Candide, workers cultivate

their garden in which the symbiotic fungus will grow. When it

starts to appear, they tend it with great care. They weed out any

hyphae that are growing too slowly, then replant these ﬁlaments

THE LIVES OF ANTS

130

of mycelium in newly prepared gardens containing vegetable

paste. They also regularly lick the surface of their crop to rid it

of any moulds of foreign vegetable species.

Leaf-cutting ants are therefore, for the most part, genuine

farmers. They set up their garden plots right in the centre of

the nest, where the queen is quartered and lays her eggs. This is

convenient for the larvae, as the nourishment they need for

growth is right beside them. And when the time comes for the

young queens to ﬂy away, they will carry in a pocket inside their

mouth cavities some ﬁlamentous strands of hyphae which will

enable them to start a new garden for their own society-to-be.

Parallel evolution

Leaf-cutting ants of the genera Acromyrmex and Atta do not just

cultivate fungi; they have really domesticated them. By replant-

ing strands of hyphae on their carpet of vegetation, they prevent

sexual reproduction and spore formation in their basidiomycete

cultivar. Spores are superﬂuous to their needs, as they do not eat

them, and so it would be pointless to let them form. Because of

this, the fungus can no longer do without the ants to achieve

asexual reproduction, just as the ants cannot get enough food

without the fungus. Their symbiosis has become obligatory.

This example of mutualism is so neat that one cannot help

wondering whether it may not have come about through co-

evolution of ant and plant, each new species of ant having as it

were ‘created’ a species of fungus adapted to its needs. But that is

in fact not the case. The basidiomycete cultivars (varieties of

fungi grown by ants) found in nests are much of a muchness with

ones that grow wild. This was established in 1998 in a very

thorough study by Ulrich Mueller and his colleagues from the

Smithsonian Tropical Research Institute (STRI) in Panama. Hav-

ing searched ants’ nests for fungi, they took samples of 550

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ATTINES AND FUNGUS GETTING ON FAMOUSLY

cultivars; and at the same time, they gathered 300 fungi growing

wild in the vicinity. They then analysed each group in an attempt

to determine their ‘genetic distance’, a measure indicating how

long ago species diverged. By dr awing up this sort of genea-

logical tree, they hoped to be able to get back to the origin of the

lineage, the very ﬁrst basidiomycete fungus domesticated by the

very ﬁrst species of farming ant. This, however, proved to be

impossible; but what they did discover was that the cultivars were

very close to the wild varieties. This ﬁnding suggests that on

several occasions during evolution, the ants must have renewed

their stock of fungus with new wild plants, which they then

redomesticated. It even appears that one ant species introduced

into Florida in the twentieth century has already acquired a crop

cultivated by an indigenous Florida ant species.

During their study, the team from STRI also observed that

present-day ants cultivate a wide variety of fungi and that they

have contrived to change their cultivars over the millennia. It has

been established that distantly related ant lineages can cultivate

the same fungus, while two colonies of the same species living

almost next door to one another can cultivate different fungi.

However, no colony ever cultivates two different varieties of

fungus at the same time.

Once they have chosen ‘their’ fungus, the ants make a sort of

contract of ﬁdelity with it and will prevent any other variety from

growing in the vicinity. This they do by using their excrement,

according to Michael Poulsen and Jacobus Boomsma of the

University of Copenhagen, who have conducted laboratory ex-

periments to show that Acromyrmex excrete compounds which

have the effect of eliminating all foreign varieties. They have even

demonstrated that the ants sweep away any excreta containing

another fungus, thus inhibiting the development of any vegetable

competitor in their garden.

THE LIVES OF ANTS

132

nage a

trois

So fungus-growing ants have developed monoculture, a unique

achievement in the animal kingdom if we leave aside human

activity and some termites which also grow fungi in their nests.

Monocultures do, however, have a drawback: they are particu-

larly prone to parasites. But Atta ants have the answer to this

problem: they use antibiotics.

During the 1970s, entomologists working on these ants no-

ticed that the surface of their cuticle was covered in a white

powder which they thought at ﬁrst was just a secretion. However,

when it was analysed by Cameron Currie and his colleagues from

the Department of Botany at the University of Toronto, it was

found to be a ﬁlamentous bacterium of the genus Pseudonocardia.

There was nothing random in this. In fact, the micro-organism

produces an antibiotic active against Escovopsis, a fungal parasite

harmful to the ant cultivar. Thus, as part of their hygiene and to

protect their crops, all species of Atta rely on the good ofﬁces of

fungicidal micro-organisms. And when the queen ﬂies away she

also takes them with her and passes them on to her future family.

In this way, ants, fungi, and bacteria have been getting on fam-

ously together probably for a very long time, in a successful sort

of me

nage a

trois.

ATTINES AND FUNGUS GETTING ON FAMOUSLY

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Part V

Bloody Pests!

Drawing 5 Invasive ants Fire ants, taken to different continents by

human activity, can overrun and devastate their new habitat.

Stand by for invaders!

There can be no doubt about it: in their own way, ants are

geniuses. They take advantage of whatever they can use

to colonize their habitat and increase their empire; if need

be, they even demonstrate an astonishing sense of innov-

ation. Just as the attines invented agriculture,

Formica

lugubris

or some species of

Lasius

were into the raising of

livestock long before our own era. However, despite the

admiration we may feel for their behaviours and ingenu-

ity, ants are not unmitigated paragons of virtue. They can

also become nuisances, cause harm, or turn into actual

pests, for instance when they invade new territories.

Most native European species, and more generally those belong-

ing to places with temperate climates, are unobtrusive and

harmless. Admittedly, they don’t mind coming into your house,

invading kitchens or rummaging about in rubbish bins. It is

especially in spring and summer that we can see these proces-

sions of little black Lasius niger or their cousins Lasius emarginatus

with their tan thorax. If they ﬁnd too few aphids to exploit, they

will look inside houses for the sweetstuffs they need for their

brood.

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